This is one of the webpages of Libarid A. Maljian at the Department of Physics at CSLA at NJIT.
New Jersey Institute of Technology
College of Science and Liberal Arts
Department of Physics
Introductory Astronomy and Cosmology
Fall 2023
Second Examination lecture notes
Introduction to the Solar System
The Solar System is the Sun
together with the planets orbiting the Sun, moons orbiting the planets,
asteroids orbiting the Sun, comets orbiting the Sun, and other minor
objects. By far, the most important
member of the Solar system is the Sun.
However, we will for the most part ignore the Sun during this survey of
the Solar System. We will study the Sun
in tremendous detail in the context of the Sun being a star, not in the context
of the Sun being a member of the Solar System.
During this survey of the Solar System, we will concentrate our
attention on the planets, their moons, the asteroids, the comets, and the other
minor objects of the Solar System.
There are eight planets
orbiting the Sun. The eight planets
listed in the correct order from closest to the Sun are Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, and Neptune furthest from the Sun. There is remarkable symmetry among the eight
planets. In particular, the four planets
closer to the Sun all share similar physical properties, the four planets
further from the Sun all share similar physical properties, and these two
groups of planets have opposite physical properties from each other. Evidently, there are two different categories
of planets, with the four inner planets being of one category and the four
outer planets being of the other category.
The four inner planets are small, while the four outer planets are
large. The four inner planets are dense,
composed primarily of metal and rock; the four outer planets are tenuous (low
density), composed primarily of hydrogen gas and helium gas. The four inner planets have no moons or
perhaps few moons, while the four outer planets each have many moons. The four inner planets have no rings; the
four outer planets do have rings. The
four inner planets have slow rotation, while the four outer planets have fast
rotation. Again, the four inner planets
all share similar physical properties, the four outer planets all share similar
physical properties, and these two groups of planets have opposite physical
properties from each other. The four
inner planets are Mercury, Venus, Earth, and Mars, and these four planets are all classified as terrestrial planets. The word terrestrial means Earth-like, but we
must be careful with interpreting the term terrestrial (Earth-like)
planet. We do not mean that Mercury,
Venus, and Mars are exactly like the Earth.
There are many differences among all four terrestrial planets. When we say a planet is a terrestrial
(Earth-like) planet, we mean that the planet is small, dense (composed
primarily of metal and rock), with no moons or perhaps few moons, with no
rings, and with slow rotation. The four
outer planets are Jupiter, Saturn, Uranus, and Neptune, and these four planets
are all classified as jovian
planets. The word jovian means Jupiter-like, but we must be careful
with interpreting the term jovian (Jupiter-like)
planet. We do not mean that Saturn,
Uranus, and Neptune are exactly like Jupiter.
There are many differences among all four jovian planets.
When we say a planet is a jovian
(Jupiter-like) planet, we mean that the planet is large, tenuous (composed
primarily of hydrogen gas and helium gas), with many moons, with rings, and
with fast rotation. Another term for jovian planets is gas-giant
planets, since they are large and tenuous (composed primarily of gas). In summary, the four inner planets are all
terrestrial planets, while the four outer planets are all jovian, gas-giant planets.
There are further remarkable
symmetries among the eight planets of the Solar System. All eight planets orbit the Sun in nearly the
same plane. The plane of the Earth’s
orbit around the Sun is called the ecliptic
plane. All eight planets orbit the Sun
in nearly the ecliptic plane. Although
the orbital planes of the planets are actually inclined relative to the
ecliptic plane, these inclinations are nevertheless small. In other words, the entire Solar System is
very flat. All eight planets have roughly
circular (but nevertheless slightly elliptical) orbits
around the Sun. All eight planets orbit
the Sun in the same direction, counterclockwise when observed from above the
plane of the Solar System. Most of the
planets rotate in that same direction while they orbit Sun. Most of the moons of most of the planets
orbit their mother planet in that same direction. Most of the moons of most of the planets
rotate in that same direction while orbiting their mother planet in that same
direction. The Sun itself rotates in
that same direction.
To explain all of these
symmetries among the planets of the Solar System, we must discuss the formation
of the Solar System. The entire Solar
System was born roughly 4.6 billion years ago from a diffuse nebula, which is a
giant cloud of gas (many light-years across) composed mostly of hydrogen and helium and very small amounts of all the other
atoms on the Periodic Table of Elements.
The gases within a diffuse nebula are pushed by many
different forces, including thermal pressures, gravitational forces, magnetic
pressures, and even cosmic rays (ultra high-energy
particles). All these different
forces are comparable in strength with each other in interstellar space (the
space between star systems). Thus, the
gases within a diffuse nebula are pushed in seemingly random directions,
causing some parts of the diffuse nebula to be more dense than average and
other parts to be less dense (or more tenuous) than average. On occasion, a small part of a diffuse nebula
may become dense enough that its self-gravity dominates over all other
forces. Thus, a small part of an ancient
diffuse nebula collapsed from its self-gravity (under its own weight). Most of the material collapsed toward the
center, eventually forming the Sun. As
the rest of the material collapsed, collisions between particles within the
collapsing cloud became more frequent, since the gravitational collapse brought
the particles closer together. These
collisions tended to be sticky, with colliding particles merging into larger
masses. By the law of conservation of
translational (linear) momentum, the resulting larger masses had less motion
along the direction of the axis defined by the total angular momentum of the
collapsing cloud, since the collisions averaged out their more random motions
in this direction. By
the law of conservation of energy, the resulting larger masses were
heated, since sticky collisions convert a significant fraction of the kinetic
energy (moving energy) of the colliding particles into thermal energy (heat
energy). By the law of conservation of
angular momentum, the resulting larger masses had more
circular orbits, since the collisions averaged out their more random
orbits, many of which were more elliptical.
In summary, the laws of physics together caused the gravitationally
collapsing material to flatten into a circular, rotating disk perpendicular to
the axis of the total angular momentum of the forming Solar System. This disk is called
the protoplanetary disk, since the planets would eventually form from this
flat, circular, rotating disk. We see
that this nebular theory explains why the Solar System is very flat and why all
eight planets orbit the Sun in the same direction with roughly circular (but nevertheless slightly elliptical) orbits. During the first couple of hundred million
years after the formation of the protoplanetary disk, there were frequent
sticky, heat-generating collisions among particles, resulting in larger and
larger masses. This is
called the heavy bombardment period in the history of the Solar
System. The inner part of the
protoplanetary disk was closer to the Sun and therefore was more subject to the
Sun’s heat. The outer part of this
protoplanetary disk was further from the Sun and therefore was less subject to
the Sun’s heat. Since the inner part of
the protoplanetary disk was warmer, only materials with hotter melting
temperatures were able to condense, in particular metal and rock. However, most of the protoplanetary disk was
composed of hydrogen and helium; metal and rock constituted only a tiny
fraction of the protoplanetary disk.
Therefore, only these small amounts of metal and rock were able to
condense closer to the Sun. Since the
outer part of the protoplanetary disk was cooler, a greater abundance of
materials in addition to metal and rock was able to condense. We see that this nebular theory explains why
we will eventually have small, dense planets closer to the Sun and large,
tenuous planets further from the Sun.
However, these condensing objects orbiting the Sun were not yet
planets. They were planetesimals,
the technical term for baby planets. As
the planetesimals orbited the Sun, they collided with
material along their orbits, which stuck to the planetesimals. Thus, the planetesimals
grew larger through accretion, the gaining of mass through sticky
collisions. The planetesimals
grew larger and larger through accretion until the planetesimals
became so large that their self-gravity became strong enough to force
themselves into spherical shapes. These
objects were now so large with so much mass and therefore strong enough
self-gravity that they began to attract even more material around them, causing
them to grow even larger. The outer planetesimals were sufficiently large with sufficiently
strong gravity that they attracted nearby hydrogen and helium, which
constituted most of the material of the protoplanetary disk. Consequently, the outer planetesimals
grew to enormous sizes as compared with the inner planetesimals. Eventually, all the planetesimals
grew large enough that they became planets.
In summary, the nebular theory explains why all the
planets orbit the Sun in nearly the same plane (the flatness of the Solar
System), why all eight planets orbit the Sun with roughly circular (but
nevertheless slightly elliptical) orbits, why all eight planets orbit the Sun
in the same direction, why the inner planets are all terrestrial planets, and
why the outer planets are all jovian, gas-giant
planets. We will reveal further
successes this nebular theory has with explaining what we observe during this
overview of the Solar System.
The Sun is
named for Sol, the ancient Roman mythological god of the Sun (Helios in
ancient Greek mythology). We use ☉ as the
astronomical symbol for the Sun. For
example, astrophysicists use M☉ to
denote the mass of the Sun, astrophysicists use R☉ to denote the radius of the Sun, and so on and so
forth. The planet Mercury is named for the ancient Roman mythological god of
messengers (Hermes in ancient Greek mythology).
Since the planet Mercury is the closest planet to the Sun, it orbits the
Sun with the fastest speed with the shortest orbital period (in the shortest
amount of time). Mercury even appears to
move back and forth quickly across the Sun as observed in our sky. This is why the ancient Greeks and Romans
believed that this object was the god of messengers Mercury (Hermes), since a
messenger must travel quickly to deliver messages promptly. We use ☿ as the astronomical symbol for planet Mercury. The planet Venus is named
for the ancient Roman mythological goddess of love (Aphrodite in ancient Greek
mythology). We use ♀ as the astronomical symbol for planet Venus. The planet Earth is named
for Terra, the ancient Roman mythological goddess of the Earth (Gaia in ancient
Greek mythology). We use ⊕ as the
astronomical symbol for planet Earth.
For example, astrophysicists use M⊕ to
denote the mass of planet Earth, astrophysicists use R⊕ to denote the radius of planet Earth, and so on and
so forth. When spelled beginning with a
lowercase letter, the word earth means ground or dirt or soil. The Earth’s Moon is named
for Luna, the ancient Roman mythological goddess of the Moon (Selene in ancient
Greek mythology). We use ☽︎ as
the astronomical symbol for the Earth’s Moon.
The planet Mars is named for the ancient Roman
mythological god of war (Ares in ancient Greek mythology). In the modern ages after the telescope was
invented, the two moons of planet Mars were discovered. These moons were named
Phobos and Deimos after the twin mythological sons of
the god of war Mars (Ares). Phobos was the ancient mythological god of fear (a phobia
is a fear), and Deimos was the ancient mythological god of panic. We use ♂ as the astronomical symbol for planet Mars. The planet Jupiter is named
for the ancient Roman mythological king of the gods (Zeus in ancient Greek
mythology). The planet Jupiter happens
to be the largest and the most massive planet in the Solar System. Therefore, naming this planet after the ancient
mythological king of the gods seems to be appropriate. However, the only way to determine the size
of a planet is with a telescope, which was not invented
until the Modern Ages. Also, the only way to calculate the mass of a planet is
using Kepler’s third law, which again was not formulated until the Modern
Ages. So, the
fact that the ancient Romans named the planet Jupiter in the sky after their
mythological king of the gods is actually a remarkable coincidence. We use ♃ as the astronomical symbol for planet Jupiter. The planet Saturn is named
for the youngest of the ancient mythological titans (Kronos in ancient Greek
mythology). The mythological titan
Saturn (Kronos) was the father of the mythological god Jupiter (Zeus). In the Modern Ages after the telescope was invented, the planet Saturn was discovered to have many
moons. The largest moon was named Titan,
since the mythological Saturn (Kronos) was the
youngest of the mythological titans. We
use ♄ as the astronomical symbol for planet Saturn. The planets Uranus and Neptune were not discovered until the Modern Ages after the
telescope was invented. We will include
Pluto in this discussion, even though Pluto is not a planet, as we will discuss
shortly. Pluto was
also not discovered until the Modern Ages after the telescope was
invented. Nevertheless, astronomers have
decided to continue this ancient tradition of naming planets after ancient
Roman mythological gods. The planet
Uranus is named for the ancient Greek mythological god
of the sky (Caelus in ancient Roman mythology).
Note that Uranus is the only planet named for an ancient Greek
mythological god instead of an ancient Roman mythological god. The mythological god Caelus (Uranus) was the
father of the mythological titans, including Saturn (Kronos). Therefore, Caelus (Uranus) was also the
grandfather of Jupiter (Zeus). We use ⛢ as the
astronomical symbol for planet Uranus.
The planet Neptune is named for the ancient
Roman mythological god of the sea (Poseidon in ancient Greek mythology). The largest moon of planet Neptune is Triton,
named for the mythological son of the god of the sea Neptune (Poseidon). The mythological god Triton carried a
trident, a three-pronged spear. We use ♆ as the
astronomical symbol for planet Neptune.
The celestial body Pluto is named for the
ancient Roman mythological god of the underworld (Hades in ancient Greek
mythology). The largest moon of Pluto is
Charon, named for the mythological boatman who ferried souls across the
mythological river Styx to the underworld.
As an extraordinary coincidence, the wife of American astronomer James
W. Christy who discovered the moon Charon was named
Charlene, who already used the nickname Char before her husband discovered the
moon Charon!
During our survey of the Solar
System, we will discuss each of the planets extensively. Generally, we will discuss the planets in
order, from closest to the Sun to furthest from the Sun. However, we will begin with the Earth-Moon
system, which we will discuss in tremendous detail. We will discuss the Earth-Moon system in
tremendous detail since we know more about planet Earth than any other planet
in the Solar System. The reason for this
is obvious: we live on planet Earth and can therefore directly study the
Earth. After we discuss the Earth-Moon
system in tremendous detail, we will then discuss the other terrestrial planets
(Mercury, Venus, and Mars) by comparing them and contrasting them with the
Earth. We will then discuss the jovian, gas-giant planets. Finally, we will conclude our survey with a
discussion of the minor objects of the Solar System.
The Earth-Moon System
Planet Earth is the third
planet from the Sun. The solid part of
planet Earth is layered. The most dense layer
of the Earth is the core at its center. The core is composed primarily of metals, such
as iron and nickel. The next layer of
the Earth surrounding the core is the mantle, which is less dense than the
core. The mantle is composed primarily
of rock, which is itself composed primarily of silicates. Silicates are minerals composed of silicon
atoms, oxygen atoms, and other atoms chemically bonded to one another. There are also fair amounts of metals in the
mantle, such as iron. Therefore, the
mantle is actually composed of iron-rich silicate rock. The outermost layer of the Earth surrounding
the mantle is the crust. The crust is
the least dense layer of the Earth and the thinnest layer of the Earth. The crust is also composed of silicate rock,
but there are fewer metals such as iron in the crust as compared with the
mantle. Therefore, the crust is composed
of iron-poor silicate rock.
The Earth’s core is itself
layered. At the very center of the Earth
is the inner core, composed primarily of metals such as iron and nickel. The temperature of the Earth’s core is very
hot, for reasons we will discuss shortly.
These hot temperatures should be sufficient to melt metals into the
molten state. However, the pressure of the
inner core is so enormous that the metals are compressed
into the solid state even though they are at temperatures where they should be
in the molten state. Since the inner
core is composed of solid metal, the inner core is also
called the solid core. The layer
around the solid (inner) core is the outer core, which is also composed
primarily of metals such as iron and nickel.
Again, the temperature of the outer core is sufficiently hot to melt
metals into the molten state. Although
the pressure of the outer core is enormous by human standards, the pressure is
nevertheless not as high as the pressure of the inner core. Therefore, the pressure of the outer core is
not sufficient to compress metals into the solid state. The outer core is therefore molten, as metals
should be at these hot temperatures.
Since the outer core is composed of molten metal, the outer core is also called the molten core. Surrounding the molten (outer) core is the
first layer of the mantle: the mesosphere.
The Greek root meso- means middle. For example, Central America is sometimes called Mesoamerica, as in Middle America. Therefore, the word mesosphere simply means
middle sphere or middle layer. The next
layer of the mantle that surrounds the mesosphere is the asthenosphere. This layer of the mantle is composed of weak
rock. Indeed, the Greek root astheno- means weak.
The asthenosphere is composed of weak rock that is still mostly solid,
although parts of the asthenosphere are composed of rock that is partially molten.
Finally, the rest of the mantle together with the entire crust is called the lithosphere.
The lithosphere is of varying thickness.
Some parts of the lithosphere are so thick that they protrude out of the
oceans. These thick parts of the
lithosphere are called continents. The parts of the lithosphere that are thin
are the ocean basins at the bottom of the ocean.
How have geophysicists
determined the layers of the Earth, their thicknesses, their compositions, and
their physical states (solid or molten)?
Have we drilled to the center of the Earth and directly studied the interior
of the Earth? No, we have come nowhere
near drilling to the center of the Earth.
We have barely drilled through the Earth’s crust, which is by far the
thinnest layer of the Earth. We will
never have technology advanced enough to drill far beyond the crust, and
reaching the core is out of the question.
Geophysicists have determined the layers of the Earth, their
thicknesses, their compositions, and their physical states (solid or molten)
using seismic waves. We will discuss
earthquakes in more detail shortly. For
now, earthquakes cause waves that propagate (travel) throughout the entire
planet Earth. These waves are called seismic waves, and a seismometer is a device that
detects seismic waves. There are
millions of earthquakes on planet Earth every day. Most earthquakes are so weak that humans
cannot feel them, but seismometers sensitive (accurate) enough can detect
incredibly weak seismic waves.
Geophysicists have placed thousands of seismometers all over planet
Earth to detect these seismic waves.
Surface seismic waves propagate (travel) along the surface of the Earth,
while body seismic waves propagate (travel) throughout the interior of the
Earth. There are two different types of
body seismic waves: pressure waves and shear-stress waves. A pressure is a force exerted directly onto
(perpendicularly onto) an area.
Consequently, a pressure wave is a longitudinal wave that may propagate
(travel) through solids, liquids, and even gases, since all that is necessary
for a longitudinal wave to propagate is for atoms or molecules to collide with
other atoms or molecules in the direction of propagation. A shear-stress is a
force exerted along (parallel across) an area.
Consequently, a shear-stress wave is a transverse wave that can
propagate (travel) through solids but not through liquids or gases, since there
must be strong chemical bonding for atoms or molecules to pull other atoms or
molecules perpendicular to the direction of propagation. Also, pressure waves
propagate faster than shear-stress waves.
Therefore, a seismometer will always detect pressure waves first. For this reason, pressure waves are also called primary waves. A seismometer will always detect shear-stress
waves second, since they propagate slower than pressure (primary) waves. For this reason, shear-stress waves are also called secondary waves. By an amazing coincidence, the words pressure
and primary both begin with the same letter.
Therefore, these waves are also called
P-waves. By another amazing coincidence,
the words shear, stress, and secondary also all begin with the same
letter! Therefore, these waves are also called S-waves.
We can calculate how distant an earthquake occurred from a seismometer
from the arrival times of the P-waves and the S-waves. If a seismometer detects the S-waves a long
duration of time after the P-waves, the earthquake must have occurred far from
the seismometer, since the P-waves had sufficient distance to propagate
(travel) far ahead of the S-waves, resulting in a long delay between them. If the seismometer detects the S-waves
immediately after the P-waves, the earthquake must have occurred near the
seismometer, since the P-waves did not have sufficient distance to propagate
(travel) that far ahead of the S-waves, resulting in a short delay between
them. In summary, we calculate the
distance the seismic waves propagated from the arrival times of the P-waves and
the S-waves. We now have the distance of
propagation, and we certainly know the time of propagation since we know when
the earthquake occurred and when the seismometer detected the seismic
waves. We can then calculate the speed
of the seismic waves, since the speed of anything equals its distance traveled
divided by its time of travel. Once we
have the speed of the seismic waves, we can determine the properties of the
materials that would cause that speed of propagation, whether the material was
solid metal, molten metal, solid rock, or molten rock. We can program a computer with all of these data,
and the computer can calculate the layers of the Earth, their thicknesses,
their compositions, and their physical states (solid or molten) from all these
data. Also, we
are certain that the interior of the Earth is at least partially molten, since
seismometers on the opposite side of planet Earth from an earthquake do not
detect S-waves; these seismometers only detect P-waves. As we discussed, S-waves cannot propagate
through liquids; they can only propagate through solids. However, P-waves can propagate through either
solids or liquids, as we discussed. In
fact, the opposite side of planet Earth from any earthquake is
called the shadow zones of that particular earthquake. This term comes from the idea that the molten
(outer) core casts a shadow, preventing those seismometers from detecting
S-waves. In actuality, the S-waves
cannot propagate through the molten (outer) core, while the P-waves can
propagate through the molten (outer) core.
Hence, seismometers in the shadow zones only detect P-waves from
earthquakes on the opposite side of planet Earth. In summary, by using thousands of
seismometers all over planet Earth that detect seismic waves from millions of
earthquakes each day and by running computer simulations, geophysicists have
determined the layers of the Earth, their thicknesses, their compositions, and
their physical states.
It cannot be accidental or
coincidental that the solid part of the Earth is layered
according to density. Why are the inner
layers more dense while the outer layers are less
dense? To explain this layering, we must
discuss the formation of the Earth. As
we discussed, the entire Solar System is roughly 4.6 billion years old, and the
Earth (indeed all the terrestrial planets) were born as small, dense planetesimals that grew larger through accretion. Again, accretion is the gaining of mass
through sticky collisions. During a
sticky collision, a significant fraction of the kinetic energy (moving energy)
of the colliding objects is converted into thermal
energy (heat energy). Thus, objects that
suffer from sticky collisions become significantly warmer. Therefore, as the Earth was forming and
growing larger through accretion, it became warmer. Eventually, the Earth became so hot that it
became almost entirely molten. While the Earth was almost
entirely molten, more dense materials were able to sink toward the
center of the planet while less dense materials were able to rise toward the
surface of the planet. Most metals are more dense than most rocks.
That is, most rocks are less dense than most metals. Therefore, most of the metals sank toward the
center of the planet, forming the core.
Most of the rocks rose toward the surface of the planet, forming the
mantle and the crust. The process by
which any planet separates materials according to density is called
differentiation, and the planet is said to be differentiated. A planet larger than the Earth would be more
severely differentiated than the Earth, since it would have more
mass and therefore stronger gravity that would pull the metals towards
the center of the planet more strongly.
A planet smaller than the Earth would be less severely differentiated
than the Earth, since it would have less mass and therefore weaker gravity that
would pull the metals toward the center of the planet less strongly. For example, planet Mars is smaller than the
Earth. Therefore, Mars has less mass and thus weaker gravity as compared with the
Earth. Hence, Mars is
less severely differentiated as compared with the Earth. Of course, planet Mars is still
differentiated. It has a dense core
composed primarily of metals such as iron, and it has less dense outer layers
composed primarily of rock.
Nevertheless, Mars is less severely differentiated
as compared with the Earth, meaning that not all of the metals sank toward the
center of Mars to form its core. This is
also the case with the Earth. Although
most of the metals sank toward the center of the Earth to form its core, there
are still fair amounts of metals in the mantle and small amounts of metals in
the crust. Since Mars is
less severely differentiated as compared with the Earth, we find more
iron on the surface of Mars as compared with the amount of iron on the surface
of the Earth. The abundance of iron on
the surface of Mars has oxidized. Iron
oxide is commonly known as rust, which has a reddish
color. This is why Mars is red. In fact, the nickname of planet Mars is the
Red Planet. Mars even appears red to the
naked eye (without the aid of a telescope or even binoculars) in our sky.
The Earth has a magnetic
field, but we do not completely understand how this magnetic field is generated.
According to older theories, the Earth’s magnetic
field is generated by its solid (inner) core. This theory may seem reasonable, since the
solid (inner) core is composed of ferromagnetic metals such as iron and
nickel. However, we now realize that
this old model is too simplistic.
According to more modern theories, the Earth’s
magnetic field is created by its molten (outer) core. This more modern theory also seems reasonable,
since the molten (outer) core is also composed of ferromagnetic metals such as
iron and nickel. These modern theories
claim that the rotation of the Earth causes circulating currents of molten
metal in the outer core. These
circulating currents of molten metal in turn generate the Earth’s magnetic
field. These more modern theories seem
reasonable, but nevertheless these theories are not
fully developed. If these models are
correct, then the two equally important variables that create a terrestrial
planet’s magnetic field is a metallic core that is at least partially
molten and reasonably rapid rotation.
Indeed, among all of the terrestrial (inner) planets of the Solar
System, the Earth has the strongest magnetic field, since it is the only
terrestrial planet that has both a partially molten
metallic core and reasonably rapid rotation.
For example, Venus probably has a partially molten
metallic core, but Venus has very slow rotation. Hence, Venus has a very weak magnetic
field. As another example, Mars has
reasonably rapid rotation, but Mars has a metallic core that is no longer partially molten.
Hence, Mars also has a very weak magnetic field. The planet Mercury has neither a partially molten metallic core nor reasonably rapid
rotation. This may lead us to conclude
that Mercury has the weakest magnetic field among all the terrestrial
planets. However, Mercury has a modest
magnetic field, stronger than the magnetic fields of Venus and Mars. We will discuss the theory to explain
Mercury’s modest magnetic field shortly.
Nevertheless, Mercury’s magnetic field is still weak as compared with
the Earth’s magnetic field. Therefore,
even Mercury’s modest magnetic field does support the theory that a terrestrial
planet’s magnetic field is created by both a partially molten
metallic core and reasonably rapid rotation.
Caution: in this discussion, we are only comparing the magnetic fields
of the terrestrial (inner) planets.
Although the Earth has the strongest magnetic field among the
terrestrial (inner) planets, its magnetic field is still weak as compared with
all of the jovian, gas-giant
(outer) planets.
The overall structure of the
Earth’s magnetic field is very much similar to the magnetic field created by a
bar magnet, such as a refrigerator magnet.
In fact, the only difference between the Earth’s magnetic field and a
bar magnet’s magnetic field is strength.
The Earth’s magnetic field is thousands of times weaker than a bar
magnet’s magnetic field. That is, a bar
magnet’s magnetic field is thousands of times stronger than the Earth’s
magnetic field. Students often cannot
believe that a small refrigerator magnet could create a stronger magnetic field
than an entire planet, but this stands to reason actually. A refrigerator magnet’s magnetic field is
strong enough to lift paper clips for example, but the Earth’s magnetic field
is not this strong. The Earth’s magnetic
field is everywhere around us, yet we do not see paper clips floating around
us! A refrigerator magnet’s magnetic
field is strong enough to lift paper clips against planet Earth’s gravitational
field, but planet Earth’s magnetic field is not strong enough to lift paper
clips against its own gravitational field.
Hence, the Earth’s magnetic field is indeed thousands of times weaker than
a bar magnet’s magnetic field.
The Earth’s magnetic field
reverses itself once every few hundred thousand years. We do not understand how or why this
occurs. We do know that this does happen
from the magnetization of iron within rocks.
The uppermost layer of rock has its iron magnetized in the same
direction as the Earth’s magnetic field; this is called
normal polarity. However, a deeper layer
of rock has its iron magnetized in the opposite direction of the Earth’s
magnetic field; this is called reverse polarity. An even deeper layer of rock has normal
polarity again, and an even more deep layer of rock
has reverse polarity again. In other
words, the magnetization of iron within rock (the polarity) alternates from
normal to reverse and back again. The
reason for this is clear. When new rock
forms, the iron within that rock will become magnetized
in whichever direction the Earth’s magnetic field happens to point at the time
of that rock’s formation. The Earth’s
magnetic field must reverse itself periodically to cause the alternating
polarity of rock layers. Therefore, we
are certain that the Earth’s magnetic field reverses itself once every few
hundred thousand years, but again we do not understand how or why this occurs.
It is a common misconception
that the Earth’s magnetic field begins at the north pole
and ends at the south pole. This is
false for a couple of reasons. Firstly,
it is a basic law of physics that magnetic field lines are
not permitted to begin or end anywhere.
Magnetic field lines must form closed loops. The magnetic field lines of a bar magnet,
such as a refrigerator magnet for example, do not begin at the north pole of
the magnet, nor do they end at the south pole of the magnet. The magnetic field lines of a bar magnet go straight
through the magnet, coming out of its north pole, circulating around to go into
its south pole, going straight through the magnet, and coming out its north
pole again. Similarly, the magnetic
field lines of planet Earth go straight through the planet, coming out one end,
circulating around to go into the opposite end, going straight through the
planet, and coming out again. Moreover,
the Earth’s magnetic field lines do not come out from nor do they go into the
terrestrial poles (the geographical poles).
The Earth’s magnetic field lines come out from and go into the magnetic
poles, which are different from the terrestrial (geographical) poles. Indeed, a magnetic compass does not point
toward terrestrial (geographical) north as is commonly believed. A magnetic compass points toward magnetic
north, which again is different from terrestrial (geographical) north. Admittedly, the Earth’s magnetic poles are
somewhat close to the planet’s terrestrial (geographical) poles, but there is
no reason to expect that the magnetic poles should be the same or even close to
the terrestrial (geographical) poles.
The solid (inner) core is literally floating within the molten (outer)
core. Therefore, the solid (inner) core
is actually detached from the rest of the planet. Consequently, there is no reason to expect
that the solid (inner) core is rotating in precisely the same direction as the
rest of the planet. In addition, there
are circulating currents of molten metal within the molten (outer) core. In brief, the rotation of the Earth is
complicated; the entire planet does not rotate together as one solid unit. Therefore, there is no reason to expect that
the magnetic poles should be the same or even close to the terrestrial
(geographical) poles. To ask why the
Earth’s magnetic poles are different from the terrestrial (geographical) poles
is a wrong question to ask, since there is no reason to expect that the
magnetic poles should be the same as the terrestrial (geographical) poles. The correct question to ask is why are the
Earth’s magnetic poles even close to the Earth’s terrestrial (geographical)
poles. We do not understand why this is
the case. Indeed, other planets have
magnetic poles that are completely different from their geographical poles.
The solar wind is a stream of
charged particles from the Sun composed primarily of protons (hydrogen nuclei),
electrons, and alpha particles (helium nuclei).
This solar wind is capable of substantially ionizing the Earth’s atmosphere
in a fairly short amount of time. Fortunately, the Earth’s magnetic field,
although weak compared with the magnetic field of bar magnets, is sufficiently
strong to deflect most of the Sun’s solar wind.
Some of the charged particles in the solar wind do
however become trapped within the Earth’s magnetic field. These charged particles execute helical
trajectories around the Earth’s magnetic field lines. These regions of the Earth’s magnetic field are called the Van Allen belts, named for the American
physicist James Van Allen who discovered these belts in the year 1958. The charged particles within the Van Allen
belts execute helical trajectories while drifting along the Earth’s magnetic
field lines toward the magnetic poles.
The charged particles eventually collide with the molecules of the
Earth’s atmosphere, surrendering their kinetic energy by emitting light. The result is gorgeous curtains of light or
sheets of light across the sky near the Earth’s magnetic poles. This is called an
aurora. Near the Earth’s north magnetic
pole it is called aurora borealis (or more commonly the northern lights), and
near the Earth’s south magnetic pole it is called aurora australis
(or more commonly the southern lights).
If the Sun happens to be less active, its solar wind would be weaker,
the resulting aurorae would appear less spectacular, and we would only be able
to enjoy them near the Earth’s magnetic poles.
If the Sun happens to be more active, its solar wind would be stronger,
the resulting aurorae would appear more spectacular, and we would be able to
enjoy them further from the Earth’s magnetic poles. For example, the Battle of Fredericksburg in
December 1862 during the American Civil War was interrupted
by the appearance of the aurora borealis in the sky, even though
Fredericksburg, Virginia is quite far from the Earth’s north magnetic pole.
The interior of the Earth is
hot due to geothermal energy. This
geothermal energy drives geologic activity, as we will discuss shortly. The source of this geothermal energy is primarily
radioactive decay. There are certain
atoms that have an unstable nucleus, since the nucleus has too much
energy. To stabilize itself, the nucleus
will emit particles to decrease its own energy.
These atoms are called radioactive atoms, and
the emission of these particles is called radioactive decay. A certain naturally occurring fraction
(percentage) of all the atoms in the universe are radioactive. A certain fraction (percentage) of the atoms
that compose planet Earth are radioactive.
When these atoms suffer from radioactive decay, the emitted particles
are themselves a source of energy. This
is the source of the Earth’s geothermal energy.
A planet larger than the Earth would have a longer geologic lifetime
than the Earth, since it would have more mass and therefore more radioactive
atoms and hence a greater supply of internal energy. A planet smaller than the Earth would have a
shorter geologic lifetime than the Earth, since it would have less mass and
therefore less radioactive atoms and hence a smaller supply of internal
energy. Billions of years from now, most
of the radioactive atoms of planet Earth will have decayed. With very little radioactive atoms remaining
to provide geothermal energy, geologic activity will end, and the Earth will
become a geologically dead planet. This
geologic death has already occurred for other planets smaller than the
Earth. The planets Mercury and Mars for
example are geologically dead planets, since they are significantly smaller
than the Earth. However, planet Venus has
almost the same size as the Earth.
Presumably, the geologic lifetime of Venus is roughly the same as the
geologic lifetime of the Earth. Indeed,
both Venus and the Earth are currently geologically alive.
The three mechanisms by which
heat (energy) is transported from one location to
another are conduction, convection, and radiation. Conduction is the transfer of heat (energy)
from one object to another because they are in direct contact with one another. For example, our hand becomes hot when we grab
a hot object because our hand is in direct contact with the hot object when we
grab it. The heat (energy) is transferred from the hot object to our hand by
conduction, since our hand is in direct contact with the hot object. Convection is the transfer of heat (energy)
from one object to another by moving materials.
It is commonly known that hot air rises and
cold air sinks. This is not just true
for air; this is true for any gas and for any liquid as well. In physics, gases and liquids are both considered
fluids. Caution: in colloquial English,
the word fluid refers to liquids only, but in physics
the word fluid may apply to both liquids and gases. Convection is the transfer of heat (energy)
by moving fluids, since hot fluids rise and cool fluids sink. For example, a heater on the first floor of a
house also warms the second floor by convection. The heater on the first floor warms the air
on the first floor; since hot fluids rise, the hot air rises to the second
floor and warms the second floor. When
the air that has risen to the second floor loses its heat and cools, it sinks
back to the first floor, where it is warmed again by the
heater. Radiation is the transfer
of heat (energy) without direct contact and without moving fluids. Radiation typically transports heat (energy)
using electromagnetic waves, which are actually composed of photons as we
discussed earlier in the course. For
example, heat (energy) is not transported from the Sun
to the Earth by conduction, since the Earth is not in direct contact with the
Sun, thank God! Heat (energy) is not transported from the Sun to the Earth by convection,
since there is no fluid in outer space that takes the Sun’s heat (energy) and
moves it to the Earth. The Sun’s heat
(energy) is transported to the Earth across outer
space by radiation, through electromagnetic waves composed of photons.
The Theory of Plate Tectonics
is the fundamental theory of the geology of planet Earth. According to the Theory of Plate Tectonics,
the Earth’s lithosphere is fractured (or broken) into roughly
twenty pieces called tectonic plates.
The adjective tectonic is derived from a Greek
word meaning construction or building.
In other words, the Theory of Plate Tectonics states that the Earth’s
lithosphere is composed of (built from or constructed from) pieces. These tectonic plates are floating on the
asthenosphere and are being pushed very slowly by
convection cells in the asthenosphere.
Mostly solid but nevertheless weak rock in the
lower layers of the asthenosphere are heated by the Earth’s geothermal
energy. Since hot fluids rise, this hot
rock rises until it reaches a tectonic plate of the lithosphere. The risen rock then pushes the tectonic plate
in a certain direction, since the thermal energy (heat energy) of the rock is transformed into the kinetic energy (moving energy) of
the tectonic plate. Since the tectonic
plate gains kinetic energy at the expense of the thermal energy of the rock,
the rock must become cooler as it pushes the tectonic plate. Since cool fluids sink, the now cooler rock
sinks back into the asthenosphere, where it is warmed
again by the Earth’s geothermal energy.
In brief, there are circulating currents of weak rock within the
asthenosphere. These are
called convection cells. These
convection cells push the tectonic plates very slowly, a few centimeters per
year on average. According to the Theory
of Plate Tectonics, much geologic activity occurs at the boundary between two
tectonic plates. There are three
different types of tectonic plate boundaries: divergent plate boundaries,
convergent plate boundaries, and transform fault boundaries.
At a divergent plate
boundary, two tectonic plates are being pushed away
from each other. The verb to diverge
means to spread out. As the tectonic plates
are pushed away from each other, hot rock within the
asthenosphere can rise and deposit itself onto the tectonic plates. As the rock cools, it may crystallize into
solid rock. Hence, this is the youngest
part of a tectonic plate, since newborn rock forms at divergent plate
boundaries. Entire mountain ranges can be built at divergent plate boundaries. For example, the Mid-Atlantic Ridge runs
along the middle of the Atlantic Ocean and assumes the same shape as the east
coast of South America and the west coast of Africa. The South American continent is actually a
part of a much larger tectonic plate called the South American plate, and the
African continent is actually a part of a much larger tectonic plate called the
African plate. The South American plate
and the African plate are being pushed away from each
other, permitting hot rock to rise out of the asthenosphere. The hot rock has deposited onto both plates,
has cooled, and has crystallized to form the Mid-Atlantic Ridge. This is why the shape of the east coast of
South America, the west coast of Africa, and the Mid-Atlantic Ridge between
them are all similar to each other. South America and Africa were
connected to each other roughly two hundred million years ago, but they
have been pushed apart from one another, thus forming the Mid-Atlantic Ridge at
the boundary between these two tectonic plates.
North America and Europe were also connected to
each other two hundred million years ago, and they are also being pushed apart
from one another. The Atlantic Ocean is
continuously becoming wider very slowly, only a few centimeters per year. The Red Sea between the Arabian peninsula and northeastern Africa is the result of another
divergent plate boundary. The Arabian peninsula was ripped off of the continent of Africa millions
of years ago, opening up the Red Sea.
The African continent is still being ripped
apart in southeastern Africa, forming the African Rift Valleys. If we wait millions of years, the African
Rift Valleys will rip open to become a narrow sea like the Red Sea. If we wait millions of more years, that narrow sea will open even wider to become a
growing ocean like the Atlantic Ocean.
At a convergent plate
boundary, two tectonic plates are being pushed toward
each other. The verb to converge means
to come together. As the tectonic plates
are pushed toward each other, they eventually
collide. The more dense tectonic plate
will sink into the asthenosphere, while the less dense tectonic plate will rise
out of the asthenosphere. Hence, the
more dense plate will sink underneath the less dense plate. As the more dense tectonic plate sinks into
the asthenosphere, it is heated by the Earth’s
geothermal energy. Eventually, the plate
becomes so hot that it melts. Hence,
this is the oldest part of a tectonic plate, since the tectonic plate is being destroyed as it melts into molten rock. Liquids are usually less dense than
solids. Hence, this less dense molten
rock may rise and collide underneath the other tectonic plate; the molten rock
may even push through the other tectonic plate, forming active volcanoes. For example, the Nazca plate at the bottom of
the Pacific Ocean and the South American plate are being
pushed toward each other. The
Nazca plate is more dense, while the South American
plate is less dense. Thus, the Nazca
plate is sinking underneath the South American plate. As the Nazca plate sinks, it is heated by the Earth’s geothermal energy. The Nazca plate melts, and the less dense
molten rock rises. The molten rock collides
underneath and even pushes through the South American plate, forming the Andes
Mountains on the west coast of South America.
Indeed, many of the mountains in the Andes are volcanically active. The Nazca plate is an example of an oceanic
plate, since it is at the bottom of the Pacific Ocean. The South American plate is an example of a
continental plate, since it is mostly the South American continent. Hence, the Andes Mountains have formed at an
oceanic-continental convergent plate boundary.
At an oceanic-oceanic convergent plate boundary, two oceanic plates
collide. Again, the more dense plate
will sink underneath the less dense plate.
The more dense plate is heated and melts. The molten rock rises and collides underneath
and may even push through the other tectonic plate, again forming active
volcanoes. These are island volcanoes,
such as the Aleutian Islands in Alaska or the Japanese Islands or the
Philippine Islands. A subduction zone is
wherever one tectonic plate sinks underneath another tectonic plate. There are two different types of subduction
zones: oceanic-continental convergence and oceanic-oceanic convergence. However, continental-continental convergence
does not result in a subduction zone, since continental plates are too thick to
permit one plate to sink underneath the other plate. If two continental plates collide and if
convection cells in the asthenosphere continue to push them toward each other,
the rock of both plates becomes folded. If we apply a large force to rock in a short duration of time, we will fault (break) the rock. However, if we apply a small force to rock
over a long duration of time, we will bend or deform
or warp the rock. The technical term for
this bending or deforming or warping is folding. If we apply a small force to rock over a long
duration of time, we will fold the rock.
Hence, two colliding continental plates become folded. The rocks are folded upward forming
non-volcanic mountains, since there is no subduction. The technical term for these non-volcanic
mountains is fold mountains. For example, millions of years ago the Indian
plate was near the South Pole. The
Indian plate was pushed northward and closed up an ancient
ocean that no longer exists. The
Indian plate eventually collided with the Eurasian plate. As the plates continue to be
pushed toward each other, they have folded upward forming the Himalayas,
the tallest mountains in the world. The
Himalayas continue to grow taller, although very slowly, as the Indian plate
and the Eurasian plate continue to be pushed toward each other. Mount Everest is in the Himalayas, is the
tallest mountain in the world, and is still growing taller, although only a few
centimeters per year. The Appalachian
Mountains are another example of fold mountains. Two hundred million years ago, the North
American plate and the Eurasian plate collided and folded upward, forming the
Appalachian Mountains. The Appalachian
Mountains were the tallest mountains in the world roughly two hundred million
years ago. However, over the past two
hundred million years, the North American plate and the Eurasian plate have
diverged from one another, and thus the Appalachian Mountains are no longer
growing taller. In fact, the Appalachian
Mountains are continuously becoming shorter as natural forces such as wind and
rain degrade (weaken and destroy) their rocks, ultimately breaking them down
into sediment and moving the sediment to other landscapes. The Ural Mountains are also fold mountains.
Hundreds of millions of years ago, Europe and Asia were two separate
continents. They collided, forming the
Ural Mountains. In summary, there are
three different types of convergent plate boundaries: oceanic-continental
convergence, oceanic-oceanic convergence, and continental-continental
convergence. At oceanic-continental
convergence, we have volcanoes on a continent such as the Andes Mountains. At oceanic-oceanic convergence, we have
island volcanoes such as the Aleutian Islands, the Japanese Islands, and the
Philippine Islands. These two cases are
subduction zones. However, at
continental-continental convergence, we do not have subduction; we have fold mountains such as the Himalayas, the Appalachians, or
the Urals.
At a transform fault boundary,
two tectonic plates slide across each other.
Tectonic plates are not smooth, since they are composed of rock. Hence, the sliding of the tectonic plates
across each other causes seismic activity, such as weak vibrations. On occasion, the two tectonic plates become
stuck, but the convection cells in the asthenosphere continue to try to push
the plates across each other. It may
take years or even decades, but eventually the plates become unstuck and slide
across each other by a tremendous amount, perhaps several meters. An enormous amount of energy is liberated from years or even decades of accumulated
stored energy within the rock, thus causing intense vibrations. This powerful type of seismic activity is called an earthquake.
For example, the San Andreas Fault is the boundary between the North
American plate and the Pacific plate.
The North American plate is being pushed south,
while the Pacific plate is being pushed north.
The sliding of these two tectonic plates across each other causes seismic
activity, including earthquakes. This
explains why there is an abundance of seismic activity, including earthquakes,
in California, since it is on the west coast of North America at the boundary
between the North American plate and the Pacific plate. This also explains why the shape of Baja
California fits into the shape of Mexico.
Millions of years ago, Baja California was connected
to Mexico. As these two tectonic plates
were pushed in two opposite directions, Baja California was ripped off of the Mexican mainland, opening up the Sea of
Cortez. If we wait millions of years,
all of Baja California together with upper California
(the State of California) will be ripped off of the North American mainland
forming an enormous island off the west coast of North America. Geologists have given this future island a
name even though it has not yet been born; it is called
the Island of California. The Island of
California will then continue to move north as the Pacific plate continues to be pushed north.
Most geologic activity occurs
at ocean basins (at the bottom of the ocean), simply because most of the world
is covered with ocean. For example, most
seismic activity such as earthquakes occurs at the bottom of the ocean. Most igneous activity such as volcanic
eruptions occurs at the bottom of the ocean.
Most mountains are at the bottom of the ocean. Even most landslides occur at the bottom of
the ocean. The San Andreas Fault is
actually an example of a continental-continental transform fault boundary. The Queen Charlotte Fault off the west coast
of Canada is an example of an oceanic-oceanic transform fault boundary that
causes earthquakes at the bottom of the Pacific Ocean and near the west coast
of Canada. Just as it is
popularly known among many Americans that the San Andreas Fault
continually causes earthquakes near their west coast, it is popularly known
among many Canadians that the Queen Charlotte Fault continually causes
earthquakes near their west coast.
According to the Theory of
Plate Tectonics, geologic activity does not just occur at plate
boundaries. Geologic activity also
occurs at hotspots, which are caused by mantle
plumes. A mantle plume is a rising mass
of hot rock within the asthenosphere.
The rising mantle plume eventually collides underneath a tectonic
plate. This collision can occur anywhere
underneath the tectonic plate depending on the location of the mantle
plume. The collision need not be located
at a plate boundary; the collision could even occur in the middle of a tectonic
plate. After the collision, the hot rock
may push through the tectonic plate, causing geologic activity at this location
on the tectonic plate. This is called a hotspot.
For example, there is volcanic activity in the Hawaiian Islands, but the
Hawaiian Islands are nowhere near a plate boundary; the Hawaiian Islands are in
the middle of the Pacific plate. The
volcanic activity in the Hawaiian Islands is caused by
a hotspot, which is itself caused by a mantle plume. Molten rock pushes out of the Pacific plate
at the bottom of the Pacific Ocean. The
extruding molten rock builds a volcano at the bottom of the Pacific Ocean. More extruding molten rock grows the volcano
taller and taller until it protrudes out of the Pacific Ocean. It is now a volcanic island. We all learn in primary (elementary) school
that Mount Everest in the Himalayas is the tallest mountain in the world. Actually, if we measure the height of the
Hawaiian Islands above their true base, which is at the bottom of the Pacific
Ocean, then the Hawaiian Islands are in fact the tallest mountains in the
world. As measured from their true base
at the bottom of the Pacific Ocean, the Hawaiian Islands are much taller than
Mount Everest. It is however strictly
correct to state that Mount Everest has the highest elevation above sea level.
The rocks that compose the
westernmost Hawaiian island are oldest, while the rocks that compose the
Hawaiian Islands further and further east are younger and younger. The rocks that compose the easternmost Hawaiian
island (commonly known as the Big Hawaiian Island) are youngest, and that is
the only Hawaiian island with active volcanism.
All the other volcanoes on all the other Hawaiian Islands are extinct. This suggests that the hotspot is moving
east, but in actuality the Pacific plate is moving west over the hotspot. As the Pacific plate moves west, the hotspot
pushes up a Hawaiian island with active volcanism. As the Pacific plate
continues to move west, that Hawaiian island moves west and off of the hotspot
causing its volcanoes to become extinct, while the hotspot pushes up a new
Hawaiian island with active volcanism.
As the Pacific plate continues to move west,
that Hawaiian island moves west and off of the hotspot causing its volcanoes to
become extinct, while the hotspot pushes up a new Hawaiian island, and so on
and so forth. If we wait millions of
years, the Big Hawaiian Island will move off of the
hotspot, its volcanoes will become extinct, and the hotspot will push up a new
Hawaiian island to the east of the easternmost Big Hawaiian Island. This is already beginning to occur. To the east of the easternmost Big Hawaiian
Island, there is a small active volcano at the bottom of the Pacific
Ocean. If we wait millions of years,
that active volcano will grow to become a new Hawaiian island.
Imagine a terrestrial globe
that depicts the shapes of the continents and the oceans. The Theory of Plate Tectonics has revealed
that the Earth has never looked this way before and that the Earth will never look
this way again. If the tectonic plates
are slowly but continuously being pushed by convection
cells in the asthenosphere, then the shapes of the continents and the oceans
are slowly but continuously changing. We
do not notice this occurring, but we would notice significant changes if we
wait millions of years. During some eras
of Earth’s history, the continents may have been more uniformly (evenly)
distributed around the planet. During
other eras in Earth’s history, the continents may have all
been together as one giant landmass called a supercontinent. A supercontinent is
surrounded by a superocean, the unity of all
the oceans around the supercontinent.
According to the supercontinent cycle, once every roughly five hundred
million years, all the continents are together as a supercontinent surrounded
by a superocean.
If planet Earth is roughly 4.6 billion years old and if there is a
supercontinent every roughly five hundred million years, this means that there
have been roughly nine or ten supercontinents thus far in Earth’s history. As we will discuss later in the course, in
roughly five billion years the Sun will swell to become a red giant, thus
destroying all four of the terrestrial (inner) planets, including the
Earth. If the Earth will be destroyed in
roughly five billion years and if there is a supercontinent every roughly five
hundred million years, then there will be roughly ten supercontinents in the
Earth’s future. Therefore, over the
entire history of planet Earth, there have been and there will be very roughly
twenty supercontinents. Many students
believe that we happen to be alive when the continents are uniformly (evenly)
distributed around the planet, but this is not the case. The continents are presently crowded together
on the opposite side of planet Earth from the Pacific Ocean, which is by far
the largest ocean in the entire world.
The continents are also presently more crowded toward the northern
hemisphere as compared with the southern hemisphere. Therefore, we happen to be alive not when the
continents are more uniformly (evenly) distributed; we happen to be alive when
the continents are rather crowded together.
This is because rather recently, roughly two hundred million years ago, all
the continents were together in one giant supercontinent called Pangaea
surrounded by a superocean called Panthalassa. It is a common misconception that when the
Earth formed, the continents were all together as Pangaea and have been
spreading apart ever since. This is false. Again, there have been roughly ten
supercontinents in Earth’s history thus far, and there will be another roughly
ten supercontinents in the Earth’s future.
Not only was Pangaea not the first supercontinent in Earth’s history, Pangaea
was in fact the most recent supercontinent in Earth’s history. There were several other supercontinents
before Pangea ever formed, and there will be several other supercontinents in
the Earth’s future.
The Theory of Plate Tectonics
has explained seismic activity, such as earthquakes in California. The Theory of Plate Tectonics has explained subductive igneous activity, such as the volcanic activity
in the Andes Mountains, the Aleutian Islands, the Japanese Islands, and the
Philippine Islands. The Theory of Plate
Tectonics has explained the formation of non-volcanic mountains, such as the
Himalayas, the Appalachians, and the Urals.
The Theory of Plate Tectonics has explained why the continents fit
together like a jigsaw puzzle, such as the east coast of South America fitting
into the west coast of Africa and the Arabian peninsula
fitting into northeastern Africa and Baja California fitting into Mexico. The Theory of Plate Tectonics has even
explained the igneous activity of the Hawaiian Islands. The Theory of Plate Tectonics is truly the
fundamental theory of the geology of planet Earth. Caution: the Theory of Plate Tectonics has
not explained all of the geology of planet Earth; there are still many examples
of geologic activity that geologists do not completely understand. Nevertheless, whenever a geologist tries to
understand any geologic activity on planet Earth, the geologist does so within
the context of the Theory of Plate Tectonics.
During eras of Earth’s
history when there was a supercontinent at the terrestrial equator, the climate
of the entire planet would be warm, since the terrestrial equator is warm
throughout the entire year. During other
eras of Earth’s history when there were continents that were relatively
isolated at one or both terrestrial poles, the climate of the entire planet
would be cold, since the terrestrial poles are cold throughout the entire
year. An ice age is a long period of time, lasting millions of years, when the climate
of the entire planet is cold. There have
been a few ice ages throughout Earth’s history thus far. It is truly remarkable how we have just
applied the Theory of Plate Tectonics to climatology. The Theory of Plate Tectonics is the
fundamental theory of the Earth’s geology, the study of the solid part of the
Earth. We might suspect that this theory
therefore has nothing to do with meteorology and climatology, which are the
study of the Earth’s atmosphere.
However, we have just applied the Theory of Plate Tectonics to explain
ice ages, a climatological phenomenon.
Therefore, the Theory of Plate Tectonics is not just the fundamental
theory of geology of planet Earth; the Theory of Plate Tectonics is one of the
fundamental theories of all the Earth Sciences.
Although the Theory of Plate
Tectonics is the fundamental theory of the geology of planet Earth, the Theory
of Plate Tectonics is not a universal law of geology. In other words, the Theory of Plate Tectonics
does not apply to every terrestrial planet in the universe. The Theory of Plate Tectonics does not even
apply to every terrestrial planet in our own Solar System. In particular, the Theory of Plate Tectonics
is not the correct theory to explain the geologic activity on Venus. Geophysicists continue to search for the
correct theory of the geology of planet Venus.
An atmosphere is a thin layer
of gas gravitationally held to a moon, a planet, or a star. The Earth’s atmosphere is roughly eighty
percent nitrogen gas, roughly twenty percent oxygen gas, and tiny amounts of
other gases. The tiny amounts of other
gases are quite important, as we will discuss shortly. Every second of every day of our lives, we
are breathing mostly nitrogen gas (roughly eighty percent) and a fair amount of
oxygen gas (roughly twenty percent).
This roughly twenty-percent abundance of oxygen gas is an enormous
fraction; other planets have nowhere nearly this much oxygen gas in their
atmospheres. Other planetary atmospheres
have only tiny amounts of oxygen gas with a large abundance of carbon dioxide
gas, as is the case with the atmospheres of planets Venus and Mars for
example. The Earth’s atmosphere has a
large fraction of oxygen gas but only a tiny amount of carbon dioxide gas. To understand why the Earth’s atmosphere is
so different from other planetary atmospheres, we must discuss the history of
the Earth’s atmosphere. When the Earth
formed roughly 4.6 billion years ago, its atmosphere was almost entirely
hydrogen gas and helium gas; this is called the
Earth’s primary atmosphere. The primary
atmosphere was almost entirely hydrogen and helium because the Earth together
with the entire Solar System was born from a nebula composed of mostly hydrogen
and helium, as we discussed. The more
massive (or heavier) an atom or molecule, the slower it moves; the less massive
(or lighter) an atom or molecule, the faster it moves. This is rather remarkable. Suppose all the air in a room is at the same
temperature, which means that all the air molecules in the room have the same
average energy. Nevertheless, the oxygen
molecules are moving slower on average since they are more massive (or
heavier), while the nitrogen molecules are moving faster on average since they
are less massive (or lighter), even though all of the air is at the same
temperature, which means both the nitrogen molecules and the oxygen molecules
have the same average energy! Hydrogen
is the least massive (lightest) atom in the entire universe, and helium is the
second least massive (second lightest) atom in the entire universe. Hydrogen and helium are so light that they
move so fast that they can escape from the Earth’s gravitational
attraction. Thus, the Earth lost its
primary atmosphere because its own gravity was too weak to hold onto hydrogen
and helium. This occurred with all four
of the terrestrial (inner) planets orbiting the Sun (Mercury, Venus, Earth, and
Mars). These four terrestrial (inner)
planets have weaker gravity since they are smaller with less mass as compared
with the four jovian, gas-giant (outer) planets
orbiting the Sun (Jupiter, Saturn, Uranus, and Neptune). These four jovian, gas-giant (outer) planets have stronger
gravity since they are larger with more mass; thus, they have retained their
primary (hydrogen and helium) atmospheres to the present day. Even the gravity of the jovian, gas-giant (outer) planets is weak compared
with the gravity of the Sun; therefore, the Sun has certainly retained its
primary (hydrogen and helium) atmosphere to the present day. To summarize, the Sun and the four jovian, gas-giant (outer) planets have retained their
primary (hydrogen and helium) atmospheres, but the four terrestrial (inner)
planets have lost their primary (hydrogen and helium) atmospheres. After the Earth lost its primary atmosphere,
this left behind a secondary atmosphere composed of an abundance of water vapor
and nitrogen gas and carbon dioxide gas.
These gases came from volcanic outgassing. Since the Earth was born almost entirely molten, volcanic eruptions everywhere on its
surface ejected not just lava but gases as well. These gases are significantly more massive
(or heavier) than hydrogen and helium.
Therefore, they did not move fast enough to escape from the Earth’s
gravitational attraction. As the Earth
cooled, the water vapor condensed into liquid water which precipitated back
down onto the planet for such a long period of time
that most of the surface of the Earth became flooded, thus forming the
oceans. At this point, the Earth may be regarded as having a normal atmosphere with an
abundance of carbon dioxide gas similar to other planets such as Venus and
Mars. However, roughly
one billion years after the Earth formed (roughly 3.6 billion years ago),
something extraordinary occurred in the oceans that to our knowledge did not
occur anywhere else in the entire universe: life appeared. The first lifeforms were primitive
unicellular microorganisms, such as bacteria and blue-green algae. Some of the carbon dioxide gas of the
atmosphere dissolved into the oceans, which these primitive lifeforms converted
to oxygen gas. Over the next roughly one
billion years, the rock at the ocean floor was oxidized by
the oxygen that these microorganisms continually synthesized. When nearly all the rock at the ocean floor was oxidized, the oxygen gas that these primitive lifeforms
continued to synthesize then began to accumulate within the oceans. Some of this accumulating oxygen gas then
dissolved back into the atmosphere.
After an additional roughly two billion years, these microorganisms actually
succeeded in extracting almost all of the carbon dioxide gas from the atmosphere,
replacing it with oxygen gas. Thus, as
of roughly 600 million years ago, planet Earth attained its tertiary atmosphere
that we enjoy to the present day: roughly eighty percent nitrogen gas, roughly
twenty percent oxygen gas, and tiny amounts of other gases. Again, the tiny amounts of other gases are
quite important, and we will discuss these trace gases shortly.
The pressure of the Earth’s
atmosphere is typically a maximum at sea level and decreases exponentially with
increasing elevation. The equation that
describes this decreasing pressure with increasing elevation is called the law
of atmospheres, but we do not need this equation to understand why this is the
case. The Earth’s gravity pulls air
downward; therefore, the air becomes thinner as we climb the atmosphere, making
the air pressure less at higher elevations.
The average air pressure at sea level is called one atmosphere of
pressure, equal to 101325 pascals
of pressure. One pascal
of pressure is one newton of force per square meter of area. This average air pressure of 101325 pascals is close enough to one
hundred thousand pascals that meteorologists have
defined another unit of air pressure: the bar.
One bar of air pressure is exactly one hundred thousand pascals of air pressure. Thus, the average air pressure at sea level
is equal to 1.01325 bars of pressure; this is also 1013.25 millibars
of pressure. When meteorologists report
the air pressure on any given day, they may report that the air pressure is
several millibars above average or several millibars below average.
A device to measure air pressure is called a
barometer. To build a primitive
barometer, we insert a long, narrow container inverted into any liquid. The air pressure will push downward onto the
liquid, thus forcing the liquid upward into the long, narrow inverted
column. If the air pressure is greater
than average, it will push downward more strongly onto the liquid, thus forcing
the liquid further upward the inverted column, making the column of liquid
taller. If the air pressure is less than
average, it will push downward less strongly onto the liquid, thus forcing the
liquid not as far upward the inverted column, making the column of liquid
shorter. Thus, by measuring the height
of the column of liquid and performing a calculation, we can determine the air
pressure that has pushed downward onto the liquid. Most barometers use the element mercury as
the liquid; at average air pressure at sea level, liquid mercury will be pushed 760 millimeters (or 29.9 inches) up the
narrow column. Thus, the average air
pressure at sea level is also equal to 760 millimeters of mercury (or 29.9
inches of mercury). When meteorologists
report the air pressure on any given day, they may report that the air pressure
is several millimeters of mercury higher than average or several millimeters of
mercury lower than average. Barometers almost always use liquid mercury because mercury is between
thirteen and fourteen times more dense than water. In other words, mercury is between thirteen
and fourteen times heavier than water; thus, gravity pulls mercury thirteen or
fourteen times more strongly than water, making the column of mercury only 760
millimeters (or 29.9 inches) tall. If a
barometer used water instead of mercury, the column of water would be between
thirteen or fourteen times taller; this would make barometers more than ten
meters (almost thirty-four feet) tall!
It is not convenient to carry such a tall device; it is much more convenient
to carry a barometer that is only 760 millimeters (or 29.9 inches) tall. This is why almost all barometers use mercury
instead of water.
Meteorologists have defined
layers of the Earth’s atmosphere based on the variation of the temperature of
the Earth’s atmosphere with elevation.
The lowest layer of the atmosphere is the troposphere. With increasing elevation, the troposphere is followed by the stratosphere, then the mesosphere, and
finally the thermosphere. Beyond the
thermosphere is the exosphere, where the air smoothly transitions from the
Earth’s atmosphere to the surrounding outer space, as we will discuss
shortly. The temperature typically cools
with increasing elevation within the troposphere, the lowest layer of the
atmosphere. However, we reach a certain
elevation at which the temperature stops becoming cooler and begins instead to
become warmer with increasing elevation.
This elevation defines the end of the troposphere and the beginning of
the stratosphere. This precise elevation
is called the tropopause. We may regard the tropopause as the boundary
between the troposphere and the stratosphere, but the tropopause is more correctly defined as the end of the
troposphere. The temperature then
typically becomes warmer with increasing elevation within the
stratosphere. The reason for this
warming is a heat source within the stratosphere that we will discuss
shortly. However, we reach a certain
elevation at which the temperature stops becoming warmer and begins instead to
become cooler with increasing elevation.
This elevation defines the end of the stratosphere and the beginning of
the mesosphere. This precise elevation is called the stratopause. We may regard the stratopause
as the boundary between the stratosphere and the mesosphere, but the stratopause is more correctly defined
as the end of the stratosphere. The
temperature then typically becomes cooler with increasing elevation within the
mesosphere. However, we reach a certain
elevation at which the temperature stops becoming cooler and begins instead to
become warmer with increasing elevation.
This elevation defines the end of the mesosphere and the beginning of
the thermosphere. This precise elevation
is called the mesopause. We may regard the mesopause
as the boundary between the mesosphere and the thermosphere, but the mesopause is more correctly defined
as the end of the mesosphere. The
temperature then typically becomes warmer with increasing elevation within the
thermosphere. The reason for this
warming is a heat source within the thermosphere that we will discuss
shortly. However, we reach a certain
elevation at which the temperature stops becoming warmer and begins instead to
become cooler with increasing elevation.
This elevation defines the end of the thermosphere and the beginning of
the exosphere. This precise elevation is called the thermopause. We may regard the thermopause
as the boundary between the thermosphere and the exosphere, but the thermopause is more correctly defined
as the end of the thermosphere. The
temperature then typically becomes cooler with increasing elevation within the
exosphere, smoothly transitioning into the very cold temperatures of the
surrounding outer space. To summarize,
the temperature typically becomes cooler with increasing elevation within the
troposphere, the mesosphere, and the exosphere, while the temperature typically
becomes warmer with increasing elevation within the stratosphere and the
thermosphere.
It may seem reasonable to ask
for the precise elevation at which the Earth’s atmosphere ends and outer space
begins, but this is in fact an ill-defined question. The concentration of gases in the Earth’s
atmosphere becomes thinner and thinner with increasing elevation until we reach
a certain elevation at which the concentration of gases matches the concentration
of gases of the surrounding outer space.
It is a common misconception that outer space is a perfect vacuum, but
this is false. There is no such thing as
a perfect vacuum; in fact, a perfect vacuum would violate the laws of
physics. In actuality, the entire
universe is filled with extremely diffuse gas. Therefore, the Earth’s atmosphere smoothly
transitions into the gases of the surrounding outer space. We may actually interpret the Earth’s atmosphere
as extending forever, filling the entire universe. The same interpretation can
be applied to all other planetary atmospheres. In other words, there is no well-defined
exopause. The tropopause is the end of
the troposphere, the stratopause is the end of the
stratosphere, the mesopause is the end of the
mesosphere, and the thermopause is the end of the
thermosphere. If there were an end of
the exosphere (which would also be the end of the entire atmosphere), that end
would be called the exopause, but there is no well-defined exopause. Nevertheless, if we insist upon a boundary
between the Earth’s atmosphere and outer space, we may arbitrarily use the
elevation of the tropopause, since the Earth’s gravity pulls most of the air in
the entire atmosphere down into the troposphere. Indeed, the vast majority of all
meteorological phenomena (commonly known as weather) occurs within the
troposphere, the lowest layer of the atmosphere. So, we may
arbitrarily regard the thickness of the Earth’s atmosphere as the elevation of
the tropopause as a rough estimate. The
elevation of the tropopause is roughly ten kilometers above sea level. Using this as a rough estimate for the
thickness of the Earth’s atmosphere, we conclude that the atmosphere is
extremely thin as compared with the average radius of the Earth, roughly 6400
kilometers. To summarize, the Earth’s
atmosphere extends indefinitely far according to strict interpretations, but
the Earth’s atmosphere is only a few kilometers thick for all practical
purposes. The Earth’s atmosphere keeps
us alive in a variety of different ways.
After we discuss all these ways the atmosphere keeps us alive, we will
be humbled. In this vast universe, we
are only able to survive within a very thin layer of air surrounding a single
planet: the atmosphere of planet Earth.
The most obvious way the
Earth’s atmosphere keeps us alive is with its abundance of oxygen. Humans and all animals must inhale oxygen gas
to survive. This is because humans and
animals must react oxygen with glucose (a simple sugar) to extract the energy
they need for their survival. Humans
ingest various sugars as well as complex carbohydrates such as bread, rice,
cereal, and pasta. Our bodies digest
complex carbohydrates as well as sugars, breaking them down into glucose (a
simple sugar). There is a tremendous
amount of energy stored within the chemical bonds of the glucose molecule,
which our bodies access by reacting it with oxygen. The human body is composed of roughly one
hundred trillion cells. Within these
cells, the following reaction occurs: glucose plus oxygen yields energy plus
carbon dioxide and water as waste products.
This reaction is called cellular respiration and is more properly
written C6H12O6 + 6 O2 → energy + 6
CO2 + 6 H2O. When we inhale, the oxygen gas that goes into
our lungs is transferred to our blood; our blood then
carries the oxygen to the trillions of cells of our bodies. The oxygen enters our cells and reacts with
glucose to yield energy. The carbon
dioxide that is produced as a waste product from the
reaction is transferred back into our blood; our blood then carries the carbon
dioxide back to our lungs, and we then exhale.
Cellular respiration not only explains why humans and animals must
inhale oxygen gas; cellular respiration also explains why humans and animals
must exhale carbon dioxide gas. Plants
inhale carbon dioxide gas to use together with water as the raw materials to
synthesize glucose. Plants use the
energy of sunlight to initiate this reaction, which is why this reaction is called photosynthesis.
The photosynthesis reaction is more properly written 6 CO2 +
6 H2O
+ energy → C6H12O6
+ 6 O2. Notice that the photosynthesis
reaction is precisely the reverse of the cellular respiration reaction. Note also that oxygen is a waste product of
this photosynthesis reaction. Plants
inhale carbon dioxide gas and exhale oxygen gas, the precise reverse of humans
and animals. Therefore, the relationship
between animals (including humans) and plants is a symbiotic relationship. Animals (including humans) exhale carbon dioxide
gas, which plants then inhale. Plants
then exhale oxygen gas, which animals (including humans) then inhale. Animals (including humans) then exhale carbon
dioxide gas, which plants then inhale, and so on and so forth.
Another way the Earth’s atmosphere
keeps us alive is by maintaining a habitable temperature for life. Based on the Earth’s distance from the Sun,
our planet should be too cold for life to exist. The temperature of our planet should be much
colder than the freezing temperature of water; not only should all the oceans
be frozen, but the continents should be frozen over as well. However, there are tiny amounts of gases
within the Earth’s atmosphere that absorb and then emit some of the heat that
our planet radiates. These gases are called greenhouse gases.
The most important greenhouse gas is water vapor; carbon dioxide gas is
a secondary greenhouse gas. These gases
absorb some of the heat that the Earth radiates, and these gases then reradiate
this heat themselves. Although these gases
reradiate some of this heat into outer space, these gases also reradiate some
of this heat back to the Earth. This
causes the temperature of planet Earth to be significantly warmer than it would
have been otherwise, based on its distance from the Sun. In fact, the Earth is
sufficiently warmed that its average surface temperature is habitable
for life. This warming is called the greenhouse effect, since it is rather like a
greenhouse that is warm even in the wintertime.
The Earth’s atmosphere is mostly nitrogen gas and oxygen gas, but
neither of these gases can absorb or radiate heat efficiently. In other words, neither nitrogen gas nor
oxygen gas are greenhouse gases. Water
vapor and carbon dioxide gas are able to absorb and radiate heat efficiently. The tiny amounts of water vapor and carbon
dioxide in the Earth’s atmosphere warm the planet to a habitable
temperature. This is a second way the
Earth’s atmosphere keeps us alive.
The Sun not only radiates
visible light; the Sun radiates all forms of electromagnetic radiation. As we discussed earlier in the course, the
Electromagnetic Spectrum is a list of all the different types of
electromagnetic waves in order as determined by either the frequency or the wavelength. Starting with the lowest frequencies (which
are also the longest wavelengths), we have radio waves, microwaves, infrared,
visible light (the only type of electromagnetic wave our eyes can see),
ultraviolet, X-rays, and gamma rays at the highest frequencies (which are also
the shortest wavelengths). The visible
part of the Electromagnetic Spectrum is actually quite narrow. Nevertheless, the visible part of the
Electromagnetic Spectrum can be subdivided.
In order, the subcategories of the visible part of the Electromagnetic
Spectrum starting at the lowest frequency (which is also the longest
wavelength) are red, orange, yellow, green, blue, indigo, and violet at the
highest frequency (which is also the shortest wavelength). As we discussed earlier in the course, we now
realize why electromagnetic waves just before visible light are
called infrared, since their frequencies (or wavelengths) are just
beyond red visible light. In other
words, infrared light is more red than red!
We also realize why electromagnetic waves just after visible light are called ultraviolet, since their frequencies (or
wavelengths) are just beyond violet visible light. In other words, ultraviolet light is more
purple than purple! The Sun radiates all
of these electromagnetic waves. For
example, the near ultraviolet from the Sun causes suntans, and too much near
ultraviolet from the Sun causes sunburns.
The far ultraviolet has even more energy, and the Sun radiates
sufficient far ultraviolet that we should be killed from its far
ultraviolet radiation. X-rays
have even greater energy; thus, the X-rays from the Sun should kill us in a fairly short amount of time.
Something must be shielding us from the Sun’s far ultraviolet and from
the Sun’s X-rays. Our atmosphere
provides these shields. The symbol for
the oxygen atom is O. Under normal
temperatures and pressures, the oxygen atom will never remain by itself; it
will always chemically bond to another atom.
If there are no other atoms nearby, the oxygen atom will chemically bond
to another oxygen atom. Two oxygen atoms
chemically bonded to each other is called the oxygen
molecule, which is written O2. Molecular oxygen is also known as normal
oxygen, since oxygen is almost always in this state
under normal temperatures and pressures.
Roughly twenty percent of the Earth’s atmosphere is molecular (normal)
oxygen for example, and this is the form of oxygen that
plants exhale as well as the form humans and all animals must
inhale. Notice this is the form of
oxygen appearing in the cellular respiration reaction and the photosynthesis
reaction written above. Whenever we use
the simple word oxygen, we are not being clear.
Do we mean atomic oxygen O or do we mean molecular
oxygen O2? We probably mean molecular oxygen, since this
is normal oxygen. If molecular oxygen
absorbs far ultraviolet, a chemical reaction will
synthesize a strange form of oxygen: three oxygen atoms chemically bonded to
each other. This strange form of oxygen is written O3 and is
called ozone. The synthesis of ozone is
more properly written 3 O2 + energy → 2
O3. Ozone is
toxic, since inhaling O3 causes severe
respiratory problems. This is ironic,
since ozone also keeps us alive. The
molecular oxygen in the Earth’s atmosphere absorbs the far ultraviolet from the
Sun, synthesizing ozone. Therefore, the
far ultraviolet from the Sun never reaches the surface of the Earth, since it is absorbed by molecular oxygen to synthesize ozone. In fact, there is a layer of ozone in the
stratosphere below which far ultraviolet does not penetrate. This layer is commonly
known as the ozone layer, but it is more correctly called the
ozonosphere. The ozonosphere is the heat
source within the stratosphere that is responsible for warming temperatures
with increasing elevation within that atmospheric layer. Much higher in the atmosphere within the
thermosphere, various atoms and molecules absorb X-rays from the Sun. X-rays have so much energy that absorbing them
strips electrons completely free from an atom or molecule. In other words, atoms or
molecules are ionized by X-rays.
Therefore, the X-rays from the Sun never reach the surface of the Earth,
since they are absorbed by atoms and molecules to synthesize
ionized atoms and molecules. In fact, there is a layer of ionized atoms and molecules in the
thermosphere below which X-rays do not penetrate. This layer is called
the ionosphere, and it is the heat source within the thermosphere that is
responsible for warming temperatures with increasing elevation within that
atmospheric layer.
Let us summarize all the ways
the Earth’s atmosphere keeps us alive.
Firstly, humans and animals would not have oxygen to react with glucose
to extract energy for their survival if unicellular microorganisms did not
remove most of the carbon dioxide gas from the atmosphere, replacing it with
molecular oxygen. Secondly, planet Earth
would be too cold for life to exist without the presence of greenhouse gases
that make the planet warm enough to be habitable for life. Thirdly, life on Earth would
be killed from the far ultraviolet from the Sun if it were not for the
ozonosphere. Fourthly, life on Earth would be killed from the X-rays from the Sun if it were not
for the ionosphere. We also discussed
that the atmosphere would be substantially ionized by
the Sun’s solar wind without the Earth’s magnetic field deflecting most of
these charged particles from the Sun that continually bombard our planet. This is a fifth way our
planet keeps us alive. If only
one of these were the case, we would not be here from the lack of the other
four. If two were the case, we would not
be here from the lack of the other three.
If three were the case, we would not be here from the lack of the other
two. If four were the case, we would not
be here from the lack of the remaining one.
The fact that all five of these are the case on the same planet is truly
miraculous. Again, we are humbled. In this vast universe, we are only able to
survive within a very thin layer of air surrounding a single planet: the
atmosphere of planet Earth.
The surface temperature of
the Earth causes the Earth to radiate heat from its surface. The explains why the troposphere becomes
cooler with increasing elevation; as we climb the troposphere, we are further
and further from the surface of the Earth and thus further and further from
this source of heat. Air in the lower
troposphere (near sea level) is often heated by the
Earth’s surface. Since hot fluids rise,
this hot air may rise to the upper troposphere (near the tropopause). This rising air may cool. Since cool air sinks, air in the upper
troposphere (near the tropopause) may sink to the lower troposphere (near sea
level), where it may be warmed again thus causing it to rise again. In summary, there is significant convection
within the troposphere caused by continually circulating air within the
troposphere. This convection
(circulation) of air within the troposphere is ultimately responsible for
meteorological phenomena (commonly known as weather). This explains why the vast majority of all
meteorological phenomena (weather) occurs within the troposphere, the lowest
layer of the atmosphere. This also
explains why the lowest layer of the Earth’s atmosphere is
called the troposphere, since the Greek root tropo- means turning. The ozonosphere is in the upper stratosphere,
near the stratopause.
This explains why the temperature warms as we climb the
stratosphere. After passing the
tropopause, we approach the ozonosphere, which serves as a heat source, thus
causing warming temperatures. After
passing the stratopause, we are further and further
from the ozonosphere. This explains why
the temperature cools as we climb the mesosphere. Note that cooler air resides in the lower
stratosphere while warmer air resides in the upper stratosphere. Since cool fluids do not rise, the cool air
in the lower stratosphere does not rise to the upper stratosphere. Conversely, since warm fluids do not sink,
the warm air in the upper stratosphere does not sink to the lower
stratosphere. Therefore, there is no
convection (circulation) of air within the stratosphere. The air in the stratosphere remains layered
based on temperature. This explains why
this layer of the atmosphere is called the
stratosphere, since the air is stratified or layered. The word stratify is derived from a Latin
word meaning layer. The air in the
mesosphere cools as we climb the mesosphere, just as air in the troposphere
cools as we climb the troposphere. This
may lead us to conclude that there is significant convection (circulation) of
air within the mesosphere just as in the troposphere, thus causing an abundance
of meteorological phenomena (weather) within the mesosphere. However, the Earth’s gravity pulls most of
the air in entire atmosphere down into the troposphere. Although there is convection (circulation) of
air within the mesosphere, the air within this layer is too thin for this
convection (circulation) to result in an abundance of meteorological phenomena
(weather) within the mesosphere. The
Greek root meso- means middle. For example, Central America is sometimes called Mesoamerica, as in Middle America. Therefore, the word mesosphere simply means
middle sphere or middle layer. The
ionosphere is in the upper thermosphere, near the thermopause. This explains why the temperature warms as we
climb the thermosphere. After passing
the mesopause, we approach the ionosphere, which
serves as a heat source, thus causing warming temperatures. After passing the thermopause,
we are further and further from the ionosphere.
This explains why the temperature cools as we climb the exosphere. Note that cooler air resides in the lower
thermosphere while warmer air resides in the upper thermosphere. Since cool fluids do not rise, the cool air
in the lower thermosphere does not rise to the upper thermosphere. Conversely, since warm fluids do not sink,
the warm air in the upper thermosphere does not sink to the lower
thermosphere. Therefore, there is no convection
(circulation) of air within the thermosphere.
The air in the thermosphere remains layered based on temperature. This is similar to the air in the
stratosphere, but note that the air in the thermosphere is much thinner than
the air in the stratosphere, since the Earth’s gravity pulls air downward. As we climb the exosphere, the air becomes
cooler and cooler, smoothly transitioning into the very cold temperatures of
the surrounding outer space. The Greek
root exo- means outside or external. For example, an exoskeleton is a skeleton
that is outside (surrounding) an organism.
Therefore, the word exosphere simply means external sphere or external
layer. The exosphere is a layer of gas
that is not strictly part of the Earth’s atmosphere but is outside
(surrounding) the Earth’s atmosphere that smoothly transitions into the gas of
the surrounding outer space.
All of us have a basic
understanding of the seasons: it is warmer in summertime and colder in
wintertime. It is a
common misconception that the seasons occur because of the varying
distance of planet Earth from the Sun. Supposedly when our planet Earth is closer to the Sun, it is
warmer causing summertime, and supposedly when our planet Earth is further from
the Sun, it is colder causing wintertime.
This argument seems reasonable, but it is completely wrong. The orbit of the Earth around the Sun is
almost a perfect circle, meaning that the Earth is roughly the same distance
from the Sun throughout the entire year.
Of course, the true shape of the Earth’s orbit around the Sun is an
ellipse; sometimes the Earth is closer to the Sun than average, and other times
the Earth is further from the Sun than average.
However, the eccentricity of the Earth’s elliptical orbit is so close to
zero that the orbit is almost a perfect circle.
The eccentricity of an ellipse quantifies the elongation of the
ellipse. When the eccentricity is zero,
the ellipse is a perfect circle, as we discussed earlier in the course. When the eccentricity is close to zero, the ellipse
is almost a perfect circle. The
eccentricity of the Earth’s elliptical orbit around the Sun is so close to zero
that its orbit is almost a perfect circle.
When the Earth is at perihelion (closest to the Sun), it is roughly 2.5
million kilometers (roughly 1.5 million miles) closer to the Sun than
average. When the Earth is at aphelion
(furthest from the Sun), it is roughly 2.5 million kilometers (roughly 1.5
million miles) further from the Sun than average. These closer or further distances may seem
large, but the Earth is on average roughly 150 million kilometers (roughly 93
million miles) from the Sun. Therefore,
these closer or further distances are less than two-percent variations from the
average distance between the Earth and the Sun, and this is not enough of a
difference to cause the seasons. There
is a spectacular piece of evidence that will forever bury the misconception
that the varying distance of the Earth from the Sun causes the seasons: the
Earth is closest to the Sun in wintertime and furthest from the Sun in
summertime! The Earth is at its
perihelion on roughly January 03rd every year, but
early January is in wintertime! The
Earth is at its aphelion on roughly July 03rd every
year, but early July is in summertime!
Therefore, it is absolutely not the Earth’s
varying distance from the Sun that causes the seasons. Caution: we do not argue that distance from
the Sun is completely irrelevant.
Obviously, if we were to move the Earth fifty million kilometers closer
to the Sun, of course the planet would become so hot that it would no longer be
habitable for life (all life, including all of us, would die). Obviously, if we were to move the Earth fifty
million kilometers further from the Sun, of course the planet would become so
cold that it would no longer be habitable for life (all life, including all of
us, would die). However, if we were to
move the Earth only a couple million kilometers closer to or further from the
Sun, this would not be enough to affect the Earth’s average temperature. The proof of this assertion is that this
already occurs; every year as the Earth orbits the Sun on its elliptical orbit,
the Earth moves roughly 2.5 million kilometers closer to the Sun at perihelion
and roughly 2.5 million kilometers further from the Sun at aphelion, and these
variations do not affect the average temperature of the planet. In fact, planet Earth is closest to the Sun
in wintertime and furthest from the Sun in summertime!
After discussing in
tremendous detail what does not cause the seasons, we must finally discuss what
does cause the seasons. The Earth’s rotational
axis is tilted from the vertical, the vertical being defined as perpendicular
to the ecliptic plane (the plane of the Earth’s orbit around the Sun). The tilt of any planet’s rotational axis is called the obliquity of the planet. The seasons are caused
by the Earth’s obliquity, the tilt of its rotational axis. The obliquity of planet Earth is roughly 23½
degrees. As the Earth orbits the Sun,
this 23½ degrees of obliquity remains fixed to an excellent approximation. Thus, as the Earth orbits the Sun, sometimes
the Earth’s northern hemisphere will be tilted toward
the Sun, causing that hemisphere to receive more direct sunlight thus causing
warmer summertime. The
warmer summertime is also caused by daytime being longer than nighttime, as we
will discuss shortly. At the same
time the Earth’s northern hemisphere is tilted toward
the Sun, the Earth’s southern hemisphere is tilted away from the Sun, causing
that hemisphere to receive less direct sunlight thus causing colder
wintertime. The colder
wintertime is also caused by nighttime being longer than daytime, as we will
discuss shortly. Six months later
when the Earth is on the opposite side of its orbit, the Earth’s northern
hemisphere will be tilted away from the Sun, causing that hemisphere to receive
less direct sunlight and causing that hemisphere to have longer nighttime than
daytime, thus causing colder wintertime.
At the same time the Earth’s northern hemisphere is tilted away from the
Sun, the Earth’s southern hemisphere is tilted toward the Sun, causing that
hemisphere to receive more direct sunlight and causing that hemisphere to have
longer daytime than nighttime, thus causing warmer summertime. This is remarkable; the seasons are reversed in the two hemispheres at the same time! As another example, when it is springtime in
the northern hemisphere, it is autumntime in the
southern hemisphere at the same time.
This means that the Earth is at perihelion (closest to the Sun) during
the southern hemisphere’s summertime, and the Earth is at aphelion (furthest
from the Sun) during the southern hemisphere’s wintertime. We may be tempted to conclude that the
southern hemisphere’s summertime is especially hot, and the southern
hemisphere’s wintertime is especially cold.
This is false; the opposite is true!
Summers are typically hotter in the northern hemisphere as compared with
summers in the southern hemisphere, and winters are typically colder in the
northern hemisphere as compared with winters in the southern hemisphere! In other words, both summers and winters are more mild in the southern hemisphere as compared with the
northern hemisphere, where both summers and winters are more severe. This is because the continents are presently
somewhat crowded together in the northern hemisphere, making the southern
hemisphere mostly covered with ocean (water).
Water has a large heat capacity, meaning that it is difficult to change
the temperature of water. Therefore, the
abundance of water in the southern hemisphere stabilizes temperatures, causing
smaller temperature variations in the southern hemisphere as compared with
larger temperature variations in the northern hemisphere. This spectacularly emphasizes that variations
in the distance from the Sun do not determine seasonal temperatures. Again, summers are more mild (less hot) in
the southern hemisphere, even though the Earth is closest to the Sun during
summertime in the southern hemisphere, while summers are more severe (more hot)
in the northern hemisphere, even though the Earth is furthest from the Sun
during summertime in the northern hemisphere!
Similarly, winters are more mild (less cold) in the southern hemisphere,
even though the Earth is furthest from the Sun during wintertime in the southern
hemisphere, while winters are more severe (more cold) in the northern
hemisphere, even though the Earth is closest to the Sun during wintertime in
the northern hemisphere! The large heat
capacity of liquid water is also responsible for moderating the temperature
difference between daytime and nighttime on our planet Earth. The daytime side of any planet faces toward
the Sun, while the nighttime side of any planet faces away from the Sun. For most planets, nighttime is much colder
than daytime, but the nighttime side of planet Earth is only slightly cooler
than its daytime side, thanks to the stabilizing effect of all the water that
covers most of the planet.
Extraordinarily, the nighttime side of planet Earth may at times become
warmer than the daytime side depending upon weather patterns. As another example of how water stabilizes
temperatures on our planet Earth, other planets have north poles and south
poles that are much colder than their equators.
Although the Earth’s poles are cold and the Earth’s equator is hot by
human standards, the difference in temperature is nevertheless moderate as
compared with other planets. Without the
abundance of water that covers our planet Earth, our poles would be too cold
and our equator would be too hot to be habitable for life.
The moment when the Earth’s
northern hemisphere is tilted the most toward the Sun
is called the summer solstice. This
occurs on average June 21st every year; some years it could occur one or two
days earlier, while other years it could occur one or two days later. Since the northern hemisphere is tilted the most toward the Sun on the summer solstice,
the Sun appears to be highest in the sky, since the northern hemisphere
receives the most direct sunrays. This
is also the longest daytime and the shortest nighttime of the year in the
northern hemisphere. The precise
duration of daytime and nighttime depends upon our latitude. At the midlatitudes,
there are roughly fifteen hours of daytime and only roughly nine hours of
nighttime on the summer solstice. Note
that the sum of fifteen hours and nine hours is twenty-four hours. Six months later when the Earth is on the
other side of its orbit around the Sun, there is a moment when the Earth’s
northern hemisphere is tilted the most away from the
Sun. This moment is called the winter
solstice, occurring on average December 21st every year; some years it could
occur one or two days earlier, while other years it could occur one or two days
later. Since the northern hemisphere is tilted the most away from the Sun on the winter solstice,
the Sun appears to be lowest in the sky, since the northern hemisphere receives
the least direct sunrays. This is also
the longest nighttime and the shortest daytime of the year in the northern
hemisphere. The precise duration of
nighttime and daytime depends upon our latitude, but it will always be the
reverse of the summer solstice. At the midlatitudes for example, there are roughly fifteen hours
of nighttime and only roughly nine hours of daytime on the winter
solstice. Note again that the sum of
fifteen hours and nine hours is twenty-four hours. Halfway in between the solstices are two
other moments called the equinoxes when the Earth’s axis is not tilted toward
or away from the Sun, resulting in equal amounts of daytime and nighttime
(twelve hours each). This is why they are called equinoxes, since there are equal amounts of
daytime and nighttime. Roughly three
months after the summer solstice (roughly three months before the winter
solstice) is the autumn equinox, occurring on average September 21st every
year; some years it could occur one or two days earlier, while other years it
could occur one or two days later. Roughly three months after the winter solstice (roughly
three months before the summer solstice) is the vernal equinox (or the spring
equinox). The vernal equinox (spring
equinox) occurs on average March 21st every year; some years it could occur one
or two days earlier, while other years it could occur one or two days later. It is a common misconception that since every
day is twenty-four hours, supposedly every day has twelve hours of daytime and
twelve hours of nighttime. This is false
for almost every day the entire year. In
fact, there are only two days of the entire year when this is the case: the
equinoxes. Once we pass the vernal
equinox (spring equinox), every day for the next six months there is more
daytime than nighttime, with maximum daytime on the summer solstice. Once we pass the autumn equinox, every day
for the next six months there is more nighttime than daytime, with maximum
nighttime on the winter solstice.
The term summer solstice has
three different yet interrelated meanings.
We may interpret the summer solstice as the location on the Earth’s
orbit where its northern hemisphere is tilted the most
toward the Sun. We may also interpret
the summer solstice as the moment in time when the Earth’s northern hemisphere
is tilted the most toward the Sun, occurring on average June 21st
every year. Thirdly, we may interpret
the summer solstice as the point on the Celestial Sphere (the point on the sky)
where the ecliptic has a positive maximum declination of +23½°,
as we discussed earlier in the course.
Obviously, the Earth is actually located at its orbital summer solstice
and the Sun appears to be located at the sky’s summer solstice at the moment of the temporal summer solstice. The terms winter solstice, vernal equinox,
and autumn equinox have similar interpretations. The term winter solstice has three different
yet interrelated meanings. We may
interpret the winter solstice as the location on the Earth’s orbit where its
northern hemisphere is tilted the most away from the
Sun. We may also interpret the winter
solstice as the moment in time when the Earth’s northern hemisphere is tilted
the most away from the Sun, occurring on average December 21st
every year. Thirdly, we may interpret
the winter solstice as the point on the Celestial Sphere (the point on the sky)
where the ecliptic has a negative maximum declination of −23½°, as we
discussed earlier in the course.
Obviously, the Earth is actually located at its orbital winter solstice
and the Sun appears to be located at the sky’s winter solstice at the moment of the temporal winter solstice. The term vernal equinox (spring equinox) has
three different yet interrelated meanings.
We may interpret the vernal equinox (spring equinox) as the location on
the Earth’s orbit where its axis is not tilted toward
or away from the Sun as it journeys from the orbital winter solstice toward the
orbital summer solstice. We may also
interpret the vernal equinox (spring equinox) as the moment in time occurring
on average March 21st every year when the Earth’s axis
is not tilted toward or away from the Sun after the temporal winter solstice
but before the temporal summer solstice.
Thirdly, we may interpret the vernal equinox (spring equinox) as the
point on the Celestial Sphere (the point on the sky) where the ecliptic
intersects the Celestial Equator and where the Sun appears to have an
increasing declination, as we discussed earlier in the course. Obviously, the Earth is actually located at
its orbital vernal equinox and the Sun appears to be located at the sky’s
vernal equinox at the moment of the temporal vernal
equinox. Finally, the term autumn
equinox has three different yet interrelated meanings. We may interpret the autumn equinox as the
location on the Earth’s orbit where its axis is not tilted
toward or away from the Sun as it journeys from the orbital summer solstice
toward the orbital winter solstice. We
may also interpret the autumn equinox as the moment in time occurring on
average September 21st every year when the Earth’s
axis is not tilted toward or away from the Sun after the temporal summer
solstice but before the temporal winter solstice. Thirdly, we may interpret the autumn equinox
as the point on the Celestial Sphere (the point on the sky) where the ecliptic
intersects the Celestial Equator and where the Sun appears to have a decreasing
declination, as we discussed earlier in the course. Obviously, the Earth is actually located at
its orbital autumn equinox and the Sun appears to be located at the sky’s
autumn equinox at the moment of the temporal autumn
equinox.
As we discussed earlier in
the course, the latitude of any location on planet Earth is
defined as its angle from the terrestrial equator, whether north or
south. The colatitude of any location on
planet Earth is defined as its angle from the north
terrestrial pole. Since there are ninety
degrees of latitude from the terrestrial equator to the north terrestrial pole,
this makes the colatitude equal to ninety degrees minus the latitude. For example, if our latitude is ten degrees
north, this means we are ten degrees of latitude north of the terrestrial equator,
making us eighty degrees from the north terrestrial pole; therefore, our
colatitude is eighty degrees. Indeed,
ninety minus ten equals eighty. As
another example, if our latitude is seventy degrees north, this means we are
seventy degrees of latitude north of the terrestrial equator, making us twenty
degrees from the north terrestrial pole; therefore, our colatitude is twenty
degrees. Indeed, ninety minus seventy
equals twenty. The only location on
planet Earth where our latitude and our colatitude equal the same number is at
forty-five degrees north latitude, since that would place us halfway between
(equidistant from) the terrestrial equator and the north terrestrial pole. Indeed, ninety minus forty-five equals
forty-five. The altitude of the Sun at
noon on the summer solstice equals our colatitude plus the obliquity. The altitude of the Sun at noon on the winter
solstice equals our colatitude minus the obliquity. The altitude of the Sun at noon on either
equinox equals our colatitude.
Everything we have discussed applies not just to planet Earth but also
to any other planet orbiting the Sun.
The obliquity of any planet is the angular tilt of its rotational axis
from the vertical direction that is perpendicular to its own orbital plane around
the Sun. The poles of any planet are
where its own rotational axis intersects the planet. The equator of any planet is halfway between
the two poles of the planet. Our
latitude on that planet would be our angle north or south from that planet’s
equator. The planet’s equator would be
0° latitude on that planet. The planet’s
north pole would be 90°N
latitude on that planet, and the planet’s south pole would be 90°S latitude on that planet. Our colatitude on that planet would be our
angle from that planet’s north pole, which would again
be ninety degrees minus our latitude.
The summer solstice of any planet is the moment when its northern
hemisphere is tilted the most towards the Sun. The winter solstice for any planet is the
moment when its northern hemisphere is tilted the most
away from the Sun. The equinoxes of any
planet is halfway between the solstices when its rotational axis is not tilted toward or away from the Sun. The altitude of the Sun at noon on each of
these dates would be the same equations: colatitude plus obliquity on the
summer solstice, colatitude minus obliquity on the winter solstice, and
colatitude on the equinoxes. The only
difference in this analysis for other planets are the actual dates of the
solstices and the equinoxes. For any
planet orbiting the Sun, the time from one solstice to the next solstice (which
is also the time from one equinox to the next equinox) is one-half of the
planet’s orbital period around the Sun.
The time from one solstice to the next equinox (which is also the time
from one equinox to the next solstice) is one-quarter of the planet’s orbital
period around the Sun. As an example,
consider a hypothetical planet with an obliquity of thirty degrees, and suppose
we live at fifty degrees north latitude on this hypothetical planet. Since our latitude is fifty degrees north,
our colatitude is forty degrees, since ninety minus fifty equals forty. Hence, the altitude of the Sun at noon on the
summer solstice would be seventy degrees, since the colatitude plus the
obliquity is forty plus thirty, which equals seventy. The altitude of the Sun at noon on the winter
solstice would be ten degrees, since the colatitude minus the obliquity is
forty minus thirty, which equals ten.
The altitude of the Sun at noon on either equinox would be forty
degrees, since that is our colatitude.
It is a common misconception
that the Sun is at the zenith (directly overhead) at noon. This misconception comes from the phrase high
noon. Of course, the Sun has the
greatest altitude (is highest in the sky) at noon, giving the phrase some
validity. Nevertheless, the Sun is never
ever at the zenith (directly overhead) at most locations on Earth. For example, suppose we live at forty degrees
north latitude. Our colatitude would be
fifty degrees, since ninety minus forty equals fifty. The highest the Sun would ever be at this
location is on the summer solstice, when its altitude at noon is 73½ degrees,
since our colatitude plus obliquity is fifty plus 23½, which is indeed 73½
degrees. Although 73½ degrees is a high
altitude, it is not at the zenith (directly overhead). As we discussed earlier in the course, the
zenith is +90° altitude. If the Sun is not at
the zenith at noon on the summer solstice, it would only be lower in the sky
every other day of the year. This shows
that the Sun is never ever at the zenith (directly overhead) at most locations
on Earth. Is there anywhere on planet
Earth where the Sun is at the zenith (directly overhead) at noon on the summer
solstice? Yes, at a latitude the same
number of degrees north of the terrestrial equator as the obliquity of planet
Earth. To prove this, suppose we live at
23½ degrees north latitude, then our colatitude would
be 66½ degrees, since ninety minus 23½ equals 66½. Thus, the altitude of the Sun at noon would
be our colatitude 66½ degrees plus the obliquity of planet Earth 23½ degrees,
but 66½ plus 23½ equals +90° altitude, the zenith! This location of 23½ degrees north latitude
is so important that it deserves a special name: the Tropic of Cancer. There is only one location on planet Earth
where the Sun is directly overhead at noon on the winter solstice: 23½ degrees
south latitude. This location is so
important that it deserves a special name: the Tropic of Capricorn. The words Cancer and Capricorn refer to astronomical
constellations of the zodiac, as we discussed earlier in the course. There is only one location
on planet Earth where the Sun is at the zenith (directly overhead) at noon on
the equinoxes: the terrestrial equator at zero degrees latitude. Thousands of years ago, primitive humans did
not understand that the Earth is a planet with a tilted rotational axis
orbiting the Sun. For thousands of
years, humans believed that the motion of the Sun was responsible for the
seasons. Although today we understand that it is actually the Earth that is orbiting the Sun, we
must also confess that when we look up into the sky, it does appear as if the
Sun is moving. Therefore, we should
understand the seasons from the frame of reference of the Earth, which was the
only understanding of humans for thousands of years. From the frame of reference of the Earth, the
Sun appears to be directly on top of the Tropic of Cancer on the summer
solstice. For the next six months, the
Sun appears to move south, arriving on top of the terrestrial equator three
months later on the autumn equinox and arriving on top of the Tropic of
Capricorn three months after that on the winter solstice. For the following six months, the Sun appears
to move north, arriving on top of the terrestrial equator three months later on
the vernal equinox (spring equinox) and arriving on top of the Tropic of Cancer
three months after that on the summer solstice.
Again, it is not the Sun that is actually moving north
and south; in actuality, the Earth is orbiting the Sun. Nevertheless, we live on planet Earth, and so
we must understand the seasons from the frame of reference of the Earth. To summarize, it is only possible for the Sun
to appear at the zenith (directly overhead) at noon if we live somewhere
between the Tropic of Cancer and the Tropic of Capricorn. If we live north of the Tropic of Cancer or
south of the Tropic of Capricorn, the Sun never ever appears to be at the
zenith (directly overhead).
The Arctic Circle is 66½
degrees north latitude, making its colatitude 23½ degrees, since ninety minus
66½ equals 23½. The altitude of the Sun
at noon at the Arctic Circle on the winter solstice would be zero degrees,
since our colatitude minus the obliquity would be 23½ minus 23½, which is
obviously zero. As we discussed earlier
in the course, 0° altitude means the Sun is on the horizon, such as during
sunrise. At even more northern
latitudes, the altitude of the Sun on the winter solstice will be a negative
number, which means it is below the horizon as we discussed earlier in the
course. In other words, we cannot see
the Sun, making it nighttime even though it is noon! Before noon or after noon, the Sun will be
even further below the horizon. Thus,
the entire day is in perpetual nighttime!
The same occurs on the Antarctic Circle at 66½ degrees south latitude:
the altitude of the Sun at noon on the summer solstice is zero degrees, meaning
that it is on the horizon. At even more
southern latitudes, the altitude of the Sun on the summer solstice will be a
negative number, which means it is below the horizon. Again, we cannot see the Sun, making it
nighttime even though it is noon! Before
noon or after noon, the Sun will be even further below the horizon. Thus, the entire day is in perpetual nighttime! These extreme northern latitudes and extreme
southern latitudes are the only places on planet Earth where the Sun may never
rise on some days of the year and may never set on other days of the year. At the north terrestrial pole, six months of
continuous nighttime occurs from the autumn equinox all the way to the vernal
equinox (spring equinox), and then six months of continuous daytime occurs from
the vernal equinox (spring equinox) all the way to the autumn equinox. The reverse occurs at the south terrestrial
pole: six months of continuous nighttime occurs from the vernal equinox (spring
equinox) all the way to the autumn equinox, while six months of continuous
daytime occurs from the autumn equinox all the way to the vernal equinox
(spring equinox). Even when the clock
time is midnight, the Sun may still be in the sky causing daytime at these
extreme latitudes. This is the origin of
the phrase midnight Sun.
By one interpretation,
nighttime begins at sunset and extends all the way to sunrise. However, from the interpretation of the
brightness of the sky, true nighttime does not immediately begin upon sunset,
nor does true nighttime extend all the way to sunrise. The Earth’s atmosphere refracts the Sun’s
light even when the Sun is below the horizon.
In fact, the sky remains somewhat bright for quite some time after
sunset, and the sky becomes somewhat bright for quite some time before
sunrise. The somewhat bright times after
sunset are called the evening twilights, while the
somewhat bright times before sunrise are called the morning twilights. After sunset, the sky is still quite bright
for several minutes. This is called the evening civil twilight, ending with civil
dusk. After civil dusk, the sky becomes
noticeably darker with some stars becoming visible, but the western sky is
still bright enough for sailors at sea to find the western horizon. For this reason, this time is
called the evening nautical twilight, ending with nautical dusk. After nautical dusk, the sky becomes dark
enough that the western horizon is no longer visible, but the sky is still
bright enough to prevent astronomers from making observations of faint (dim)
objects in the sky. For this reason,
this time is called the evening astronomical twilight,
ending with astronomical dusk. It is
only after astronomical dusk that true nighttime begins. Before sunrise, these three twilights are reversed. First
is astronomical dawn followed by the morning astronomical twilight, when the
sky is still dark enough that the eastern horizon is still not visible yet
sufficiently bright that astronomers can no longer observe faint (dim) objects
in the sky.
This morning astronomical twilight ends with nautical dawn followed by
the nautical twilight, when the sky is still dark with some stars still
visible, but the eastern sky becomes bright enough for sailors at sea to find
the eastern horizon. This morning
nautical twilight ends with civil dawn followed by the morning civil twilight,
when the sky becomes quite bright.
Finally, the morning civil twilight ends with sunrise. Civil dusk and civil dawn are
strictly defined as the moments when the Sun has an altitude of −6°
(six degrees below the horizon).
Nautical dusk and nautical dawn are strictly defined
as the moments when the Sun has an altitude of −12° (twelve degrees below
the horizon). Astronomical dusk and
astronomical dawn are strictly defined as the moments
when the Sun has an altitude of −18° (eighteen degrees below the
horizon). The durations of time of all
of these twilights depends upon our latitude and the time of year. At the midlatitudes,
these twilights (civil, nautical, and astronomical) are each roughly thirty
minutes in duration on average. Thus, at
the midlatitudes, there are roughly ninety minutes on
average of evening twilight and roughly ninety minutes on average of morning
twilight. Therefore, at the midlatitudes, there are roughly three hours of total
twilight on average. As we discussed,
the vernal equinox (spring equinox) and the autumn equinox each have equal
amounts of daytime and nighttime (twelve hours each); this is why they are
called equinoxes. However, we now
realize there are only roughly nine hours of true nighttime at the midlatitudes because of the evening twilights and the
morning twilights, at least at the midlatitudes. Every other day of the year also has a
correspondingly shorter true nighttime, because of the evening twilights and
the morning twilights.
As we discussed, at extreme
latitudes either north of the Arctic Circle at 66½°N
latitude or south of the Antarctic Circle at 66½°S
latitude, we may have continuous daytime (no nighttime) or continuous nighttime
(no daytime), depending on the time of year.
At every other latitude on planet Earth, there will be some daytime and
some nighttime every day throughout the entire year. However, we now realize that all of these nighttimes may not necessarily be true nighttimes. At the extreme latitudes north of the Arctic
Circle at 66½°N latitude or south of the Antarctic
Circle at 66½°S latitude, continuous nighttime may
actually be continuous twilight, depending on the time of year. Moreover, there are even less extreme
latitudes where true nighttime may not occur, depending on the time of year. Between 60½° latitude and 66½° latitude (in
both the northern hemisphere and the southern hemisphere), there may be some
days of the year when daytime is followed by civil twilight followed by daytime
again. In other words, nautical
twilight, astronomical twilight, and true nighttime never occur. Between 54½° latitude and 60½° latitude (in
both the northern hemisphere and the southern hemisphere), there may be some
days of the year when daytime is followed by civil twilight, then nautical
twilight, then civil twilight, then finally daytime again. In other words, astronomical twilight and
true nighttime never occur. Between 48½°
latitude and 54½° latitude (in both the northern hemisphere and the southern
hemisphere), there may be some days of the year when daytime is followed by
civil twilight, then nautical twilight, then astronomical twilight, then
nautical twilight, then civil twilight, then finally daytime again. In other words, true nighttime never
occurs. At every other latitude on
planet Earth (from 48½°N latitude to 48½°S latitude), there is some daytime, some twilights (of
all six varieties) and some true nighttime every day of the entire year.
As we discussed earlier in
the course, the Sun appears to wander around the Celestial Sphere (around the
sky). The giant circle that the Sun
appears to wander around the Celestial Sphere (around the sky) is called the ecliptic, and it takes the Sun one year to
appear to take one complete journey around the ecliptic. The constellations along the ecliptic are called the zodiac constellations. Thousands of years ago, the Sun appeared to
be wandering through Cancer (the crab) on the summer solstice, when the Sun
appears to be on top of 23½ degrees north latitude. This is why 23½ degrees north latitude is called the Tropic of Cancer. Thousands of years ago, the Sun appeared to
be wandering through Capricornus (the sea goat) on the winter solstice, when
the Sun appears to be on top of 23½ degrees south latitude. This is why 23½ degrees south latitude is called the Tropic of Capricorn. Also thousands of years
ago, the Sun appeared to be wandering through Aries (the ram) on the vernal
equinox (spring equinox), and the Sun appeared to be wandering through Libra
(the scales) on the autumn equinox. As
we will discuss shortly, the Earth’s rotational axis suffers from a very slow
precession, causing the entire Celestial Sphere to slowly
shift over thousands of years thus causing the Sun to appear to wander
through different zodiac constellations on the solstices and the equinoxes over
the course of many thousands of years.
Today, the Sun appears to be wandering through Gemini (the twins) on the
summer solstice, not Cancer (the crab).
Perhaps 23½ degrees north latitude should be renamed the Tropic of
Gemini! Today, the Sun appears to be
wandering through Sagittarius (the centaur archer) on the winter solstice, not
Capricornus (the sea goat). Perhaps 23½
degrees south latitude should be renamed the Tropic of Sagittarius! Also today, the Sun appears to be wandering
through Pisces (the fish) on the vernal equinox (spring equinox) instead of
Aries (the ram), and the Sun appears to be wandering through Virgo (the virgin)
on the autumn equinox instead of Libra (the scales). Thousands of years from now, the Sun will
appear to be wandering through Taurus (the bull) on the summer solstice. Perhaps 23½ degrees north latitude should
then be renamed the Tropic of Taurus!
Thousands of years from now, the Sun will appear to be wandering through
Scorpius (the scorpion) on the winter solstice.
Perhaps 23½ degrees south latitude should then be renamed the Tropic of
Scorpius! Also
thousands of years from now, the Sun will appear to be wandering through
Aquarius (the water bearer) on the vernal equinox (spring equinox), and the Sun
will appear to be wandering through Leo (the lion) on the autumn equinox. This precession of the solstices and the
equinoxes completes one full cycle around the ecliptic in roughly twenty-six
thousand years, as we will calculate shortly.
Therefore, it is convenient to leave the names of 23½ degrees north
latitude and 23½ degrees south latitude as the Tropic of Cancer and the Tropic
of Capricorn, respectively, since the Sun will return to wandering through
those zodiac constellations on the solstices every twenty-six thousand years.
There are
several religious holidays that
have their origins in the solstices and the equinoxes. For example, Christmas Day is observed on
December 25th every year. Notice that this is shortly after the winter
solstice. Before Christmas Day became
the celebration of the birth of Jesus Christ, this was a pagan holiday
celebrating the winter solstice. Why
would pagans celebrate the day when the Sun appeared to be lowest in the sky at
noon with the most number of nighttime hours?
Primitive humans observed the Sun appear lower and lower in the sky
after the summer solstice; perhaps many primitive humans were terrified that
the Sun would continue to move downward until it disappeared below the
horizon. However, by simply paying attention
every year, we observe that the Sun stops moving downward on the winter
solstice, and then begins to move upward.
This was a reason to celebrate for many ancient pagans. This pagan celebration became the celebration
of the birth of Jesus Christ, since early Christians saw the birth of Jesus
Christ as bringing more and more light into a spiritually dark world. In actuality, Jesus was not born on December 25th. We will never
be certain of the exact day of the birth of Jesus. Most of the details of the vast majority of
persons throughout human history, including Jesus, were never
recorded. We will also never be
certain of the exact year of the birth of Jesus. Although it is commonly believed that the
year of the birth of Jesus was anno Domini 1, in
actuality no one recorded the year that Jesus was born. Again, most of the details of the vast
majority of persons throughout human history, including Jesus, were never recorded.
The people of the world did not declare the year to be anno Domini 1
when Jesus was born. The ancient Romans
designated years using a few different numbering schemes, among which was the
number of years since the founding of the city of Rome, which was more than
seven centuries before the birth of Jesus.
Using this particular convention, the ancient Romans designated years using either of the Latin phrases “ab urbe condita” or “anno urbis
conditae,” meaning “in the year since the founding of the city.” Either Latin phrase was abbreviated AUC. This numbering
scheme continued to be used by Europeans even after
the fall of the Western Roman Empire.
More than five centuries after the birth of Jesus and roughly fifty
years after the Western Roman Empire fell, the Christian monk Dionysius Exiguus tried to determine the year that Jesus was
born. This monk declared that AUC 1278 should be reset to A.D. 525, where A.D. is the
abbreviation for the new Latin phrase “anno Domini” meaning “in the year of our
Lord,” effectively meaning the number of years since the birth of Jesus. Over the next few centuries, Europeans
retroactively changed the years of historical dates from AUC
to A.D. based on this declaration by Dionysius Exiguus. Europeans even changed the years of
historical dates before the birth of Jesus from AUC
to B.C., which is simply an abbreviation for “before Christ.” Modern scholarship has revealed that the year
that Jesus was born as determined by Dionysius Exiguus
is probably a few years in error. Modern
scholarship estimates that 6 B.C. is a more accurate estimate for the year of
the birth of Jesus. Many students are offended by this assertion. Students claim that it would be contradictory
for Jesus to have been born in the year 6 B.C., since B.C. is the abbreviation
for before Christ and no one can be born before the year of their own birth! Again, we must remember that no one recorded
the date that Jesus was born. We must
remember that the belief that the year of the birth of Jesus
was A.D. 1 is based upon an estimate by a monk who lived roughly five centuries
after Jesus was born. In fact, we
should marvel that the estimate made by Dionysius Exiguus
for an unrecorded event that occurred roughly five centuries earlier was only a
few years in error! As another example
of a religious holiday that has its origin in the solstices and the equinoxes,
Easter is always the first Sunday after the first Full Moon after the vernal
equinox (spring equinox) every year.
Since Jesus was Jewish, what is traditionally called
the Last Supper was in actuality a Passover celebration. The Jewish calendar is a lunar calendar, and
hence Jewish holy days are determined by the cycles of the Moon. Placing Easter on the first Sunday after the
first Full Moon after the vernal equinox (spring equinox) is an attempt to keep
the date of Easter as close as possible to the date of Passover.
A thermometer is a device to
measure temperature. The operation of a
thermometer is based on the principle of thermal
expansion and thermal contraction. Most
substances expand when they become warmer, and most substances contract when
they become colder. To build a primitive
thermometer, we take any object and measure its length at one temperature, and
we measure its different length at a different temperature. We draw marks at each of these lengths, and
we draw other marks between these two marks.
To determine the temperature, we simply read off whichever mark the end
of the object meets based on its length at that temperature. Unfortunately, most substances expand and
contract by only tiny amounts when their temperature changes. Hence, the marks are often
too close together, making differences in length difficult to
measure. However, the element mercury
expands by quite a noticeable amount as it becomes warmer and contracts by
quite a noticeable amount as it becomes colder.
Therefore, most thermometers use liquid mercury, since the marks are
then well separated and easy to read. An
actinometer is a device that measures solar
radiation. To build a primitive actinometer, we take any object and use a thermometer to
measure its initial temperature. Then,
we place the object in sunlight for a certain amount of time, perhaps one
hour. As the object absorbs solar
radiation, it becomes hotter. We use a
thermometer to measure its hotter temperature afterwards, and from the
difference in temperature between its hotter final temperature and its colder
initial temperature, we can calculate the amount of solar radiation the object
absorbed. Note that we must wrap the
object in black cloth to ensure that it absorbs all of the solar
radiation. Otherwise, the object will
only become hotter by some of the solar radiation that it absorbed, since the
object will reflect the rest of the solar radiation. It is convenient to use a bucket of water as
the object, since we know the heat capacity of water. An actinometer
would measure the greatest amount of solar radiation on the summer solstice,
and an actinometer would measure the least amount of
solar radiation on the winter solstice.
However, the summer solstice is almost never the hottest day of the
year; a thermometer measures the hottest air temperature roughly a month later,
in late July in the northern hemisphere.
Similarly, the winter solstice is almost never the coldest day of the
year; a thermometer measures the coldest air temperature roughly a month later,
in late January in the northern hemisphere.
These delays occur because the Earth is mostly covered
with water, which has a large heat capacity.
In other words, it is difficult to change the temperature of water. The northern hemisphere receives the most
direct sunrays on the summer solstice around June 21st, but it still takes
another month for the air to warm to maximum temperature sometime in late
July. In fact, it takes yet another
month for the ocean waters to warm to maximum temperature, in late August in
the northern hemisphere. This is why
most people in the northern hemisphere take their summer vacations in August,
so that they may enjoy swimming in the ocean when it is warmest. The northern hemisphere receives the least
direct sunrays on the winter solstice around December 21st, but the oceans have
retained so much heat from the summertime that it still takes another month for
the air to cool to minimum temperature sometime in late January. In fact, it takes yet another month for the ocean
waters to cool to minimum temperature, in late February in the northern
hemisphere.
The time it takes the Earth
to complete one rotation is one day, but we must clearly define what we mean by
one day. Suppose it is noon; thus, the Sun
is at its greatest altitude (appears highest in the sky). In the time it takes
the Earth to complete one full rotation, it has moved a small amount in its
orbit around the Sun. Therefore, it is
not yet noon after this full rotation.
We must wait a short time longer for the Earth to rotate a little bit
more to bring the Sun to greatest altitude (highest in the sky) again. We now realize that the time it takes the
Earth to complete one rotation is less than the time it takes the Sun to go
from noon to noon. In other words, the
time it takes the Sun to go from noon to noon is longer than the time it takes
the Earth to complete one rotation. The
time it takes the Sun to go from noon to noon is called a solar day, and this
is equal to twenty-four hours. However,
this twenty-four-hour solar day is longer than the time it takes the Earth to
rotate once. The time it takes the Earth
to rotate once is called the sidereal day, which is
shorter than the twenty-four-hour solar day.
We can roughly calculate the duration of time of the sidereal day. There are roughly 365 days in a year,
although even this requires clarifications that we will discuss shortly! For now, we will use 365 days as a rough
estimate. Recall that there are 360
degrees in a full circle, and notice that these two numbers 365 and 360 are
roughly equal to each other. This means
that every day the Earth moves roughly one degree in its orbit around the
Sun. Thus, after one complete rotation,
the Earth has moved roughly one degree along its orbit. Therefore, we must calculate the time it
takes the Earth to rotate an additional one degree, which is a 1/360 fraction
of a complete rotation. As a rough
estimate, we will use 24 hours or 1440 minutes as a complete rotation. Therefore, the time it takes the Earth to
turn one degree is roughly 1/360 of 1440 minutes. Remarkably, 1440 minutes divided by 360 is a
simple whole number: four minutes.
Therefore, the sidereal day is roughly four minutes shorter than the
solar day. In other words, the solar day
is roughly four minutes longer than the sidereal day. In conclusion, the solar day is twenty-four
hours, but this is roughly four minutes longer than the sidereal day, which is
the true amount of time it takes the Earth to complete one rotation. Whenever we hear anyone claim that it takes
the Earth twenty-four hours to rotate once, we should correct their error. The time
it takes the Earth to rotate once is roughly twenty-three hours and fifty-six
minutes, the sidereal day!
The time it takes the Earth
to complete one orbit around the Sun is one year, but we must clearly define
what we mean by one year. After 365
solar days, the Earth does not return precisely to where it began in its orbit. We must wait an additional roughly six hours
before the Earth returns to where it began in its orbit. In other words, one year is not an integer
number (not a whole number) of solar days.
It is difficult to construct a calendar based on this orbit. Are we supposed to have a calendar with 365
solar days followed by six hours of limbo before we begin the new year? Many of us
would enjoy such a calendar, since this would give us an additional six hours
to party every New Year’s Eve!
Nevertheless, we cannot construct a calendar this way. If we had six hours of limbo after midnight
ending New Year’s Eve before declaring that it is a new midnight beginning New
Year’s Day, then the Sun would be preparing to rise. Thus, the Sun would rise at roughly midnight
on average throughout that new year. Also notice that the Sun would culminate
(reach its highest altitude in the sky) at roughly 06:00 a.m. on average
throughout that new year, and the Sun would set at roughly noon on average
throughout that new year. This is
unacceptable. We want the Sun to rise at
06:00 a.m. on average, we want the Sun to culminate (reach its highest
altitude) at noon, and we want the Sun to set at 06:00 p.m. on average every
year indefinitely. We can accomplish
this by using a calendar with 365 solar days without adding six hours of limbo
between New Year’s Eve and New Year’s Day, but then the Earth would be roughly
six hours behind from where it began on its orbit one year earlier. After an additional 365 solar days, the Earth
would then be roughly twelve hours behind from where it began on its orbit two
years earlier. After a third 365 solar
days, the Earth would then be roughly eighteen hours behind from where it began
on its orbit three years earlier. After
a fourth 365 solar days, the Earth would be roughly twenty-four hours behind
from where it began on its orbit four years earlier, but twenty-four hours is
one full solar day! This inspires us to
construct a calendar as follows. One
calendar year will have 365 solar days, but once every four years we will add
an extra solar day to the calendar. This
brings the Earth back to where it began on its orbit four years earlier. This fourth year is
commonly known as the leap year, and the extra day we add is commonly
known as the leap day. Many primitive
humans across many ancient cultures thousands of years ago constructed their
calendars in this way. They did not
understand that the Earth orbited the Sun, but they did notice that 365 solar days after the summer solstice for example,
the Sun would not return to exactly where it appeared in the sky one year
earlier. After another 365 solar days,
they noticed that the Sun still did not return to exactly where it appeared in
the sky two years earlier. After a third
365 solar days, they noticed that the Sun still did not return to exactly where
it appeared in the sky three years earlier.
After a fourth 365 solar days, many ancient humans noticed that after
one additional solar day, the Sun finally returned to where it appeared in the
sky four years earlier.
The ancient Roman Republic used
a calendar with not only 365 solar days plus one extra
solar day every four years, but they also divided their calendar into ten
months. Some of these months were named for ancient Roman mythological gods, such as
March for Mars (Ares), the god of war.
Several of these months were named simply after
their numerical month of the calendar, such as October for the eighth
month. The Greek root octo- means eight.
For example, an octopus has eight legs, an octagon has eight sides and
eight angles, and octuplets are eight babies born from the same pregnancy. Although ancient Rome was born
a kingdom that did evolve into a republic, ancient Rome nevertheless
degenerated from a republic to a dictatorship by the first century before
Christ. The last of the ancient Roman
dictators was Julius Caesar, who declared himself perpetual dictator. Several Roman senators conspired to
assassinate him, including one of his closest friends Brutus. After the assassination of Julius Caesar, the
Roman dictatorship plunged into a civil war that ended when Julius Caesar’s
great-nephew Octavius Augustus declared himself the first emperor of Rome. This completed the degeneration of ancient
Rome from a republic to a dictatorship to an empire. Also during this time, two new months were inserted into the calendar, turning the ten-month
ancient Roman calendar into a twelve-month ancient Roman calendar. One of the new months that was inserted into
the calendar was named July after Julius Caesar, and the other new month that
was inserted into the calendar was named August after Octavius Augustus. This new calendar was named
the Julian calendar, after Julius Caesar.
The Julian calendar was used by more persons
for a longer period of time than any other calendar in all of human history. This Julian calendar was used throughout the
roughly 500-year (half a millennium) imperial history of ancient Rome, and even
after the Western Roman Empire fell Europeans continued to use this Julian
calendar for another 1000 years (a millennium).
This Julian calendar lasted for 1500 years, a millennium and a
half! Again, more people used this
Julian calendar for a longer period of time than any
other calendar in all of human history.
For this reason, we will now discuss the Julian calendar in tremendous
detail, even though this is not the calendar we use presently. According to the Julian calendar, the
calendar year is divided into twelve months. In the correct order, these
months are January with thirty-one solar days, February with twenty-eight solar
days (except for leap years, which we will discuss shortly), March with
thirty-one solar days, April with thirty solar days, May with thirty-one solar
days, June with thirty solar days, July with thirty-one solar days, August with
thirty-one solar days, September with thirty solar days, October with
thirty-one solar days, November with thirty solar days, and December with
thirty-one solar days. Notice how
difficult it is to memorize this calendar.
The word September means seventh month, since the Latin root sept- means
seven. For example, septuplets are seven
babies born from the same pregnancy.
However, in the Julian calendar, September is the ninth month instead! The word October means eighth month, since
the Greek root octo- means eight. Again, an octopus has eight legs, an octagon
has eight sides and eight angles, and octuplets are eight babies born from the
same pregnancy. However, in the Julian
calendar, October is the tenth month instead!
The word November means ninth month, since the Latin root nov- means nine.
However, in the Julian calendar, November is the eleventh month
instead! The word December means tenth
month, since the Greek root dec- means ten. For example, a decimal is one-tenth, there
are ten decimeters in a meter, and a decagon has ten sides and ten angles. However, in the Julian calendar, December is
the twelfth month instead! Finally, the
Julian calendar adds the leap day once every four years to February, meaning
that February actually has 28 and ¼ solar days.
If we add all the solar days in all twelve months of the Julian
calendar, we obtain 365 and ¼ solar days, which is roughly the number of solar
days in one year.
If one Earth year were
exactly 365 and ¼ solar days, the Julian calendar would be a perfect calendar. However one Earth
year is roughly but not exactly equal to 365 and ¼ solar days. Therefore, even if we add one solar day every
four years, the Earth does not return to precisely where it began in its orbit
four years earlier. There is a small
error, and this error will accumulate, becoming larger
and larger as decades, centuries, and millennia pass. After 1500 years of using this calendar, the
Earth was roughly ten days displaced from where it was
1500 years earlier. Hence, the dates of
the solstices and the equinoxes were also displaced by
roughly ten days. If an additional 1500
years had passed while we continued to use the Julian calendar, the dates of
the solstices and the equinoxes would be roughly twenty days displaced. If a third 1500 years had passed while we
continued to use the Julian calendar, the dates of the solstices and the
equinoxes would be displaced by roughly thirty
days. Thus, using the Julian calendar
for 4500 years would cause the dates of the solstices and the equinoxes to drift
by roughly one month. Using the Julian
calendar for 27000 years would cause the dates of the solstices and the
equinoxes to become displaced by roughly six
months. This is unacceptable. A proper calendar should ensure that
summertime (in the northern hemisphere) remains in June, July, and August
indefinitely and wintertime (in the northern hemisphere) remains in December,
January, and February indefinitely. In
the year 1582, Pope Gregory XIII realized these problems would arise from
continuing to use the Julian calendar.
To correct these problems, Pope Gregory XIII firstly erased ten solar
days from the year 1582. In particular,
the day after October 04, 1582, was the day October 15, 1582, skipping October 05th through October 14th that year, by decree of the
pope. This returned the Earth to where
it belonged in its orbit, thus resetting the dates of the solstices and the
equinoxes. However, Pope Gregory XIII
realized that this was only a temporary solution. Again, the dates of the solstices and the
equinoxes would continue to drift if we continue to use the Julian
calendar. The pope ordered astronomers
to construct a more accurate calendar that would maintain the dates of the
solstices and the equinoxes indefinitely.
This new calendar was named the Gregorian
calendar, after Pope Gregory XIII. This
is the calendar that Catholics have used for more than four hundred years, and
this is also the calendar the entire world uses for
civil (nonreligious) purposes. We will
now discuss the Gregorian calendar in tremendous detail, since all of us must
be familiar with the calendar that we actually use! According to the Gregorian calendar, the year
is still divided into the same twelve months in the
same order as the Julian calendar. This
unfortunately still leaves illogically-named months in
the calendar, such as October for the tenth month instead of the eighth
month. According to the Gregorian
calendar, the number of solar days in each month is also the same as the number
of solar days in each month of the Julian calendar, including February which still has twenty-eight solar days on non-leap years
but twenty-nine solar days on leap years.
The Gregorian calendar at first glance seems to be identical to the
Julian calendar. Indeed, the only
difference between the Gregorian calendar and the Julian calendar is the
leap-year calculation. The Julian
calendar leap-year calculation is simple; there is a leap year once every four
years. However, the leap-year
calculation in the Gregorian calendar is more complex than the leap-year
calculation in the Julian calendar to ensure that the dates of the solstices
and the equinoxes do not drift.
According to the Gregorian calendar, the leap year calculation is as
follows. If the year is divisible by
four, then it is a leap year, unless the year is divisible by one hundred in
which case it is not a leap year, unless the year is divisible by four hundred
in which case it is a leap year. In
other words, February has 28 plus 1/4 minus 1/100
plus 1/400 solar days according to the Gregorian
calendar. If we add the total number of
solar days from all the months in the Gregorian calendar, we obtain a number of
solar days so close to the true amount of time it takes the Earth to orbit the
Sun that the dates of the solstices and the equinoxes will not drift for
countless thousands of years. Therefore,
we should continue use the Gregorian calendar for many thousands of years. Let us work through some examples using the
Gregorian calendar. The years 1629,
1781, and 1857 were not leap years, since those years are not divisible by
four. The years 1648, 1732, and 1816
were leap years, since those years are divisible by four. The years 1700, 1800, and 1900 may be
divisible by four, but they are also divisible by one hundred; therefore, those
years were not leap years. The years
1600 and 2000 are divisible by four and are divisible by one hundred, but they
are also divisible by four hundred!
Therefore, the years 1600 and 2000 were leap years. The first time this four-hundred rule was ever invoked was the year 1600, and the second time this
four-hundred rule was ever invoked was the year 2000. The third time this four-hundred rule will ever be invoked will be the year 2400. Leap years happen to coincide with
presidential election years in the United States, but there have been two
exceptions thus far. The year 1800 was a
United States presidential election year (Thomas Jefferson versus John Adams),
but the year 1800 was not a leap year.
The year 1900 was a United States presidential election year (William
McKinley versus William Jennings Bryan), but the year 1900 was not a leap
year. Note that the year 2000 was a
United States presidential election year (George W. Bush versus Albert Gore,
Jr.), and the year 2000 was indeed a leap year.
The next United States presidential election that will not be a leap
year will be the year 2100. This assumes
that the United States will last for another seventy-seven years, which based
on student performance in this course is not
likely! To memorize the months and the
number of solar days in each month for both the Julian calendar and the
Gregorian calendar, we may memorize the following poem. “Thirty days hath September, April, June, and
November; All the rest have thirty-one, Excepting February alone, Which has
twenty-eight days clear, But twenty-nine in each leap year.”
For most of the history of
planet Earth, the hot temperatures at the terrestrial equator and the cold
temperatures at the terrestrial poles have been moderated
by the oceans due to the relatively large heat capacity of water. However, the moving tectonic plates of the
lithosphere slowly change the configuration of the continents and the oceans
over enormous timescales (millions of years).
If there happens to be relatively isolated continents at the terrestrial
poles, the relatively small heat capacity of these landmasses will permit the
temperatures at the terrestrial poles to become extremely cold. The result is an ice age, an extremely long
period of time (millions of years) when the temperature at the terrestrial
poles is so cold that enormous icecaps cover these landmasses. To our knowledge, there have been only five
ice ages throughout the entire history of planet Earth, each lasting many
millions of years. The Current Ice Age
began roughly thirty million years ago and still
continues to the present day. The
Current Ice Age will last many more millions of years as long as the continent
Antarctica remains relatively isolated at the south terrestrial pole. There are only two
scenarios that can end the Current Ice Age. In one scenario, Antarctica may move off of the south terrestrial pole, which would make it less
cold. In the other scenario, another
continent may move to the south terrestrial pole and collide with
Antarctica. This would end the isolation
of Antarctica, also making it less cold.
Whether Antarctica moves away from the south terrestrial pole or another
continent moves toward the south terrestrial pole, it takes millions of years
for tectonic plates to move significantly, as we discussed. Therefore, the Current Ice Age will continue
for millions of more years to come.
During the Current Ice Age, the southern icecap covers the continent
Antarctica, and the northern icecap covers Greenland, a giant island near (but
not exactly at) the north terrestrial pole.
Within the
Current Ice Age, there have been many periods of time when the Earth has become
even colder. These are glacial periods of the Current Ice
Age. Between two glacial periods is an
interglacial period when the Earth is not as cold. The Earth becomes so cold during a glacial
period that the icecaps expand beyond the terrestrial poles and advance onto
other continents. A major glacial period
lasts roughly one hundred thousand years, while a minor glacial period lasts
between roughly twenty-five thousand years and roughly fifty thousand
years. We are currently within an
interglacial period of the Current Ice Age.
This interglacial period began roughly twelve thousand years ago at the
end of a major glacial period that lasted roughly one hundred thousand
years. Plate tectonics cannot be
responsible for the alternation between glacial periods and interglacial
periods within the Current Ice Age, since tectonic plates do not move
appreciably over timescales of thousands of years. The alternation between glacial periods and
interglacial periods within the Current Ice Age is caused
by the Milanković cycles, named for the Serbian
climatologist and astronomer Mulutin Milanković who formulated this theory. The Earth’s orbit around the Sun is presently
almost a perfect circle. In other words,
the eccentricity of the Earth’s orbit around the Sun is close to zero, but this
has not always been the case nor will it always be the case. Gravitational perturbations (tugs) from the
other planets, primarily Jupiter, change the eccentricity of the Earth’s
orbit. When the Earth’s orbit is
perturbed to become more elliptical, the Earth will be
significantly further from the Sun, causing the Earth to become significantly
colder thus causing a major glacial period of the Current Ice Age. Calculations show that the eccentricity of
the Earth’s orbit changes once every one hundred thousand years (roughly),
which is roughly the duration of time of a major glacial period within the
Current Ice Age. Minor
glacial periods are caused by the precession and the nutation of the Earth’s
rotational axis. Precession is
the turning of an axis around another axis.
Nutation is the nodding of an axis resulting in a change in obliquity. The Earth’s rotational axis precesses and nutates, and this changes the amount of sunlight the Earth receives
from the Sun, which in turn causes minor glacial periods within the Current Ice
Age. Calculations show that the Earth’s
rotational axis precesses once every twenty-six
thousand years (roughly), and the Earth’s rotational axis nutates
once every forty-one thousand years (roughly).
These are roughly equal to the duration of time of minor glacial periods
within the Current Ice Age.
Long-term variations in
global temperature (over millions of years) are caused by slowly moving
tectonic plates, resulting in ice ages when continents happen to be relatively
isolated at or near the terrestrial poles, as with the Current Ice Age. Intermediate-term variations in global
temperature (over thousands of years) cause glacial periods and interglacial
periods within the Current Ice Age.
These intermediate-term variations in global temperature (over thousands
of years) are caused by the Milanković
cycles (the variations of the eccentricity of the Earth’s orbit around the Sun,
the precession of the Earth’s rotational axis, and the nutation of the Earth’s
rotational axis). However, there are
also short-term variations in global temperature, from centuries to decades,
and even as short as a few years.
Extremely short-term variations in global temperature (over a few years)
are caused by violent igneous eruptions. A single igneous eruption can cause global
cooling for a few years, since igneous eruptions eject ash into the atmosphere,
reducing the amount of incoming sunlight to the Earth. Short-term variations in global temperature
(from decades to centuries) are caused by a
combination of the Pacific Decadal Oscillation (PDO),
the Atlantic Multidecadal Oscillation (AMO), and variations in solar activity. The Pacific Decadal Oscillation (PDO) is the alternation between decades of El Niño
domination in the Pacific Ocean and decades of La Niña domination in the
Pacific Ocean. The Atlantic Multidecadal Oscillation (AMO) is
a similar oscillation in the Atlantic Ocean.
Although direct observations of the PDO and
the AMO only stretch back several decades,
measurements of the radioactive isotope carbon-fourteen within trees have revealed that the PDO and the
AMO each undergo roughly sixty-year cycles, where one
complete PDO for example consists of roughly three
decades of El Niño domination followed by roughly three decades of La Niña
domination. Quantitative observations of
solar activity stretch back a few centuries, giving us a stronger understanding
of how variations in solar activity affect global temperatures. At times, the Sun is more active, radiating
more energy that warms planet Earth; at other times, the Sun is more quiet, radiating less energy that cools planet
Earth. These variations in solar
activity manifest themselves through sunspots, regions on the surface of the
Sun where magnetic field strengths are particularly strong, as we will discuss
later in the course. These sunspots have been directly observed for roughly four hundred years,
since the invention of the telescope.
Astronomers have observed that the number of sunspots goes through a
roughly eleven-year cycle. In one complete
cycle, the number of sunspots increases then decreases over a time period of roughly eleven years. For example, the Sun experienced a period of
increasing activity during the last few years of the twentieth century and the
first few years of the twenty-first century (the current century). This period of increasing solar activity was followed by a period of decreasing solar activity. During that particular period of decreasing
solar activity, some of the coldest temperatures over the last one hundred
years occurred. Furthermore,
measurements of the radioactive isotope carbon-fourteen
within trees have revealed that this roughly
eleven-year solar cycle itself goes through a roughly
two-hundred-year cycle. This is the de
Vries cycle, named for the Dutch physicist Hessel de Vries, one of the pioneers
of radiocarbon dating. According to the de Vries
cycle, the Sun gradually increases in activity to what is
called a solar maximum then gradually decreases in activity to what is
called a solar minimum. Caution: the
eleven-year solar cycles continue to occur throughout each two-century de Vries cycle. Since
one complete de Vries cycle lasts for roughly two
centuries, each solar maximum and each solar minimum lasts for roughly one
hundred years. Over the past twelve
thousand years (since the beginning of the current interglacial period of the
Current Ice Age), there have been roughly sixty complete de Vries
cycles, with each de Vries cycle having one solar
maximum and one solar minimum. The
Modern Maximum occurred throughout most of the twentieth century, and the
Modern Minimum began toward the beginning of the twenty-first century (the
current century). These de Vries cycles have caused variations in global temperatures
over the past several thousand years.
For example, a solar maximum contributed to the Roman Warm Period,
lasting from the ancient Late Roman Republic to the ancient Early Roman
Empire. As another example, another
solar maximum contributed to the Medieval Warm Period during the High Middle
Ages. As yet
another example, a solar minimum contributed to the Little Ice Age, lasting
from the Late Middle Ages to the Early Modern Ages. Most recently, the Modern Maximum that
occurred throughout most of the twentieth century contributed to the warming
temperatures of that century, and the Modern Minimum that began toward the
beginning of the twenty-first century (the current century) has already caused
cooling temperatures that will continue for the rest of the current
century. By combining the roughly
sixty-year PDO, the roughly sixty-year AMO, the roughly eleven-year solar cycle, and the roughly
two-century de Vries cycle, we obtain a nearly
perfect model of global temperature variations over the past couple of thousand
years. As we just discussed, this model
predicts that there will be gradual global cooling during the current
century. Caution: an unexpected violent
igneous eruption may cause additional global cooling in addition to these
cyclic variations.
To summarize the climate of
planet Earth, the average temperature of the atmosphere is determined primarily
by the Earth’s distance from the Sun together with the concentration of
greenhouse gases in the atmosphere, primarily water vapor, which warms the
Earth sufficiently so that it is habitable for life. Temperatures are hot near the terrestrial
equator, temperatures are cold near the terrestrial poles, and temperatures
vary at the midlatitudes based on the seasons (warmer
in the summertime and cooler in the wintertime). The abundance of liquid water that covers the
Earth (the oceans) stabilizes global temperatures due to the large heat
capacity of water. Long-term variations
in global temperature (over millions of years) are caused by slowly moving
tectonic plates, resulting in ice ages when continents happen to be relatively
isolated at or near the terrestrial poles, as with the Current Ice Age that
began roughly thirty million years ago and continues to the present day. Intermediate-term variations in global
temperature (over thousands of years) cause glacial periods and interglacial
periods within the Current Ice Age.
These intermediate-term variations in global temperature (over thousands
of years) are caused by the Milanković
cycles (the variations of the eccentricity of the Earth’s orbit around the Sun,
the precession of the Earth’s rotational axis, and the nutation of the Earth’s
rotational axis). We are currently in an
interglacial period of the Current Ice Age, and this interglacial period began
roughly twelve thousand years ago.
Short-term variations in global temperature (over a few decades or a few
centuries) are caused by the roughly eleven-year solar
cycle, the roughly two-century de Vries cycle, the
roughly sixty-year PDO, and the roughly sixty-year AMO. We just began a
century of gradual global cooling resulting from these short-term solar cycles
and oceanic cycles. Finally, extremely
short-term variations in global temperature (over a few years) may result from
powerful igneous eruptions.
The very slow precession and
nutation of the Earth’s rotational axis also causes the entire Celestial Sphere
to very slowly wobble along with the Earth’s
rotational axis, since the celestial poles and the celestial equator are all
defined as projections of the terrestrial poles and the terrestrial equator
onto the sky. As we discussed earlier in
the course, the star α (alpha) Ursae Minoris is presently the north star,
since the Earth’s rotational axis almost perfectly points toward that
star. However, this has not always been
the case nor will it always be the case.
Thousands of years ago, a different star was the north
star, since the Earth’s rotational axis pointed in a different direction
toward a different star in the northern half of the Celestial Sphere. After thousands of years of precession and
nutation, the Earth’s rotational axis presently points toward α (alpha) Ursae Minoris. After thousands of more years of precession
and nutation, yet another star will become the new north star. We also discussed earlier in the course that
presently there is no south star, since the Earth’s rotational axis does not
point toward any particularly bright star in the southern half of the Celestial
Sphere. However, this has not always
been the case nor will it always be the case.
Thousands of years ago, there may have been a south star, since the
Earth’s rotational axis pointed in a different direction toward a bright star
in the southern half of the Celestial Sphere.
After thousands of years of precession and nutation, the Earth’s
rotational axis presently does not point toward any particularly bright star in
the southern half of the Celestial Sphere.
After thousands of more years of precession and nutation, perhaps
another star will become a new south star.
In summary, as the Earth’s rotational axis slowly precesses and nutates over
thousands of years, during some centuries there will be a north star but no
south star (as is presently the case), during other centuries there will be a
south star but no north star, during yet other centuries there will be both a
north star and a south star, while during yet other centuries there will be
neither a north star nor a south star.
The precession and the
nutation of the Earth’s rotational axis causes another complication. The time it takes the Earth to complete one
orbit around the Sun is one year, but in that time the Earth’s axis has precessed and nutated by a tiny
amount. Therefore, the orientation of
its rotational axis relative to its orbit around the Sun has already passed its
original orientation by a tiny amount.
This discrepancy manifests itself in the precession of the equinoxes and
the solstices. The time it takes the
Earth to complete one orbit around the Sun is strictly
defined as the sidereal year, while the time it takes the Earth to go
from the vernal equinox to the next vernal equinox is strictly defined as the
tropical year. The tropical year is
slightly shorter than the sidereal year.
In other words, the sidereal year is slightly longer than the tropical
year. The difference between these two
years is roughly twenty minutes. We can
use this difference to estimate the time for one complete cycle of the
precession of the equinoxes and the solstices.
One complete precession of the Earth’s rotational axis would cause the
Sun to appear to precess around the entire ecliptic,
but every year the Sun also appears to wander around the entire ecliptic. Therefore, the time for one complete
precession of the equinoxes and the solstices must be the ratio between one year and twenty minutes. If we regard one year as 365 and ¼ solar days
as a rough estimate, then one year is also 8766 hours, which is also 525960
minutes. The ratio between 525960
minutes and roughly 20 minutes of precession per year is roughly 26000 years
for one complete precession around the entire ecliptic. This finally justifies our earlier assertion
that the Earth’s rotational axis precesses once every
roughly 26000 years. Somewhat
more accurate calculations reveal that the tropical year is 365.24219
solar days, while the sidereal year is 365.25636 solar days, the difference
being 20.4 minutes. Note that human
calendars are almost always based upon the tropical
year, not the sidereal year. The
tropical year is defined in terms of the equinoxes and
the solstices, which in turn determine the seasons. The seasons directly affect life on Earth,
including human life. Thus, the seasons
have a direct affect upon human history, human societies, and human
cultures. The Julian year is exactly 365
and ¼ solar days, since the Julian year is defined by
the Julian calendar. Note that one
Julian year is 365.25 solar days expressed as a
decimal. The Gregorian year is exactly
365 plus 1/4 minus 1/100 plus 1/400
solar days, since the Gregorian year is defined by the
Gregorian calendar. Note that one
Gregorian year is 365.2425 solar days expressed as a
decimal. Notice that the Gregorian year
(365.2425 solar days) is much closer to the tropical year (365.24219 solar
days) than the sidereal year (365.25636 solar days). The difference between the Gregorian year and
the tropical year is less than thirty seconds.
This finally justifies our earlier assertion that using the Gregorian
calendar will prevent the dates of the solstices and the equinoxes from
drifting for countless thousands of years.
The Earth has one moon,
called the Moon. The Moon has a diameter
roughly one-fourth of the Earth’s diameter.
No other planetary moon in the Solar System is this enormous as compared
with its mother planet. All the other
planetary moons in the Solar System are very small as compared with the size of
their mother planet. The Moon is so
enormous that it is not even strictly correct to say that the Moon is orbiting
the Earth. More correctly, the Earth and
the Moon are both orbiting their common center of mass, while that common
center of mass orbits the Sun. For this
reason, many astrophysicists do not even consider the Moon to be a moon. Instead, many astrophysicists regard the
Earth-Moon system to be a double planet orbiting the Sun. If we regard Mercury as the first planet from
the Sun and Venus as the second planet from the Sun, we should regard the Earth
as planet 3A and the Moon as planet 3B. Mars would then
remain the fourth planet from the Sun, and so on and so forth.
Although the Moon is very
large as compared with the size of other planetary moons relative to their
mother planets, the Moon is nevertheless only one-fourth the diameter of the
Earth. Being significantly smaller than
the Earth, the Moon was born with much less radioactive atoms (as compared with
the Earth) to provide itself with internal energy for geologic processes. Therefore, the Moon was geologically alive
for only a brief amount of time, perhaps a few hundred million years. For most of its lifetime, the Moon has been
geologically dead. While the Moon was
geologically alive, it did have a partially molten
metallic core, and thus the young ancient Moon generated a magnetic field. However, since the Moon has been geologically
dead for most of its lifetime, its metallic core is no longer even partially molten.
Therefore, the Moon has not generated a substantial magnetic field for
most of its lifetime. Also due to its
small size and small mass, the Moon’s gravity is sufficiently weak that it
cannot retain an atmosphere. Without
geologic activity for most of its lifetime to provide volcanic outgassing to
replenish a lost atmosphere, the Moon has had virtually no atmosphere for most
of its lifetime. At best, there is an exosphere that begins at the surface of the Moon
that smoothly transitions into the very cold temperatures of the surrounding
outer space. Since the Moon has been
geologically dead for most of its lifetime, there has been no geologic activity
for most of its history to cover or eradicate craters caused by asteroid impacts
and comet impacts. Hence, the Moon is much more cratered as compared with the Earth.
We are not certain how the
Moon formed. The jovian, gas-giant (outer) planets are large enough
with sufficient mass and therefore strong enough gravity to capture moons, as
we will discuss shortly. However, the
terrestrial (inner) planets are small enough with insufficient mass and
therefore insufficient gravity to capture moons. This explains why both Mercury and Venus have
no moons. (The terrestrial planet Mars
has two moons that it probably captured, and we will discuss this exception
shortly.) Since the Earth is a
terrestrial planet and is therefore small as compared with the jovian, gas-giant planets, it is
unlikely that the Earth captured the Moon, especially considering the large
size of the Moon. It is also unlikely
that the young, forming Earth fragmented itself to form the Moon. If the Moon was not
captured by the Earth and if the Moon did not form from the Earth
through self-fragmentation, then where did the Moon come from? Our strongest theory for the formation of the
Moon is the violent-collision theory.
When the Earth was still forming, it was almost
entirely molten, as we discussed.
During this time, a large planetesimal
accidentally collided with the Earth, ripping huge chunks of molten rock from
the outer layers of the Earth. Most of
these chunks of molten rock did not have enough energy to escape from the
Earth’s gravitational attraction. Thus,
these chunks of molten rock remained in orbit around the Earth. These chunks of molten rock then suffered
sticky collisions with each other, coalescing into a larger and larger mass
through accretion. The growing mass
eventually became large enough that its self-gravity forced itself into a
spherical shape, becoming the Moon. The
damaged Earth also had enough self-gravity to restore itself back to its
original spherical shape. One piece of
evidence in favor of this violent-collision theory is that the composition of
the Moon is similar to the composition of the outer layers of the Earth, namely
iron-poor silicate rock. The Apollo
astronauts who landed on the Moon more than fifty years ago returned to the
Earth with Moon rocks, and these rocks are indeed composed of iron-poor
silicate rock. We may argue that these
rocks should be composed of iron-poor silicate rock regardless, since the Moon
should have differentiated itself into a dense metallic core, a less dense
mantle, and a least dense outer crust while it first formed. However, the Apollo astronauts also left
seismometers on the surface of the Moon.
Although the Moon is geologically dead, gravitational tidal forces from
the Earth nevertheless cause weak moonquakes on the Moon that these
seismometers have measured. Just as the
detection of earthquakes by seismometers on Earth together with computer
simulations have revealed the layers of the Earth, their thicknesses, their
compositions, and their physical states, the detection of moonquakes by
seismometers on the Moon together with computer simulations have revealed the
layers of the Moon, their thicknesses, their compositions, and their physical
states. These measurements and
calculations have revealed that the Moon does indeed have a dense metallic
core, a less dense mantle, and a least dense outer crust. However, the Moon’s dense metallic core is
unusually small for a terrestrial planet with the Moon’s overall size. This is a strong piece of evidence in favor
of the violent-collision theory for the Moon’s formation. The Moon has a very small metallic core since
it formed from a violent collision ripping chunks of molten rock from the outer
layers of the Earth, which are composed of iron-poor silicate rock.
It takes roughly one month
for the Earth and the Moon to orbit their common center of mass. In fact, the word month is
derived from the word moon. If we
take the word month, remove the suffix -th,
and insert an extra letter o, we obtain the word moon! The Moon always faces the same side toward
the Earth as the Earth and the Moon orbit each other. This side of the Moon is called
the near side of the Moon, and the side of the Moon that always faces away from
the Earth is called the far side of the Moon.
Therefore, for most of human history, the far side of the Moon remained
a complete mystery. Beginning in the
mid-to-late twentieth century, humans have sent satellites to orbit the
Moon. These satellites have taken
photographs of every part of the Moon, including the far side. All of us can observe the near side of the
Moon in the sky. Without a telescope and
even without binoculars, we can see with the naked eye that the near side of
the Moon has some landscapes lighter in color and other landscapes darker in
color. When Galileo Galilei observed the
near side of the Moon through his primitive telescope more than four hundred
years ago, the darker landscapes looked so smooth and flat to him that he
believed they were bodies of water. In
fact, Galileo Galilei named these darker areas maria, which is plural for the singular mare, the
Latin word for sea. Today, we know that these
lunar maria are not bodies
of water; they are low-elevation plains that are actually hardened lava
flows. When the Moon was young, it had a
molten interior. Collisions from
asteroids and comets penetrated the outer layers of the Moon, permitting this
molten rock from the interior to extrude out and flow onto the surface of the
Moon. This molten rock cooled and
crystallized into solid rock, forming the lunar maria. The parts of the near side of the Moon that
appear lighter in color are mountainous highlands. Craters on any planet or moon are caused by asteroid impacts and comet impacts. The lunar maria are more sparsely cratered, while the lunar mountainous
highlands are more heavily cratered.
This stands to reason. The lunar maria formed later than the lunar mountainous
highlands. Since the lunar maria are younger, there has been less time for impact
cratering to occur, making them sparsely cratered. The lunar mountainous highlands are
older. Therefore, there has been more time
for impact cratering to occur, making them heavily cratered. In fact, since the early Solar System was the
heavy bombardment period as we discussed, the lunar mountainous highlands are heavily cratered in an even greater proportion than
their age as compared with the lunar maria. The far side of the Moon has fewer sparsely-cratered maria and more
heavily-cratered mountainous highlands as compared with the near side of the
Moon, which has more sparsely-cratered maria and
fewer heavily-cratered mountainous highlands as compared with the far side of
the Moon. Again, this topographical
difference between the near side of the Moon and the far side of the Moon was not discovered until the mid-to-late twentieth century,
when humans began to send artificial satellites to the Moon to orbit the Moon
and photograph every part of the Moon, including the far side.
It is a
common misconception that the Moon must not be rotating, since it always faces the same side toward the
Earth. This is false. The Moon does rotate, and this rotation is
essential in keeping the same side of the Moon pointed toward the Earth. The time it takes the Moon to rotate once is
exactly equal to the time it takes the Earth and the Moon to orbit each other
once. In other words, the rotational
period of the Moon is exactly equal to its orbital period. More plainly, the Moon turns at the same rate
at which it orbits, thus keeping the same side always facing toward the
Earth. This is not a coincidence or an
accident. Millions of years ago, the
rotational period of the Moon was not equal to its orbital period. Hence, the Moon did not always face the same
side toward the Earth. However,
gravitational tidal forces from the Earth exerted different forces on different
parts of the Moon, distorting the shape of the Moon by giving it small tidal
bulges. More plainly, the Earth’s
gravitational attraction stretches the Moon into a slightly elongated
shape. Since the Moon’s tidal bulges are
slightly closer to the Earth with each lunar rotation, the Earth’s gravitational
attraction pulls on the Moon’s tidal bulges more strongly. If the ancient Moon was
rotating faster than its current rotation, then the Earth’s greater
gravitational attraction of the Moon’s tidal bulges would have slowed down the
Moon’s rotation. If the ancient Moon was rotating slower than its current rotation, then the
Earth’s greater gravitational attraction of the Moon’s tidal bulges would have
sped up the Moon’s rotation. In either
case, the Earth’s gravitational attraction adjusted the rotational period of
the Moon over millions of years until one of its tidal bulges was forced to always face toward the Earth. This mechanism is called
tidal locking, and we say that the Moon is tidally locked to the Earth. Tidal locking occurs frequently throughout
the Solar System; many moons are tidally locked to
their mother planet, meaning that their rotational periods are equal to their
orbital periods causing them to always face the same side toward their mother
planet. In particular, both of the moons
of Mars are tidally locked to Mars, the four largest moons of Jupiter are
tidally locked to Jupiter, the seven largest moons of Saturn are tidally locked
to Saturn, the five largest moons of Uranus are tidally locked to Uranus, and
the two largest moons of Neptune are tidally locked to Neptune. Pluto and its largest moon Charon are both tidally locked to one another. Not only does Charon always face the same
side toward Pluto, but Pluto always faces the same
side toward Charon. This stands to
reason, since gravitation is universal; everything in the universe exerts a
gravitational force on everything else in the universe, as we discussed earlier
in the course. Not only has Pluto
adjusted the rotational period of Charon until it became
tidally locked to Pluto, but Charon has adjusted the rotational period
of Pluto until it became tidally locked to Charon. The same will eventually occur with the
Earth-Moon system in many millions of years.
The Earth has already succeeded in causing the Moon to be tidally locked to the Earth, but the Moon will someday
succeed in causing the Earth to be tidally locked to the Moon. The Earth’s rotation is presently slowing
down by tiny unnoticeable amounts.
Eventually, the Earth will become tidally locked
to the Moon, meaning that in millions of years there will be a near side of the
Earth that always faces toward the Moon and a far side of the Earth that always
faces away from the Moon. When this
finally occurs millions of years from now, anyone who happens to live on the
far side of the Earth would never ever see the Moon, and anyone who happens to
live on the near side of the Earth would always see the Moon in the sky. This is already the case on the Moon. Anyone who happens to live on the far side of
the Moon never ever sees the Earth, and anyone who happens to live on the near
side of the Moon always sees the Earth in the sky. As the Earth’s rotation slows, this
rotational angular momentum must be transferred back
to the Moon, by the law of conservation of angular momentum. If the Moon is gaining angular momentum, then
the size of its orbit must grow, meaning that the Moon must be drifting away
from the Earth. The Apollo astronauts
who landed on the Moon more than fifty years ago left mirrors on the Moon so
that NASA could send laser signals to the Moon that would reflect off of those mirrors.
By timing how long it takes a laser signal to
propagate to the Moon and for its reflection off of those mirrors to return to
the Earth, we have measured the distance to the Moon within an accuracy of a
few centimeters! These measurements
reveal that the Moon is indeed drifting away from the Earth very slowly, a few
centimeters per year. This recessional
motion of the Moon will not continue until the Moon escapes from the Earth
however. This gradual recession of the
Moon from the Earth will cease when the Earth becomes tidally
locked to the Moon. The angular
momentum transfer will then end, and the orbit of the Earth-Moon system will
attain a stable (unchanging) equilibrium.
It is a common misconception
that the far side of the Moon is in perpetual darkness. This is false. This misconception comes from confusing the
far side of the Moon with the dark side of the Moon. Again, the far side of the Moon faces away
from the Earth. The dark side or more
correctly the nighttime side of the Moon faces away from the Sun. In fact, the nighttime side of any planet or
moon in the Solar System is the side that faces away from the Sun, while the
daytime side of any planet or moon in the Solar System is the side that faces
toward the Sun. The nighttime side of
the Moon faces away from the Sun, but this is different from the far side of
the Moon, which faces away from the Earth.
Similarly, the daytime side of the Moon faces toward the Sun, but this
is different from the near side of the Moon, which faces toward the Earth. We emphasize this again: the far side of the
Moon and the nighttime (dark) side of the Moon are two different things, just
as the near side of the Moon and the daytime side of the Moon are two different
things. The far side of the Moon is not
in perpetual nighttime (darkness), just as the near side of the Moon is not in
perpetual daytime. Since
it takes roughly one month for the Earth and the Moon to orbit their common
center of mass as that common center of mass orbits the Sun and since the
rotational period of the Moon is also roughly one month, every part of the Moon
(both the near side and the far side) experiences roughly two weeks of
continuous daytime followed by roughly two weeks of continuous nighttime.
For thousands of years,
humans noticed that the appearance of the Moon in the sky appears to change
daily, even hourly. These are called the
phases of the Moon. We use the word
crescent throughout the time when the Moon appears to be less than
half-illuminated, and we use the word gibbous throughout the time when the Moon
appears to be more than half-illuminated.
We use the word waxing throughout the time when the Moon appears to be
growing in illumination, and we use the word waning throughout the time when
the Moon appears to be shrinking in illumination. The verb to wax means to grow or to become stronger, and the verb to wane means to shrink or to become
weaker. The entire lunar cycle lasts for
roughly one month. Beginning with New Moon which is when we cannot see the Moon, gradually over a
period of roughly one week the Moon appears to grow in illumination, although
it still appears to be less than half-illuminated. This is Waxing Crescent Moon, since it
appears growing in illumination (waxing) but still appears less than
half-illuminated (crescent). At the end
of this first week, the Moon appears to be half-illuminated. This is First Quarter Moon, since we have
completed the first quarter of the entire lunar cycle. Gradually over the period of the second week,
the Moon continues to grow in illumination, now appearing more than
half-illuminated. This is Waxing Gibbous
Moon, since it continues to appear growing in illumination (waxing) and now
appears more than half-illuminated (gibbous).
At the end of this second week, the Moon appears to be completely
illuminated. This is Full Moon. Gradually over the period of the third week,
the Moon appears to shrink in illumination, although it still appears to be
more than half-illuminated. This is
Waning Gibbous Moon, since it appears shrinking in illumination (waning) but
still appears more than half-illuminated (gibbous). At the end of this third week, the Moon
appears to be half-illuminated. This is
Third Quarter Moon, since we have completed the third quarter of the entire
lunar cycle. Gradually over the period
of the fourth week, the Moon continues to shrink in illumination, now appearing
less than half-illuminated. This is
Waning Crescent Moon, since it continues to appear shrinking in illumination
(waning) and now appears less than half-illuminated (crescent). Finally at the end
of the fourth week, we have returned to New Moon. Notice that during the first half of the
lunar cycle (roughly two weeks), the Moon is waxing, while during the second
half of the lunar cycle (also roughly two weeks), the Moon is waning. While the Moon is waxing, the west side of
the Moon appears illuminated. This is
because the illumination of the Moon is actually sunlight that the Moon
reflects, and the Sun is west of the Moon (the Moon is east of the Sun) while
the Moon is waxing. Conversely, while
the Moon is waning, the east side of the Moon appears illuminated. Again, the illumination of the Moon is
actually sunlight that the Moon reflects, and the Sun is east of the Moon (the
Moon is west of the Sun) while the Moon is waning.
The phases of the Moon occur
because the Moon orbits the Earth while the Earth orbits the Sun. When the Moon is approximately between the Sun
and the Earth, the far side of the Moon is facing toward the Sun, making it
daytime on the far side. The near side
of the Moon must therefore be facing away from the Sun, making it nighttime on
the near side, but this means that anyone on the Earth looking at the near side
of the Moon sees its nighttime side, which means we cannot see the Moon! This is New Moon, when we cannot see the
Moon. Over the following two weeks, more
and more of the near side of the Moon enters into daytime. This is why the Moon appears waxing during
this time. When the Moon has arrived on
the opposite side of its orbit, the Earth is approximately between the Sun and
the Moon. Hence, the near side of the
Moon is facing toward the Sun, making it daytime on the near side. This means that anyone on the Earth looking
at the near side of the Moon sees its entire face illuminated. This is Full Moon. Note that the Full Moon is the only phase of
the Moon when the far side of the Moon coincides with the nighttime side of the
Moon. Over the following two weeks, more
and more of the near side of the Moon enters into nighttime. This is why the Moon appears waning during
this time. Eventually, the Moon returns
to where it began in its orbit, meaning that we have returned to New Moon. Note that the near side of the Moon is
entirely nighttime and the far side of the Moon is entirely daytime during the
New Moon.
When the Moon is crescent
(either waxing or waning), less than half of its near side is in daytime and
more than half of its near side is in nighttime. However, the sunlight that reflects off of the daytime side of the Earth does illuminate this
nighttime part of the near side of the Moon.
This is called ashen light or earthshine. We can always see the nighttime part of the near
side of the Moon when the Moon is crescent, thanks to this ashen light or
earthshine.
Many people boast about being
able to determine the time of day from the position of the Sun in the sky, but
our discussion enables us to do something more impressive. We now discuss how we can determine the time
of day from the position of the Moon in the sky together with its phase. During Full Moon for example, the Earth is
approximately between the Sun and the Moon, as we discussed. Therefore, the Sun and the Moon appear to be
on opposite sides of the sky. If we
observe the Full Moon rising in the east, the Sun must be on the opposite side
of the sky. In other words, the Sun must
be setting in the west, making the clock time roughly 06:00 p.m. on average
over the course of the entire year. If
we observe the Full Moon culminating (highest altitude in the sky while
crossing the Celestial Meridian, as we discussed earlier in the course), the
Sun must be on the opposite side of the sky.
In other words, the Sun must be crossing the Celestial Meridian below
the horizon, making the clock time roughly midnight. If we observe the Full Moon setting in the
west, the Sun must be on the opposite side of the sky. In other words, the Sun must be rising in the
east, making the clock time roughly 06:00 a.m. on average over the course of
the entire year. As another example of
this exercise, the First Quarter Moon is halfway through the waxing part of the
lunar cycle, as we discussed. Therefore,
the Sun is west of the Moon and the angle formed by the Sun, the Earth, and the
Moon is approximately a right angle. In
other words, the Sun will appear to be roughly ninety degrees to the west of
the Moon in the sky. If we observe the
First Quarter Moon rising in the east, the Sun must be roughly ninety degrees
to its west. In other words, the Sun
must be culminating (highest altitude in the sky while crossing the Celestial
Meridian), making the clock time roughly noon on average over the course of the
entire year. If we observe the First
Quarter Moon culminating (highest altitude in the sky while crossing the
Celestial Meridian), the Sun must be roughly ninety degrees to its west. In other words, the Sun must be setting in
the west, making the clock time roughly 06:00 p.m. on average over the course
of the entire year. If we observe the
First Quarter Moon setting in the west, the Sun must be roughly ninety degrees
to its west. In other words, the Sun must
be crossing the Celestial Meridian below the horizon, making the clock time
roughly midnight on average over the course of the entire year. As yet another
example of this exercise, the Third Quarter Moon is halfway through the waning
part of the lunar cycle, as we discussed.
Therefore, the Sun is east of the Moon and the angle formed by the Sun,
the Earth, and the Moon is approximately a right angle. In other words, the Sun will appear to be
roughly ninety degrees to the east of the Moon in the sky. If we observe the Third Quarter Moon rising
in the east, the Sun must be roughly ninety degrees to its east. In other words, the Sun must be crossing the
Celestial Meridian below the horizon, making the clock time roughly midnight on
average over the course of the entire year.
If we observe the Third Quarter Moon culminating (highest altitude in
the sky while crossing the Celestial Meridian), the Sun must be roughly ninety
degrees to its east. In other words, the
Sun must be rising in the east, making the clock time roughly 06:00 a.m. on average
over the course of the entire year. If
we observe the Third Quarter Moon setting in the west, the Sun must be roughly
ninety degrees to its east. In other
words, the Sun must be culminating (highest altitude in the sky while crossing
the Celestial Meridian), making the clock time roughly noon on average over the
course of the entire year. We can also
apply this logic to determine the time of day from the position of the Moon in
the sky when its phase is Waxing Crescent Moon, Waxing Gibbous Moon, Waning
Gibbous Moon, and Waning Crescent Moon.
If the plane of the Moon’s
orbit around the Earth were the same as the ecliptic plane (the plane of the
Earth’s orbit around the Sun), then whenever the Moon was New Moon it would be
directly between the Sun and the Earth.
Thus, it would cast a shadow upon the Earth. Also in this case,
whenever the Moon is Full Moon then the Earth would be directly between the Sun
and the Moon. Thus, the Earth would cast
a shadow upon the Moon. However, the plane
of the Moon’s orbit around the Earth is not the same as the ecliptic plane (the
plane of the Earth’s orbit around the Sun).
The plane of the Moon’s orbit around the Earth is inclined to the
ecliptic plane. Hence, when the Moon is
New Moon, it does not cast a shadow on the Earth on most occasions, and when
the Moon is Full Moon, the Earth does not cast a shadow on the Moon on most
occasions. However, if the Moon happens
to be New Moon while passing through the ecliptic plane, it will cast a shadow
upon the Earth. This is
called a solar eclipse. If the Moon
happens to be Full Moon while passing through the ecliptic plane, the Earth
will cast a shadow upon the Moon. This is called a lunar eclipse.
If the plane of the Moon’s orbit around the Earth were the same as the
ecliptic plane, then a solar eclipse would occur with every New Moon and a
lunar eclipse would occur with every Full Moon.
In other words, both solar eclipses and lunar eclipses would occur much
more frequently if the plane of the Moon’s orbit around the Earth were the same
as the ecliptic plane.
A lunar eclipse occurs when
the Moon happens to be Full Moon while passing through the ecliptic plane. Thus, the Earth casts a shadow upon the
Moon. Only those who happen to live on
the nighttime side of the Earth witness a lunar eclipse occurring. Anyone who happens to live
on the daytime side of the Earth will not witness the lunar eclipse as
it occurs. The entire lunar eclipse
lasts for several hours. During a lunar
eclipse, the Moon begins as a Full Moon.
Then, the Earth’s penumbra gradually passes over the Full Moon,
darkening the Moon somewhat. The less
dark part of any shadow is its penumbra, while the more dark part of any shadow
is its umbra. After the Full Moon has
become somewhat darkened by the Earth’s penumbra, the Earth’s umbra then
gradually passes over the already darkened face of the Moon. Eventually, the entire face of the Moon is in
the Earth’s umbra. This is called the totality of the lunar eclipse, which lasts for
roughly one hour, although the duration of totality may be longer or shorter
depending on the particular lunar eclipse.
After totality, the Earth’s umbra gradually recedes off
of the Moon, followed by the Earth’s penumbra gradually receding off of
the Moon, returning the Moon to Full Moon.
Again, the entire lunar eclipse lasts for several hours. Everything we have described is actually called a total lunar eclipse. At times, the Moon is Full Moon while only
partially passing through the ecliptic plane.
Hence, the Earth’s shadow does not completely cover the Full Moon. The Earth’s shadow only covers a part of the
Full Moon. In other words, totality
never occurs. This is
called a partial lunar eclipse.
On most occasions, the Full Moon misses the ecliptic plane entirely, and
no lunar eclipse occurs.
A solar eclipse occurs when
the Moon happens to be New Moon while passing through the ecliptic plane. Thus, the Moon casts a shadow on the
Earth. Only those who happen to live
within that small shadow on the daytime side of the Earth will witness a solar
eclipse occurring. Anyone
who happens to live on the nighttime side of the Earth and even all those who
happen to live on the daytime side of the Earth but outside of the Moon’s
shadow will not witness the solar eclipse as it occurs. Even when a solar eclipse does occur, on most
occasions the Moon’s shadow lands in the middle of the ocean, since the Earth is mostly covered with water. Therefore, although lunar eclipses and solar
eclipses occur with roughly the same frequency, it is much more
rare for a human to witness solar eclipses as compared to witnessing
lunar eclipses. For those who witness a
solar eclipse, the Moon appears to gradually move in
front of the Sun. This causes greater
and greater darkness until the Moon completely eclipses the Sun. This is called the
totality of the solar eclipse, which lasts for only a few minutes, although the
duration of totality may be longer or shorter depending on the particular solar
eclipse. During totality, the sky
becomes nearly as dark as nighttime; even stars become visible in the sky. Now we must briefly digress and discuss an
amazing coincidence. The physical
diameter of the Sun is roughly four hundred times the physical diameter of the
Moon, but the Sun happens to be roughly four hundred times further than the
Moon, causing the Sun to appear four hundred times smaller than it would appear
at the same distance as the Moon. Stated
the other way around, the physical diameter of the Moon is roughly four hundred
times smaller than the physical diameter of the Sun, but the Moon happens to be
roughly four hundred times closer than the Sun, causing the Moon to appear four
hundred times larger than it would appear at the same distance as the Sun. It is a remarkable coincidence that the ratio
of the physical diameters of the Sun to the Moon is equal to the ratio of the
distances of the Sun and the Moon from the Earth. As a result of this
remarkable coincidence, the Moon and the Sun appear to be the same size in the
sky as seen from the Earth. More
technically, the Moon and the Sun have the same angular diameter, roughly
one-half of one degree, as observed from the Earth. Therefore, during the totality of a solar
eclipse, the Moon eclipses the Sun so perfectly that we can see the solar
corona around the Moon. As we will
discuss later in the course, the solar corona is the Sun’s atmosphere. Millions of years ago, the Moon was closer to
the Earth, and the Moon has been drifting further and further from the Earth
due to the transfer of the Earth’s rotational angular momentum to the Moon, as
we discussed. Since the Moon was closer
to the Earth millions of years ago, the Moon appeared to be larger than the Sun
in the sky and therefore eclipsed not just the Sun but its corona as well. Millions of years from now, the Moon will be
further from the Earth and therefore will appear smaller than the Sun in the
sky. Therefore, totality will not
occur. We happen to be alive when the
Moon is at the ideal distance from the Earth to appear to have the same angular
diameter of the Sun as seen in the sky, thus causing a totality that still
leaves the solar corona visible around the Moon. After the totality of the solar eclipse, the
Moon appears to gradually move off of the Sun, and
daylight gradually returns. Everything
we have described is actually called a total solar
eclipse. At times, the Moon is New Moon
while only partially passing through the ecliptic plane. Hence, the Moon does not appear to pass
directly in front of the Sun; the Moon appears to partially
miss the Sun. In other words,
totality never occurs. It does become
darker and darker as the Moon moves in front of the Sun, but the sky does not
become as dark as nighttime. This is called a partial solar eclipse. Note that even during a total solar eclipse,
only those who happen to be within the Moon’s umbra (the more dark part of its
shadow) will witness the total solar eclipse; those who happen to be within the
Moon’s penumbra (the less dark part of its shadow) just outside of the Moon’s
umbra will witness a partial solar eclipse instead. There is a third type of solar eclipse called
the annular solar eclipse. The Moon’s
orbit around the Earth is an ellipse.
Hence, the distance between the Moon and the Earth varies. The Moon’s closest position to the Earth is
the Moon’s perigee, while the Moon’s furthest position from the Earth is the
Moon’s apogee. Suppose the Moon happens
to be at its apogee when it happens to be New Moon while it happens to be
passing through the ecliptic plane. The
result is still a solar eclipse, but the Moon would appear to be smaller than
usual (smaller than the Sun in the sky), since the Moon is at its apogee
(furthest from the Earth). Hence,
totality never occurs. At best, a
smaller-appearing Moon would be in front of the larger-appearing Sun. Thus, the Sun will appear to be a ring around
the Moon. The technical term for a ring
is an annulus. Hence, the Sun will
appear to be an annulus around the Moon.
This is why this is called an annular solar
eclipse. To summarize, there are three
different types of solar eclipses: total solar eclipses, partial solar
eclipses, and annular solar eclipses.
Millions of years ago when the Moon was closer to the Earth, the Moon
always appeared larger than the Sun in the sky.
Therefore, there were no annular solar eclipses millions of years ago;
there were only total solar eclipses (that also eclipsed the solar corona in
addition to the Sun itself) as well as partial solar eclipses. Millions of years from now when the Moon is
further from the Earth, the Moon will always appear smaller than the Sun in the
sky. Therefore, there will no longer be
total solar eclipses millions of years from now; there will only be annular
solar eclipses as well as partial solar eclipses. We happen to be alive when the Moon is at the
ideal distance from the Earth for us to enjoy all three varieties of solar
eclipses: total, partial, and annular. Note that today on most occasions, the New Moon misses the ecliptic
plane entirely, and no solar eclipse of any type occurs. Eclipse hunters are persons who are wealthy
enough to travel wherever necessary to be within the Moon’s shadow during a
solar eclipse to witness the solar eclipse as it occurs. Ideally, eclipse hunters travel wherever
necessary to be within the Moon’s umbra during a total solar eclipse. On most occasions, the Moon’s shadow lands in
the middle of the ocean. Therefore, the
most wealthy eclipse hunters have their own yachts that they take into the
ocean to wherever the Moon’s shadow happens to be located during a solar
eclipse. We conclude the topic of solar
eclipses with a stern warning about the Sun in general, not just during solar
eclipses. Never ever
observe the Sun through a telescope.
Never ever observe the Sun through
binoculars. Never ever
observe the Sun even with the naked eye.
Solar observations done incorrectly causes permanent blindness. It is a common misconception that during a
solar eclipse, it is supposedly somewhat safe to look at the Sun, since some of its light is being eclipsed by the Moon. This is false. The part of the Sun that has
not been eclipsed by the Moon is just as dangerous to observe as during
any other circumstance. The only
exception to this stern warning is during the totality of a total solar
eclipse. The Moon fits so perfectly over
the Sun in the sky that during these brief few minutes, it is safe to observe
with the naked eye the solar corona around the Moon.
Let us discuss eclipses
purely from the frame of reference of the Earth. As we discussed, the plane of the Earth’s
orbit around the Sun is called the ecliptic
plane. As a result
of the Earth’s orbit around the Sun, the Sun appears to wander around the
Celestial Sphere (around the sky) along a circle called the ecliptic, which is
actually the projection of the ecliptic plane onto the Celestial Sphere (onto
the sky). Since the plane of the Moon’s
orbit around the Earth is inclined to the ecliptic plane, the Moon appears to
wander around the sky along a different circle, the projection of its orbit
onto the sky. However, this circle does
intersect the ecliptic at two points.
When the Moon happens to pass through one of these intersections while
the Sun happens to be at the other intersection on the opposite side of the
sky, we have a lunar eclipse. When the Moon
happens to pass through one of these intersections while the Sun happens to be
at the same intersection, we have a solar eclipse. This finally justifies why the Sun’s apparent
path around the sky is called the ecliptic. The word ecliptic is
derived from the word eclipse.
The Sun is always on the ecliptic by definition, but eclipses occur when
the Moon happens to pass through the ecliptic while the Sun is either at the
same intersection or at the opposite intersection of the Sun’s apparent path
around the sky (the ecliptic) and the Moon’s apparent path around the sky.
As we discussed, the orbital
period of the Moon is one month, but we must clearly define what we mean by one
month. Suppose it is New Moon, which is
when the Moon is approximately between the Sun and the Earth. In the time it takes
the Moon to complete one full orbit, the Earth has moved significantly in its
orbit around the Sun. Therefore, it is
not yet New Moon after this full lunar orbit.
We must wait somewhat longer for the Moon to move somewhat more so that
it is again New Moon (again approximately between the Sun and the Earth). We now realize that the time it takes the
Moon to complete one orbit is less than the time it takes the Moon to go from
New Moon to New Moon. In other words,
the time it takes the Moon to go from New Moon to New Moon is longer than the
time it takes the Moon to complete one orbit.
The time it takes the Moon to go from New Moon to New Moon is called a
synodic month. However, this synodic
month is longer than the time it takes the Moon to complete one orbit. The time it takes the Moon to complete one
orbit is called the sidereal month, which is shorter
than the synodic month. We can roughly
calculate the duration of time of the sidereal month. We will regard a synodic month as roughly
thirty days as a rough estimate.
Although a year is somewhat accurately 365 and ¼ solar days, this is
close enough to 360 solar days that we will use this as a rough estimate. If we divide 360 days by roughly thirty days
for each synodic month, we conclude that there are roughly twelve synodic
months in one year. Notice that both the
Julian calendar and the Gregorian calendar divide the year into twelve months. If there are roughly twelve synodic months in
one year and if there are 360 degrees around the complete orbit of the Earth,
then the Earth moves roughly one-twelfth of 360 degrees in its orbit with each
synodic month. One-twelfth of 360
degrees is thirty degrees. Therefore,
the Earth moves roughly thirty degrees in its orbit with each synodic
month. This stands to reason. We have already estimated that the Earth
moves roughly one degree in its orbit with each passing day. If there are roughly thirty days in a synodic
month and if the Earth moves roughly one degree in its orbit in each day, then
the Earth moves roughly thirty degrees in its orbit with each synodic
month. Therefore, we must calculate the
time it takes the Moon to move an additional thirty degrees in its orbit, which
is a 30/360 fraction of a complete orbit.
The fraction 30/360 reduces to the fraction 1/12. Therefore, we must calculate the time it
takes the Moon to move an additional 1/12 of a complete orbit. As a rough estimate, we will use 30 days as a
complete orbit. Therefore, the time it takes
the Moon to move 1/12 of its orbit is 1/12 of thirty days, or 2.5 days. Therefore, the sidereal month is between two
and three days shorter than the synodic month.
In other words, the synodic month is between two and three days longer
than the sidereal month. Somewhat more accurate calculations reveal that the synodic
month is roughly 29.5 solar days, and the sidereal month is roughly 2.2 solar
days shorter than the synodic month. In
other words, the sidereal month is roughly 27.3 solar days.
Just as geography is the
naming of topographical features on Earth such as oceans, mountains, and
rivers, selenography is the naming of topographical features on the Moon such
as maria and craters. The largest mare on the Moon is Oceanus Procellarum, which is Latin for the Ocean of Storms. The first humans who stepped foot on the Moon
landed on Mare Tranquillitatis, which is Latin for
the Sea of Tranquility. The three
astronauts of the Apollo 11 mission were Neil Armstrong, Buzz Aldrin, and
Michael Collins. After traveling to the
Moon and entering into lunar orbit in the command module named the Columbia, Neil Armstrong and Buzz Aldrin
climbed into the small lunar module named the Eagle that detached from the Columbia. Michael Collins remained in the command
module Columbia while orbiting the
Moon. Hence, Michael Collins never
stepped foot on the Moon. Neil Armstrong
and Buzz Aldrin piloted the Eagle and
landed in Mare Tranquillitatis (the Sea of
Tranquility) on Sunday, July 20, 1969.
Neil Armstrong’s exact words upon landing on the Moon were “The Eagle has landed,” since the name of the
lunar module was the Eagle. After spending several hours checking
instruments and taking measurements, Neil Armstrong opened the door of the Eagle, climbed down a ladder, and became
the first human being to step foot on the Moon.
His exact words upon stepping foot on the Moon were “This is one small
step for a man, but one giant leap for mankind.” Then, Buzz Aldrin climbed down the ladder and
became the second human being to step foot on the Moon. They planted an American flag, gathered some
Moon rocks, climbed back into the Eagle,
and launched off of Mare Tranquillitatis
(the Sea of Tranquility). They piloted
the Eagle to lunar orbit, reconnected
with Michael Collins in the command module Columbia,
and all three of these astronauts returned safely to the Earth. The same procedure was
carried out by the astronauts of the Apollo 12, Apollo 14, Apollo 15, Apollo
16, and Apollo 17 missions. The same procedure should have been carried out by the Apollo 13
astronauts, but an accident occurred while traveling to the Moon. All three astronauts of the Apollo 13 mission
traveled to the Moon, orbited the Moon, and returned safely to the Earth;
however, none of them were able to step foot on the Moon. Thus, only twelve human beings have stepped
foot on the Moon. There have been twelve
human beings who have traveled to the Moon and who have orbited the Moon but
who have not stepped foot on the Moon.
Note that this includes the astronauts of the Apollo 8 and Apollo 10
missions, two earlier missions that sent astronauts to orbit the Moon without
landing on the Moon. Therefore, there
have been twenty-four human beings in all of human history who have traveled to
the Moon and who have orbited the Moon.
Half of those twenty-four astronauts have actually stepped foot on the
Moon.
The Terrestrial Planets and their Moons
During the young forming
Solar System, the inner part of the protoplanetary disk was closer to the Sun
and therefore was more subject to the Sun’s heat. Since the inner part of the protoplanetary
disk was warmer, only materials with hotter melting temperatures were able to
condense, in particular metal and rock.
However, most of the protoplanetary disk was composed of hydrogen and
helium; metal and rock constituted only a small fraction of the protoplanetary
disk. Therefore, only these small
amounts of metal and rock were able to condense closer to the Sun. Therefore, small and dense planetesimals condensed in the inner protoplanetary disk
closer to the Sun. These planetesimals grew larger and larger through accretion
until the planetesimals became so large that their
self-gravity became strong enough to force themselves into spherical
shapes. These objects were now so large
with so much mass and therefore strong enough self-gravity that they began to
attract even more material around them, causing them to grow even larger. Eventually, they became the four terrestrial
planets closer to the Sun: Mercury, Venus, Earth, and Mars.
As these metallic and rocky planetesimals grew to become terrestrial planets, they
continuously collided with material in the protoplanetary disk during the heavy
bombardment period of the young Solar System.
These collisions were sticky collisions, converting a significant
fraction of the kinetic energy (moving energy) of the colliding objects into
thermal energy (heat energy). Therefore,
the terrestrial planets became warmer and warmer as they formed. Eventually, the terrestrial planets became so
hot that they became almost entirely molten. We conclude that accretion is a source of
internal energy. However, accretion only
served as an important source of internal energy during the heavy bombardment
period of the young Solar system. When
most planetesimals had accreted together to form
planets, few planetesimals remained to collide with
planets, thus ending the heavy bombardment period. While the terrestrial planets were almost
entirely molten, they differentiated themselves as more dense metals sank
toward their centers while less dense rock rose toward their surfaces,
ultimately forming dense metallic cores surrounded by less dense mantles
surrounded by even less dense and thin crusts.
Differentiation is also a source of internal energy. As more dense materials sink, gravitational
energy is converted to thermal energy (heat
energy). Differentiation is completed when materials have mostly separated based on
their densities. The differentiation of
the terrestrial planets was completed in a relatively
short amount of time, roughly one hundred million years. After this brief early history of the Solar
System, neither accretion nor differentiation served as significant sources of
internal energy for the terrestrial planets.
For most of the history of the Solar System, the terrestrial planets
derived most of their internal energy from radioactive decay. Any given radioactive sample becomes less and
less radioactive over time, since more and more of the radioactive parent atoms
are transmuted into daughter atoms leaving fewer and
fewer radioactive atoms to decay.
Therefore, any given radioactive sample was more radioactive in the
past. Consequently, the terrestrial
planets derived even more internal energy from radioactive decay billions of
years ago while they were still forming.
In summary, the most important source of internal energy for the
terrestrial planets is radioactive decay.
This is the only source of internal energy for most of the geologic
lifetime of a terrestrial planet, and it is an even greater source of internal
energy when a young terrestrial planet is forming, when it also derives
internal energy from accretion and from differentiation.
The internal energy within a
terrestrial planet drives its geologic activity, and the most important source
of internal energy is radioactive decay.
The internal energy heats the interior of the terrestrial planet,
causing some layers of rock to be weak, perhaps even
partially molten. A large terrestrial
planet would have a long geologic lifetime, since it would have more mass and
therefore more radioactive atoms and hence a greater supply of internal
energy. A small terrestrial planet would
have a short geologic lifetime, since it would have less mass and therefore
less radioactive atoms and hence a smaller supply of internal energy. Over billions of years, more and more
radioactive atoms composing a terrestrial planet decay, leaving a smaller and
smaller supply of internal energy. The
interior of the planet cools, resulting in more and more of the interior of the
planet becoming more and more rigid, resulting in less and less geologic
activity. Eventually, most of the
radioactive atoms of the terrestrial planet will have decayed. With very little radioactive atoms remaining
to provide internal energy, the interior of the terrestrial planet becomes
nearly entirely solid. Geologic activity
ends, and the terrestrial planet becomes geologically dead. Since a large terrestrial planet has a long
geologic lifetime, there is abundant geologic activity to cover or eradicate
craters caused by asteroid impacts and comet impacts. Hence, large terrestrial planets do not have
heavily cratered surfaces. Since a small
terrestrial planet has a short geologic lifetime, there is no geologic activity
for most of its history to cover or eradicate craters caused by asteroid
impacts and comet impacts. Hence, small
terrestrial planets have heavily cratered surfaces.
A terrestrial planet
generates a magnetic field from circulating currents of molten metal in its
core caused by the rotation of the planet.
Therefore, the two equally important variables that create a terrestrial
planet’s magnetic field is a metallic core that is at least partially
molten and reasonably rapid rotation.
Again, internal energy from radioactive decay is responsible for heating
the interior of the terrestrial planet, causing part of the metallic core to be
molten. A large terrestrial planet will
maintain its magnetic field over its long geologic lifetime assuming it has
reasonably rapid rotation, while a small terrestrial planet can only maintain
its magnetic field over its short geologic lifetime assuming it even has
reasonably rapid rotation. Eventually,
most of the radioactive atoms of the terrestrial planet will have decayed. With very little radioactive atoms remaining
to provide internal energy, the interior of the terrestrial planet becomes
nearly entirely solid. As the metallic
core gradually solidifies, the terrestrial planet’s magnetic field weakens and
ultimately dies, even if the planet has reasonably rapid rotation.
An atmosphere is a thin layer
of gas gravitationally held to a moon, a planet, or a star. Although terrestrial planets are born with
primary atmospheres composed of almost entirely hydrogen gas and helium gas,
the gravitational attraction of a terrestrial planet is too weak to retain this
primary atmosphere. A terrestrial
planet’s secondary atmosphere is composed of an abundance of water vapor and
nitrogen gas and carbon dioxide gas.
These gases come from volcanic outgassing. Since terrestrial planets are born almost entirely molten, the volcanic eruptions everywhere on
the surface of the planet eject not just lava but gases as well, thus forming
the secondary atmosphere. Note that
water vapor and carbon dioxide are greenhouse gases. Thus, a terrestrial planet’s secondary
atmosphere causes the planet to become warmer than it would have been
otherwise, based on its particular distance from the Sun.
As we discussed, the geologic
history of a terrestrial planet is determined by its
size. Similarly, the atmospheric history
of a terrestrial planet is strongly, but not exclusively, determined by its
size. A small terrestrial planet has less mass and therefore weak gravity, insufficient to retain
even its secondary atmosphere. In
addition, a small terrestrial planet has a short geologic lifetime; thus, a
small terrestrial planet cannot maintain its secondary atmosphere, since there
is no ongoing geologic activity that would result in volcanic outgassing. Moreover, a small terrestrial planet can only
maintain its magnetic field over its short geologic lifetime, assuming it even
has reasonably rapid rotation. After its
magnetic field dies, a small terrestrial planet can no longer protect its
atmosphere from the Sun’s solar wind, which will substantially ionize any weak
atmosphere that the small terrestrial planet may have. After only a few hundred million years, a
small terrestrial planet is left with virtually no
atmosphere. At best,
there will be an exosphere that begins at the surface of the small terrestrial
planet that smoothly transitions into the very cold temperatures of the
surrounding outer space. A large
terrestrial planet has more mass and therefore strong
gravity, sufficient to retain its secondary atmosphere. In addition, a large terrestrial planet has a
long geologic lifetime; thus, a large terrestrial planet can maintain its
secondary atmosphere, since there is ongoing geologic activity that results in
volcanic outgassing. Moreover, a large
terrestrial planet can maintain its magnetic field over its long geologic
lifetime, although this assumes the planet has reasonably rapid rotation. The large terrestrial planet’s magnetic field
will protect its atmosphere from significant ionization from the Sun’s solar
wind. In summary, a large terrestrial
planet will have a substantial atmosphere over its long geologic lifetime. The lowest atmospheric layer of a large
terrestrial planet is the troposphere, which becomes cooler with increasing
elevation, since we are further and further from the surface of the planet and
thus further and further from the heat radiated from its surface. Since there may be
significant convection (circulation) of air in the troposphere and since the
large terrestrial planet’s gravity pulls most of its atmosphere down into its
troposphere, the vast majority of all meteorological phenomena (weather) on the
planet occurs within its troposphere, the lowest layer of its atmosphere, although
this generalization may be altered by the rotation of the planet. The next atmospheric layer of most large
terrestrial planets is the thermosphere, which becomes warmer with increasing
elevation, since we are closer and closer to the ionosphere, the layer within
the upper thermosphere composed of various atoms and molecules that have
absorbed X-rays from the Sun, thus shielding the surface of the large
terrestrial planet from the Sun’s X-rays.
There is no convection (circulation) of air within a planet’s
thermosphere, since the air in the thermosphere remains layered based on
temperature. Beyond the planet’s
thermosphere is the exosphere, where the air becomes cooler and cooler,
smoothly transitioning into the very cold temperatures of the surrounding outer
space. Note that planet Earth is an
extraordinary exception to this generalization.
Due to the presence of life on Earth, there is an
abundance of oxygen in the Earth’s atmosphere, which absorbs far ultraviolet
from the Sun forming an ozonosphere, which not only shields the surface of the
Earth from the Sun’s far ultraviolet but is also responsible for two additional
layers of its atmosphere: the stratosphere and the mesosphere both sandwiched
between the troposphere and the thermosphere. No other terrestrial planet has an abundance
of oxygen in its atmosphere; therefore, no other terrestrial planet’s
atmosphere shields the planet from the Sun’s far ultraviolet,
nor does any other terrestrial planet’s atmosphere have a stratosphere or a
mesosphere.
In addition to its size, the
atmospheric history of a terrestrial planet is strongly determined by its
distance from the Sun. If a large
terrestrial planet is sufficiently far from the Sun, the planet will be sufficiently
cool that the water vapor of its secondary atmosphere may freeze to solid water
(ice), subtracting this important greenhouse gas from the atmosphere, making
the planet even cooler. If a large
terrestrial planet is sufficiently close to the Sun, the planet will be
sufficiently warm that the water vapor of its secondary atmosphere will remain
in the gaseous state, leaving this important greenhouse gas in the atmosphere,
making the planet even warmer. If a
large terrestrial planet is at an intermediate distance from the Sun, the planet
may be at the ideal temperature for some water vapor to freeze into solid water
(ice), for some water vapor to condense into liquid water, and for the
remaining water vapor to remain in the gaseous state. As a result, the terrestrial planet remains
at an intermediate temperature where water can exist in all three of its
physical states: solid water (ice), liquid water, and water vapor. Note that the distance from the Sun would be
a less important variable for the atmospheric history of a small terrestrial
planet, since the short geologic lifetime of such a planet would prevent it
from maintaining a substantial atmosphere for longer than a few hundred million
years. Even a large terrestrial planet
would deviate from this analysis if it had slow rotation resulting in a weak
magnetic field, which would expose the planet’s atmosphere to substantial
ionization from the Sun’s solar wind.
The first terrestrial (inner)
planet is Mercury, the closest planet to the Sun. The orbital period of Mercury around the Sun
is roughly three months. Mercury is the
smallest of the eight planets. Because
of its small size, Mercury presumably had very little radioactive atoms to
provide internal energy. Therefore,
Mercury was geologically alive for only a brief amount of time, perhaps a few
hundred million years. For most of its
history, Mercury has been a geologically dead planet. Also due to its small size and small mass,
Mercury’s gravity is sufficiently weak that it cannot retain an
atmosphere. Without geologic activity
for most of its history to provide volcanic outgassing to replenish a lost
atmosphere, Mercury has had virtually no atmosphere for most of its
lifetime. At best, there
is an exosphere that begins at the surface of Mercury that smoothly
transitions into the very cold temperatures of the surrounding outer
space. Since Mercury has been
geologically dead for most of its lifetime, there has been no geologic activity
for most of its history to cover or eradicate craters caused by asteroid
impacts and comet impacts. Hence, the
surface of Mercury is much more cratered as compared
with the surface of the Earth. Much of
what we have described about planet Mercury is similar to the Earth’s
Moon. In fact, Mercury’s actual
appearance is also similar to the appearance of the Earth’s Moon.
Whereas the Earth’s Moon has
an unusually small metallic core as compared with its overall size, Mercury has
an unusually large metallic core as compared with its overall size. This usually large metallic core may explain
why Mercury generates a modest magnetic field.
The dominant theory to explain Mercury’s unusually large metallic core
is as follows. Mercury was actually born
a somewhat larger terrestrial planet, roughly the size of Mars but not as large
as Venus or Earth. This would make
Mercury’s metallic core more reasonably proportioned as compared with its
original larger size. While the young
Mars-sized Mercury was still geologically alive, a violent collision with a
large planetesimal stripped off its outer layers, making
the entire planet smaller and leaving it with a disproportionately large
metallic core. As we discussed, a
violent collision between the young Earth and a large planetesimal
ripped huge chunks of molten rock from the outer layers of the Earth. In the case of the Earth however, these
chunks of molten rock did not have enough energy to escape from the Earth’s
gravitational attraction. Thus, these
chunks of molten rock remained in orbit around the Earth, eventually forming
the Earth’s Moon. The young Mercury was
already Mars-sized before it suffered from its own violent collision. Hence, Mercury was already smaller than the
Earth, and so Mercury already had less mass and therefore weaker gravitational
attraction as compared with the Earth.
Immediately after the violent collision, Mercury lost a significant
fraction of its mass, making its gravitational attraction even weaker. Therefore, the chunks of molten rock ripped
from Mercury’s outer layers were able to escape its very weak gravitational
attraction. Therefore, this violent
collision did not form a Mercurian moon.
Since Mercury lost a large fraction of its mass from this violent
collision, it also lost a large fraction of its radioactive atoms to supply it
with internal energy to keep it geologically alive. Therefore, Mercury suffered from sudden
internal cooling, as it abruptly transformed from a geologically alive
terrestrial planet to a geologically dead terrestrial planet. Most substances contract when they cool. If this theory is
correct, the already smaller Mercury would have suddenly contracted to an even
smaller size, not only giving it an even more disproportionately large metallic
core but also resulting in Mercury becoming the smallest of the eight planets
of the Solar System. This sudden
contraction would have fractured (broken) Mercury’s outer layers. Indeed, there are scarps (steep cliffs)
throughout the surface of planet Mercury, which geophysicists believe are
actually fractures (breaks) caused when Mercury suffered from this sudden
thermal contraction as it transformed from a geologically alive terrestrial
planet to a geologically dead terrestrial planet. In summary, a violent
collision between a young Mars-sized Mercury with a large planetesimal
explains why Mercury is the smallest of the eight planets of the Solar System,
explains why Mercury has an unusually large metallic core as compared with its
present overall size, explains why Mercury has no moons despite suffering from
a violent collision, and explains the abundance of scarps (steep cliffs)
throughout the surface of Mercury.
It is a common misconception
that Mercury is the hottest planet in the Solar System, since it is the closest
planet to the Sun. This is false. As we will discuss shortly, Venus is the
hottest planet in the Solar System. Of
course, Mercury’s daytime side (facing toward the Sun) is very hot. However, Mercury has virtually no atmosphere
and has no liquid water to regulate the temperature between its daytime side
and its nighttime side, as is the case with planet Earth. Therefore, the nighttime side of Mercury
(facing away from the Sun) is very cold.
Indeed, Mercury is one of the hottest planets and one of the coldest planets
at the same time! The poles of Mercury
receive very little sunlight, and some regions near the poles are in permanent
darkness due to shadows cast by nearby landforms. As a result, these shadowed regions remain
perpetually near absolute zero temperature, on the closest planet to the Sun!
The second terrestrial
(inner) planet is Venus, the second planet from the Sun. The orbital period of Venus around the Sun is
roughly seven months. Venus is almost
exactly the same size as the Earth, and Venus has almost exactly the same mass
as the Earth. With almost the same size
and mass, Venus therefore has almost exactly the same density and gravity as
the Earth. For all these reasons, Venus
is sometimes considered Earth’s twin sister planet. However, the similarities with the Earth end
here. Venus has no moons, unlike the
Earth. Venus has an incredibly thick
atmosphere that completely covers the planet.
The thick atmosphere of Venus reflects so much sunlight that Venus
appears to be the brightest object in the Earth’s sky, aside from the Sun and
the Moon of course. Since Venus is
closer to the Sun than the Earth, Venus always appears near the Sun in the
Earth’s sky. Since the Sun is in the sky
in the daytime, our only hope of observing Venus in the Earth’s sky is in the
west shortly after sunset or in the east shortly before sunrise. Venus appears so bright in the west shortly
after sunset that it is often called the evening star,
and Venus appears so bright in the east shortly before sunrise that it is often
called the morning star. Again, the
evening star and the morning star are not stars; they are actually the planet
Venus. The atmosphere of Venus so
thoroughly covers the planet that we cannot observe Venusian surface features
through our telescopes. We knew nothing
about the topography of Venus until we sent artificial satellites to orbit
Venus. These probes used radar to
penetrate the thick Venusian atmosphere.
Thus, we have successfully mapped the topography of Venus using radar
from the satellites that we have sent to orbit Venus.
Since Venus is almost the
same size as the Earth, it has roughly the same amount of radioactive atoms and
thus roughly the same supply of internal energy as the Earth. Therefore, Venus has a geologic lifetime
similar to the Earth. Indeed, Venus is
still geologically alive, as is the Earth.
For example, there is evidence of both igneous activity and seismic
activity on Venus. There are also
mountains on Venus. The tallest mountain
on Venus is Maxwell Montes. Although
Venus is still geologically alive, the Theory of Plate Tectonics is not the
correct theory to explain the geologic activity on Venus, and geophysicists
have not yet come to an agreement on the correct theory to explain the geologic
activity of Venus. Although Venus is
still geologically alive and therefore has a partially molten
metallic core, Venus also has very slow rotation. Hence, Venus has a very weak magnetic
field. As a result, the Venusian
atmosphere is substantially ionized by the Sun’s solar
wind. As we would expect, the Venusian
atmosphere begins with a troposphere that cools with increasing elevation
followed by a thermosphere that warms with increasing elevation as we approach
the ionosphere in the upper thermosphere.
Beyond the thermosphere is the exosphere, where the temperature smoothly
transitions into the very cold temperatures of the surrounding outer
space. Without life to provide oxygen,
the Venusian atmosphere has no ozonosphere and therefore no stratosphere and
mesosphere sandwiched between the troposphere and the thermosphere. Without an ozonosphere, the surface of Venus
is not shielded from far ultraviolet from the Sun.
Since it is almost the same
size as the Earth, Venus should have outgassed a secondary atmosphere similar
to the Earth’s secondary atmosphere.
However, since Venus is closer to the Sun than the Earth, hotter
temperatures caused more water vapor and carbon dioxide to be
extracted from Venusian volcanic activity. Since both water vapor and carbon dioxide are
greenhouse gases, this caused the Venusian atmosphere to trap more heat, making
the planet even hotter. This caused even
more water vapor and carbon dioxide to be extracted
from Venusian volcanic activity, causing the Venusian atmosphere to trap even
more heat, making the planet hotter still, and so on and so forth. The result was a runaway greenhouse effect,
causing Venus to become the hottest planet in the Solar System, even though it
is the second planet from the Sun. This
also explains the incredibly thick atmosphere of Venus. The Venusian atmosphere is so thick that the
air pressure on the surface of Venus is enormous as compared with the Earth’s
air pressure. Note however that very
little water vapor remains in the Venusian atmosphere. Since Venus does not have a magnetic field to
shield its atmosphere from the Sun’s solar wind, most of the outgassed water
vapor was attacked by the Sun’s solar wind, stripping hydrogen atoms off of the water molecules.
The hydrogen atoms then escaped from the gravitational attraction of
Venus. There are clouds within the
Venusian atmosphere, but these are not like clouds in the Earth’s
atmosphere. Clouds in
the Earth’s atmosphere are composed of and precipitate (rain) liquid water. However, the clouds within
the Venusian atmosphere are composed of and precipitate (rain) sulfuric acid,
which is poisonous. It is so hot
on Venus that most of the sulfuric acid rain evaporates before reaching the
ground, although there may be gaseous sulfuric acid near the surface of
Venus. If we were to stand on the
surface of Venus, we would be crushed by the air pressure,
poisoned by the sulfuric acid, burned to a crisp by the greenhouse effect, and
irradiated by the far ultraviolet from the Sun all at the same time! A lack of oxygen thus causing suffocation
would be the least of our worries!
The third terrestrial planet
is the Earth-Moon system, the third planet from the Sun. However, we have already discussed this third
planet in tremendous detail. Therefore,
we now discuss the fourth and last of the terrestrial (inner) planets: Mars,
the fourth planet from the Sun. The
orbital period of Mars around the Sun is roughly two Earth years. Since Mars is smaller than the Earth, it has less mass and thus weak gravity as compared with the
Earth. Hence, Mars is
less severely differentiated as compared with the Earth, resulting in a
greater abundance of metals such as iron on the surface of Mars as compared
with the Earth. The abundance of iron on
the surface of Mars has reacted with oxygen forming iron oxide (rust), which
has a reddish color. This is why Mars is
red. In fact, the nickname of Mars is
the Red Planet. Mars even appears red to
the naked eye (without the aid of a telescope or even binoculars) as seen in
the Earth’s sky. Mars has an obliquity
(axial tilt) similar to the Earth, causing seasons on Mars similar to the
seasons on Earth. Mars has polar ice
caps similar to the Earth’s polar ice caps.
Mars also rotates once in roughly twenty-four hours, just like the Earth. For all of these reasons, Mars is sometimes regarded as being Earth’s twin brother
planet. However, the similarities with
the Earth end here. Mars is smaller than
the Earth and Venus, but Mars is not as small as Mercury or the Earth’s Moon. Therefore, Mars had more
radioactive atoms and thus a greater supply of internal energy than
Mercury and the Earth’s Moon, giving Mars a longer geologic lifetime than
Mercury or the Earth’s Moon. However,
Mars did not have as much radioactive atoms and thus did not have as great a
supply of internal energy as the Earth or Venus, giving Mars a shorter geologic
lifetime than the Earth or Venus. Mars
was probably geologically alive for a couple billion years, and Mars has
probably been geologically dead for a couple billion years. In other words, Mars is the only terrestrial
(inner) planet that has been both geologically alive for a significant duration
of time and geologically dead for a significant duration of time. Therefore, studying Mars may offer insight
into what Earth will become billions of years from now after it has exhausted
its own geothermal energy. This would
imply that Mars may have been similar to the Earth in
the distant past.
While
it was still geologically alive, Mars outgassed a secondary atmosphere similar
to the Earth’s secondary atmosphere, composed primarily of water vapor,
nitrogen gas, and carbon dioxide gas. Also while Mars was still geologically alive, it still had a
partially molten metallic core. Together
with a rotation rate that is roughly equal to Earth’s rotation rate, ancient
Mars generated a substantial magnetic field while it was geologically
alive. This magnetic field shielded the
ancient Martian atmosphere from substantial ionization from the Sun’s solar
wind. Since water vapor and carbon
dioxide are greenhouse gases, the ancient Martian atmosphere caused Mars to
become warmer than it would have been otherwise, based on its distance from the
Sun. The temperature of ancient Mars may
have permitted water to exist in all three of its physical states: solid water
(ice), liquid water, and water vapor. Hence, some of the water vapor in the ancient
Martian atmosphere could condense into liquid water, precipitating (raining)
onto the planet. Indeed, there is
evidence that there were rivers of running water, lakes of water, and perhaps
even an ocean of water on ancient Mars.
Although the condensation of water vapor into liquid water subtracted
this greenhouse gas from the ancient Martian atmosphere, the abundance of
carbon dioxide remaining in the ancient Martian atmosphere still maintained a
warm ancient Martian temperature, based on its distance from the Sun. Also, the continuing
geologic activity caused volcanic outgassing, replenishing the atmosphere with
both water vapor and carbon dioxide, both greenhouse gases that helped to
maintain the warm ancient Martian temperature.
Ancient Mars was indeed similar to the Earth, both geologically and
atmospherically. To our knowledge
however, there was never life on Mars.
Therefore, the Martian atmosphere (both ancient and present) begins with
a troposphere that cools with increasing elevation followed by a thermosphere
that warms with increasing elevation as we approach the ionosphere in the upper
thermosphere. Beyond the thermosphere is
the exosphere, where the temperature smoothly transitions into the very cold
temperatures of the surrounding outer space.
Without life to provide oxygen, the Martian atmosphere (both ancient and
present) has no ozonosphere and therefore no stratosphere and mesosphere
sandwiched between the troposphere and the thermosphere. Without an ozonosphere, the surface of Mars
(ancient and present) is not shielded from far ultraviolet
from the Sun.
Since Mars is smaller than
Earth and Venus, it had fewer radioactive atoms and thus did not have as great
a supply of internal energy as Earth or Venus, giving Mars a shorter geologic
lifetime than Earth or Venus. The
Earth-like conditions we have described on ancient Mars could
only be maintained for a couple billion years. As Mars exhausted its supply of internal
energy, its interior cooled, resulting in more and more of the interior of the
planet becoming more and more rigid, resulting in less and less geologic
activity and a weaker and weaker magnetic field. With less and less geologic activity, there
was less and less volcanic outgassing to replenish the Martian atmosphere. With a weaker and weaker magnetic field, Mars
could no longer shield its atmosphere from substantial ionization from the
Sun’s solar wind. Also
since Mars is smaller than Earth and Venus, it has less mass and therefore
weaker gravity, insufficient to retain its dwindling atmosphere. As the Martian atmosphere became thinner and
thinner, its greenhouse effect became weaker and weaker, making the planet
colder and colder. Eventually, Mars
became so cold that water could no longer exist in the liquid state. Most of the water froze into solid water
(ice) at the Martian poles, forming the Martian ice caps. Although the present Martian atmosphere is
composed mostly of carbon dioxide which is a
greenhouse gas, the present Martian atmosphere is too thin to cause a
significant greenhouse effect.
Therefore, Mars is presently significantly colder than the Earth, which
is appropriate since Mars is further from the Sun than the Earth. Also as a result of
the weakening Martian magnetic field, any remaining water vapor in the Martian
atmosphere was attacked by the Sun’s solar wind, stripping hydrogen atoms off
of the water molecules. The hydrogen
atoms then escaped from the gravitational attraction of Mars. The remaining oxygen atoms bonded with each
other to form molecular oxygen, which then reacted with the abundance of iron
on the surface of Mars to form iron oxide (rust), giving the surface of Mars is
famously red color. Over the past couple
billion years, Mars has had no geologic activity to cover or eradicate craters
caused by asteroid impacts and comet impacts.
Hence, the surface of Mars is more cratered as
compared with the surfaces of Earth or Venus.
However, the surface of Mars is not as cratered as the surface of Mercury
or the Earth’s Moon, since those terrestrial planets have been geologically
dead for much longer than Mars. In terms
of size, mass, gravity, geologic lifetime, atmospheric pressure, and impact
cratering, Mars is intermediate between two extremes. At one extreme, Earth and Venus have larger
sizes, are more massive, have stronger gravity, have long geologic lifetimes,
have substantial atmospheric pressure, and have fewer impact craters. At the other extreme, Mercury and the Earth’s
Moon have smaller sizes, are less massive, have weaker gravity, have short
geologic lifetimes, have virtually zero atmospheric pressure, and have an
abundance of impact craters. Between
these two extremes, Mars has intermediate size, is intermediate in mass, has
intermediate gravity, has an intermediate geologic lifetime, has intermediate
(thin but appreciable) atmospheric pressure, and has an intermediate number of
impact craters.
While the young Mars was
still geologically alive, it built the largest geologic features in the entire
Solar System. For example, Olympus Mons
is not only the largest, tallest mountain on Mars,
Olympus Mons is also the largest, tallest mountain in the entire Solar
System. It is much larger and much
taller than Mount Everest on Earth for example.
Olympus Mons is actually an extinct volcano that formed from igneous
activity when the young Mars was still geologically alive. Valles Marineris is
not only the longest, widest, deepest canyon on Mars,
Valles Marineris is also the longest, widest, deepest
canyon in the entire Solar System. It is
much longer, wider, and deeper than the Grand Canyon on Earth for example. Canyons on Earth form from rivers of running
water; the Grand Canyon was carved by the Colorado
River for example. Although we discussed
that there were rivers of running water on ancient Mars, Valles Marineris is too enormous to have been
carved by a river. Valles Marineris probably formed from an enormous seismic
disruption of the Martian crust when the young Mars was still geologically
alive. Utopia Planitia
is not only the largest
impact basin on Mars, Utopia Planitia is also the
largest impact basin in the entire Solar System. Utopia Planitia is
so enormous that astronomers formerly believed that it was simply a plain,
which is why it was named Utopia Planitia.
Mars has two moons: Phobos and Deimos.
Both of these moons are very small.
They were probably asteroids in the asteroid belt that Jupiter’s gravity
ripped out of the asteroid belt, as we will discuss shortly. Since the orbit of Mars is right next to the
asteroid belt, it would be somewhat likely for the gravity of Mars to capture
an asteroid that Jupiter’s gravity ripped from out of the asteroid belt. Both Phobos and
Deimos are tidally locked to Mars, meaning that their
rotational periods are equal to their orbital periods around Mars. Hence, both of these moons always face the
same side toward Mars as they orbit Mars.
The Jovian, Gas-Giant Planets, their Moons, and their Rings
During the formation of the
Solar System, the frost line was the precise distance from the Sun beyond which
volatile ices were able to condense in addition to metal and rock. Volatile ices are hydrogen compounds, such as
methane CH4, ammonia NH3,
and water H2O. Closer to the
Sun than the frost line, the protoplanetary disk was
sufficiently warmed by the Sun that only materials with hot melting
temperatures were able to condense, such as metal and rock. Further from the Sun than the frost line, the
protoplanetary disk was sufficiently cool that volatile ices with cooler
melting temperatures were able to condense in addition to metal and rock. As we will discuss later in the course, the
very early universe was composed entirely of hydrogen and helium, and stars
later synthesized small amounts of carbon, nitrogen, and oxygen and even
smaller amounts of silicon, iron, and nickel.
As a result, even later generations of star systems, such as our own
Solar System, were born from a nebula that was mostly hydrogen and helium with
small amounts of volatile ices and even smaller amounts of metal and rock. Only metal and rock were able to condense in
the warmer inner parts of the protoplanetary disk, but metal and rock only
accounted for tiny fractions of the composition of the Solar System. This is why the terrestrial (inner) planets
are small as compared with the jovian,
gas-giant (outer) planets. Volatile
ices, which are more abundant than metal and rock, were able to condense in the
cooler, outer parts of the protoplanetary disk in addition to metal and
rock. Therefore, large planetesimals composed of volatile ices in addition to
metal and rock condensed in the outer Solar System. These large planetesimals
grew even larger through accretion, eventually becoming large enough with
sufficient self-gravity to force themselves into spherical shapes. These planetesimals
began to differentiate themselves, with the most dense
metals sinking toward their centers, the least dense volatile ices rising
toward their surfaces, and moderately dense rock settling in the intermediate
layer between the metallic core and the icy outer layers. As these planetesimals
continued to grow through accretion, they eventually became so massive that
they began to gravitationally attract the surrounding
hydrogen and helium, which accounted for the vast majority of the mass of the
protoplanetary disk. Consequently, these
outer planets grew to enormous sizes as compared with the terrestrial (inner) planets. These outer planets ultimately became
gas-giant planets, but note that this term is misleading. Although the outer layers of these so-called
gas-giant planets are indeed mostly hydrogen gas and helium gas, these
so-called gas-giant planets have sufficiently strong gravity to compress these
gases to exotic densities in deep (interior) layers. Beneath the hydrogen gas and helium gas are
layers of dense liquid hydrogen, at even deeper layers are higher density
electrically conducting hydrogen (commonly known as metallic hydrogen), and at
deeper layers still are even higher density volatiles ices. Beneath the volatile ices, we would find a
very high-density layer of rock before we would finally encounter the most dense metallic core of a so-called gas-giant
planet. In summary, small terrestrial
planets formed in the protoplanetary disk closer to the Sun than the frost
line, while large jovian
planets formed in the protoplanetary disk further from the Sun than the frost
line. The frost line was obviously
somewhere between the orbit of Mars and the orbit of Jupiter. More precisely, the frost line was somewhere
between the asteroid belt and the orbit of Jupiter. We will discuss the asteroid belt shortly.
As we discussed, the final
stage of formation of these jovian,
gas-giant planets was the gravitational attraction of large amounts of hydrogen
gas and helium gas that fell onto the forming planet. The collapsing material rotated faster,
ensuring that the angular momentum of the gas remained conserved (remained
constant). This explains why the jovian, gas-giant (outer) planets
have fast rotations as compared with the terrestrial (inner) planets. The fast rotations have caused each of the jovian, gas-giant (outer) planets
to be severely oblate: bulged out at the equator and pushed in at the
poles. The terrestrial (inner) planets
also rotate, causing their shapes to be oblate as well. However, the terrestrial (inner) planets
rotate slowly as compared with the jovian,
gas-giant (outer) planets. Hence, the oblateness of each of the terrestrial (inner) planets is
very small. To the naked eye, the shape
of the four terrestrial (inner) planets seems to be perfectly spherical. The jovian,
gas-giant (outer) planets rotate sufficiently fast that they are so severely
oblate that they are noticeably oblate in photographs or when observed through
telescopes. The fast rotations of the
gas-giant (outer) planets also cause circulating currents of electrically
conducting hydrogen (metallic hydrogen) in deeper layers of the planet. These circulating currents of electrically
conducting hydrogen (metallic hydrogen) in turn create extraordinarily powerful
magnetic fields. Indeed, the magnetic
fields of all the jovian,
gas-giant (outer) planets are much stronger than even Earth’s magnetic field,
which is the strongest magnetic field among the terrestrial (inner) planets of
the Solar System.
The material that fell toward
young forming jovian, gas-giant (outer) planets was
sufficiently abundant that the final stages of formation of each of these
planets was a microcosm of the formation of the entire Solar System. Most of the material collapsed to form the
outer layers of the planet. As the rest
of the material collapsed, collisions between particles within the collapsing
cloud became more frequent, since the gravitational collapse brought the
particles closer together. These
collisions tended to be sticky, with colliding particles merging into larger
masses. By the law of conservation of
translational (linear) momentum, the resulting larger masses had less motion
along the direction of the axis defined by the total angular momentum of the
collapsing cloud, since the collisions averaged out their more random motions
in this direction. By
the law of conservation of energy, the resulting larger masses were
heated, since much of the kinetic energy (moving energy) of the colliding
particles was converted to thermal energy (heat energy). By the law of conservation of angular
momentum, the resulting larger masses had more circular
orbits, since the collisions averaged out their more random orbits, many
of which were more elliptical. In
summary, the laws of physics together caused the gravitationally collapsing
material to flatten into a circular, rotating disk perpendicular to the axis of
the total angular momentum of the forming jovian,
gas-giant planet. This disk is a
microcosm of the protoplanetary disk that formed the planets orbiting the
Sun. We will call this smaller disk
around a forming jovian,
gas-giant planet a protolunar disk, since large-sized moons and even
medium-sized moons would eventually form from this flat, circular, rotating
disk. First, small-sized baby moons
condensed from the protolunar disk. As
these baby moons orbited the forming planet, they grew larger through accretion,
the gaining of mass through sticky collisions.
As the baby moons orbited the forming planet, they collided with
material along their orbits, which stuck to the baby moons. Thus, the baby moons grew larger and larger
through accretion until they became so large that their self-gravity became
strong enough to force themselves into spherical shapes, thus becoming
medium-sized moons and even large-sized moons.
This explains why most of the large-sized and medium-sized moons of the jovian, gas-giant (outer) planets
orbit their mother planet in nearly the same plane with roughly circular (but
nevertheless slightly elliptical) orbits in the same direction around the
planet. This is also enough to explain
why these planets have many moons. In
addition, these jovian,
gas-giant (outer) planets are so massive that they can gravitationally capture
asteroids that Jupiter rips out of the asteroid belt, as we will discuss
shortly. These captured asteroids become
small-sized moons of the planet, explaining why these jovian, gas-giant (outer) planets have dozens of
small-sized moons. The medium-sized
moons and certainly the large-sized moons that formed around the jovian planets had sufficient mass and hence sufficiently
strong self-gravity to differentiate themselves as they formed, with the most
dense metals sinking toward their centers surrounded by a layer of less dense
rock. Most of these large-sized moons
and medium-sized moons have icy outer layers that surround the rocky layers.
The first of the jovian, gas-giant (outer) planets
is Jupiter, the fifth planet from the Sun.
The orbital period of Jupiter around the Sun is roughly twelve Earth
years. Jupiter is the largest planet in
the Solar System. Jupiter is also the
most massive planet in the Solar System.
In fact, Jupiter is more massive than the rest of the material in the
Solar System combined, excluding the Sun of course. The Sun is itself one thousand times
Jupiter’s mass. In other words, Jupiter
is more massive than the rest of the planets and their moons and the asteroids and the meteoroids and the comets and the dust all
combined together. Stated another way,
roughly 99.9% of the mass of the Solar System is the Sun, the remaining 0.1% of
the mass of the Solar System is Jupiter, and the rest of the material in the
Solar System is insignificant by comparison.
Since Jupiter has tremendous mass, Jupiter exerts tremendous
gravity. Caution: since Jupiter’s mass
is one-thousandth of the Sun’s mass, Jupiter’s gravity is still weak as
compared with the Sun’s gravity. The
Sun’s gravitational attraction is the one and only one thing that holds the
Solar System together. Nevertheless,
Jupiter’s gravity is very strong as compared with the other planets. Hence, Jupiter exerts sufficiently strong
gravity that nearly all perturbative (small but noticeable) gravitational
effects in the Solar System are the result of Jupiter’s influence. For example, Jupiter’s gravity perturbs the
orbits of the other planets, causing their orbits to alternate from more elliptical
to less elliptical and back again. In
particular, Jupiter’s gravity causes the Earth’s orbit to alternate in
eccentricity once every roughly one hundred thousand years causing major
glacial periods of the Earth’s Current Ice Age, as we discussed. Jupiter’s gravity also causes the elliptical
orbits of the other planets to turn very slowly. The slow turning of an elliptical orbit is called orbital precession, which we will discuss again
later in the course. Jupiter’s gravity
can also change the orbit of a comet that passes close enough to Jupiter. Astronomers witnessed a spectacular example
of this in the year 1992 when Jupiter’s gravity not only changed the orbit of
comet Shoemaker-Levy but also ripped that comet apart into more than twenty
comet fragments. These comet fragments
did not have enough energy to escape Jupiter’s gravity. Thus, all of these comet fragments collided
with Jupiter in the year 1994. Jupiter’s
gravity also rips asteroids out of the asteroid belt. We will discuss the asteroids and the
asteroid belt in detail shortly. For
now, most asteroids orbit the Sun in the asteroid belt, which is between the
orbit of Mars and the orbit of Jupiter.
Since these asteroids in the asteroid belt are closer to the Sun than
Jupiter, their orbital periods around the Sun are shorter than Jupiter’s
orbital period around the Sun. Suppose
an asteroid in the asteroid belt has an orbital period around the Sun
commensurate with Jupiter’s orbital period, meaning that the ratio of the
asteroid’s orbital period to Jupiter’s orbital period is a simple ratio (a
simple fraction), such as one-to-two or two-to-five or four-to-seven. In this case, the asteroid would come near
Jupiter at regular intervals. With each
encounter, Jupiter’s gravity would pull on the asteroid. After many such periodic tugs, Jupiter’s
gravity would eventually succeed in ripping the asteroid out of the asteroid
belt. The jovian, gas-giant (outer) planets have sufficiently
strong gravity to capture these asteroids that Jupiter rips out of the asteroid
belt. The captured asteroids would thus
become small-sized moons of these planets.
This explains why all of the jovian,
gas-giant (outer) planets have many small-sized moons. The terrestrial (inner) planets by contrast
have insufficient gravity to capture asteroids that Jupiter rips out of the
asteroid belt. This explains why the
terrestrial (inner) planets should have no moons. Mars is an exception since its orbit is right
next to the asteroid belt; this gives Mars a somewhat higher probability of
capturing asteroids that Jupiter has ripped out of the asteroid belt. We have already discussed that the Earth’s
Moon is also an exception, since it formed from a violent collision between a planetesimal and the young forming Earth. If we graph the number of asteroids in the
asteroid belt as a function of their orbital periods around the Sun, we see
gaps in the graph at the orbital periods where there are no asteroids in the
asteroid belt. Indeed, these gaps in
this graph are at orbital periods that are commensurate (simple ratios) with
Jupiter’s orbital period. These gaps are called the Kirkwood gaps, named for the American
astronomer Daniel Kirkwood who discovered and correctly explained the origin of
these gaps. There is however
a peak in this graph at one simple fraction, the ratio of one-to-one. If the ratio is one-to-one, these asteroids
have the same orbital period around the Sun as Jupiter’s orbital period around
the Sun, but this implies that these asteroids orbit the Sun on Jupiter’s own
orbit around the Sun. Indeed, there are
two groups of asteroids orbiting the Sun on Jupiter’s own orbit. These are the Trojan asteroids. One group of Trojan asteroids orbits the Sun
sixty degrees ahead of Jupiter, and the other group of Trojan asteroids orbits
the Sun sixty degrees behind Jupiter.
More technically, the Trojan asteroids are locked
orbiting the Sun at the fourth and fifth Lagrangian
points of the combined gravitational fields of the Sun and Jupiter.
Jupiter is the fastest rotating
planet in the Solar System, rotating more than twice as fast as planet
Earth. This rapid rotation has caused
Jupiter to be severely oblate, as is the case with all the jovian, gas-giant (outer) planets. Jupiter’s fast rotation also generates a strong
magnetic field, as is the case with all the jovian, gas-giant (outer) planets. In fact, among all the planets, Jupiter
generates the strongest magnetic field in the entire Solar System. Jupiter’s rapid rotation has also divided its
atmosphere into many prevailing winds, which appear as cloud bands. There is a giant storm in Jupiter’s southern
hemisphere called the Great Red Spot.
The Great Red Spot is enormous; it is roughly two Earth diameters across
and roughly one Earth diameter wide! The
Great Red Spot was discovered through powerful telescopes in the year 1830, but
there is evidence that it may have been discovered by smaller
telescopes more than a century earlier.
The Great Red Spot seems to have existed for at least as long as
telescopes have been powerful enough to observe it, for at least two or perhaps
three centuries. This suggests that
giant storms are long-lived phenomena on jovian,
gas-giant planets.
Jupiter has a weak ring
system. Since its ring system is weak,
we will defer a discussion of rings until we discuss Saturn, which has a
spectacular ring system. Jupiter has
roughly one hundred moons, but most of them are small-sized moons. They may have been asteroids that Jupiter
itself ripped out of the asteroid belt, and then Jupiter itself captured them
gravitationally. However, four of
Jupiter’s moons are enormous as compared with the other one hundred. These four moons are so enormous that Galileo
Galilei discovered them through his primitive telescope more than four hundred
years ago. Hence, they are called the Galilean moons in his honor. The correct order to list the moons of any
planet is from closest to the mother planet to furthest from the mother planet,
just as the correct order to list the planets is from closest to the Sun to
furthest from the Sun. The four Galilean
moons in the correct order from closest to Jupiter are Io, Europa, Ganymede,
and Callisto furthest from Jupiter. These four Galilean moons are
named for various lovers of the ancient mythological god Jupiter
(Zeus). All four of these moons are tidally locked to Jupiter, meaning that their rotational
periods are equal to their orbital periods.
Hence, they always face the same side towards Jupiter as they orbit
Jupiter. All four of these moons have
sufficient mass and therefore sufficiently strong self-gravity to have
differentiated themselves when they formed.
Hence, all four of these moons have a metallic core surrounded by less
dense rock. Most of them, with Io being
the exception, have even less dense icy outer layers surrounding the rocky
layers. All four of these moons orbit
Jupiter in nearly the same plane. All
four of these moons orbit Jupiter in the same direction, counterclockwise if
observed from above the plane of the Solar System. All four of these moons rotate in that same
direction as they orbit Jupiter in that same direction. Jupiter itself rotates in that same
direction. We see that the Jovian System
is a microcosm of the entire Solar System.
In this analogy, Jupiter itself is analogous to the Sun, and Jupiter’s
large-sized moons are analogous to the planets.
As we discussed, this is because these four moons formed from a
protolunar disk around Jupiter while Jupiter was first forming, just as the
planets formed from a protoplanetary disk around the Sun while the Sun was
first forming. This analogy between the
Jovian System and the entire Solar System is even stronger than this
however. Io and Europa are similar to
each other, Ganymede and Callisto are similar to each
other, and these two subsets of moons are somewhat opposite to each other in
character. This is remarkable; it is as
if there are two different categories of moons.
Both inner moons (Io and Europa) are of one category, and both outer
moons (Ganymede and Callisto) are of the other
category, just as there are two different categories of planets, with all four
inner planets (Mercury, Venus, Earth, and Mars) of one category and all four
outer planets (Jupiter, Saturn, Uranus, and Neptune) of the other category. This analogy between the Jovian System and
the Solar System is not perfect however.
The four inner planets are similar to one another because they were more
subject to the Sun’s intense heat as they were forming, while the four outer
planets are similar to one another because they were less subject to the Sun’s
intense heat as they were forming. The
key variable that resulted in two different categories of the Galilean moons is
not Jupiter’s intense heat but Jupiter’s intense
gravity. The two inner moons (Io and Europa)
are similar to each other because they formed closer to Jupiter and hence were
more subject to Jupiter’s intense gravity, while the two outer moons (Ganymede
and Callisto) are similar to one another because they
formed further from Jupiter and hence were less subject to Jupiter’s intense
gravity.
The large-sized inner moon Io
orbits so close to Jupiter that Jupiter’s intense gravity continuously tries to
rip Io apart. Io is large enough with
sufficient mass and therefore sufficient self-gravity to hold itself together
in a spherical shape however.
Nevertheless, Jupiter’s gravity melts rocks on Io, pulls this molten
rock into the interior of Io, and pulls this molten rock out of the interior of
Io again. As a result, Io is the most
volcanically active world in the entire Solar System. There is continuous igneous activity such as
lava extrusions and even volcanic eruptions nearly everywhere upon the surface
of Io. There are no impact craters
visible anywhere on the surface of Io.
Although asteroids and comets do collide with Io and these impacts do form craters, frequent lava eruptions almost
immediately cover these craters. The
mechanism to explain the geologic activity on Io is obviously not plate
tectonics; it is the strong gravitational tidal forces from Jupiter. We will call this mechanism strong tidal
heating. The large-sized inner moon
Europa is also more subject to Jupiter’s intense gravity as compared with the
outer moons. However, Europa is somewhat
further from Jupiter than Io. Therefore,
Europa is less subject to Jupiter’s gravity as compared with Io, making Europa
nowhere nearly as geologically active as Io.
Nevertheless, Jupiter’s gravity has succeeded in fracturing (cracking)
the ice that covers Europa. Some
astrophysicists speculate that Jupiter’s gravity has melted some of the ice
into liquid water beneath the fractured (cracked) ice that covers Europa. Moreover, there is evidence that Jupiter’s
gravity pushes large slabs of ice that float upon this deeper layer of liquid
water, rather like Earth’s moving tectonic plates that float upon the deeper
asthenosphere. There is even evidence of
cryovolcanoes on Europa that erupt ice instead of
molten rock. In other words, the Theory
of Plate Tectonics may apply to Europa but for giant slabs of floating ice
instead of giant slabs of floating rock!
We will use the term weak tidal heating for the mechanism to explain the
fractured (cracked) ice that covers Europa, the possible liquid water beneath that
fractured (cracked) ice, the possible moving tectonic ice plates floating upon
that deeper layer of liquid water, and the possible cryovolcanism
on Europa. If there is liquid water
beneath the fractured (cracked) ice that covers Europa, there might be very
primitive lifeforms, such as unicellular microorganisms, within that deep
liquid water. This however is unlikely,
since Europa is far enough from the Sun that it should be too cold for life to
exist. Nevertheless, if weak tidal
heating from Jupiter can provide sufficient heat to melt ice, that weak tidal
heating may also provide enough heat to maintain a sufficiently warm
temperature for life to exist.
The large-sized outer moon
Ganymede is much less subject to Jupiter’s gravity as compared with the inner
moons. Hence, Ganymede is geologically
dead, rather like planet Mercury and the Earth’s Moon. Since Ganymede is geologically dead, there is
very little geologic activity to remove or cover impact craters resulting from
collisions from asteroids and comets.
Hence, Ganymede is heavily cratered, again rather like planet Mercury
and the Earth’s Moon. Ganymede is
Jupiter’s largest moon. In fact,
Ganymede is the largest moon in the entire Solar System. Ganymede is so enormous that it is even
larger than planet Mercury! Hence, it is
possible for the moon of one planet to be larger than an entire other
planet. Note however that the mass of
Ganymede is significantly less than the mass of planet Mercury. A larger size and a significantly smaller
mass together make Ganymede much less dense than planet Mercury. Nevertheless, Ganymede is sufficiently large
with sufficient mass that it was born with a sufficient amount of radioactive
atoms to provide itself with internal energy for geologic processes for a few
hundred million years, just like planet Mercury and the Earth’s Moon. After most of the radioactive atoms decayed
and with insufficient tidal heating from Jupiter due to Ganymede’s far distance
from Jupiter, Ganymede has been geologically dead for most of its history. Much of what we have described about Ganymede
is similar to planet Mercury and the Earth’s Moon. In fact, Ganymede actually has an appearance
similar to planet Mercury and the Earth’s Moon.
Ganymede has heavily cratered older regions, just like the Earth’s Moon,
and Ganymede has sparsely cratered younger regions, just like the Earth’s
Moon. However, unlike planet Mercury and
the Earth’s Moon, Ganymede has icy outer layers that surround a deeper layer of
rock surrounding a metallic core. It is these icy outer layers that are heavily cratered. It is also these icy outer
layers that give Ganymede a significantly lower density than planet
Mercury. The large-sized outer moon Callisto is the furthest of the four Galilean moons. Hence, Callisto is
the least subject to Jupiter’s gravity among these four Galilean moons, making Callisto even more geologically dead than Ganymede. Therefore, Callisto
is even more heavily cratered than Ganymede. Nearly every square kilometer of Callisto is covered with impact
craters. In fact, Callisto
is the most heavily cratered world in the entire Solar System. Callisto has icy
outer layers surrounding a deeper layer of rock surrounding a metallic core,
similar to Europa and Ganymede. It is Callisto’s icy outer layers that
are heavily cratered, as is the case with Ganymede.
The second of the jovian, gas-giant (outer) planets
is Saturn, the sixth planet from the Sun.
The orbital period of Saturn around the Sun is roughly thirty Earth
years. Saturn is the second largest planet
in the Solar System, second only to Jupiter.
Saturn is the second most massive planet in the Solar System, second
only to Jupiter. Saturn is tenuous (low
density) since it is composed of hydrogen gas and helium gas surrounding a
deeper layer of volatiles ices surrounding an even deeper layer of rock
surrounding a metallic core, as are all of the jovian,
gas-giant (outer) planets. In fact,
Saturn is the most tenuous (least dense) planet in the entire Solar
System. The density of Saturn is
actually less than the density of liquid water.
Saturn would float in water if we had a bathtub large enough to hold
that much water! Note that the most dense planet in the Solar System happens to be planet
Earth. Saturn suffers from fast
rotation, causing Saturn to be severely oblate, as is the case with all the jovian, gas-giant (outer)
planets. Saturn’s fast rotation also
generates a strong magnetic field, as is the case with all the jovian, gas-giant (outer)
planets. In fact, among all the planets,
Saturn generates the second strongest magnetic field in the entire Solar
System, second only to Jupiter. Saturn’s
fast rotation has also divided its atmosphere into many prevailing winds that
appear as cloud bands. Saturn has more
than one hundred moons, but these are mostly small-sized moons. They may have been asteroids that Jupiter’s
gravity ripped out of the asteroid belt.
Saturn’s gravity is sufficiently strong that it could have captured
these asteroids after Jupiter’s gravity ripped them out of the asteroid
belt. Saturn does have one large-sized
moon named Titan and six medium-sized moons.
These seven moons listed in the correct order from closest to Saturn are
Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus furthest from Saturn. Again, Titan is the only large-sized moon,
while the other six moons are medium-sized moons. All seven of these moons are tidally-locked to Saturn.
All seven of these moons have sufficient mass and therefore sufficiently
strong self-gravity to have differentiated themselves when they formed. Hence, all seven of these moons have a
metallic core surrounded by less dense rock surrounded by even less dense icy
outer layers. All seven of these moons
orbit Saturn in nearly the same plane.
All seven of these moons orbit Saturn in the same direction. All seven of these moons rotate in that same
direction as they orbit Saturn in that same direction. Saturn itself also rotates in this same
direction. We see that the Saturnian System is a microcosm of the entire Solar System,
with Saturn itself being analogous to the Sun and Saturn’s moons being
analogous to the planets. As we
discussed, this is because these seven moons formed from a protolunar disk
around Saturn while Saturn was first forming, just as the planets formed from a
protoplanetary disk around the Sun while the Sun was first forming.
Mimas and Enceladus are
medium-sized inner moons of Saturn.
Therefore, these moons are more subject to Saturn’s gravity as compared
with the outer moons of Saturn. In
particular, Saturn’s gravity has fractured (cracked) the ice that covers
Enceladus. There is also evidence that
Saturn’s gravity has melted some of the ice into liquid water beneath the
fractured (cracked) ice that covers Enceladus.
Moreover, there are cryovolcanoes on Enceladus
that erupt ice instead of molten rock.
In summary, weak tidal heating from Saturn explains the fractured
(cracked) ice that covers Enceladus, the possible liquid water beneath that
fractured (cracked) ice, and the cryovolcanism on
Enceladus, just as weak tidal heating from Jupiter is responsible for similar
processes on Jupiter’s moon Europa. If
there is liquid water beneath the fractured (cracked) ice that covers
Enceladus, there might be very primitive lifeforms, such as unicellular
microorganisms, within that deep liquid water.
This however is unlikely, since Enceladus is far enough from the Sun
that it should be too cold for life to exist.
Nevertheless, if weak tidal heating from Saturn can provide sufficient
heat to melt ice, that weak tidal heating may also provide enough heat to
maintain a sufficiently warm temperature for life to exist. Again, we see that Saturn’s moon Enceladus is
very much similar to Jupiter’s moon Europa.
The icy outer layers of Enceladus surround a deeper layer of rock
surrounding a metallic core, again similar to Europa. Since Mimas is even closer to Saturn than
Enceladus, we expect Mimas to suffer from strong tidal heating from Saturn,
making Mimas similar to Jupiter’s moon Io.
However, this is not the case.
Firstly, Mimas has icy outer layers that surround deeper layers of rock
surrounding a metallic core, while Io lacks icy outer layers. Moreover, the ice that covers Mimas is heavily cratered similar to an outer moon, which implies
that Mimas is geologically dead as an outer moon should be, not an inner
moon. Astrophysicists continue to debate
why Mimas is geologically dead even though it is the innermost medium-sized
moon of Saturn. Mimas has an enormous
crater called the Herschel crater, named for the British astronomer William
Herschel who discovered Mimas. The
Herschel crater is nearly the size of the entire moon Mimas. Whatever object collided with Mimas to form
the Herschel crater would have completely obliterated this moon if that object
had been moving just a little bit faster or if that object had been somewhat
more massive. Therefore, the Herschel
crater on Mimas reveals that collisions can occur in the Solar System that are
powerful enough to obliterate medium-sized moons and certainly small-sized
moons. The Herschel crater gives Mimas
the appearance of the Death Star from the Star
Wars mythology. Mimas is also
roughly the size of the Death Star from the Star
Wars mythology. Therefore, the
nickname of Mimas is the Death Star Moon, and the nickname of the Herschel
crater is Darth Crater, a fanciful name based upon one of the most popular
characters from the Star Wars
mythology, Darth Vader.
Tethys, Dione, and Rhea are
medium-sized outer moons of Saturn.
Therefore, these moons are less subject to Saturn’s gravity as compared
with Mimas and Enceladus, making Tethys, Dione, and Rhea geologically dead
moons. Hence, there is very little
geologic activity on these moons to remove or cover impact craters resulting
from collisions from asteroids and comets.
Therefore, Tethys, Dione, and Rhea are heavily
cratered. In fact, Tethys has an
enormous crater called the Odysseus crater that is nearly the size of the
entire moon Tethys. Whatever object
collided with Tethys to form the Odysseus crater may have completely
obliterated this moon if that object had been moving just a little bit faster
or if that object had been somewhat more massive. Just as the Herschel crater on Mimas reveals
that collisions can occur in the Solar System that are powerful enough to
obliterate medium-sized moons and certainly small-sized moons, the Odysseus
crater on Tethys is further evidence that collisions can occur in the Solar
System that are powerful enough to obliterate medium-sized moons and
small-sized moons. Tethys, Dione, and
Rhea have icy outer layers that surround a deeper layer of rock surrounding a
metallic core. It is
these icy outer layers that are heavily cratered.
Titan is Saturn’s only
large-sized moon. This is an appropriate
name for Saturn’s largest moon, since the ancient mythological figure Saturn
(Kronos) was the youngest of the mythical titans. Titan is the second-largest moon in the
entire Solar System, second only to Ganymede (Jupiter’s largest moon). In fact, Titan is so enormous that it is even
larger than planet Mercury! Again, we
see that the moon of one planet can be even larger than an entire other
planet. Note however that the mass of
Titan is significantly less than the mass of planet Mercury. A larger size and a significantly smaller
mass together make Titan much less dense than planet Mercury. Titan has the most substantial atmosphere out
of all the moons in the entire Solar System, but Titan’s atmosphere is not
habitable for life. Although Titan’s
atmosphere is mostly nitrogen like Earth’s atmosphere, Titan’s atmosphere also
has an abundance of volatile ices and virtually no oxygen. Also, Titan is so
distant from the Sun that the temperature is too cold to sustain life. Titan’s atmosphere surrounds its icy surface which surrounds a deeper layer of rock surrounding a
metallic core.
Iapetus is the outermost
medium-sized moon of Saturn. Therefore,
Iapetus is much less subject to Saturn’s gravity as compared with inner moons,
making Iapetus a geologically dead moon.
Hence, there is very little geologic activity on Iapetus to remove or
cover impact craters resulting from asteroid impacts and comet impacts. Therefore, Iapetus is
heavily cratered. Iapetus has icy
outer layers surrounding a deeper layer of rock surrounding a metallic
core. It is the icy
outer layers of Iapetus that are heavily cratered. Iapetus has the most severe difference in
color between its two hemispheres than any other object in the Solar
System. In particular, the near side of
Iapetus (the hemisphere that always faces toward Saturn) is very dark in color,
while the far side of Iapetus (the hemisphere that always faces away from
Saturn) is very light in color.
Moreover, Iapetus has a steep ridge along its equator. Astrophysicists continue to debate why
Iapetus has such an extreme difference in color between its two hemispheres and
why it has such a steep ridge along its equator.
All four of the jovian, gas-giant (outer) planets,
Jupiter, Saturn, Uranus, and Neptune, have rings. However only Saturn has a spectacular ring
system. Jupiter, Uranus, and Neptune
have weak ring systems. Through a small
telescope, Saturn’s rings appear to be solid.
However, through a more powerful telescope, a large gap is visible in
the rings called the Cassini division.
Using an even more powerful telescope, we discover more gaps within the
rings, such as the Colombo gap, the Maxwell gap, the Huygens gap, the Encke gap, and the Keeler gap. Through an even more powerful telescope, we
see so many gaps that we realize that the rings are not solid at all. Saturn’s rings are actually composed of
thousands of individual ringlets. The
ringlets themselves are not solid. The
entire ring system is actually billions of individual chunks of metal, rock,
and volatile ices, and each chunk is on its own orbit around Saturn in accord
with Kepler’s laws. The light from all
these billions of chunks of metal, rock, and volatile ices blends
together, causing the rings to appear to be solid from the Earth through
a small telescope. If we interpret each
of these chunks of metal, rock, and volatile ices as a moon (they orbit Saturn
after all), then Saturn actually has billions of moons! The rings are thousands of kilometers across,
but the rings are also extremely thin, less than one kilometer thick! Saturn’s rings are so thin that they seem to
disappear when we happen to be looking at the rings edge on. The rings of all the jovian, gas-giant planets are caused by their own
intense gravity. When any object such as
an asteroid or a comet or even one of its own moons
ventures too close to the planet, the planet’s gravity rips the object apart
into many small fragments. The fragments
remain in orbit around the planet, becoming rings. Indeed, Saturn’s rings are closer to Saturn
than any of its seven medium-to-large-sized moons, implying that if an object
of such size were to approach Saturn closer, the object would
be ripped apart by Saturn’s gravity and become part of Saturn’s ring
system. The closest distance that a
small object of a given size can approach a large gravitating object before being torn apart by the gravity of the large object is
called the Roche limit, named for the French astrophysicist Édouard
Roche who first performed this calculation.
Saturn’s rings are closer to Saturn than the Roche limit of
medium-to-large-sized moons. This also
explains why all the jovian,
gas-giant (outer) planets have rings, while none of the terrestrial (inner)
planets have rings. The jovian, gas-giant (outer) planets
are large with more mass and therefore strong gravity as compared with the
terrestrial (inner) planets. Therefore, jovian, gas-giant planets can
capture objects that they can rip apart to form rings. The terrestrial (inner) planets are small
with less mass and therefore weak gravity as compared with the jovian, gas-giant (outer)
planets. Therefore, terrestrial planets
are much less likely to capture objects that they would be able to rip apart to
form rings. The rings of gas-giant planets
are thin and circular for the same reason the protoplanetary disk and
protolunar disks are thin and circular.
Various laws of physics together caused orbiting material to flatten
into a thin, circular, rotating disk perpendicular to the axis of the total
angular momentum of the planet. Over
time, fragments within rings continue to collide with each other. The collisions cause the fragments to lose
orbital speed. This in turn causes the
fragments to approach the planet and to eventually collide
with the planet. As a result, a
spectacular ring system such as Saturn’s rings only lasts a few hundred million
years. All of the jovian, gas-giant planets have had weak rings for
most of the history of the Solar System, including Saturn. On rare occasions, a jovian, gas-giant planet happens to capture and rip
apart a large number of asteroids and comets over a period of a few hundred
million years, resulting in a spectacular ring system. During some eras in the history of the Solar
System, perhaps Jupiter had a spectacular ring system while the other three
gas-giant planets, including Saturn, had weak ring systems. During other eras in the history of the Solar
System, perhaps Uranus had a spectacular ring system while the other three
gas-giant planets, including Saturn, had weak ring systems. We happen to be alive when Saturn happens to
have a spectacular ring system, while the other three gas-giant planets have
weak ring systems. As we will discuss
shortly, Neptune will have a spectacular ring system a few hundred million
years from now, while the other three gas-giant planets, including Saturn, will
have weak ring systems. Although it is
possible for more than one gas-giant planet in the Solar System to have spectacular
ring systems at the same time, this is sufficiently improbable that it most
likely never occurred.
What causes the gaps within
Saturn’s rings? If we use Kepler’s third
law to determine the orbital period around Saturn of a hypothetical object in
the Cassini division, we calculate that the object would have an orbital period
around Saturn exactly half the orbital period of Mimas, one of Saturn’s
medium-sized moons. Before the Cassini
division existed, certain chunks of metal, rock, and volatile ices within
Saturn’s rings encountered Mimas at regular intervals. The gravity of Mimas tugged on these bodies
regularly with each encounter.
Eventually, the gravity of Mimas ripped these bodies out of the ring
system, leaving the Cassini division within the rings. The other ring gaps formed similarly, from
gravitational encounters from other moons of Saturn. This is precisely the same mechanism that
caused the Kirkwood gaps within the asteroid belt. This mechanism is important enough to deserve
a name: orbital resonance. The Kirkwood
gaps are caused by orbital resonances with Jupiter,
the Cassini division is caused by orbital resonances with Mimas, and the other
gaps within Saturn’s rings are caused by orbital resonances with other moons of
Saturn. If the Saturnian
System is analogous to the entire Solar System, then Saturn’s rings would be
analogous to the asteroid belt, and all of the ring gaps would be analogous to
the Kirkwood gaps.
The two small-sized moons Telesto and Calypso orbit Saturn on the same orbit as the
medium-sized moon Tethys. Telesto orbits Saturn sixty degrees ahead of Tethys, while
Calypso orbits Saturn sixty degrees behind
Tethys. Telesto
and Calypso are locked orbiting Saturn at the fourth
and fifth Lagrangian points of the combined
gravitational fields of Saturn and Tethys.
The two small-sized moons Helene and Polydeuces
orbit Saturn on the same orbit as the medium-sized moon Dione. Helene orbits Saturn sixty degrees ahead of
Dione, while Polydeuces orbits Saturn sixty degrees behind Dione.
Helene and Polydeuces are locked
orbiting Saturn at the fourth and fifth Lagrangian
points of the combined gravitational fields of Saturn and Dione. These small-sized moons are beautifully
analogous to the Trojan asteroids orbiting the Sun on Jupiter’s own orbit. Therefore, we will call the small-sized moons
Telesto, Calypso, Helene, and Polydeuces
the Trojan moons of Saturn. To
summarize, the Saturnian System is a microcosm of the
entire Solar System. In this analogy,
Saturn itself is analogous to the Sun, Saturn’s moons are analogous to the
planets, Saturn’s rings are analogous to the asteroid belt, the ring gaps are
analogous to the Kirkwood gaps, and the Trojan moons of Saturn are analogous to
the Trojan asteroids.
All four of the jovian, gas-giant (outer) planets,
Jupiter, Saturn, Uranus, and Neptune, all share several physical
properties. They are all large as
compared with the terrestrial (inner) planets.
They are all tenuous, being composed of hydrogen gas and helium gas
surrounding a deeper layer of volatiles ices surrounding an even deeper layer
of rock surrounding a metallic core.
They all have many moons. They
all have rings. They all suffer from
fast rotation, generating strong magnetic fields and causing them to be
severely oblate. However, there are
significant differences among these planets, compelling us to subdivide the jovian, gas-giant planet category
into two subcategories. Jupiter and
Saturn are in one subcategory, while Uranus and Neptune are in the other
subcategory. Uranus and Neptune are composed
of significantly less outer layers of hydrogen gas and helium gas as compared
with Jupiter and Saturn. As a result,
deeper layers of volatile ices are more exposed with Uranus and Neptune as
compared with Jupiter and Saturn. The
exposed volatile ices give Uranus and Neptune exotic colors: greenish for
Uranus and bluish for Neptune. Some
astrophysicists go so far as to claim that only Jupiter and Saturn are truly
gas-giant planets. These astrophysicists
place Uranus and Neptune in a different category: ice-giant planets. We will not adopt this extreme
interpretation. We will regard all four
of the outer planets as gas-giant planets.
We will regard Jupiter and Saturn as one type of gas-giant planet, and
we will regard Uranus and Neptune as another type of gas-giant planet. Astrophysicists continue to debate why Uranus
and Neptune are composed of significantly less outer layers of hydrogen gas and
helium gas as compared with Jupiter and Saturn.
The third of the jovian, gas-giant (outer) planets
is Uranus, the seventh planet from the Sun.
The orbital period of Uranus around the Sun is roughly eighty-four Earth
years. Uranus is tenuous (low density),
being composed of hydrogen gas and helium gas surrounding a deeper layer of
volatiles ices surrounding an even deeper layer of rock surrounding a metallic
core. However, Uranus has less outer
layers of hydrogen gas and helium gas as compared with Jupiter and Saturn. As a result, deeper layers of volatile ices are exposed, giving Uranus a greenish color. Although Uranus is slightly larger in size than Neptune, the mass of Uranus is slightly
less than the mass of Neptune. A
slightly larger size and a slightly smaller mass together make Uranus
noticeably less dense than Neptune.
Uranus has a weak ring system and more than two dozen moons. Most of the moons of Uranus are small-sized
moons. They may have been asteroids that
Jupiter’s gravity ripped out of the asteroid belt. Uranus has sufficient gravity to capture them
after Jupiter ripped them out of the asteroid belt. Although Uranus does not have any large-sized
moons, Uranus does have five medium-sized moons. All five of these moons are
tidally locked to Uranus. All
five of these moons have sufficient mass and therefore sufficiently strong
self-gravity to have differentiated themselves when they formed. Hence, all five of these moons have a
metallic core surrounded by less dense rock surrounded by even less dense icy
outer layers. All five of these moons
orbit Uranus in nearly the same plane.
All five of these moons orbit Uranus in the same direction and rotate in
that same direction as they orbit Uranus.
Uranus itself rotates in that same direction. We see that the Uranian
System is a microcosm of the entire Solar System, with Uranus itself being analogous
to the Sun, the moons of Uranus being analogous to the planets, and the rings
of Uranus being analogous to the asteroid belt.
As we discussed, this is because these five moons formed from a
protolunar disk around Uranus while Uranus was first forming, just as the
planets formed from a protoplanetary disk around the Sun while the Sun was
first forming. These five moons listed
in the correct order from closest to Uranus are Miranda, Ariel, Umbriel, Titania, and Oberon furthest from Uranus. The largest of these five moons is Titania, although Titania is
still a medium-sized moon. Most of the
moons of Uranus, both medium-sized and small-sized, are named
for fictional characters from the plays of William Shakespeare, the greatest
writer of the English language.
Miranda and Ariel are
medium-sized inner moons of Uranus.
Therefore, these two moons are more subject to the gravity of Uranus as
compared with the outer moons of Uranus.
Since Miranda is the innermost medium-sized moon of Uranus, Miranda is therefore
the most subject to the gravity of Uranus as compared with the other
medium-sized moons of Uranus. The
gravitational forces from Uranus are so strong that the icy outer layers of
Miranda are severely fractured with countless canyons, scarps (steep cliffs),
and ridges. In fact, the tallest cliff
in the entire Solar System, named Verona Rupes, is on
Miranda. Verona Rupes
is roughly twenty kilometers high! If a
cliff of this height were on Earth, it would take roughly one minute to fall
from the top to the bottom of that cliff, hitting the ground at more than two
thousand kilometers per hour. However,
since Miranda is only a medium-sized moon, its surface gravity is more than one
hundred times weaker than the Earth’s surface gravity. As a result, it would take roughly twelve
minutes to fall from the top to the bottom of Verona Rupes,
hitting the icy surface of Miranda at only two hundred kilometers per
hour! The severe fracturing of the icy
outer layers of Miranda is also caused by cryovolcanism, which is itself also caused by the strong
gravitational forces from Uranus. In
summary, strong tidal heating from Uranus is responsible for the severe
fracturing of the icy outer layers of Miranda and the cryovolcanism
on Miranda. The fracturing of the icy
outer layers of Miranda is so severe that astronomers formerly believed that a
violent collision shattered Miranda into many pieces which
then accreted back together to recreate Miranda. However, this violent
collision model has been replaced by the modern strong tidal heating model to
explain the severe fracturing of the icy outer layers of Miranda. The medium-sized inner moon Ariel is also
more subject to the gravity of Uranus as compared with the outer moons of
Uranus. However, Ariel is somewhat further
from Uranus than Miranda. Therefore,
Ariel is less subject to the gravity of Uranus as compared with Miranda, making
the surface of Ariel nowhere nearly as fractured as the surface of Miranda. Nevertheless, the gravity of Uranus has
succeeded in fracturing the icy outer layers of Ariel. There is an abundance of canyons, scarps
(steep cliffs), and ridges throughout the icy outer layers of Ariel. There is even evidence of cryovolcanism
on Ariel. In summary, weak tidal heating
from Uranus causes the fracturing of the icy outer layers of Ariel and the cryovolcanism on Ariel, just as the large-sized moon Europa
suffers from weak tidal heating from Jupiter and the medium-sized moon
Enceladus suffers from weak tidal heating from Saturn.
Umbriel, Titania,
and Oberon are medium-sized outer moons of Uranus. Therefore, these three moons are less subject
to the gravity of Uranus as compared with Miranda and Ariel, making Umbriel, Titania, and Oberon geologically dead moons. Hence, there is very little geologic activity
on these three moons to remove or cover impact craters resulting from
collisions from asteroids and comets.
Therefore, the icy outer layers of Umbriel, Titania,
and Oberon are heavily cratered. As we would expect, we find little fracturing
of the icy outer layers of these three moons that would have been caused by
tidal heating from Uranus. Hence, we
find few canyons, few scarps (steep cliffs), and few ridges within the icy
outer layers of Umbriel, Titania, and Oberon. As we would also expect, we find little
evidence of cryovolcanism on these three moons. Since Oberon is the outermost medium-sized
moon of Uranus, Oberon is therefore the least subject to the gravity of Uranus
as compared with the other medium-sized moons of Uranus, making Oberon the most
geologically dead medium-sized moon of Uranus.
As we would expect, Oberon is the most heavily cratered moon of Uranus
with the least amount of cryovolcanism and the least
number of canyons, scarps (steep cliffs), and ridges.
Uranus has an obliquity
(axial tilt) that is slightly greater than ninety degrees. If the obliquity were ninety degrees, we
would say that Uranus rotates on its side.
Since the obliquity is slightly greater than ninety degrees, Uranus
rotates mostly on its side and in the opposite direction from its orbital
motion around the Sun. Uranus and Venus
are the only two planets in the Solar System that rotate in the opposite
direction of the planet’s orbital motion around the Sun. Warning: all eight planets, including Venus
and Uranus, orbit the Sun in the same direction. More plainly, Venus and Uranus orbit the Sun
in the same direction as all the other planets while rotating in the opposite
direction of their orbital motion around the Sun. Since Uranus rotates mostly on its side, its
northern hemisphere and its southern hemisphere are mostly within its orbital
plane instead of mostly above and below its orbital plane, as is the case with
all the other planets in the Solar System.
Consequently, Uranus experiences the most extreme
seasonal variations among all the planets in the Solar System. For roughly forty-two Earth years (half of
its orbital period around the Sun), Uranus’s northern hemisphere experiences
nearly continuous daytime while Uranus’s southern
hemisphere experiences nearly continuous nighttime. During the other half of its orbit (also
roughly forty-two Earth years), Uranus’s northern hemisphere experiences nearly
continuous nighttime while Uranus’s southern hemisphere experiences nearly
continuous daytime. The strange
obliquity of Uranus may have been caused by a violent
impact. Presumably, Uranus had a more
reasonable obliquity before the violent impact, but the impact was so violent
that the rotational axis of Uranus was tipped
sideways. Further evidence that this
severe obliquity was caused by a violent impact is
Uranus’s strange magnetic field. Uranus
suffers from fast rotation, generating a strong magnetic field, as is the case
with all the jovian,
gas-giant (outer) planets. However, the
magnetic poles of Uranus are severely displaced from
its rotational poles. The magnetic
center of Uranus is also severely displaced from its
gravitational center. Presumably, Uranus
generated a more reasonable magnetic field before the violent impact, with
magnetic poles that were reasonably close to its rotational poles and a
magnetic center reasonably close to its gravitational center. The violent impact gave Uranus a mostly
sideward rotation, rotational poles severely displaced from its magnetic poles,
and a gravitational center severely displaced from its magnetic center.
It seems that nearly every
unexpected deviation from dominant patterns in the Solar System was caused by a violent collision. For example, a violent collision explains why
Mercury is the smallest of the eight planets of the Solar System, explains why
Mercury has an unusually large metallic core as compared with its present
overall size, and explains the abundance of scarps (steep cliffs) throughout
the surface of Mercury. As another
example, a violent collision explains the existence of the Earth’s
unusually large Moon, explains why the composition of the Earth’s Moon is
similar to the composition of the outer layers of the Earth, and explains why
the Earth’s Moon has an unusually small metallic core as compared with its
overall size. As yet another example, a
violent collision explains the enormous Herschel crater on the Saturnian moon Mimas and the enormous Odysseus crater on
the Saturnian moon Tethys, thus revealing that
collisions can occur in the Solar System that are powerful enough to obliterate
medium-sized moons and certainly small-sized moons. As a fourth example, a violent collision gave
Uranus a mostly sideward rotation, gave Uranus rotational poles severely
displaced from its magnetic poles, and gave Uranus a gravitational center
severely displaced from its magnetic center.
The fourth and last of the jovian, gas-giant (outer) planets
is Neptune, the eighth and final planet from the Sun. The orbital period of Neptune around the Sun
is roughly one hundred and sixty-five Earth years. Neptune is tenuous (low density), being
composed of hydrogen gas and helium gas surrounding a deeper layer of volatiles
ices surrounding an even deeper layer of rock surrounding a metallic core. With less outer layers of hydrogen gas and
helium gas, deeper layers of volatile ices are exposed,
giving Neptune a bluish color. Although
Neptune is slightly smaller in size than Uranus, the
mass of Neptune is slightly greater than the mass of Uranus. A slightly smaller size and a slightly
greater mass together make Neptune noticeably more dense
than Uranus. Neptune suffers from fast
rotation, causing it to be severely oblate, as is the case with all the jovian, gas-giant (outer)
planets. Neptune’s fast rotation also
generates a strong magnetic field, as is the case with all the jovian, gas-giant (outer)
planets. Neptune’s fast rotation has
also divided its atmosphere into many prevailing winds that appear as cloud
bands.
Neptune has a weak ring
system and more than one dozen moons.
Most of the moons of Neptune are small-sized moons, but Neptune does
have one large-sized moon named Triton and two medium-sized moons. These three moons listed in the correct order
from closest to Neptune are Proteus, Triton, and Nereid furthest from
Neptune. Again, Triton is the only
large-sized moon, while the other two moons are medium-sized moons. Since Proteus is a medium-sized moon, it
should have sufficient mass and therefore sufficient self-gravity to force
itself into a spherical shape. However,
Proteus is highly non-spherical in shape.
In fact, Proteus is the largest non-spherical object in the entire Solar
System. Triton is the only large-sized
moon of Neptune. Triton is the only
large-sized moon in the entire Solar System that orbits its mother planet in
the opposite direction as its mother planet’s rotation. This implies that Triton could be a captured
moon. Perhaps Triton was formerly a
short-period comet orbiting the Sun within the neighboring Kuiper belt, which
we will discuss shortly. Perhaps
Triton’s previous orbit around the Sun brought it too close to Neptune on one
occasion. Since Neptune is a jovian, gas-giant planet, it had
sufficient gravity to capture Triton, turning Triton from a short-period comet
orbiting the Sun within the Kuiper belt to Neptune’s only large-sized
moon. The medium-sized moon Nereid has
the most severely elliptical orbit among all the medium-to-large-sized moons in
the entire Solar System. Also, Nereid is the only medium-sized moon in the Solar
System that is not tidally locked to its mother planet. In other words, the rotational period of
Nereid is not equal to its orbital period around Neptune. Both Proteus and Triton are
tidally locked to Neptune however.
Just as the Earth’s Moon is slowing down the Earth’s rotation, Triton is
slowing down Neptune’s rotation. This
lost rotational angular momentum is transferred back to Triton,
just as the Earth’s lost rotational angular momentum is transferred back to the
Earth’s Moon. The angular momentum
transfer from the Earth to its Moon causes its Moon to drift further and
further from the Earth, as we discussed.
There is also an angular momentum transfer from Neptune to Triton. However, Triton orbits Neptune in the
opposite direction as Neptune’s rotation.
Thus, instead of drifting away from Neptune, Triton is instead drifting
toward Neptune. In a few hundred million
years, Triton will approach its Roche limit from Neptune, and hence Neptune’s
gravity will rip Triton apart into billions of individual chunks of metal,
rock, and volatile ices. Triton is
sufficiently large that when it is eventually ripped
apart, Neptune will have a spectacular ring system like Saturn’s rings. A few hundred million years from now, it will
be Neptune with a spectacular ring system while the other three gas-giant
planets, including Saturn, will have weak ring systems.
In the 1970s,
NASA launched two probes to study the jovian,
gas-giant (outer) planets. These probes were named Voyager 1
and Voyager 2. These probes revolutionized our understanding
of the jovian, gas-giant
(outer) planets. For example, these two
probes discovered dozens of moons around all four of these planets and
discovered the weak ring systems around Jupiter, Uranus, and Neptune. The Voyager
2 probe flew by Neptune in the year 1989 and discovered a giant storm storm
on Neptune beautifully analogous to the Great Red Spot on Jupiter. Astronomers named this giant storm on Neptune
the Great Dark Spot. Many
astrophysicists believed that the Great Dark Spot served as strong evidence
that giant storms are long-lived phenomena on jovian, gas-giant planets. However, recent telescopic observations of
Neptune have not found the Great Dark Spot.
This giant storm seems to have ended.
Therefore, the Great Dark Spot seems to serve as evidence that giant
storms on jovian, gas-giant
planets are temporary, not long-lived.
Astrophysicists continue to debate the nature of giant storms on jovian, gas-giant planets, with
the Great Red Spot on Jupiter being evidence in favor of their longevity and
with the Great Dark Spot formerly on Neptune being evidence against their
longevity.
The Minor Objects of the Solar System
By far, the most important
member of the Solar System is the Sun, but during this survey of the Solar System we are for the most part ignoring the Sun. We will study the Sun in tremendous detail,
but in the context of it being a star, not in the context of it being a member
of the Solar System. If we are for the
most part ignoring the Sun and if we have completed a discussion of the eight
planets and their moons and rings, then what remains to be
discussed is the minor objects of the Solar System. The minor objects of the Solar System are the
asteroids, the meteoroids, the comets, and the dust. Most of these minor objects are not
spherically shaped. At sizes larger than
roughly four hundred kilometers in diameter, most objects have enough mass and
therefore strong enough self-gravity to force themselves into spherical
shapes. This is why the Sun, all eight
planets, all the large-sized moons of the Solar System, and most of the
medium-sized moons of the Solar System are nearly spherically shaped. However, small-sized moons, most asteroids,
all meteoroids, and most comets are smaller than roughly four hundred
kilometers in diameter. Thus,
small-sized moons, most asteroids, all meteoroids, and most comets are
irregularly shaped. This argument also
applies to even smaller objects, such as mountains, houses, cars, tables,
chairs, and mobile telephones. These
objects are so small with so little mass that they have insufficient
self-gravity to force themselves into spherical shapes. (Although baseballs and some humans are
spherically shaped, they are spherical for reasons different from
self-gravity.) Dust particles,
molecules, and atoms have such minuscule self-gravity that they are not
spherical as well.
Our provisional definition of
an asteroid is a small chunk of metal and rock orbiting the Sun. However, this definition is not
satisfactory. Aren’t
the inner planets small chunks of metal and rock orbiting the Sun? By this definition, Mercury, Venus, Earth,
and Mars would be considered asteroids. We could refine our provisional definition by
claiming that an asteroid is a very small chunk of metal and rock orbiting the
Sun. Although this is a better
definition, we will reveal the true definition of an asteroid shortly. Most asteroids are
irregularly shaped, since they are small with insufficient mass and
hence insufficient self-gravity to force themselves into spherical shapes. Most asteroids orbit the Sun within the
asteroid belt between the orbit of Mars and the orbit of Jupiter. The asteroid belt should have been the orbit
of a fifth terrestrial planet after Mars.
However, Jupiter is so massive with such intense gravity that it
perturbed the metallic and rocky planetesimals at
that orbit, ripping many planetesimals out of that
orbit and even preventing the remaining planetesimals
from accreting together to form a fifth terrestrial planet. This reveals the true definition of an
asteroid. An asteroid is
defined as a metallic and rocky planetesimal
leftover from the formation of the Solar System. Presumably, the total mass of the asteroid
belt was once equal to that of a terrestrial planet, but Jupiter’s gravity has
ripped the overwhelming majority of the planetesimals
out of the original asteroid belt.
Today, the total mass of all the asteroids in the asteroid belt combined
together is barely equal to that of a single
large-sized moon. Many asteroids that
Jupiter ripped out the asteroid belt collided with forming planets during the
heavy bombardment period of the young Solar System. Jupiter’s gravity continues to rip asteroids
out of the asteroid belt today through orbital resonances, as we
discussed. Many of these asteroids have
been captured by the jovian,
gas-giant (outer) planets and in addition Mars to become small-sized
moons. Some asteroids are actually
orbiting the Sun on Jupiter’s orbit; these are the Trojan asteroids. Some asteroids orbit the Sun on random orbits
outside of the asteroid belt. Some of
the orbits of these asteroids intersect the Earth’s orbit around the Sun. Therefore, an asteroid could collide with the
Earth. If an asteroid were to collide
with the Earth, the collision would unleash roughly one thousand times the
combined nuclear arsenal of the entire world!
This much liberated energy would completely
obliterate all life at and near the point of impact. In addition, this much energy would shatter
the asteroid into innumerable extremely hot fragments that would shoot outward
from the point of impact and then rain back downward, igniting global forest
fires and heating most of the Earth’s atmosphere to inhospitable temperatures,
eradicating life over an even larger area.
Most significantly, this much energy would pulverize rock at the point
of impact into dust and ash, and the force of the collision would eject that
dust and ash into the atmosphere, surrounding the entire planet and blocking
most sunlight for several months, perhaps even a couple of years. Without sunlight, most plants would die,
since plants require the energy of sunlight to synthesize their own food
(photosynthesis). Most herbivorous
animals would then die, since herbivores eat plants. Most carnivorous animals would then die,
since carnivores eat herbivores. The
result is an extinction level event, when most of the organisms across the
entire planet suddenly become extinct.
The most recent extinction level event was roughly sixty-six million
years ago when most of the organisms on planet Earth suddenly became extinct,
including the dinosaurs. In the 1970s, a crater was discovered in
Yucatán in Mexico, named the Chicxulub crater. This crater is roughly the correct size that would be caused by an asteroid impact. The Chicxulub crater is also the correct age,
having formed at the rock layer that is roughly sixty-six million years
old. Finally, this rock layer is rich in
dense elemental metals such as iridium and osmium that are less abundant on the
surface of the Earth but more abundant in asteroids. Therefore, the extinction level event that
occurred roughly sixty-six million years ago that caused most life on Earth to
suddenly become extinct, including the dinosaurs, was definitely caused by an
asteroid impact in Yucatán in modern-day Mexico.
A meteoroid is an extremely
small chunk of metal and rock orbiting the Sun.
Meteoroids come from asteroids.
When two asteroids happen to collide with each other, they chip pieces off of each other.
These chipped pieces of asteroids are meteoroids. These meteoroids fill the entire Solar
System, and countless meteoroids continuously fall toward all planets and moons. Millions of meteoroids fall toward planet
Earth every day. Most of them completely
burn up in Earth’s atmosphere as they fall toward Earth. While the chunk of metal and rock is in outer
space, it is called a meteoroid, but while burning in
Earth’s atmosphere, the chunk of metal and rock is called a meteor. It is also called a
falling star or a shooting star, since it appears to be as bright as a star
while falling from the sky. We can see
some meteors (falling stars or shooting stars) every day of the year. On any given night that is not cloudy, we can
see meteors (falling stars or shooting stars) if we stare continuously up at
the night sky over the course of a few hours.
A large number of meteors (falling stars or shooting stars) over the
course of a few nights is called a meteor shower,
which we will discuss shortly. If the
original meteoroid was large enough, it may survive the burning as a meteor
(falling star or shooting star) and then crash into the Earth. This is called a
meteorite. Meteorites crash into the
Earth rather often, but most fall into the ocean since the Earth is mostly covered with water. Although houses and cars
have been damaged by meteorites falling from the sky, no human being has
ever been killed by a meteorite falling from the sky. The largest meteorite in captivity in the
entire world is at the American Museum of Natural History in New York
City. To summarize, an extremely small
chunk of metal and rock in outer space is called a meteoroid, while it burns in
the Earth’s atmosphere as it falls toward the Earth it is called a meteor (or
falling star or shooting star), and if it crashes into the Earth it is called a
meteorite. Why should we use three
different terms for the same chunk of metal and rock? In fact, different names are
often used for the same thing in colloquial languages. While they are living animals, chickens,
turkeys, and ducks are called fowl, but after they
have been slaughtered and prepared as food they are called poultry. While they are living animals, bulls and cows
are called cattle, but after they have been
slaughtered and prepared as food they are called beef. While they are living animals, pigs are
called swine, but after they have been slaughtered and prepared as food they are called pork.
While they are living animals, deer, elk, moose, and reindeer are called cervids, but after they
have been slaughtered and prepared as food they are called venison. The Spanish word for living fish is pez, but the Spanish word for fish that is
slaughtered and prepared as food is pescado. There are countless other examples in
colloquial languages. As a technical
example, molten rock deep within the Earth is called
magma, but molten rock that has extruded out of the Earth is called lava.
As we discussed, volatile
ices in addition to metal and rock were able to condense beyond the frost line
of the protoplanetary disk of the young Solar System. These planetesimals
differentiated themselves, with the most dense metals sinking toward their
centers, the least dense volatile ices rising toward their surfaces, and
moderately dense rock settling in the intermediate layer between the metallic
core and the icy outer layers. Many of
these icy planetesimals accreted together to form the
four jovian, gas-giant
planets Jupiter, Saturn, Uranus, and Neptune.
These planets are so massive with such intense gravity that they
perturbed all the remaining icy planetesimals in the
outer Solar System that had not yet accreted together. In many cases, the gas-giant planets,
particularly Jupiter, flung icy planetesimals in
random directions out to enormous distances from the Sun, almost ejecting them
entirely from the Solar System. In other
cases, the gas-giant planets, particularly Jupiter, prevented the remaining icy
planetesimals from accreting together to form a fifth
jovian planet.
In either case, there remains to the present day millions of these icy planetesimals orbiting the Sun. These are comets, icy planetesimals
leftover from the formation of the Solar System. Note however that it is the
outer layers of a comet that are volatile ices. Beneath these volatile ices is higher density
rock, and the core of a comet is very dense metal. As a result, most comets are larger than most
asteroids, since asteroids are metallic and rocky planetesimals
leftover from the formation of the Solar System that formed closer to the Sun
than the frost line where volatile ices could not condense. Although most comets are larger than most
asteroids, most comets are nevertheless not spherically
shaped, since they have such little mass and therefore such weak
self-gravity that they cannot force themselves into spherical shapes. Hence, most comets are
irregularly shaped, just as most asteroids are irregularly shaped.
We divide comets into two
categories: short-period comets and long-period comets. Short-period comets take a short amount of
time to orbit the Sun, between a few decades and a few centuries. The most famous comet, the Halley comet, is
certainly a short-period comet since it has an orbital period around the Sun of
seventy-six years. The orbits of
short-period comets are more or less within the plane of the Solar System,
although the orbits of some short-period comets have somewhat significant
inclinations with the plane of the Solar System. Most short-period comets orbit the Sun in the
Kuiper belt, a belt of comets mostly in the plane of the Solar System just
beyond Neptune’s orbit. The Kuiper belt is named for the Dutch astronomer Gerard Kuiper who
theorized that there should be comets orbiting the Sun beyond Neptune’s
orbit. The Kuiper belt is beautifully
analogous to the asteroid belt. This is
further evidence that comets are icy planetesimals
leftover from the formation of the Solar System, just as asteroids are metallic
and rocky planetesimals leftover from the formation
of the Solar System. Just as the
asteroid belt should have become a fifth terrestrial planet, the Kuiper belt
should have become a fifth jovian, gas-giant planet
with less hydrogen and helium outer layers thus exposing volatile ices, similar
to Uranus and Neptune. The total mass of
the Kuiper belt was once roughly equal to the mass of Uranus or Neptune, but
the gas-giant planets, particularly Jupiter, have ripped the overwhelming
majority of the icy planetesimals out of the original
Kuiper belt. Today, the total mass of
all the short-period comets in the Kuiper belt combined together is barely equal to that of a single large-sized moon. Many short-period comets that Jupiter ripped
out the Kuiper belt collided with forming planets during the heavy bombardment
period of the young Solar System.
Gravitational perturbations from Jupiter continue to rip short-period
comets out of the Kuiper belt.
Therefore, not all short-period comets orbit the Sun from within the
Kuiper belt. There are short-period
comets orbiting the Sun on random orbits.
The Halley comet is an example of a short-period comet that orbits the
Sun but not from within the Kuiper belt.
Some short-period comets intersect the Earth’s orbit. Therefore, a comet could collide with the
Earth. The result of such a collision
would be an extinction level event similar to an extinction level event caused
by an asteroid impact.
Long-period comets take such
a long period of time to orbit the Sun that
astronomers are not certain of their orbital periods. Perhaps their orbital periods are several
million years. Perhaps some of them are
on unbound orbits (parabolic orbits or hyperbolic orbits), meaning that they
will never return after passing by the Sun once. The orbits of long-period comets are not confined to the plane of the Solar System. A long-period comet may come from above the
plane of the Solar System or from below the plane of the Solar System, pass
around the Sun once, and then move away from the Sun, possibly never to
return. The source of the long-period
comets is the Oort cloud, an enormous spherical cloud
of comets around the entire Solar System, possibly one light-year in size. The Oort cloud is named for the Dutch astronomer Jan Oort
who theorized its existence. Oort cloud comets are so distant from the Sun that they are barely held by the Sun’s gravitational attraction. Gravitational attractions from other stars in
other star systems can perturb the orbit of a long-period comet, causing it to
fall to the inner Solar System. Caution:
the inner Solar System relative to the Oort cloud
would include the four inner planets, the asteroid belt, the four outer
planets, and even the Kuiper belt of short-period comets. The Oort cloud must be spherically shaped, since the orbits of long-period
comets are not confined to the plane of the Solar System. The Oort cloud
formed when the jovian, gas-giant planets,
particularly Jupiter, flung most of the icy planetesimals
of the young Solar System in random directions out to enormous distances from
the Sun, almost ejecting them entirely from the Solar System.
As any comet (short-period or long-period) approaches the Sun, the Sun’s
heat will melt its volatile ices.
Actually, the very low pressures of outer space causes the volatile ices
to turn from the solid state (ice) directly into the gaseous state, skipping
the liquid state. In other words, the
Sun’s heat does not melt the volatile ices; the Sun’s heat sublimes the
volatile ices. As the volatile ices
sublime, dust particles that were trapped within the
ice are liberated. The result is a halo
of sublimed volatile ices and dust particles around the comet. This is the coma of the comet, which is
rather like an atmosphere around the comet.
The nucleus of the comet is the volatile ices that remain in the solid
state around the core of metal and rock.
The coma forms just where we would expect: at
the frost line between the orbit of Mars and the orbit of Jupiter. After passing around the Sun and then moving
away from the Sun, the coma condenses back into solid ice when the comet again
crosses the frost line as it returns to the outer Solar System. Throughout the comet’s orbit, the Sun’s light
will push the liberated dust particles away from the Sun. The force that light exerts is called
radiation pressure, which is so weak that we do not notice it in our daily
lives. Although the radiation pressure
of the Sun’s light does push planets and asteroids and
comets away from the Sun, this radiation pressure is so weak that it can be
completely ignored as compared with the gravitational attraction of the Sun
keeping all of these bodies in their orbits.
However, a tiny dust particle is so small with such a minuscule mass
that the gravitational attraction it feels from the Sun is so weak that the
radiation pressure of the Sun’s light may balance or even exceed the Sun’s
gravitational attraction. Thus, the tiny
dust particles liberated from comets are pushed away
from the Sun by the radiation pressure of the Sun’s light. Hence, the comet grows a dust tail that
points away from the Sun. Actually, the
comet grows two tails that both point away from the Sun. The dust tail is caused
by the radiation pressure of the Sun’s light.
The ion tail (or the plasma tail) is caused by
the Sun’s solar wind. Although both
comet tails point away from the Sun, the dust tail is curved while the ion tail
(plasma tail) is straight, pointing directly away from the Sun. These two tails always point away from the
Sun, whether the comet is moving toward the Sun or moving away from the
Sun. When the comet is moving toward the
Sun, the comet’s two tails point behind it.
When the comet is moving away from the Sun, the comet’s two tails are
ahead of it, preceding its own motion.
It is theoretically possible to build a spacecraft that would be propelled by the radiation pressure of the Sun’s
light and the Sun’s solar wind. Although
these forces are relatively weak, an enormous panel attached to a spacecraft
would suffer a significant force from the radiation pressure of the Sun’s light
and from the Sun’s solar wind as both continuously bombard the panel’s large
surface area. If the spacecraft together
with the enormous panel had a sufficiently small mass, the spacecraft together
with its enormous panel would suffer a significant acceleration away from the
Sun as the radiation pressure of the Sun’s light and the Sun’s solar wind both
push the enormous panel attached to the spacecraft. This theoretical enormous panel is called a solar sail, since it is rather like the sail of
a sailboat that propels the sailboat as the Earth’s atmospheric winds push upon
the sail.
Each time a short-period
comet approaches the Sun closer than the frost line between the orbit of Mars
and the orbit of Jupiter, some of its volatile ices will sublime. Although some of these sublimed ices will
condense back into solid ice when the comet moves away from the Sun further
than the frost line, some of the sublimed ices completely escape the weak gravitational
attraction of the comet. Therefore, a
short-period comet that has part of its orbit closer to the Sun than the frost
line will continually lose more and more of its volatile ices with each
orbit. Ultimately, the short-period
comet may lose its entire outer layer of volatile ices, leaving only its core
of metal and rock. This is a dead
comet. The comet Phaethon is an example
of a dead comet. Moreover, each time a
short-period comet approaches the Sun closer than the frost line, dust
particles will be liberated whenever the comet’s ice
sublimes. Since short-period comets
orbit the Sun cyclically (over and over again) on roughly the same orbit, the
orbit of a short-period comet that periodically approaches the Sun closer than
the frost line is littered with dust particles that were liberated by the Sun’s
heat each time the short-period comet approached the Sun. Although Jupiter’s gravity perturbs the
orbits of planets and asteroids and comets, these perturbations are small. Therefore, planets and asteroids and
short-period comets orbit the Sun cyclically (over and over
again) on roughly the same orbit. If the
orbit of a short-period comet intersects Earth’s orbit, then the Earth will
pass through the orbit of that comet at roughly the same time every year. As a result, all of the dust particles at
that part of the comet’s orbit will fall toward the Earth, burning within the
Earth’s atmosphere and appearing as falling stars or shooting stars. This is called a
meteor shower. A meteor shower is named
for a particular constellation in the sky, but not because the meteors only
appear within that constellation. The
meteors (falling stars or shooting stars) actually appear everywhere throughout
the entire sky. A meteor shower is named for a particular constellation because that particular
constellation appears to be the extrapolated source of all the meteors
throughout the entire sky. In actuality,
the Earth is moving toward that particular constellation as the Earth moves
through a particular location within the comet’s orbit, causing that
constellation to appear to be the extrapolated source of all the meteors
throughout the entire sky. This is
similar to driving into a snowstorm. The
snowflakes appear to diverge away from the point in front of us, the point in
the direction toward which we are driving.
Similarly, the meteors during a meteor shower appear to diverge away
from the constellation in the direction toward which the Earth is moving. Examples of meteor showers
include the Lyrids named for the constellation Lyra
(the harp) and occurring in late April every year, the Perseids
named for the constellation Perseus (the hero) and occurring in mid-August
every year, the Draconids named for the constellation
Draco (the dragon) and occurring in early October every year, the Orionids
named for the constellation Orion (the hunter) and occurring in late October
every year, the Leonids named for the constellation
Leo (the lion) and occurring in mid-November every year, and the Geminids named
for the constellation Gemini (the twins) and occurring in mid-December every
year.
The entire plane of the Solar
System is filled with dust. We can see this dust from the Earth, although
barely. At extremely dark locations, far
from any city lights and other light pollution, we can see a faint glow along
the zodiac constellations. This is called zodiac light.
In actuality, zodiac light is sunlight reflected off
of the dust that fills the plane of the Solar System. As we discussed earlier in the course, the
zodiac constellations are along the ecliptic, which is the apparent path of the
Sun around the sky. In actuality, the
Earth orbits the Sun in a plane called the ecliptic plane. The ecliptic is actually the projection of
the ecliptic plane onto the Earth’s sky.
Hence, the zodiac constellations are also in the ecliptic plane, but the
stars within constellations are far beyond our Solar System. Although, the stars within constellations are
still within our Milky Way Galaxy. The
galaxies within constellations are far beyond the stars within our Milky Way
Galaxy.
Pluto is the final object in
the Solar System that we must discuss.
The orbital period of Pluto around the Sun is roughly two hundred and
fifty Earth years. Pluto should not be considered a planet for several reasons. Firstly, Pluto’s orbit around the Sun is
moderately inclined to the plane of the Solar System, while the eight planets
all have orbits around the Sun nearly in the plane of the Solar System. Secondly, Pluto’s orbit around the Sun is
significantly elliptical, while the eight planets all have orbits around the
Sun that are roughly circular (but nevertheless
slightly elliptical). More correctly,
the eight planets all have orbits around the Sun that are ellipses with small
eccentricities, while Pluto has an orbit around the Sun with a significant
eccentricity. Thirdly, Pluto’s orbit
around the Sun intersects Neptune’s orbit around the Sun, while the eight
planets all have orbits that do not intersect each other. Although Pluto’s orbit intersects Neptune’s
orbit, Pluto will not collide with Neptune.
The gravity of Neptune has adjusted Pluto’s orbital motion around the
Sun using orbital resonance to ensure that Pluto will never collide with
Neptune. Since Pluto’s orbit intersects
Neptune’s orbit, sometimes Pluto is closer to the Sun than Neptune. Fourthly, Pluto’s orbit is mostly just beyond
Neptune’s orbit. If Pluto were a planet,
then it should be a jovian,
gas-giant planet, since it is in the outer Solar System. However, Pluto is smaller than Mercury, the
smallest of the eight planets! Also, Pluto was not a terrestrial planet that was
accidentally ejected from the inner Solar System to the outer Solar
System. If this were the case, Pluto
would be composed of metal and rock.
Although Pluto probably does have a core composed
of metal and rock, Pluto is primarily composed of volatile ices surrounding
that metallic and rocky core. For all of
these reasons, Pluto should not be regarded as a
planet. After discussing at length that
Pluto is not a planet, we must now explain what Pluto actually is. Since Pluto is a chunk of volatile ices (with
a metallic-rocky core) orbiting the Sun just beyond Neptune’s orbit, would this
not make Pluto a short-period comet? The
answer is yes: Pluto is actually a short-period comet
in the Kuiper belt. Pluto’s largest moon
Charon is practically the same size as Pluto itself. Charon is so large that it should
not even be regarded as a moon of Pluto.
The Pluto-Charon system should be regarded as a short-period,
double comet system orbiting the Sun within the Kuiper belt.
Many students are offended
that Pluto is no longer considered a planet. These students argue that Pluto was considered the ninth planet from the Sun for more than
seventy-five years, from its discovery in the year 1930 until the year
2006. These students argue that we
cannot demote Pluto from a planet to a Kuiper belt short-period comet after it
was considered to be a planet for more than
three-quarters of a century. However,
this type of demotion has occurred before.
In the year 1801, an object was discovered
orbiting the Sun between the orbit of Mars and the orbit of Jupiter. This object was named
Ceres after the ancient Roman mythological goddess of agriculture (Demeter in
ancient Greek mythology). The ancient
Roman mythological goddess Ceres (Demeter) is depicted
on the flag and the great seal of the State of New Jersey, representing
prosperity. When the astronomical body
Ceres was discovered in the year 1801, it was declared to be the fifth planet. Ceres was even given
the astronomical symbol ⚳ that astronomers continue to use to the present day, just as the other
planets each have their own astronomical symbol, as we discussed. Notice Ceres was even named
for an ancient mythological Roman figure, as any planet should be named. Jupiter was then
reclassified as the sixth planet, Saturn was reclassified as the seventh
planet, and Uranus was reclassified as the eighth planet. Note that Neptune was not
yet discovered. However, Pallas was then discovered in the year 1802 orbiting the Sun
between the orbit of Mars and the orbit of Jupiter. Juno was then discovered
in the year 1804 orbiting the Sun between the orbit of Mars and the orbit of
Jupiter. Vesta
was then discovered in the year 1807 orbiting the Sun
between the orbit of Mars and the orbit of Jupiter. Hygiea was discovered
in the year 1849 orbiting the Sun between the orbit of Mars and the orbit of
Jupiter. As telescopes became more and
more powerful, more and more of these objects were discovered
orbiting the Sun between the orbit of Mars and the orbit of Jupiter. Also, more powerful
telescopes began to reveal that these objects were much, much smaller than the
other planets. Several decades later,
astronomers realized that none of these objects should be regarded
as planets. After roughly fifty years of
regarding Ceres as a planet, Ceres and Pallas and Juno and Vesta and Hygiea and all these other objects were
demoted to minor planets. The term minor
planet was used to appease millions of people who
considered Ceres to be a planet for decades.
After this demotion, Jupiter was then restored
as the fifth planet, Saturn was restored as the sixth planet, and Uranus was
restored as the seventh planet. After
several more decades, there was no one left alive who remembered that Ceres was ever regarded as a planet. At that point, Ceres and
Pallas and Juno and Vesta and Hygiea and all
these other objects were renamed as asteroids.
The historical classification of Pluto nearly perfectly parallels the
historical classification of Ceres. In
the year 1930, Pluto was discovered just beyond
Neptune’s orbit, and Pluto was regarded as the ninth planet for decades. Pluto was even given
the astronomical symbol ♇ that astronomers continue to use to the present day, just as the other
planets each have their own astronomical symbol, as we discussed. Notice Pluto was even named
for an ancient mythological Roman figure, as any planet should be named. However, Varuna was discovered in the year 2000 orbiting the Sun just beyond
Neptune’s orbit. Ixion was then discovered in the year 2001 orbiting the Sun just
beyond Neptune’s orbit. Quaoar and its
moon Weywot were discovered
in the years 2002 and 2007, respectively, orbiting the Sun just beyond
Neptune’s orbit. Sedna was discovered in the year 2003 orbiting the Sun just beyond
Neptune’s orbit. Orcus and its moon Vanth were discovered in the years
2004 and 2005, respectively, orbiting the Sun just beyond Neptune’s orbit. As telescopes became more and more powerful,
more and more of these objects were discovered
orbiting the Sun just beyond Neptune’s orbit.
Also, more powerful telescopes began to reveal
that these objects were much, much smaller than the other planets. In particular, Pluto is not only smaller than
planet Mercury (the smallest of the eight planets), but Pluto is even smaller
than all seven of the large-sized moons in the Solar System (Ganymede, Titan, Callisto, Io, Earth’s Moon, Europa, and Triton). Eris and its moon Dysnomia
were discovered in the year 2005 orbiting the Sun just
beyond Neptune’s orbit, and Eris is roughly the same size and roughly the same
mass as Pluto. If we were to regard
Pluto as a planet, shouldn’t we regard Eris as a
planet as well? Suppose dozens of more
objects are someday discovered just beyond Neptune’s orbit that are slightly
larger and slightly more massive than Pluto. Should all these dozens of
objects be regarded as planets?
After more than seventy-five years of regarding Pluto as a planet, Pluto-Charon and Varuna and Ixion and
Quaoar-Weywot and Sedna and Orcus-Vanth
and Eris-Dysnomia and all these other objects
were demoted to dwarf planets. The term
dwarf planet was used to appease millions of people
who considered Pluto to be a planet for decades. After several more decades, there will be no
one left alive who will remember that Pluto was ever regarded
as a planet. At that point, everyone
will agree that Pluto-Charon and Varuna
and Ixion and Quaoar-Weywot and Sedna and Orcus-Vanth and Eris-Dysnomia
and all these other objects are short-period comets orbiting the Sun within the
Kuiper belt. Just as astronomers
eventually realized that Ceres is not a planet but instead was the first
asteroid of the asteroid belt that was ever discovered,
astronomers have now realized that Pluto is not a planet but instead was the
first short-period comet of the Kuiper belt that was ever discovered.
Libarid A. Maljian homepage at the Department of Physics at CSLA at NJIT
Libarid A. Maljian profile at the Department of Physics at CSLA at NJIT
Department of Physics at CSLA at NJIT
College of Science and Liberal Arts at NJIT
New Jersey Institute of Technology
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