Giant Planets

When learned in Lecture 8 that the outer planets, Jupiter, Saturn, Uranus, and Neptune, form a class by themselves.  They owe their size and makeup to the fact that they formed beyond the "frost line" of the solar nebula from which the solar system formed.  Recall that because ice flakes could form, along with the rock and metallic flakes, these planets could grow much larger than the terrestrial planets.  In fact, they grew so large that they could attract and retain hydrogen and helium, the stuff that made up 99% of the solar nebula, so they ended up consisting of a relatively small rocky core with thick outer layers of hydrogen and helium.

Let's take a brief look at each one, as seen by the Hubble Space Telescope:

Jupiter
Saturn
Uranus (IR image)
Neptune (rotation movie)

From these images, we can see the surfaces of the planets (not really a solid surface at all, just cloud tops).  All four of the jovian planets (gas giants) rotate rapidly: Jupiter 9h 50m;

Time lapse movie showing Jupiter rotation over about 1 hour,
taken on 2004 Apr 20. Note the moon to the right, and the great red spot.

Saturn 10h 20m; Uranus 16h 33m; Neptune 17h 16m. Because they have no solid surface, they all experience differential rotation (the equators rotate more rapidly than the poles).
Lecture Question #1
But what are they like inside?  In fact, we do not really know very much for certain about the interiors of the giant planets.  We only have a few clues to help us to guess.  Here is how it is done:

• We know the original makeup of the solar nebula. We know the masses and sizes (and hence, the densities) of the planets. We know how matter behaves at high densities and pressures, from laboratory experiments and basic physics. We can measure the planets' energy input (from the Sun) and energy output (reprocessed energy from the Sun, along with some internally generated energy), so we can get the temperature.
• For one planet, Jupiter, we sent a probe into the atmosphere.

Piecing together all of the clues, here are the models that are currently accepted (but they may be wrong!):

• What looks like the outer edge of the planets is not a solid surface--it is just the cloud tops. The clouds are made of solid particles (like snow flakes), droplets (like rain), and gases.  But the stuff is hydrogen compounds (methane, ammonia, and water).  Although these clouds are 75% hydrogen and 24% helium, the trace amounts of these compounds is responsible for all of the cloud colors and other visible features. The distances of the planets from the Sun are very different, each one being almost twice as far as the previous one
• Jupiter -- 5 AU Saturn -- 10 AU Uranus -- 20 AU
• Neptune -- 30 AU
Earlier we discussed how distance from the Sun governs how hot the planets are.  Jupiter's cloud tops are 125 K, which is too warm for its distance.  Jupiter must have some source of heat--perhaps left over heat from its formation.
• The relative sizes and makeup of the interiors is like this:

• Note the inner layers of metallic hydrogen -- a form of hydrogen that occurs with the tremendous pressures found inside Jupiter and Saturn, where the hydrogen is so compressed that the atoms share electrons.  The electrons can flow freely from one atom to the next, just as in metals, so that it is electrically conducting.  This is what generates the strong magnetic fields of Jupiter, and also Saturn.
• Notice that Neptune and Uranus are too small and cold to have metallic hydrogen.

Lecture Question #2

Atmospheres of the Gas Giants

Galileo is an orbiter spacecraft circling Jupiter until recently (it was crashed into Jupiter in September, 2003).

• Jupiter Probe

The Galileo orbiter had an atmospheric probe that entered Jupiter's atmosphere in 1995 and radioed back information about the upper layers of clouds.  An overview of the cloud geometry is shown here.  It entered into a relatively dry part of Jupiter's atmosphere, and descended into the hotter layers beneath.  Jupiter's cloud tops are the lighter areas, called zones, and are colder. Darker areas, called belts, are deeper layers seen through gaps in the whiter clouds, and hence are hotter.
• Jupiter Pictures (great red spot)

Cassini is an orbiter spacecraft now at Saturn -- it arrived on 1 July 2004.

• Huygens Probe

The Cassini orbiter has a probe that landed on Saturn's largest moon, Titan.  Titan is so large that it can retain an atmosphere of methane.  We will discuss the moons and rings of the giant planets in the next lecture.

• Saturn Pictures

One of the most obvious differences in the gas giants is their colors.  These colors are due to the trace hydrogen compounds that make up a small fraction of their atmospheres, but nevertheless dominate the colors.

• Jupiter -- reds and browns (ammonia, sulfer compounds, methane)
• Saturn -- reds and yellows (ammonia, sulfer compounds, methane)
• Uranus -- blues and greens (mostly from methane gas)
• Neptune -- blue (from methane gas)

The environment of Jupiter is a dangerous one, due to a large amount of radiation trapped in the strong magnetic fields, which are generated by the metallic hydrogen of the interior.  These strong magnetic fields make Jupiter act like a huge magnet, just like the magnetic field of the Earth only about 20,000 times stronger.  If our eyes could see the region of magnetic fields around Jupiter (its magnetosphere), it would be larger than the full moon in the sky.

Here is a figure from the text, showing a comparison of the magnetospheres of the giant planets:

From Bennett, Donahue, Schneider, Vogt, The Cosmic Perspective, 2nd edition. Notice that the magnetic fields are dragged out in the direction away from the Sun, due to the solar wind.

The particles trapped in these magnetic fields cause auroras as seen from the Hubble Space Telescope:
Jupiter, Saturn.