Physics 202
Intro to Astronomy:  Lecture #11
Prof. Dale E. Gary

Jovian Worlds of the Solar System

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:

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:

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

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). Cassini is an orbiter spacecraft now at Saturn -- it arrived on 1 July 2004. 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.
Magnetospheres of the Gas Giants
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.