CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 495
evidence of two processes. Th e orbits of some moons may have
been modifi ed by interactions with other moons, so that they
now revolve around their planet in mutual resonances. Th e same
process may allow moons to aff ect the orbital motions of particles
in planetary rings.
Th e second process allows tides to heat the interiors of some
moons and produce geological activity on their surfaces, includ-
ing volcanoes and lava fl ows. You have learned that the entire
solar system received a heavy bombardment after the planets
formed, and heavily cratered surfaces are old, so when you see a
section of a moon’s surface, or an entire moon, that has few cra-
ters, you know that moon must have been geologically active
since the end of the heavy bombardment.Atmospheres and Interiors
All the Jovian worlds have hydrogen-rich atmospheres fi lled with
clouds. On Jupiter and Saturn, you can see that the clouds form
dark belts and light zones that circle the planets like the stripes
on a child’s ball. Th is form of atmospheric circulation is called
belt–zone circulation. You will fi nd traces of belts and zones on
Uranus and Neptune, but they are not very distinct. All the
Jovian worlds also have giant circulating storms, the primary
example of which is Jupiter’s Great Red Spot, which is more than
twice the size of Earth. Th ese Jovian storms are comparable to
Terrestrial hurricanes, but they can last for centuries. Th e Great
Red Spot storm has been going strong at least since the fi rst tele-
scope observations of Jupiter 400 years ago.
Th e gaseous atmospheres of the Jovian planets are not very
deep. Jupiter’s atmosphere makes up only about one percent of
its radius. Below that, Jupiter and Saturn are composed of liquid
hydrogen, so an older term for these planets, the gas giants,
refl ects a Common Misconception. In fact they are made
mostly of liquid rather than gas and could more correctly be
called the liquid giants. Only near their centers could these
worlds contain dense material with the composition of rock and
metal, but the sizes of these cores are not well known.
Uranus and Neptune are sometimes called the ice giants
because they contain a great deal of water, much of which is
probably in a solid form. Like Jupiter and Saturn, Uranus and
Neptune contain denser material in their cores.
On your visits to the Jovian planets, notice that they are low-
density worlds that are rich in hydrogen. Jupiter and Saturn are
mostly liquid hydrogen, and even Uranus and Neptune contain
a much larger proportion of hydrogen than does Earth. Recall
from Chapter 19 that these worlds are hydrogen-rich and low
density because they formed in the outer solar nebula where
water vapor could freeze to form tremendous amounts of tiny ice
particles. Th ese hydrogen-rich ice particles accreted to begin
forming the planets; and, once the growing planets became
massive enough, they could draw in more hydrogen gas directly
by gravitational collapse.
Satellite Systems
All of the Jovian worlds have large satellite systems. Around each
Jovian planet, the moons can be classifi ed into two groups:
(1) the regular satellites, which tend to be large and orbit in the
prograde direction, relatively close to their parent planet, with
low inclinations to the planet’s equator; versus (2) the irregular
satellites, which tend to be smaller than the regular satellites,
sometimes have retrograde and/or highly inclined orbits, and are
generally far from their parent planet. Astronomers have evi-
dence that the regular satellites formed approximately where they
are now as the planets formed but that the irregular satellites are
mostly, if not all, captured objects.
As you focus on the moons of the Jovian worlds, look for
SCIENTIFIC ARGUMENT
Why do you expect the outer planets to be low-density worlds?
To build this scientifi c argument, you need to think about how the
planets formed from the solar nebula. In Chapter 19, you discovered
that the inner planets could not incorporate ice when they formed
because it was too hot near the sun; but, in the outer solar nebula,
the growing planets could accumulate lots of ice. Eventually they
grew massive enough to grow by gravitational collapse, and that
pulled in hydrogen and helium gas. That makes the outer planets
low-density worlds.
The outer planets may be unearthly, but they are understandable.
For example, extend your argument. Why do you expect the outer
planets to have rings and moons?Jupiter
Jupiter is the most massive of the Jovian planets, containing
more material than all of the other planets combined. Th is high
mass accentuates some processes that are less obvious or nearly
absent on the other Jovian worlds. Just as you used Earth as the
basis of comparison for your study of the Terrestrial planets, you
can examine Jupiter in detail and use it as a standard in your
comparative study of the other Jovian planets.Surveying Jupiter
Jupiter is extreme because it is big, massive, mostly liquid hydro-
gen, and very hot inside. Th e preceding facts are common
knowledge among astronomers, but you should demand an
explanation of how they know these facts. Often the most inter-
esting thing about a fact isn’t the fact itself but how it is known.
At its closest point to Earth, Jupiter is about eight times
farther away than Mars, but even a small telescope will reveal that
the disk of Jupiter appears more than twice as big as the disk of
Mars. If you use the small-angle formula (see Chapter 3), you
can compute the diameter of Jupiter—1.4 105 km, which is
about 11 times Earth’s diameter (Celestial Profi le 7).23-2