510 PART 4^ |^ THE SOLAR SYSTEM
moons formed slowly, over perhaps 100,000 years, and were not
heated severely by in-falling material. Th e inner moons, however,
were cooked by tidal heating—possibly enhanced when an
orbital resonance developed between Ganymede, Europa, and Io.
Th e innermost moon, Io, was heated so much it lost all of its
water, and Europa retained only a small amount. Ganymede was
heated enough to diff erentiate but retained much of its water.
Callisto, orbiting far from Jupiter and avoiding orbital reso-
nances, was never heated enough to diff erentiate. Th e Galilean
moons as they appear today seem to be the result of a combina-
tion of slow formation and tidal heating.
Th e Galilean satellite system is full of clues to the history of
the solar system. Understanding that history prepares you to
explore farther from the sun.interrelated in that their densities are related to their distance
from Jupiter (■ Table 23-2).
From all the evidence, astronomers propose that the four
moons formed in a disk-shaped nebula around Jupiter—a mini-
solar nebula—in much the same way the planets formed from
the solar nebula around the sun. As Jupiter grew massive, it
would have formed a hot, dense disk of matter around its equa-
tor. Th e moons could have condensed inside that disk with the
innermost moons, Io and Europa, forming from rocky material
and the outer moons, Ganymede and Callisto, incorporating
more ice. Th is hypothesis follows the same condensation sequence
that led to rocky planets forming near the sun and ice-rich
worlds forming farther away.
Th ere are objections to this hypothesis. Th e disk around
Jupiter would have been dense and hot, and moons would have
formed rapidly, perhaps in only 1000 years. If the moons formed
quickly, the heat of formation released as material fell into the
moons would not have leaked away quickly, and they would have
grown so hot they would have lost their water. Ganymede and
Callisto are rich in water. Furthermore, Callisto has never been
hot enough to diff erentiate and form a core. Also, mathematical
models show that moons orbiting in the dense disk would have
swept up debris and lost orbital momentum; they would have
spiraled into Jupiter within a century.
A newer hypothesis proposes that Jupiter’s early disk was
indeed dense and hot and may have created moons, but those
moons spiraled into the planet and were lost. Only later, as the disk
grew thinner and cooler, did the present Galilean moons begin to
form. Additional material may have dribbled slowly into the disk,
and the moons could have formed slowly enough to retain their
water and avoid spiraling into Jupiter. In this scenario, many large
moons may have accreted around Jupiter. Th e Galilean satellites
you observe now would thus be only the last batch, formed when
the disk of construction material around Jupiter had become thin
enough that they did not fall into Jupiter and disappear. Th e same
migration and destruction process may apply, on a larger scale, to
planets forming in extrasolar planetary systems (see Chapter 19).
You can combine this hypothesis with what you know about
tidal heating to understand the interiors of the moons. Th e
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■ Table 23-2 \ The Galilean Satellites*Radius Density Orbital Period
Name (km) (g/cm^3 ) (days)
Io 1821 3.528 1.769
Europa 1561 3.014 3.551
Ganymede 2631 1.942 7.155
Callisto 2410 1.8344 16.689
*For comparison, the radius of Earth’s moon is 1738 km, and its density is 3.36 g/cm 3SCIENTIFIC ARGUMENT
What produces Io’s internal heat?
Scientifi c arguments commonly draw on basic principles that are
well understood. In this case, you understand that small worlds
lose their internal heat quickly and become geologically inactive.
Io is only slightly larger than Earth’s moon, which is cold and dead,
but Io is full of energy fl owing outward.
Clearly, Io must have a powerful source of heat inside, and that
heat source is tides. Io’s orbit is slightly eccentric (noncircular), so
it is sometimes closer to Jupiter and sometimes farther away. This
means that Jupiter’s powerful gravity sometimes squeezes Io more
than at other times, and the fl exing of the little moon’s interior
produces heat through friction. Such tides would rapidly force Io’s
orbit to become circular, and then tidal heating would end and
the planet would become inactive—except that the gravitational
tugs of the other moons keep Io’s orbit eccentric. Thus, it is the
infl uence of its companions that keeps Io in such an active state.
Io has almost no impact craters, but Callisto has many. Build
a new scientifi c argument drawing on a different principle. What
does the difference in crater distributions on the Galilean
satellites tell you about their history?Saturn
Saturn has played second fiddle to its own rings since
Galileo fi rst saw it in 1610. He didn’t recognize the rings for
what they are, but today they are instantly recognizable as one
of the wonders of the solar system. Nevertheless, Saturn itself,
not quite 10 times Earth’s diameter (Celestial Profi le 8), is
a fascinating planet with a few mysteries of its own. Your explo-
ration of Saturn and its rings can make use of the principles you
have learned from Jupiter.Surveying Saturn
Th e basic characteristics of Saturn reveal its composition and inte-
rior. Only about a third of the mass of Jupiter and 16 percent smaller
in radius, Saturn has an average density of 0.69 g/cm^3. It is less dense
than water—it would fl oat! Spectra show that its atmosphere is rich23-4