504 PART 4^ |^ THE SOLAR SYSTEM
rocky, and many are probably captured asteroids. Four of the
moons, those discovered by Galileo and now called the Galilean
satellites, are large and have interesting geologies (■ Figure 23-6).
Your study of the moons of Jupiter will illustrate three
important principles in comparative planetology. First, a body’s
composition depends on the temperature of the material from
which it formed. Th is is illustrated by the prevalence of ice as a
building material in the outer solar system, where sunlight is
weak. You are already familiar with the second principle: that
cratering can reveal the age of a surface. Also, as you have seen in
your study of the Terrestrial planets, internal heat has a powerful
infl uence over the geology of these larger moons.
Callisto: An Ancient Surface
Th e outermost of Jupiter’s four large moons, Callisto, is half
again as large in diameter as Earth’s moon. Like all of Jupiter’s
larger satellites, Callisto is tidally locked to its planet, keeping the
same side forever facing Jupiter. From its gravitational infl uence
on passing spacecraft, astronomers can calculate Callisto’s mass,
and dividing that mass by its volume shows that its density is
1.8 g/cm^3. Ice has a density of about 1 and rock 2.5 to 4 g/cm^3 ,
so Callisto must be a mixture of rock and ice.
Images from the Voyager and Galileo spacecraft show that
the surface of Callisto is dark, dirty ice heavily pocked with cra-
ters (■ Figure 23-7). Old, icy surfaces in the solar system become
dark because of dust added by meteorites and because meteorite
impacts vaporize water, leaving any dust and rock in the ice
behind to form a dirty crust. You may have seen the same thing
happen to a city snowbank. As the snow evaporates over a few
days, the crud in the snow is left behind to form a dirty rind.
Break through that dirty surface, and the snow is much cleaner
underneath.
Spectra of Callisto’s surface show that in most places it is a
50:50 mix of ice and rock, but some areas are ice free. Nevertheless,
the slumped shapes of craters suggests that the outer 10 km of
■ Figure 23-6
The Gallilean satellites of Jupiter from left to right are Io, Europa, Ganymede,
and Callisto. The circle shows the size of Earth’s moon. (NASA)
Size of Earth’s moon
Visual-wavelength images■ Figure 23-7
The dark surface of Callisto is dirty ice marked by craters in these images.
The youngest craters look bright because they have dug down to cleaner ice.
Valhalla is the 4000 km (2500 mi) diameter scar of a giant impact feature,
one of the largest in the solar system. Valhalla is so large and old that the icy
crust has fl owed back to partially heal itself, and the outer rings of Valhalla
are shallow troughs marking fractures in the crust. (NASA)ValhallaVisual-wavelength imagesthis moon is mostly frozen water; ice isn’t very strong, so big piles
of it tend to slump under their own weight. Th e disagreement
between the spectra and the shapes of craters can be understood
when you recall that the spectra contain information about only
the outer 1 mm of the surface, which can be quite dirty, while
the shapes of craters tell you about the outermost 10 km, which
appear to be rich in ice.
Delicate measurements of the shape of Callisto’s gravita-
tional fi eld were made by the Galileo spacecraft as it fl ew by.
Th ose measurements show that Callisto has never fully diff erenti-
ated to form a dense core and a lower-density mantle. Its interior
is a mixture of rock and ice rather than having distinct layers of
diff erent composition. Th is is consistent with the observation
that Callisto has only a weak magnetic fi eld of its own. A strong
magnetic fi eld could be generated by the dynamo eff ect in a liq-
uid convecting core, and Callisto has no core. It does, however,