506 PART 4^ |^ THE SOLAR SYSTEM
other moons could have pushed Ganymede into a more eccentric
orbit. Tidal forces due to Jupiter’s gravity would have deformed
the moon, and as Ganymede followed its orbit, varying in
distance from Jupiter, tides would have fl exed it, and friction
would have heated it. Such an episode of tidal
heating might have been enough to drive a
dynamo to produce a magnetic fi eld and break
the crust to make the bright terrain.
Th e second process that aff ects Ganymede is
the inward focusing of meteorites. Because
massive planets like Jupiter draw debris inward,
the closer a moon orbits to the planet, the more
often it will be struck by meteorites (Figure 23-9b).
You should expect such a moon to have lots of
craters, but the bright terrain on Ganymede has
few craters. Th at part of Ganymede’s surface must
be only about 1 billion years old, and this should
alert you that the Galilean moons are not just
dead lumps of rock and ice. Th e closer you get to
Jupiter, the more active the moons are.
Europa: A Hidden Ocean
Th e next Galilean moon inward is Europa, which
is a bit smaller than Earth’s moon (Figure 23-6).
Europa has a density of 3.0 g/cm^3 , so it must be
mostly rock and metal. Yet its surface is ice.
Europa lies closer to Jupiter than Ganymede,
so it should be exposed to more meteorite impacts
than Callisto or Ganymede, yet the icy crust of
■ Figure 23-9
Two effects on planetary satellites. (a) Tidal heating occurs when changing
tides cause friction within a moon. (b) The focusing of meteoroids exposes
satellites in inner orbits to more impacts than satellites in outer orbits
receive.
MoonMoonMoonMoonJupiterabJupiterPwyllaccc Visual-wavelength imagesVisual-wavelength imagesVisual-wavelength imagesbEuropa is almost free of craters. Recent craters such as Pwyll are
bright, but most are hardly more than blemishes in the ice
(■ Figures 23-10). Evidently the surface of Europa is active and
erases craters almost as fast as they form. Th e number of impact
scars on Europa suggests that the average age of its surface is only
10 million years. Other signs of activity include long cracks in
the icy crust and regions where the crust has broken into sections
that have moved apart as if they were icebergs fl oating on water
(Figure 23-10c).
Europa’s clean, bright face also tells you its surface is young.
Th e albedo of the surface is 0.69, meaning that it refl ects
69 percent of the light that hits it. Th is high albedo is produced
by clean ice. You have discovered that old, icy surfaces tend to be
very dark, so Europa’s high albedo means the surface is active,
covering older surfaces with fresh ice.
Europa is too small to have retained much heat from its
formation or from radioactive decay, and the Galileo spacecraft■ Figure 23-10
(a) The icy surface of Europa is shown here in natural color. Many faults
are visible on its surface, but very few craters. The bright crater is Pwyll,
a young impact feature. (b) This circular bull’s-eye is 140 km (90 mi) in
diameter. It is the remains of an impact by an object estimated to have been
about 10 km in diameter. Notice the younger cracks and faults that cross
the older impact feature. (c) Like icebergs on the Arctic Ocean, blocks of
crust on Europa appear to have fl oated apart. Spectra show that the blue ice
is stained by salts such as those that would be left behind by mineral-rich
water welling up from below and evaporating. White areas are ejecta from
the impact that formed crater Pwyll. (NASA)