The Solar System

CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 505

interact with Jupiter’s magnetic fi eld in a way that suggests it has
a layer of liquid water roughly 10 km thick about 100 km below
its icy surface. Slow radioactive decay in Callisto’s interior may
produce enough heat to keep this layer of water from freezing.

Ganymede: An Obscure Past

Th e next Galilean moon inward is Ganymede, larger than
Mercury, and over three-quarters the diameter of Mars. In fact,
Ganymede is the largest moon in the solar system. Its density is
1.9 g/cm^3 , and its infl uence on the Galileo spacecraft reveals that
it is diff erentiated into a rocky core, an ice-rich mantle, and a
crust of ice 500 km thick. It may even have a small inner iron
core. Ganymede is large enough for radioactive decay to have
melted its interior when it formed, allowing rock and iron to sink
to its center.
Ganymede’s surface hints at an active past. Although a third
of the surface is old, dark, and cratered like Callisto’s, the rest is
marked by bright parallel grooves. Because this bright grooved
terrain (■ Figure 23-8a) contains fewer craters, it must be
younger.
Observations show that the bright terrain was produced
when the icy crust broke and water fl ooded up from below and
froze. As the surface broke over and over, sets of parallel groves
were formed. Some low-lying regions are smooth and appear to
have been fl ooded by water. Spectra reveal concentrations of salts
such as those that would be left behind by the evaporation of
mineral-rich water. Also, some features in or near the bright ter-
rain appear to be caldera formed when subsurface water drained
away and the surface collapsed (Figure 23-8b).

■ Figure 23-8

(a) This color-enhanced image of Ganymede shows the frosty poles at top and bottom, the old dark terrain, and the brighter grooved

terrain. (b) A band of bright terrain runs from lower left to upper right, and a collapsed area, a possible caldera, lies at the center in this

visual-wavelength image. Calderas form where subsurface liquid has drained away, and the bright areas do contain other features probably

due to fl ooding by water. (NASA)

ab

Visual-wavelength imagesVisual-wavelength images

Th e Galileo spacecraft found that Ganymede has a magnetic

fi eld about 10 percent as strong as Earth’s. It even has its own

magnetosphere inside the larger magnetosphere of Jupiter.

Mathematical models calculated by planetary scientists do not

predict that a magnetic fi eld this strong should arise from the

dynamo eff ect in a liquid water mantle layer with the size and

location of the one in Ganymede, and there does not appear to

be enough heat in Ganymede for it to have a molten metallic

core. Th us, the cause of Ganymede’s unique magnetic fi eld

remains a puzzle. One hypothesis is that the magnetic fi eld is left

over and frozen into the rock from a time when Ganymede was

hotter and more active.

Ganymede’s magnetic fi eld fl uctuates with the 10-hour

period of Jupiter’s rotation. Th e rotation of the planet sweeps its

tilted magnetic fi eld past the moon, and the two fi elds interact.

Th at interaction reveals that the moon has a layer of liquid water

about 170 km (110 mi) below its surface. Th e data indicate that

the water layer is about 5 km (3 mi) thick. It is possible that the

water layer was thicker and closer to the surface long ago when

the interior of the moon was warmer. Th at might explain the

fl ooding that appears to have formed the bright grooved

terrain.

Ganymede orbits rather close to massive Jupiter, and that

exposes it to two unusual processes that many worlds never expe-

rience. Tidal heating, the frictional heating of a body by chang-

ing tides (■ Figure 23-9a), could have heated Ganymede’s interior

and added to the heat generated by radioactive decay. In its cur-

rent nearly circular orbit, this moon experiences little or no tidal

heating. But, at some point in the past, interactions with the