Compared to the inner satellites of Jupiter, Ganymede and Callisto have old surfaces of ice that are heavily cratered
Ganymede has several notable characteristics
It is the largest satellite in the solar system -- in fact, it is larger than the planet Mercury!
The surface of Ganymede can be classified into two types of terrain: dark and bright
The dark terrain has more craters, hence it is older; the bright terrain has fewer craters, hence it is younger
Some of the dark terrain contains long, deep furrows, such as the circular area called Galileo Regio -- based on crater counting, this terrain is billions of years old
A finer set of very long grooves is present on the bright terrain -- this terrain is at least a billion years old
The difference in brightness might be caused by the exposure of the icy surface to sunlight, or possibly micrometeorites, for long periods of time
However, part of the difference seems to originate in numerous fractures within the bright terrain -- these actually enhance the brightness of the ice
The furrows and grooves indicate some sort of geological activity took place in Ganymede's distant past
Although the satellite today looks mostly dead on the outside, it still possesses a strong magnetic field of its own (see the Galileo Legacy web site illustrating "The Song of Ganymede")
To generate such a magnetic field, there must be a moving, electrically conducting medium -- hence some models of the interior favor a partially molten iron core
In addition, Ganymede has an induced magnetic field (like Europa) -- suggesting a subsurface ocean is present!
Callisto also is unusual
Although heavily cratered and very old, there are fewer small craters than expected
The surface seems to covered by thin covering of "sooty" material of unknown origin
Similar to Europa, the planet Jupiter induces a magnetic field in Callisto -- this implies there must be some sort of electrically conducting medium in Callisto
The best models of the interior have a mixture of ice and rock in the center
Since Callisto doesn't appear to have much of an iron core, might the induced magnetic field signal the presence of a subsurface ocean?