(See the Ganymede Gallery)
Ganymede is the largest satellite in the solar system. It is larger in diameter than Mercury but only about half its mass. Ganymede is much larger than Pluto. Data now suggests that Callisto has a uniform composition while Ganymede is differentiated into a three layer structure: a small molten iron or iron/sulfur core surrounded by a rocky silicate mantle with a icy shell on top. In fact, Ganymede may be similar to Io with an additional outer layer of ice.
Ganymede's surface is a roughly equal mix of two types of terrain: very old, highly cratered dark regions, and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges. Their origin is clearly of a tectonic nature, but the details are unknown. In this respect, Ganymede may be more similar to the Earth than either Venus or Mars though there is no evidence of recent tectonic activity.
Extensive cratering is seen on both types of terrain. Unlike the Moon, however, the craters are quite flat. This is probably due to the relatively weak nature of Ganymede's icy crust which can flow over geologic time and soften the relief. Ancient craters whose relief has disappeared leaving only a "ghost" of a crater are known as "palimpsests."
Galileo's first flyby of Ganymede discovered that Ganymede has its own magnetosphere embedded inside Jupiter's huge one. This is probably generated in a similar fashion to the Earth's as a result of motion of conducting material in the interior.