Ganymede ("GAN uh meed") is the seventh and largest of Jupiter's known satellites. Ganymede is the third of the Galilean moons.

Ganymede's Surface
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 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. The density of cratering indicates an age of 3 to 3.5 billion years, similar to the Moon. Craters both overlay and are cross cut by the groove systems indicating the the grooves are quite ancient, too. Relatively young craters with rays of ejecta are also visible.

Unlike the Moon, however, the craters are quite flat, lacking the ring mountains and central depressions common to craters on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust which can flow over geologic time and thereby soften the relief. Ancient craters whose relief has disappeared leaving only a "ghost" of a crater are known as palimpsests.