orbit: 1,070,000 km from Jupiter diameter: 5262 km mass: 1.48e23 kgGanymede was a Trojan boy of great beauty whom Zeus carried away to be cupbearer to the gods.
Discovered by Galileo and Marius in 1610.
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.
Ganymede and Callisto seem to be composed of a rocky core surrounded by a large mantle of water or water ice with an ice surface. Titan and Triton are similar.
Ganymede's surface is a roughly equal mix of two types of terrain: very old, highly cratered dark regions (left), and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges (right). 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).
HST has recently found evidence of a very tenuous oxygen atmosphere on Ganymede very similar to the one found on Europa (note that this is definitely NOT evidence of life).
Similar ridge and groove terrain is seen on Enceladus, Miranda and Ariel. The dark regions are similar to the surface of Callisto.
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 (left).
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 (right).
Galileo's first flyby of Ganymede discovered that Ganymede has its own magnetic field embedded inside Jupiter's huge one.