|Discovered by||S. Marius
|Mean radius||1.07×106 km|
|Revolution period||7d 3h 42.6m|
|Is a satellite of||Jupiter|
|Equatorial diameter||5268 km|
|Surface area||87 million km2|
|Mean density||1.936 g/cm3|
|Surface gravity||0.35 m/s2|
|Rotation period||7d 3h 42.6m|
Ganymede is Jupiter's largest moon, and indeed the largest moon in the entire solar system; it is larger in diameter than Mercury but only about half its mass. Ganymede is much larger than Pluto. It was discovered in 1609 by Galileo Galilei and named for the cup-bearer of the Greek gods, beloved of Zeus.
Ganymede is composed of silicate rock and water ice, with an ice crust floating over a slushy mantle that may contain a layer of liquid water. Preliminary indications from the Galileo probe data suggest that 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. This metallic core suggests a greater degree of heating at sometime in Ganymede's past than had previously been proposed. 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; Ganymede's crust appears to be divided into separate plates which, like Earth's tectonic plates, are able to move independently and interact along fracture zones producing mountain ranges. Features reminscent of old lava flows have also been observed. 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). 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 that 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.
The largest feature on Ganymede is a dark plain named Galileo Regio, as well as a series of concentric ridges that are remnants of an ancient impact crater long since obscured by subsequent geological activity.
Evidence for a tenuous oxygen atmosphere on Ganymede, very similar to the one found on Europa, has been found recently by the Hubble space telescope. Note that this is NOT evidence of life; it is thought that the oxygen is produced when water ice on Ganymede's surface is split into hydrogen and oxygen by radiation and then the hydrogen is lost due to its low atomic mass.
The Galileo probe's first flyby of Ganymede discovered that Ganymede has its own magnetosphere field 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. It is thought that this conductive material may be a layer of liquid water with a high salt concentration, or it may originate in Ganymede's metallic core.