Ganymede (IPA: /ˈgænɪˌmiːd/, gan'-i-meed, Greek Γανυμήδης) 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. Ganymede is the only planetary satellite (besides the Moon) that can be seen by the naked eye - but only with very good eyesight during ideal seeing conditions. Therefore it is no surprise that it was discovered only in 1610 by Galileo Galilei and named after the cup-bearer of the Greek gods, beloved of Zeus (see Ganymede (mythology)). Although the name "Ganymede" was suggested by Simon Marius soon after its discovery, this name and the names of the other Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, it is simply referred to by its Roman numeral designation as Jupiter III or as the "third satellite of Jupiter". Ganymede is the only Galilean moon of Jupiter named after a male figure.
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 orbiter 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 an icy shell on top. This metallic core suggests a greater degree of heating at some time in Ganymede's past than had previously been proposed. In fact, Ganymede may be similar to Io with an additional outer layer of ice.
The sharp boundary between the dark Nicholson Regio and the bright Harpagia Sulcus
Interior of Ganymede
The Ganymedean 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 reminiscent 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 on the Moon, however, Ganymedean 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.
In the mid-1980s, a team of Indian and American astronomers working at Indonesia's Lembang Observatory detected a thin atmosphere around Ganymede during an occultation when Jupiter passed in front of a star. 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 necessarily 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 orbiter'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. Ganymede is the only moon known to have a magnetosphere.
Ganymede in fiction
List of craters on Ganymede
The asteroid 1036 Ganymed
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Jupiter's natural satellites
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