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Jupiter's largest moon is called Ganymede.
Ganymede is the biggest moon of the planet Jupiter. Ganymede is the largest moon in the entire Solar System. Ganymede is larger in diameter than Mercury but only about half its mass. 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 conditions. Ganymede may have been discovered by Chinese Astronomer Gan De in 364 BC. However, discovery of the moon is generally credited to Galileo Galilei who documented its existence in 1610. The name Ganymede was soon after suggested by Simon Marius, for the cup-bearer of the Greek gods, beloved of Zeus (see Ganymede (mythology)). 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.
Physical characteristics of Ganymede.
Internal structure of Ganymede. See image top.
Ganymede is composed of silicate rock and water ice, with an ice crust floating over a warmer ice mantle that may contain a layer of liquid water. Indications from the Galileo orbiter data suggest that Ganymede is differentiated into a three layer structure: a small partially 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 in its internal structure to Io.
Surface features of Ganymede.
The Ganymedean surface is a 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, probably formed by the extension, stretching, and faulting of the icy crust. Analogous tectonic features form the ridges and valleys in the Basin and Range province of the southwestern United States. Features reminiscent of old lava flows have also been observed. 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 4 billion years for the dark terrain, similar to the highlands of the Moon, and a somewhat younger age for the bright grooved terrain (but how much younger is uncertain). Craters both overlay and are cross cut by the groove systems indicating that some of the grooves are quite ancient. 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 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 grooves, or furrows, that are remnants of an ancient impact crater long since obscured by subsequent geological activity.
Atmosphere of Ganymede.
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 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.
Magnetosphere of Ganymede.
The Galileo orbiter's first flyby of Ganymede discovered that Ganymede has its own magnetic field, embedded inside Jupiter's huge field. Ganymede is the only moon known to have a magnetosphere. Ganymede's intrinsic magnetic field is probably generated in a similar fashion to the Earth's: as a result of conducting material moving in the interior, likely originating in its metallic core. Ganymede also has an induced magnetic field component, indicating that the satellite contains a subsurface layer that acts as a conductor. It is thought that this conductive material is a layer of liquid water containing salt, located at about 150 km depth and sandwiched between layers of different density forms of ice.
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