Ganymede (moon)

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**Ganymede**
Discovery
Discovered by	G. Galilei S. Marius
Discovered on	January 11, 1610
Orbital characteristics
Mean radius	1,070,400 km (0.007155 AU)
Eccentricity	0.0011
Periapsis	1,069,200 km (0.007147 AU)
Apoapsis	1,071,600 km (0.007163 AU)
Revolution period	7.15455296 d (0.019588 a)
Orbital circumference	6,725,500 km (0.045 AU)
Orbital velocity	max: 10.892 km/s mean: 10.880 km/s min: 10.868 km/s
Inclination	2.21° (to the ecliptic) 0.20° to Jupiter's equator)
Is a satellite of	Jupiter
Physical characteristics
Mean diameter	5262.4 km (0.413 Earths)
Surface area	87 million km² (0.171 Earths)
Volume	7.6 km³ (0.0704 Earths)
Mass	1.4819 kg (0.025 Earths)
Mean density	1.942 g/cm³
Surface gravity	1.428 m/s² (0.146 g)
Escape velocity	2.741 km/s
Rotation period	synchronous
Axial tilt	zero
Albedo	0.43
Apparent Magnitude	4.6
Surface temp.

min	mean	max
K	~109 K	K

Atmospheric characteristics Atmospheric pressure trace Oxygen 100%

Ganymede (gan'-ə-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 conditions. It might have been first seen by Chinese astronomer Gan De in 364 BC. 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

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.

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.

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.

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 a layers of different density forms of ice.

Ganymede in fiction

In the book On Two Planets by Kurd Lasswitz (1897), a powerful, benevolent alien race from Ganymede decides to assist humanity.
In the short stories "Not Final" (1941) and "Victory Unintentional" (1942) by Isaac Asimov, a conflict arises between humans living on Ganymede and the inhabitants of Jupiter.
Robert A. Heinlein's novel Farmer in the Sky (1950) depicts the terraforming and colonization of Ganymede. In Heinlein's novel Space Cadet (1948), one of the characters is listed as being a third generation colonist on Ganymede.
Leigh Brackett's short story The Dancing Girl of Ganymede (1950) is set on a volcanic, jungle-covered Ganymede.
Ganymede is referred to in virtually all of Philip K. Dick's novels from the 50s and 60s, although it seldom receives more than a brief mention. In "Clans of the Alphane Moon" (1964), we are introduced to Lord Running Clam, a species of intelligent slime mold from Ganymede.
In the The Original Series episode "By Any Other Name" (1968), Scotty asserts a green beverage was obtained on Ganymede, but he might be mistaken.
The German movie "Operation Ganymed" (1977) - made for TV - tells the story of five astronauts returning from an expedition to Ganymede. They find a seemingly desolate Earth and are trying to find out what happened while they were in space.
The Gentle Giants of Ganymede (1978) by James P. Hogan ISBN 0345314697. An alien spaceship is found on Ganymede.
In Arthur C. Clarke's novel Odyssey Three (1987), Ganymede is warmed by the new sun Lucifer and contains a large equatorial lake. It is the centre of human colonization of the Jovian system.
In the 32nd Millennium of the Warhammer 40,000 (1987) Universe, Ganymede is destroyed during a Warp Core experiment that goes horribly wrong.
The 1991 science fiction novel Buddy Holly Is Alive and Well on Ganymede, by Bradley Denton (ISBN 0688108229 and ISBN 0380718766), begins when television sets throughout the world suddenly begin broadcasting a concert by an apparently living Buddy Holly, who says he is on Ganymede.
In the Power Rangers TV series (1993-), Ganymede is the hiding place chosen by Zordon as the hiding space for a fleet of Zords known as the Mega Vehicles, which combine to form the Mega Voyager. The Space Rangers locate these Zords after winning Key Cards from Darkonda in a card game. ("Flashes of Darkonda", "The Rangers' Mega Voyage").
In the science fiction TV series Babylon 5 (1994-1998), the Shadows bury a ship under the surface of Ganymede, which is dug up during the third season of the show (the Messages From Earth episode, 1996).
The majority of events in "The Ganymede Club" (1995), a science fiction mystery by Charles Sheffield, take place on Ganymede.
In the animated television series The King of Braves GaoGaiGar (1997), many of the 31 Machine Primevals fuse with Jupiter's satellites as their last stand against heroic Gutsy Galaxy Guard. The Primeval to fuse with Ganymede is Lungs Primeval (ZX-17), which creates a "Klein Space" within itself (a pocket dimension bearing spatial geometry similar to that of a klein bottle) in an attempt to seal GGG away upon their arrival in Jovian orbit.
Ganymede is portrayed in the anime series Cowboy Bebop (1998). It is depicted as an aquatic planet, a terraformed world that is entirely covered in water. Animal life, most likely created by human intervention, is also depicted, such as the Ganymede searat, a species of rodent-like seal. About 7 million people live on it in floating colonies which are supported on large barge-type foundations. The character Jet Black was a police officer on Ganymede, and presumably was born there. Jet also has a watch that is apparently from Ganymede.
In the novel Orphanage (2004) by Robert Buettner, an alien race uses Ganymede as a staging area for a war against Earth.
In the DOS game 2097, the prize for victory in the tournament is the development right to Ganymede, which is also the home of the Angels, who seek to prevent their home being developed.
In the PC game FOM , Ganmede is a space colony which has been terraformed to support humanity.

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Physical characteristics

Ganymede in fiction

See also