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Ganymede


Ganymede is the seventh and largest of Jupiter's known satellites. Ganymede is the third of the Galilean moons. It is the largest satellite in the solar system: larger in diameter than Mercury but only about half its mass, and much larger than Pluto.

Ganymede has a small molten iron core surrounded by a rocky silicate mantle with a icy shell on top. It has a very tenuous oxygen atmosphere very similar to the one found on Europa (note that this is definitely NOT evidence of life).

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, 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). 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. 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.


Orbital Distance:   1,070,000 km (average)
Eccentricity:       0.002
Local Year:         7.155 days (7d 3h 42m 33.4s)
Orbital Velocity:   10.875 km/s
Inclination:        0.195°
Local Day:          7.155 days (7d 3h 42m 33.4s)
Axial Tilt:         n/a

Diameter:           5,262 km
Composition:        Basaltic rock and dust
Density:            1.93 g/cm3
Mass:               1.48 x 1020 tonnes (0.025 earths)
Gravity:            0.145 G
Escape Velocity:    2.738 km/s
Area:               87.2 million km2
Surface Material:   H2O ice and dust
Atmosphere:         O2
Pressure:           1.339 9.869 x 10-11 atm
Albedo:             0.42
Temperature:        ???°C

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