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Callisto


Callisto is the eighth of Jupiter's known satellites and the second largest. It is the outermost of the Galilean moons. Callisto is only slightly smaller than Mercury but only a third of its mass. Unlike Ganymede, Callisto seems to be a more or less uniform mixture of ice and rock. Titan and Triton are similar.

Callisto's surface is covered entirely with craters. The surface is very old, like the highlands of the Moon and Mars. Callisto has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years. The largest craters are surrounded by a series of concentric rings which look like huge cracks but which have been smoothed out by eons of slow movement of the ice. The largest of these has been named Valhalla. 4000 km in diameter, Valhalla is a dramatic example of a multi-ring basin, the result the result of a massive impact. Other examples are Callisto's Asgard, Mare Orientale on the Moon and Caloris Basin on Mercury. Like Ganymede, Callisto's ancient craters have collapsed. They lack the high ring mountains, radial rays and central depressions common to craters on the Moon and Mercury.

Another interesting feature is Gipul Catena, a long series of impact craters lined up in a straight line. This was probably caused by an object that was tidally disrupted as it passed close to Jupiter (much like Comet SL 9) and then impacted on Callisto.

Unlike Ganymede, with its complex terrains, there is little evidence of tectonic activity on Callisto. While Callisto is very similar in bulk properties to Ganymede, it apparently has a much simpler geologic history. The different geologic histories of the two has been an important problem for planetary scientists; (it may be related to the orbital and tidal evolution of Ganymede). "Simple" Callisto is a good reference for comparison with other more complex worlds and it may represent what the other Galilean moons were like early in their history.


Orbital Distance:   1,883,000 km (average)
Eccentricity:       0.007
Local Year:         16.689 days (16d 16h 32m 11.3s)
Orbital Velocity:   8.205 km/s
Inclination:        0.281°
Local Day:          16.689 days (16d 16h 32m 11.3s)
Axial Tilt:         n/a

Diameter:           4,800 km
Composition:        Basaltic rock and dust
Density:            1.86 g/cm3
Mass:               1.08 x 1020 tonnes (0.018 earths)
Gravity:            0.127 G
Escape Velocity:    2.449 km/s
Area:               72.6 million km2
Surface Material:   Rocky duct and some ice
Atmosphere:         None
Pressure:           None
Albedo:             0.20
Temperature:        ???°C

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