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Jupiter's Moon Callisto
Jupiter's Moon Callisto -- With a diameter of over 4,800 km (2,985 miles), Callisto is the third largest satellite in the solar system and is almost the size of Mercury. Callisto is the outermost of the Galilean satellites, and orbits beyonds Jupiter's main radiation belts.
It has the lowest density of the Galilean satellites (1.86 grams/cubic centimeter). Its interior is probably similar to Ganymede except the inner rocky core is smaller, and this core is surrounded by a large icy mantle. Callisto's surface is the darkest of the Galileans, but it is twice as bright as our own Moon.
Callisto is the most heavily cratered satellite in the solar system. Its crust is very ancient and dates back 4 billion years, just shortly after the solar system was formed.
Callisto lacks any large mountains. This is probably due to the icy nature of its surface. Impact craters and associated concentric rings are about the only features to be found on Callisto. The largest craters have been erased by the flow of the icy crust over geologic time.
Two enormous concentric ring, impact basins are found on Callisto. The largest impact basin is Valhalla. It has a bright central region that is 600 kilometers in diameter, and its rings extend to 3000 kilometers in diameter. The second impact basin is Asgard. It measures about 1600 kilometers in diameter.
Callisto has the lowest density (1.86 gm/cm3) of the Galilean satellites. The moon is probably made of a large rocky core surrounded by water and ice, giving it a dark color. It is composed of approximately equal proportions of ice-water and rock.
Meteorites have punctured holes in the crust, causing water to spread over the surface and forming bright rays and rings around the crater. Callisto has a thin and tenuous carbon dioxide atmosphere.
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NASA
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