C allisto is the second largest moon of Jupiter, the third largest in the solar system, and is about the same size as Mercury. It was discovered by Simon Marius and Galileo Galilei in 1610. It orbits just beyond Jupiter's main radiation belt. 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.

C allisto 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 3,000 kilometers in diameter. The second impact basin is Asgard, It measures about 1600 kilometers in diameter.

C allisto 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 no known atmosphere.