Callisto

Callisto, the second-largest moon of Jupiter, has a diameter of approximately 4,821 km and orbits Jupiter at an average distance of 1,883,000 km. Its surface is the oldest and most heavily cratered in the Solar System, with no signs of geological activity, and is composed of approximately equal amounts of rock and ice. Callisto's surface features include multi-ring structures, impact craters, and chains of craters, and is thought to have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than 100 km. The moon is tidally locked to its orbit around Jupiter and has a thin atmosphere composed of carbon dioxide and possibly molecular oxygen. Callisto's gradual accretion and lack of tidal heating led to partial differentiation and the formation of a subsurface ocean and small rocky core. The moon's conditions are thought to be less favorable for life than nearby Europa, but its low radiation levels make it a suitable destination for future crewed missions to study the Jovian system.learn more on wikipedia