Callisto is the outermost of the four Galilean moons, the second-largest moon of Jupiter, and the third-largest in the Solar System. Its surface is the most heavily cratered of any known body — essentially at the saturation limit, where each new crater destroys an older one — indicating that Callisto has experienced very little geological resurfacing in 4 billion years.

Because it orbits outside the Laplace resonance that drives the intense tidal heating of Io, Europa and Ganymede, Callisto has stayed geologically cold. It is also only partially differentiated: a rocky core, icy-rocky mantle, and icy crust, without the sharp iron/mantle/crust separation of its siblings. Magnetometer data from the Galileo orbiter nevertheless suggest the presence of a ~100-200 km deep salty subsurface ocean.

Because Callisto orbits well outside Jupiter’s most intense radiation belts, it is the Jovian moon on which a future human outpost would be most feasible. ESA’s JUICE mission will make 21 flybys of Callisto in 2031-2034.