Object Analysis

Callisto is the second-largest moon of Jupiter and the third-largest moon in the Solar System. It was discovered by Galileo Galilei in 1610. Callisto is about 99% the diameter of the planet Mercury but only about a third of its mass.

Callisto has the most heavily cratered surface of any moon in the Solar System. Its surface is extremely old, dating back about 4 billion years, and shows no sign of any geological resurfacing or plate tectonics. It is essentially a frozen fossil of the early solar system.

Despite its dead surface, magnetic field data suggests Callisto may harbor a subsurface ocean of salty water at depths greater than 100 km. Unlike Ganymede or Europa, this ocean is likely sandwiched between layers of ice rather than touching a rocky core, making it less likely to support life.