The most heavily cratered moon is the outermost of Jupiter’s four large moons, Callisto, which is an ancient relic whose surface is 100% covered with impact craters. No evidence of any other geological processes have been discovered.

Its distance of 1,869,000 to 1,897,000 km from Jupiter means the ambient radiation is low for the Jovian system. Coupled with the geological stability, this means Callisto is a likely first target for future human exploration of the outer Solar System.