Callisto

Callisto is the outermost of Jupiter's four large Galilean moons, discovered by Galileo Galilei in 1610. It is Jupiter's second-largest moon and the third-largest moon in the Solar System, roughly the size of Mercury but much less massive. Callisto is tidally locked, so the same hemisphere faces Jupiter as it completes each orbit, which takes about 16.7 Earth days.

Callisto's surface is composed mainly of ice and rock and is densely marked by impact craters. Its visible terrain is estimated to be about four billion years old, among the oldest and most heavily cratered surfaces known in the Solar System. Compared with the more geologically active inner Galilean moons, Callisto shows little evidence of ongoing resurfacing. Bright areas often occur around craters, where exposed water ice is thought to be common.

Measurements by the Galileo spacecraft found variations in Callisto's magnetic environment that can be explained in part by electrical currents induced in a conductive layer beneath the surface. A salty liquid ocean is one possible explanation. However, Callisto's ionosphere can also contribute to the magnetic signal, so the evidence is not definitive. A 2025 peer-reviewed analysis of Galileo flyby measurements concluded that a combination of a conductive subsurface ocean and ionosphere was a more plausible explanation than the ionosphere alone. The existence and properties of any ocean remain subjects of scientific investigation; no life has been detected on Callisto.

NASA's Galileo spacecraft studied Callisto during its exploration of the Jovian system from 1995 to 2003, providing much of the evidence about the moon's surface, composition and possible interior ocean. ESA's Jupiter Icy Moons Explorer, JUICE, launched in April 2023 and is travelling to Jupiter. Its planned investigations include detailed observations of Callisto, Europa and Ganymede, treating them as both planetary bodies and possible habitats. ESA currently lists JUICE's arrival at Jupiter as July 2031.