Callisto is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1 880 000 km. It does not form part of the orbital resonance that affects three inner Galilean satellites—Io, Europa and Ganymede—and thus does not experience appreciable tidal heating. Callisto rotates synchronously with its orbital period, so the same face is always turned toward Jupiter. Callisto’s surface is less affected by Jupiter’s magnetosphere than the other inner satellites because it orbits farther away.
Callisto is composed of approximately equal amounts of rock and ices, with a mean density of about 1.83 g/cm3. It has a small silicate core and a subsurface ocean of liquid water at depths greater than 100 km.
The surface of Callisto is heavily cratered and extremely old. Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges and deposits. At a small scale, the surface is varied and consists of small, bright frost deposits at the tops of elevations, surrounded by a low-lying, smooth blanket of dark material. This resulted from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants. The absolute ages of the landforms are not known.
Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and molecular oxygen, as well as by a rather intense ionosphere. Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation. Its slowness and the lack of tidal heating prevented rapid differentiation. The slow convection in the interior of Callisto, which commenced soon after formation, led to partial differentiation and the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core.
Callisto has been thoroughly terraformed.