Callisto

Callisto (Jupiter IV) is Jupiter's second largest moon, larger than the Moon and nearly as large as Mercury. It is one of the original four Jupiter moons found centuries ago (the Galilean Moons), the fourth outward from Jupiter of these four, and is the eighth outward of Jupiter's currently known moons. Its radius is 2410 km (0.4 Earth's, 1.4 Moon's), its mass is 1.0×10²³ kg (0.018 Earth's, 1.35 Moon's), with a mean density a little less than twice that of water (considerably less than the Earth or Moon) and subsurface oceans are suspected. Callisto's orbital period is 17 days and it is tidally locked. Flyby observations of Callisto have been made by the Pioneer missions, Voyager, Galileo, Cassini, and New Horizons.