Callisto (Jupiter IV) The third largest of the Galilean satellites, with the lowest density, the body is believed not to have differentiated into a core and mantle, consisting throughout of a mixture of rock and ice. Its surface is the darkest of the Galileans (albedo 0.20; though twice as bright as that of the Moon) and Callisto is the most heavily cratered body in the solar system. There is believed to be an almost complete absence of geologic activity at the surface, which has an age of about 4 billion years and shows no sign of any extensive resurfacing. The surface is of dirty ice and there are no large mountains. Surface craters and rings are shallow; the largest structures are Valhalla, a bright patch about 600 × 3000 km, and Asgard, a ring about 1600 km across. The surface temperature is about −45 °C. Callisto was discovered on 7 January 1610 by Galileo. Its diameter is 4806 km; mass 1.077 × 10²³ kg; density 1851 kg/m³; surface gravity 0.127 (Earth = 1); mean distance from Jupiter 1.883 × 10⁶ km; mean distance from Sun 5.203 AU; orbital period 16.68902 days; rotational period 16.68902 days.

Callisto in Greek mythology, a nymph, an attendant of Artemis, who was the lover of Zeus and mother of his son Arcas; she was turned into a bear either by Zeus to save her from the anger of his wife Hera or by Hera and Artemis in vengeance; later she was placed as a constellation in the heavens by Zeus (see also Ursa Major).

Callisto Second-largest and outermost of Jupiter's Galilean satellites, with a diameter of 4800km (3000mi). It is the most heavily cratered object known. As well as the dark dense craters, there are large, multi-ringed impact features, the largest of which is Valhalla, with a diameter of 4000km (2500mi).