Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation. Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation. Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt. Callisto's rotation is tidally locked to its orbit around Jupiter, so that it always faces the same direction, making Jupiter appear to hang directly overhead over its near-side. It is not in an orbital resonance like the three other Galilean satellites- Io, Europa, and Ganymede-and is thus not appreciably tidally heated. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than 100 km. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates, and organic compounds. The absolute ages of the landforms are not known.Ĭallisto is composed of approximately equal amounts of rock and ice, with a density of about 1.83 g/cm 3, the lowest density and surface gravity of Jupiter's major moons. This is thought to result 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. At a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters ( catenae) and associated scarps, ridges and deposits. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of impacts. Its surface is completely covered with impact craters. The surface of Callisto is the oldest and most heavily cratered object in the Solar System. ![]() It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes, being visible from Earth with common binoculars. Callisto is, with a diameter of 4821 km, roughly a third larger than the Moon and orbits Jupiter on average at a distance of 1 883 000 km, which is about six times further out than the Moon orbiting Earth. In the Solar System it is the third-largest moon after Ganymede and Saturn's largest moon Titan, and as large as the smallest planet Mercury, though only about a third as massive. Callisto ( / k ə ˈ l ɪ s t oʊ/, kə- LIST-oh), or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede.
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