However, conditions are thought to be less favorable than on nearby Europa. The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. The slow convection in the interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core. 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. The absolute ages of the landforms are not known.Ĭallisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere.

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 consists of small, bright frost deposits at the tops of elevations, 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, and is thought to have evolved predominantly under the influence of impacts. The surface of Callisto is heavily cratered and extremely old. 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. Callisto's surface is less affected by Jupiter's magnetosphere than the other inner satellites because it orbits farther away.Ĭallisto is composed of approximately equal amounts of rock and ices, with a mean density of about 1.83 g/cm3. Callisto rotates synchronously with its orbital period, so the same hemisphere always faces (is tidally locked to) Jupiter. 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. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1,880,000 km. Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. It was discovered in 1610 by Galileo Galilei. Callisto is a moon of the planet Jupiter, (pronounced /kəˈlɪstoʊ/, named after the Greek mythological figure of Callisto, Greek: Καλλιστώ).

Space pioneer callisto