It wasn't until the Voyager probe flybys in 1979 that scientists got a close look at the moon's old, cratered surface of ice. More recently, the Galileo probe returned data about Callisto, including its internal structure.
Photo: Callisto taken by the New Horizons probe (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)
Jupiter's second largest moon has an ancient surface.
Sir Patrick Moore is amazed by the detail of Voyager 1's images.
Early in Voyager 1's mission, Sir Patrick Moore and his guest Dr Garry Hunt of University College London discussed some of the probe's findings, which included a ring around Jupiter and images of Jupiter's moons Amalthea, Callisto, Io, Europa and Ganymede. Since Sir Patrick and Dr Hunt spoke, Jupiter has been found to have three faint rings.
Jupiter's moons aren't the cold, dead worlds the experts expected.
Voyager scientists thought Jupiter's moons would be cold, dead worlds. They were amazed when the first close-up images from the spacecraft revealed four moons, each different from the next. The probes were launched in 1977.
Callisto /kəˈlɪstoʊ/ (Jupiter IV) is a moon of the planet Jupiter. It was 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, and the largest object in the solar system not to be properly differentiated. At 4821 km in diameter, 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 1880000 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's rotation is tidally locked to its revolution around Jupiter, so that the same hemisphere always faces inward; Jupiter appears to stand nearly still in Callisto's sky. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside the gas giant's main radiation belt.
Callisto is composed of approximately equal amounts of rock and ices, with a mean density of about 1.83 g/cm3, the lowest density and surface gravity of Jupiter's major moons. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates, and organic compounds. 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.
The surface of Callisto is the oldest and most heavily cratered in the whole solar system. 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. 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 made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. 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. The absolute ages of the landforms are not known.
Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably 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. Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation. 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.
The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa. Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto. Because of its low radiation levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system.