CALLISTO (JUPITERS MOON) Tuesday, June 16, 2009

Callisto is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei.[1] It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. 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 1 880 000 kilometers.[2] 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.[9] Callisto rotates synchronously with its orbital period, so the same face is always turned toward Jupiter. Callisto's surface is less affected by Jupiter's magnetosphere than the other inner satellites because it orbits farther away.[10]
Callisto is composed of approximately equal amounts of rock and ices, with a mean density of about 1.83 g/cm3. Compounds detected spectrally 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 kilometers.[11][12]
The surface of Callisto is heavily cratered and extremely old. It does not show any signatures of subsurface processes such as plate tectonics, earthquakes or volcanoes, and is thought to have evolved predominantly under the influence of impacts.[13] Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges and deposits.[13] 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.[4] 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.[14] The absolute ages of the landforms are not known.
Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide[6] and probably molecular oxygen,[7] as well as by a rather intense ionosphere.[15] Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation.[16] Its slowness and the lack of tidal heating prevented 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 kilometers and a small, rocky core.[17]
The likely presence of an ocean within Callisto indicates that it can or could harbor life. However, this is less likely than on nearby Europa.[18] Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied the moon. Callisto has long been considered the most suitable place for a human base for future exploration of the system of Jupiter.[19]

Discovery and naming

Callisto was discovered by Galileo in January 1610 along with three other large Jovian moons—Ganymede, Io, and Europa.[1] Callisto is named after one of Zeus's many lovers in Greek mythology. Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis.[20] The name was suggested by Simon Marius soon after the moon's discovery.[21] Marius attributed the suggestion to Johannes Kepler.[20] However, the names of the Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral designation, a system introduced by Galileo, as Jupiter IV or as "the fourth satellite of Jupiter".[22] In scientific writing, the adjectival form of the name is Callistoan,[23] pronounced /ˌkælɨˈstoʊən/, or Callistan.[14]

ORBIT AND ROTATION
Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1 880 000 km (26.3 times the 71 398 km radius of Jupiter itself).[2] This is significantly larger than the orbital radius—1 070 000 km—of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in the mean-motion resonance—in which the three inner Galilean satellites are locked—and probably never has.[9]
Like most other regular planetary moons, Callisto's rotation is locked to be synchronous with its orbit.[3] The length of the Callistoan day, simultaneously its orbital period, is about 16.7 Earth days. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. The ranges of change are 0.0072–0.0076 and 0.20–0.60°, respectively.[9] These orbital variations cause the axial tilt (the angle between rotational and orbital axes) to vary between 0.4 and 1.6°.[24]
The dynamical isolation of Callisto means that it has never been appreciably tidally heated, which has had important consequences for its internal structure and evolution.[25] Its distance from Jupiter also means that the charged-particle flux from the planet's magnetosphere at its surface is relatively low—about 300 times lower than, for example, that at Europa. Hence, unlike the other Galilean moons, charged-particle irradiation has had a relatively minor effect on the Callistoan surface.[10]

PHYSICAL characteristics

COMPOSITION

The average density of Callisto, 1.83 g/cm3,[3] suggests a composition of approximately equal parts of rocky material and water ice, with some additional volatile ices such as ammonia.[11] The mass fraction of ices is between 49–55%.[11][17] The exact composition of Callisto's rock component is not known, but is probably close to the composition of L/LL type ordinary chondrites, which are characterized by less total iron, less metallic iron and more iron oxide than H chondrites. The weight ratio of iron to silicon is 0.9:1.3 in Callisto, whereas the solar ratio is around 1.8.[11]
Callisto's surface has an albedo of about 20%[4]. Its surface composition is thought to be broadly similar to its composition as a whole. Near-infrared spectroscopy has revealed the presence of water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers.[4] Water ice seems to be ubiquitous on the surface of Callisto, with a mass fraction of 25–50%.[12] The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed various non-ice materials: magnesium- and iron-bearing hydrated silicates,[4] carbon dioxide,[26] sulfur dioxide,[27] and possibly ammonia and various organic compounds.[4][12] Spectral data indicate that the moon's surface is extremely heterogeneous at the small scale. Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material.[4][13]
The Callistoan surface is asymmetric: the leading hemisphere—the hemisphere facing the direction of the orbital motion[g]—is darker than the trailing one. This is different from other Galilean satellites, where the reverse is true.[4] The trailing hemisphere[g] of Callisto appears to be enriched in carbon dioxide, while the leading hemisphere has more sulfur dioxide.[28] Many fresh impact craters like Lofn also show enrichment in carbon dioxide.[28] Overall, the chemical composition of the surface, especially in the dark areas, may be close to that seen on D-type asteroids,[13] whose surfaces are made of carbonaceous material.