CALLISTO

in Greek mythology, a nymph, or else a daughter of either Lycaon of Arcadia or of Nycteus or Ceteus. Callisto was one of the goddess Artemis' huntress companions and swore to remain unwed. But she was loved by Zeus and, in several variations of the legend, was turned into a she-bear either by Zeus (to conceal his deed from Hera) or by Artemis or Hera (who were enraged at her unchastity). Callisto was then killed during the chase by Artemis, who, owing to the machinations of the jealous Hera, mistook Callisto for a real bear. Zeus then gave Arcas, his child with Callisto, to the Titaness Maia to raise. He then placed Callisto among the stars as the constellation Ursa Major (Great Bear). An alternative legend has it that Arcas was transformed into the constellation Arctophylax just as he was about to kill his mother during a hunt. Approximate-natural-colour (left) and false-colour (right) pictures of Callisto, one of Jupiter's outermost of the four Galilean satellites of Jupiter. Discovered by Italian astronomer Galileo Galilei in 1610, it was named by German astronomer Simon Marius after a figure in Greek mythology. It has a diameter of 4,800 km (2,980 miles) and orbits the planet at a mean distance of about 1,880,000 km (1,170,000 miles). The bulk density of Callisto is only 1.83 times the density of water, or about one-half that of the Earth's moon, which indicates that Callisto is about half rock and half ice. The surface of the Jovian satellite, however, does not exhibit large amounts of pure water ice. Near-infrared spectra of the surface show water-ice bands, but they are very weak. Moreover, Callisto's surface is comparatively dark, as if the ice were contaminated with much dark material. Detail of a scarp on Callisto. This and many similar ringlike fractures on the satellite's icy Callisto appears to be the most heavily cratered of all of Jupiter's satellites, and an analysis of the distribution of its craters supports the hypothesis that they were produced about four billion years ago, when all bodies of the solar system came under heavy meteoroid bombardment. Internal activity has not substantially altered the surface cratering on Callisto as it has in the case of the other Galilean satellites. Besides a large number of intermediate-sized craters (with diameters of a few tens of kilometres), Callisto's most prominent landforms are multiringed structures that measure many hundreds of kilometres in diameter. These structures are probably caused by very large impacts; analogous features are found on Mercury and the Earth's Moon, but with important differences resulting from different crustal composition. The surface of Callisto is basically devoid of significant relief, probably because of thermal softening of the ice-rich crust early in its history.