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Definition: Callisto from Philip's Encyclopedia

Second-largest and outermost of Jupiter's Galilean satellites, with a diameter of 4800km (3000mi). It is the most heavily cratered object known. As well as the dark dense craters, there are large, multi-ringed impact features, the largest of which is Valhalla, with a diameter of 4000km (2500mi).

Summary Article: Callisto
from Collins Dictionary of Astronomy

The second largest of the four giant Galilean satellites of Jupiter, with a radius of 2400 km and a density of 1.86 g cm–3. It is the faintest of the Galileans, having an albedo of 0.2. It is the most heavily cratered object so far discovered in the Solar System, exhibiting a number of ray systems, i.e. craters from which bright streaks radiate. Some major systems of concentric ring mountains are prominent, particularly in the neighborhood of the huge Valhalla Basin, which is located slightly north of the equator. The basin is a bright circular region 600 km in diameter and is surrounded by concentric rings 50–200 km apart; the outermost ring is 3000 km across. This basin is comparable with Orientale Basin on the Moon, the Caloris Basin on Mercury, and Hellas Planitia on Mars. A second huge basin on Callisto, Asgard Basin, has a total diameter of 1600 km. Prior to the Galileo mission, Callisto was thought to have a thick crust of ice and rock extending to a depth of 200 to 300 km, beneath which there was believed to be a mantle of 1000 km of convecting water or soft ice. But preliminary analysis of the Galileo data has suggested that Callisto's internal structure is relatively undifferentiated, although the proportion of rock to ice may increase toward the center. Callisto, the outermost of the Galilean satellites, is beyond the major charged particle environment of Jupiter. It appears to be geologically inactive, its surface cratering being due to innumerable impacts during the early history of the Solar System. Scientists using the Hubble Space Telescope have detected a tenuous oxygen atmosphere on Callisto, probably caused by the interaction between charged solar particles and water molecules in the satellite's icy crust. See also Jupiter's satellites; Table 2, backmatter.

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