Abstract
Telescopic inspection and polarization, and albedo measurements indicate that Callisto differs from the other three Galilean satellites of Jupiter in that its surface consists of a layer of exposed rocks; also, the polarization characteristics reveal that the leading hemisphere (in its orbital motion phase-locked to Jupiter) is characteristic of a lunar-type regolith, whereas the following hemisphere is characteristic of clean, dust-free rock. This suggests that the leading hemisphere has been bombarded by meteoroids, whereas the following hemisphere has not. This bombardment asymmetry can best be explained by assuming that the dominant bombardment of Callisto has been by the meteoroids in orbit around Jupiter. As Callisto may have an icy crust above a liquid water mantle, the presence of an upper layer of rocks could be explained if the crust consisted of ice with stones embedded in it: as the exposed surface of ice slowly evaporated, rocks would become exposed at the surface. This explanation implies that the initial accretion product was a mixture of ice and rocks, and that the upper crust has never melted completely. The satellite Ganymede has a surface characteristic of ice or snow patches, probably because it has a crust of ice and rocks thinner than that of Callisto. The crust is more easily broken by impacts, which might therefore cause extrusion of water to form an ice field on the surface. The satellite Europa is almost entirely covered with ice; the absence of silicates on its surface may be explained by the ice crust having been at some time completely melted, or broken, by impacts.
Published Version
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