On Callisto the density of craters is very high. In some places they are packed as closely as one can imagine, particularly for craters several tens of kilometers in diameter. Although no one knows the exact rate of formation of impact craters on the Jovian satellites, geologists on the Voyager Imaging Team estimate that it would require several billion years to accumulate the number of craters found on Callisto. They therefore conclude that Callisto has been geologically inactive almost since the time of its formation. Although superficially similar to the heavily cratered surfaces of the Moon and Mercury, Callisto is far from identical to these rocky worlds. One of the most obvious differences is a lack of craters larger than about 150 kilometers on Callisto, together with a tendency for large craters to have much shallower depths. Apparently the ice-rock composition of Callisto alters the ability of the crust to retain large craters. Geologists speculate that the ice flows over many millions of years, filling in crater floors and gradually obliterating the largest craters. There is also a conspicuous absence of mountains on Callisto, again suggestive of a weak, icy crust. [Illustration: The IRIS instrument measured the temperature of spots on the surface of Callisto as each Voyager sped past. The measurements shown here were all made at equatorial latitudes (between -10° and 25°). Shown are very low predawn temperatures (-190° C) followed by an increase to a noon-time maximum of about -120° C, and then a drop again as the Sun sets. [260-735]] The most prominent features in the Voyager pictures are the ghost remnants of what must have been immense impact basins. The largest of these, the “bullseye” of the Voyager 1 images, has been named Valhalla, for the home of the Norse gods. These ghost basins have lost nearly all their vertical relief. What remains is a central, light-colored zone (probably the location of the original crater), surrounded by numerous concentric rings of subdued, bright ridges. Such features had never been seen before on any planet, and they appear to be the characteristic geologic feature of an ice-rock planet. Little is known about the composition of Callisto’s surface, the material from which sunlight is reflected. It appears to be primarily dark rock or soil, but it lacks diagnostic spectral features, except for one infrared band due to water molecules bound in the soil. The many lighter spots and arcs that outline craters in the high-resolution pictures may be regions in which the ice is showing through, but these cover only a very small fraction of the exposed surface. (It should be noted that, although Callisto is the darkest of the Galilean satellites, the term “dark” is relative, for even Callisto is brighter than Earth’s moon.)