The daytime surface temperature of Callisto, observed both from the ground and by Voyager, is about -118° C. The Voyager infrared interferometer spectrometer also determined the minimum temperature, reached just before dawn, of -193° C. No atmosphere is expected at these cold temperatures, and none was seen. Analysis of Voyager images provided an improved diameter for Callisto of 4840 kilometers, yielding an average density of 1.8 grams per cubic centimeter. As noted previously, it is this low density that leads to the conclusion that ice or water is an important component of the interior of Callisto. The ice has never been detected directly, but the peculiar nature of the craters seen by Voyager adds strong circumstantial support to this conclusion. Callisto, with its heavy cratering, is the most familiar-looking of the Galilean satellites; if all of them had turned out to be as geologically dead as Callisto, planetary geologists would certainly have been disappointed. However, each satellite, progressing in toward Jupiter, presents increasing evidence of internal activity. Ganymede The largest of the Galilean satellites (5270 kilometers in diameter), Ganymede was expected to be similar to Callisto in many ways. Both have low densities (for Ganymede, 1.9 grams per cubic centimeter), indicating a bulk composition of about half rocky materials and half water. In addition, their diameters differ by only eight percent, and both are far enough from Jupiter to escape the severe pounding Io receives from magnetospheric charged particles. Thus it was with great interest that Voyager scientists looked at the differences that emerged between these two satellites. [Illustration: Shaded relief map of Ganymede. [260-673]] The surface of Ganymede as revealed by the Voyager cameras is one of great diversity, indicating differing periods of geologic activity. At one extreme there are numerous dark areas that resemble the surface of Callisto in both albedo (reflectivity) and crater density. The largest of these, Regio Galileo, stretches from the equator to latitude +45° and is 4000 kilometers across, nearly as large as the continental United States. This ancient terrain even preserves the remnants of a Callisto-type impact basin in the form of a system of parallel, curving, subdued ridges about 10 kilometers wide, 100 meters high, and spaced about 50 kilometers apart. The central part of this ghost basin is missing, however; it was presumably destroyed by subsequent geologic activity.