Many of the geologic features seen on Ganymede appear to have been caused by breaking, faulting, or spreading of the crust. In a few cases, there even seem to be indications of transverse, or sideways, motion along faults. This evidence is extremely exciting to geologists, since similar crustal motion on Earth is associated with the drift of continental plates, drawn by convection currents deep in the mantle. Such activity has never been seen before on another planet. Astronomers on Earth had known since 1971 that about half the surface of Ganymede was covered with exposed water ice and about half with darker rock. An examination of the albedo variations in the Voyager pictures suggests that the ice is exposed near large craters and, to a lesser extent, in the grooved terrain, but no direct measurements were made by Voyager of the composition of different parts of the surface. The presence of ice on the surface suggested to many astronomers that Ganymede might have a very tenuous atmosphere of water vapor or oxygen, which might be released by the breakdown of water vapor by sunlight. During the Voyager 1 flyby, a sensitive test for an atmosphere was made by the ultraviolet instrument from observations of the star Kappa Centauri as it was occulted by Ganymede. No dimming of the starlight was seen, yielding an upper limit for the surface pressure of the gases oxygen, water vapor, or carbon dioxide of 10⁻¹¹ bar, or one hundred-billionth the atmospheric pressure at Earth. The differences between the geologic histories of Ganymede and Callisto are surprisingly large. No one knows the reason. Perhaps only a small increase in internal temperature is necessary to initiate geologic activity in an icy planet, and for some reason Ganymede crossed this threshold for a part of its history, whereas Callisto did not. [Illustration: The complex patterns of the grooved terrain on Ganymede are apparent in high-resolution images. This picture, taken by Voyager 1 on March 5, has a resolution of about 3 kilometers and shows a region about the size of the state of Pennsylvania. The mountain ridges are spaced about 10 to 15 kilometers apart and rise about 1000 meters, similar to many of the mountains of Pennsylvania. The transections of different mountain systems indicate that they formed at different times. A degraded crater near the left center of the picture is crossed by ridges, indicating that it predated the period of crustal deformation and mountain building. [P-21279]]