[Illustration: Ray systems of exposed water-ice are visible in this high-resolution mosaic of Ganymede, obtained by Voyager 2 on July 9 at a range of about 100 000 kilometers. The rough mountainous terrain at lower right is the outer portion of a large fresh impact basin that postdates most of the other terrain. At the bottom, portions of grooved terrain transect other portions, indicating an age sequence. The dark patches of heavily cratered terrain (right center) are probably ancient icy material formed prior to the grooved terrain. The large rayed crater at upper center is about 150 kilometers in diameter. [P-21770B/W]] [Illustration: The many variants of smooth and grooved terrain on Ganymede suggest a complex geologic history for this satellite. Four high-resolution views by Voyager 2 are grouped together. [260-678A]] [Illustration: A band of low mountain ridges has apparently been cut and offset by a fault.] [Illustration: Multiple sets of mountain ridges transect at nearly right angles. Some impact craters were formed before, and some after, the grooved terrain.] [Illustration: Many short parallel ridges butt into each other, making a crazy-quilt pattern.] [Illustration: In the center of this frame is an unusual smooth area, perhaps the result of flooding of the surface by material that filled in the grooves.] Europa As one proceeds inward through the Galilean satellites, these worlds become less and less familiar to the planetary geologist. This was an unexpected effect. Callisto and Ganymede were expected to have unusual properties as a result of their large percentage of ice. The densities of Europa and Io are more normal for the smaller, terrestrial-type planets, and before Voyager many scientists expected these two satellites might look much like the Moon, which they resemble in size. Europa, with a diameter of 3130 kilometers, is about 15 percent smaller than the Moon. Its density is 3.0 grams per cubic centimeter, indicating a basically rocky composition. However, cosmic mixtures of rocky and metallic materials are often a bit denser than this, leaving room for a substantial component of ice or water. Calculations indicate that if all the ice were at the surface, it might form a crust up to 100 kilometers thick. [Illustration: Shaded relief map of Europa. [260-659]] Telescopic observations of Europa demonstrated long before Voyager that this satellite is almost completely covered with ice. It is a white, highly reflecting body, looking, from a great distance, like a giant snowball. In the early Voyager pictures, Europa always showed a bland, white disk, in striking contrast to the spottiness of Ganymede or the brilliant colors of Io.