Europa’s surface is probably a thin ice crust overlying water or softer ice (slush) about 100 kilometers (60 miles) thick that covers a silicate interior. The tectonic processes on Europa’s surface create patterns that are drastically different from the fault systems seen on Ganymede’s surface, where pieces of the crust have moved relative to each other. On Europa, the crust evidently fractures, but the pieces remain roughly in their original position. This Voyager 2 picture is composed of three images.] [Illustration: 7/9/79 240,000 km (150,000 mi) Long linear fractures or faults which crisscross Europa’s surface in various directions are over 1000 kilometers (600 miles) long in some places. Large fractures are 200 to 300 kilometers (125 to 185 miles) wide, wider than the crust is thick. Also visible are somewhat darker mottled regions that appear to have a slightly pitted appearance. No large craters (more than five kilometers in diameter) are identifiable in this Voyager 2 picture, indicating that this satellite has a very young surface relative to Ganymede and Callisto, although perhaps not as young as Io’s surface. Scientists believe that the surface is a thin ice crust overlying water or softer ice and that the fracture systems are breaks in the crust. Resurfacing processes, such as the production of fresh ice or snow along the cracks and cold glacier-like flows, have probably removed evidence of impact events (cratering). Europa, therefore, appears to have many properties similar to Ganymede and Io.] [Illustration: 7/9/79 240,000 km (150,000 mi) Complex narrow ridges, seen as curved bright streaks 5 to 10 kilometers (3 to 6 miles) wide and typically 100 kilometers (60 miles) long, characterize the surface topography of this view of Europa. The dark bands also visible in this Voyager 2 photo are 20 to 40 kilometers (12 to 25 miles) wide and up to thousands of kilometers long. The fractures on the icy surface are filled with material from beneath, probably as a result of internal tidal flexing which continually heats the thin outer ice crust. A few features are suggestive of degraded impact craters.] [Illustration: 3/4/79 2.6 million km (1.6 million mi) Ganymede, Jupiter’s largest satellite, is about one and one-half times the size of our Moon but only about half as dense and is composed of about 50 percent water or ice and the rest rock. The bright surface of Ganymede is a complex montage of ancient, relatively dark and cratered terrain, grooved terrain that resulted from a dramatic history of tectonic movement in the icy crust, and bright young ray craters that expose fresh ice. This photograph was taken by Voyager 1.] [Illustration: 7/7/79 1.2 million km (750,000 mi)