It has a rocky core and a surface of dirty ice. About half the surface has been smoothed over by eruptions of relatively bright material. This material flowed into a global network of long valleys, creating bright lanes. The lanes separate regions that are older, darker, and more heavily cratered. Almost all the brighter material is wrinkled in a pattern of parallel ridges and hollows known as grooved terrain.
Scientists believe that extensional tectonics created the network and wrinkles.
The middle-sized icy satellites have small, rocky cores covered with ice. Five of these moons, Saturn's Dione and Tethys and Uranus's Miranda, Ariel, and Titania, show geologic activity within the last 2 billion to 3 billion years. Their surfaces are covered with vast flows of ice broken by long, straight or curved furrows that probably resulted from extensional tectonics. Geologic activity may have begun with radioactive decay of interior material or with tidal heating. Both Miranda and Ariel have regions of grooved terrain like that of Ganymede.