The surface of Ganymede, the largest moon in the Solar System, is composed of half water ice and half rock. This satellite of Jupiter is unique for its long, curved grooves in the crust, that make it unlike any other planetary body ever seen. Areas of dark and light terrain with differing densities of impact craters suggest that the moon has gone through several periods of crust formation.
When seen from 1.2 million kilometers (750,000 miles) as in this Voyager 1 image taken in July 1979, both dark and light colored terrain are pockmarked by bright impact craters and their ejecta. The large circular area at the top of this image is known as Galileo Regio, part of the ancient crust of the satellite.
Close-up views of the dark terrain of Ganymede show that the surface is cut by large, concentric furrows. These furrows in Galileo Regio and elsewhere on the moon are concentric to a point near Ganymede's equator, and may have resulted from an ancient, large impact event that fractured more than half the crust of the satellite.