Ganymede is the third Galilean satellite, in terms of increasing distance from Jupiter, and it is the largest moon in the solar system (about 1.5 times the size of Earth’s Moon and bigger than the planet Mercury). This Voyager 2 photograph from July 1979 shows the major terrain types present on Ganymede: dark areas with both craters and palimpsests (circular features having little relief) and brighter terrain with many parallel sets of grooves (not visible in this photograph but documented in many others). The dark areas are cut by the bright grooved terrain, leading to the interpretation that the dark areas are remnants of the oldest crust of Ganymede. The palimpsests may be impact craters whose topography has disappeared by the slow relaxation of the uplifted crust, thought to be very ice-rich. The complex patterns of grooves in the bright areas indicates that a dynamic process may have been involved in the breakup of the older crust. Bright frost is present at both polar regions.
Voyager 2 image (Press Release P-21751).