Ganymede is roughly evenly divided between dark terrain (the triangular-shaped regions) and broad swaths of bright terrain. Dark terrain is older and more heavily cratered than bright terrain. Bright terrain probably formed when extensional graben that crossed dark terrain were partially filled with bright material, possibly erupted flows of liquid water. The central pit crater Isis is 75 kilometers wide and roughly 2 kilometers deep. Central pits, which have low-relief rims, are common in large craters on the large icy satellites Ganymede and Callisto but also occur on Mars [compare this crater to complex craters on other planets (slides #3, #16, and #36)]. The origin of central pits is not fully understood, but may be related to the ice-rich composition of Ganymede's crust. Topographic relief on Ganymede, including the largest impact basins, rarely exceeds 2 kilometers. In contrast, the largest impact basins on the Moon are 12 kilometers deep. The low relief on Ganymede is probably related to the ice-rich composition of Ganymede’s outer layers. Ice is much weaker than ordinary rock and cannot support high topography on Ganymede, which has a higher surface gravity than any other icy satellite.
2 images 20636.38, 20640.33.