Jupiter's great moon, Ganymede, is larger than the planet Mercury and beats Saturn's moon Titan by just a few miles for the title of largest moon in the solar system. Like many objects in the outer solar system it sweeps up a dark coating, probably carbon, so the older an area is, the darker it is.
Ganymede has a large rocky core and a thick outer layer of ice, possibly with liquid water deep within. The crust of Ganymede consists of large rafts of dark old icy crust separated by lighter areas of new ice. In places the old patches of crust can be pieced together like a jigsaw puzzle. Ganymede and Earth are the only two bodies known in the solar system where the crust has moved horizontally for long distances. However, Ganymede does not have earthlike plate tectonics. Instead, it appears that the crust broke up like polar pack ice on the earth back when Ganymede was younger and less frozen.
The large circular region at top is crossed by numerous curving fractures, possibly remnants of a large multiple ring impact basin. Some of the small fragments nearby appear to have split away along these fractures. The craters on Ganymede start out as brilliant white because they expose fresh ice. The newest craters are surrounded by bright rays. Older craters lose their rays and become progressively darker, though still lighter than their surroundings.
Created 17 December 2007, Last Update 25 May 2011
Not an official UW Green Bay site