It is crisscrossed with thin lines, some several thousand kilometers long, that remind some observers of a kind of modern art. The lines may be the traces of a shifting icy crust, but we have no idea of what forces have acted, and continue to act, to keep this world so flat. Ganymede, the largest of Jupiter's moons, looks a little more familiar. Like our own Moon, its surface is divided into light and dark regions, and we can see craters. There the similarity ends.
Bright rays extend from the craters, perhaps consisting of fresh ice.
The light areas of Ganymede display long bands of parallel grooves, unique and puzzling structures which indicate that internal forces have shaped the surface. Callisto, the outermost Galilean moon, is a heavily cratered, brownish world. It seems to be a snowball moon like Ganymede, but its surface is uniformly cratered and preserves a long history without any disruption. Callisto may have the oldest surface yet observed in the solar system, its craters perhaps dating back to an ancient era of bombardment, which ended about 4 billion years ago. Even the space around Jupiter turns out to be exciting.