In contrast, the largest satellite, Ganymede, is encrusted in dirty ice. Galileo images show a complex network of fault zones and 'grooves' criss-crossing the surface. The icy arteries may be geological stress marks, places where Ganymede's brittle crust expanded and cracked. At Europa, Galileo has provided tantalising evidence for liquid water beneath a layer of ice. The ice is fractured by long cracks where slush has oozed up from below.
Pictures show city-size blocks that have drifted like icebergs, then frozen in new positions.
The first step in answering that question will come with Europa Orbiter, set for launch in 2003. The probe will use radar soundings to confirm the ocean's existence. Callisto is another enigma. Scientists know that it, too, is encased in crater-saturated ice. But recently Galileo detected a magnetic field around Callisto.