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. This is best explained by the flow of electrical energy through salt water. In other words, Callisto might be hiding an ocean below its surface as well. Saturn is almost as big as Jupiter and boasts prettier rings. Its 18 moons are mostly small and heavily cratered.