Cruising past the moons of reigning gas giant Jupiter, Voyager and Galileo have returned tantalizing evidence for a liquid water ocean beneath the surface of Europa . Now researchers are reporting telltale indications that the battered Jovian moon Callisto may also harbor a subsurface ocean. This cutaway view of Callisto shows a whitish 200 kilometer thick band of ice just beneath the moon's surface. Magnetic measurements made during Galileo flybys so far indicate Callisto's magnetic field is variable, analogous to results during Europa passes , and a plausible explanation is that Callisto too has a subsurface liquid layer. If the liquid were salt water it could easily carry electrical currents and produce the changing magnetic field .
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