... Credit: JPL , NASA Explanation: 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 . ... 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. ... Publications with keywords: Jupiter - ice - Callisto - ocean . ...