EVIDENCE FOR A SUBSURFACE OCEAN ON EUROPA

Ground-based spectroscopy of Jupiter's moon Europa, combined with grav ity data, suggests that the satellite has an icy crust roughly 150 km thick and a rocky interior(1-4). In addition, images obtained by the v oyager spacecraft revealed that Europa's surface is crossed by numerou s intersecting ridges and dark bands (called lineae) and is sparsely c ratered, indicating that the terrain is probably significantly younger than that of Ganymede and Callisto(5). It has been suggested that Eur opa's thin outer ice shell might be separated from the moon's silicate interior by a liquid water layer, delayed or prevented from freezing by tidal heating(6-10); in this model, the lineae could be explained b y repetitive tidal deformation of the outer ice shell(11-13). However, observational confirmation of a subsurface ocean was largely frustrat ed by the low resolution (>2 km per pixel) of the Voyager images(14). Here we present high-resolution (54 m per pixel) Galileo spacecraft im ages of Europa, in which we find evidence for mobile 'icebergs'. The d etailed morphology of the terrain strongly supports the presence of li quid water at shallow depths below the surface, either today or at som e time in the past. Moreover, lower-resolution observations of much la rger regions suggest that the phenomena reported here are widespread.