Flooding of Ganymede's bright terrains by low-viscosity water-ice lavas

Large regions of the jovian moon Ganymede have been resurfaced, but the mea ns has been unclear(1,2). Suggestions have ranged from volcanic eruptions o f liquid water(3-5) or solid ice(6) to tectonic deformation(7-9), but defin itive high-resolution morphological evidence has been lacking. Here we repo rt digital elevation models of parts of the surface of Ganymede, derived fr om stereo pairs combining data from the Voyager and Galileo spacecraft, whi ch reveal bright, smooth terrains that lie at roughly constant elevations 1 00 to 1,000 metres below the surrounding rougher terrains. These topographi c data, together with new images that show fine-scale embayment and burial of older features(10), indicate that the smooth terrains were formed by flo oding of shallow structural troughs by low-viscosity water-ice lavas. The o ldest and most deformed areas (the 'reticulate' terrains) in general have t he highest relative elevations, whereas units of the most common resurfaced type-the grooved terrain-lie at elevations between those of the smooth and reticulate terrains. Bright terrain, which accounts for some two-thirds of the surface, probably results from a continuum of processes, including cru stal rifting, shallow flooding and groove formation. Volcanism plays an int egral role in these processes, and is consistent with partial melting of Ga nymede's interior(11,12).