Grooved Terrain on Ganymede: First Results from Galileo High-Resolution Imaging

allowing extrapolation of the observed brightness and topographic relationships to the rest of the imaged area. Distinct High-resolution Galileo imaging has provided important geological units are apparent across the region, and ridges and insight into the origin and evolution of grooved terrain on grooves are ubiquitous within these units. The stratigraphically Ganymede. The Uruk Sulcus target site was the first imaged lowest and most heavily cratered units (‘‘lineated grooved terat high resolution, and considerations of resolution, viewing rain’’) generally show morphologies indicative of horst-andgeometry, low image compression, and complementary stereo graben-style normal faulting. The stratigraphically highest imaging make this region extremely informative. Contrast varigroove lanes (‘‘parallel ridged terrain’’) exhibit ridges of roughly ations in these low-incidence angle images are extreme and triangular cross section, suggesting that tilt-block-style normal give the visual impression of topographic shading. However, faulting has shaped them. These extensional-tectonic models photometric analysis shows that the scene must owe its characare supported by crosscutting relationships at the margins of ter to albedo variations. A close correlation of albedo variations to topography is demonstrated by limited stereo coverage, groove lanes. Thus, a change in tectonic style with time is