Evidence from Voyager II photometry for early resurfacing of Umbriel

THE uranian satellite UmbriePs dark, heavily cratered surface is remarkable for its apparent uniformity in Voyager II images¹. Its most conspicuous geological feature is a comparatively high-albedo, annulus-shaped deposit which covers the floor of the 40-km diameter crater Wunda¹. Here we present new Voyager II albedo maps of Umbriel which reveal that its surface is subdivided into low-contrast, crudely polygonal areas ranging in size from tens to hundreds of kilometres (Fig. 1). The largest polygons are elongate with systematically trending northeast-southwest boundaries. Some of the polygonal areas form topographic depressions several kilometres in depth. We suggest that this newly discovered global albedo pattern is a relic of the early tectonic disruption of Umbriel's surface.