Oberon, Uranus' most distant and second largest satellite, has extensive, heavily cratered terrain interrupted by canyons (rift valleys) and scarps. Some craters are surrounded by bright ray systems; others show dark matter on their floors. A prominent feature on Oberon's limb is a large mountain 7 mi (11 km) high and 28 mi (45 km) wide. Oberon shows the least evidence from modification of its cratered terrain by geologic activity of any Uranian satellite. Image resolution is poor due to Oberon's distance.