The largest Uranian satellites are also icy, but are much darker and also denser than Saturn's satellites, presumably due to having dirtier surfaces and rockier interiors. Nonetheless, they probably have differentiated to the point that they have distinctly icy crusts and mantles and rocky cores. The ices, in addition to that of water, may be more exotic, including ammonia hydrate, methane-water mixtures, and others. These other chemicals, when they occur with water, could allow for thick, ice-lava flows, even at the extremely cold temperatures (-200 oC) in that part of the Solar System. The carbonaceous nature of these ice-mixtures would cause them to darken on exposure to solar radiation and cosmic rays. Darkening also probably occurs from accumulation of meteoritic material. All of the large Uranian satellites now seem to be geologically inactive, but most still show signs of past activity.Umbriel, Titania, and Oberon have such a high density of impact craters that we must conclude that geological activity on these satellites ceased much earlier than on Miranda and Ariel. Umbriel is very heavily cratered and has the darkest, grayest surface of any icy satellite except the dark hemisphere of Iapetus. Geologic activity ceased so long ago that any signs of geologic features have been battered beyond recognition by impacts. Titania's canyons, similar to those on Ariel, are signs of ancient geologic activity. Bright craters again suggest cleaner ice beneath the dark surface. Titania's surface is slightly older than Ariel's, as determined by the greater number of impact craters on Titania. Oberon shows intriguing signs of very ancient geologic processes, including dark crater floors and a mountain peak 20 km high.