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.Ariel is Miranda's nearest neighbor, but it exhibits nothing like the coronae of Miranda. Deep canyons cut the moderately cratered surface. The canyons are a significant part of the landscape (up to 5 km deep, 100 km wide, and at least 600 km long). Cryovolcanic flows as thick as 1 km erupted into the canyons. Flow edges of these thicknesses suggest the eruptions may have acted more like glaciers than lava flows. Fewer craters are seen within the canyons, suggesting that a long period of time elapsed between the formation of the inter-canyon plateaus and the canyons. Craters on the inter-canyon plateaus have bright ejecta, indicating that fairly clean ice exists below the surface.