Ariel

Ariel is the brightest moon of Uranus, discovered along with Umbriel in 1851 by William Lassell. Ariel orbits Uranus at a mean distance of 119,000 miles (191,000 km) over a period of 2.52 days.

With a diameter of 720 miles (1160 km), Ariel is the second-smallest of Uranus' five major moons, after Miranda. The first and so far only close-up observations of Ariel's surface were made in 1986 by the Voyager 2 spacecraft during its Uranus flyby. Ariel's surface is pock-marked with craters, with ejecta of fresh frost radiating from particularly young impacts. Ariel's most outstanding surface features are long rift valleys with floors that appear to have been been smoothed by some kind of liquid. This liquid could not have been water, because water is hard as steel at the temperatures found on the Uranian moons; rather, the fluid might have been a "slush" of ammonia, methane, or carbon monoxide.

Ariel's composition is roughly 70% ice (of water, carbon dioxide, and possibly methane), and 30% silicate rock. Its past geological activity is believed to have been driven by tidal heating at a time when Ariel was captured in a 4:1 orbital resonance with Titania, from which it has subsequently escaped.