Umbriel

Umbriel is the darkest satellite of Uranus. It was discovered in 1851 by William Lassell, at the same time as Ariel. Umbriel orbits Uranus at a distance of 165,000 miles (266,000 km), with an orbital period of 4.14 days.

Umbriel's diameter is 730 miles (1170 km) - slightly greater than Ariel's - but its surface reflects only half as much light. Its surface is far more heavily cratered than Ariel's, and is the most stable (i.e. the least geologically active) of any Uranian moon. Umbriel's most prominent surface feature is a large ring of bright material near its equator named Wunda. Dubbed the "fluorescent cheerio", Wunda is presumably the floor of some kind of crater.

Umbriel is composed mainly of water ice, with the balance made up other ices such as methane, and silicate rock.