Umbriel is a
moon of Uranus discovered on October 24, 1851, by
William Lassell. It was discovered at the same time as
Ariel and named after a character in
Alexander Pope's poem
The Rape of the Lock. Umbriel consists mainly of
ice with a substantial fraction of
rock, and may be differentiated into a rocky
core and an icy
mantle. The surface is the darkest among Uranian moons, and appears to have been shaped primarily by impacts. However, the presence of canyons suggests early
endogenic processes, and the moon may have undergone an early endogenically driven resurfacing event that obliterated its older surface.