Umbriel is the third-largest moon of Uranus. It was discovered on October 24, 1851, by William Lassell. Named after a character in the 1712 poem The Rape of the Lock by Alexander Pope, Umbriel is composed mainly of ice with a substantial fraction of rock. It may be differentiated into a rocky core and an icy mantle. Its surface, the darkest among Uranian moons, appears to have been shaped mostly by impacts, but the presence of canyons suggests early endogenic processes. This shows Umbriel may have undergone an early endogenically driven resurfacing event that erased its older surface. Covered by numerous impact craters reaching 210 km (130 mi) in diameter, Umbriel is the second-most heavily cratered satellite of Uranus after Oberon. Like all moons of Uranus, Umbriel likely formed from an accretion disk that surrounded the planet just after its formation. The only close study of Umbriel was conducted in January 1986 by Voyager 2, which captured images of about 40 percent of its surface during the spacecraft's flyby of Uranus. (Full article...)
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