Umbriel

Umbriel is the third nearest of the five major moons of Uranus and the one having the darkest and oldest surface of the group. Its discovery is attributed to the English astronomer William Lassell in 1851, although the English astronomer William Herschel, who discovered Uranus and its two largest moons, may have glimpsed it more than a half century earlier. Umbriel was named by Herschel's son, John, for a character in Alexander Pope's poem The Rape of the Lock.

It orbits Uranus once every 4.144 days at a mean distance of 265,970 km (165,270 miles). Umbriel has a diameter of 1,170 km (727 miles) and a density of about 1.4 g/cm3 (0.81 oz/in3).

The only images of Umbriel's surface have come from the U.S. Voyager 2 spacecraft's flyby encounter with the Uranian system in 1986. These show that Umbriel is distinct from the other major moons of Uranus in having no evidence of past tectonic activity, as shown by the presence of many large impact craters. The most notable feature of the hemisphere imaged by Voyager is a bright ring, dubbed Wunda, that appears to line the floor of a crater 40 km (25 miles) across.

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