- Umbriel
-
/um"bree el'/, n. Astron.a moon of the planet Uranus.
* * *
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 (Lassell, William) in 1851, although the English astronomer William Herschel (Herschel, Sir William), 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 (Herschel, Sir John, 1st Baronet), for a character in Alexander Pope's poem The Rape of the Lock.Moons of UranusIt 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 grams per cubic cm. The moon appears to be composed of equal parts water ice and rocky material, intermixed with small amounts of frozen methane. (For comparative data about Umbriel and other Uranian satellites, see the table (Moons of Uranus).)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. Its surface is uniformly covered with impact craters, most of them large, measuring 100–200 km (60–120 miles) across. Craters of this size could only have been produced early in the history of the solar system, when planetesimal-size impacting bodies existed. Their presence on Umbriel indicates that the moon's surface was never subsequently reworked by internal processes. 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.Andrew P. Ingersoll* * *
Universalium. 2010.
