Umbriel is the darkest of Uranus’ largest moons. It reflects only 16 percent of the light that strikes its surface, a feature similar to the highland areas of Earth’s Moon. Other Uranian moons are much brighter. The process by which Umbriel’s ancient cratered surface was darkened remains a mystery.
Umbriel has a diameter of about 1,200 km (750 miles). Images taken by Voyager 2 in 1986 revealed a curious bright ring about 140 km (90 miles) in diameter on the moon’s dark surface. It is unclear what created the distinctive ring, although it may be frost deposits associated with an impact crater.
Umbriel was discovered on 24 October 1851 by English astronomer William Lassell.
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“Umbriel is the darkest of the large satellites and appears uniform in color, albedo, and general surface features. Although Voyager was unable to take photographs of the entire surface of Umbriel, the parts that were seen uniformly cratered with a high proportion of large impact scars, indicative of an ancient surface.”
The Compact NASA Atlas of the Solar System by Ronald Greeley and Raymond Batson