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.
How Umbriel Got its Name:
Umbriel is named for a malevolent spirit in English poet Alexander Pope's 18th century poem "Rape of the Lock."
Moons of Uranus are named for characters in William Shakespeare's plays and from Alexander Pope's "Rape of the Lock."