umbriel

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 750 miles (1,200 kilometers). Images taken by Voyager 2 in 1986 revealed a curious bright ring about 90 miles (140 kilometers) 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.