Umbriel and Ariel are each about 725 miles (1170 km) in diameter. Umbriel orbits at about 165,000 miles (266,000 km) from Uranus. Ariel is about 119,000 miles (191,000 km) from the planet. Both are covered with ice.
Umbriel is the darkest of all Uranus's large moons. Most of its surface is dark, except for one bright crater. The crater is about 70 miles (113 km) in diameter. It probably contains ice that flowed to the surface as water following a meteorite strike. Then water froze in the basin, making it very reflective.
