Umbriel is the darkest of Uranus’ major moons, an ancient, crater-saturated world that reflects only about a fifth of the sunlight falling on it. Its one flash of brilliance is a mystery: a bright ring of material roughly 130 km across on the floor of the crater Wunda.
It reflects only about 21% of incoming sunlight - the darkest of Uranus’ five major moons.
Umbriel and Ariel are nearly twins in size, yet Umbriel is only about half as bright.
The bright ring inside the crater Wunda may be a trap where carbon-dioxide frost accumulates.
Why so dark?
Umbriel is nearly the same size as bright Ariel, yet reflects only half as much light. Its surface has sat unchanged for billions of years while radiation and micrometeorites slowly processed its ices into dark, carbon-rich residue - old surfaces in the outer Solar System tend to darken like old snow. Wunda’s bright ring may be the exception that proves the rule: a cold trap where fresh carbon-dioxide frost collects and stays clean.
Physical characteristics
Diameter
1,169 km
Mass
1.170e+21 kg
Surface gravity
0.23 m/s²
Density
1.4 g/cm³
Tidally locked
Yes
Mean temperature
-200 °C
Geometric albedo
0.21
Atmosphere and composition
Atmosphere
None detected.
Composition
Water ice and rock beneath an unusually dark, ancient surface layer.
Discovery
Discovered by
William Lassell
Year
1851
How
24-inch reflector at his private observatory
Missions
Voyager 2NASA · Flyby probe · 1977 · active
Launched 16 days before its twin, Voyager 2 is the only spacecraft ever to visit all four giant planets - Jupiter, Saturn, Uranus and Neptune. It entered interstellar space in 2018 and is still returning data.