The dark moon with a halo
Umbriel
Moon of Uranus · discovered 1851 by William Lassell
Umbriel is the darkest of Uranus' five major moons, a heavily cratered world about 1,170 kilometres across. Its surface reflects only a small fraction of the sunlight that reaches it, giving it a gloomy, charcoal-grey appearance next to its brighter neighbour Ariel.
Almost nothing seems to have disturbed Umbriel for billions of years — except for one puzzling exception. Near its equator sits Wunda, a large crater crowned with a brilliant ring of bright material that looks, in Voyager 2's images, like a pale halo on a dark face.
Key facts
- Diameter
- 1,169 km34% of Earth's Moon
- Mass
- 1.27 × 10²¹ kg
- Surface gravity
- 0.2 m/s²0.020 g
- Density
- 1,539 kg/m³
- Distance from Uranus
- 266,000 km≈ 10.4 Uranus radii
- Orbital period
- 4.14 daysSame as its rotation (tidally locked)
- Discovered
- 1851William Lassell
- Surface
- Dark, ancient water ice with carbon-rich material
- Atmosphere
- None detected
- Surface temperature
- Roughly −190 °C to −210 °C
- Rotation
- Synchronous — always shows the same face to Uranus
- Named after
- A melancholy sprite in Pope's The Rape of the Lock
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
Umbriel's surface is crowded with impact craters, many of them tens of kilometres wide, and it shows few of the canyons or smooth flooded plains seen on Ariel and Titania. That density of craters suggests its surface is among the oldest in the Uranus system, perhaps little changed since the heavy bombardment of the early Solar System.
The most eye-catching feature is Wunda, a crater roughly 130 kilometres across located near the equator. A bright ring of material lies on its floor; its origin is uncertain, though some researchers suggest it could be a deposit of carbon dioxide frost or freshly exposed ice. Other named craters include Vuver, with its bright central peak, and Skynd.
Why Umbriel is so dark is still not fully settled. One idea is that its surface ice has been darkened over aeons by radiation and dust, while its neighbours were refreshed by later geological activity that Umbriel never experienced. Its craters are named after dark and evil spirits from various mythologies.
Interior and composition
Umbriel is thought to be made of roughly equal parts water ice and rock, likely separated into a rocky core and icy mantle. Despite its sleepy surface, 2023 NASA modelling suggested it is one of four Uranian moons — with Ariel, Titania and Oberon — that could in principle hold a thin layer of salty liquid water at depth, though there is no direct evidence yet.
Spectra show water ice and some carbon dioxide ice, the CO2 being concentrated on the trailing hemisphere as on Ariel, but in smaller amounts.
Orbit and rotation
Umbriel orbits Uranus in a little over four days on a nearly circular path close to the planet's equatorial plane. It is tidally locked, keeping the same hemisphere facing the planet.
Thanks to Uranus' sideways tilt of about 98 degrees, Umbriel's poles alternate between roughly 42 years of continuous sunlight and 42 years of darkness. During the 1986 flyby, its southern hemisphere was in the middle of its long summer.
Origin
Like the other major Uranian moons, Umbriel probably formed from a disc of gas, ice and dust around the young planet. Its apparent lack of later resurfacing may be because it was never caught in the kind of strong orbital resonances that could have heated its neighbours, though the history of these moons' orbits is still poorly known.
Exploration
Voyager 2 is still the only spacecraft to have visited Umbriel. It passed at a greater distance than it did for Miranda or Ariel, so Umbriel is one of the least well-imaged of the major moons, and only its southern hemisphere was seen.
Discovery (1851)
William Lassell discovered Umbriel on 24 October 1851, the same night he found Ariel. Its name, suggested by John Herschel, comes from the gloomy sprite in Pope's mock-epic poem.
Voyager 2 (1986)
In January 1986, Voyager 2 photographed Umbriel's dark, cratered southern hemisphere and revealed the bright ring in Wunda crater for the first time.
Telescopes today
Infrared observations from ground-based telescopes and the James Webb Space Telescope study the ices on Umbriel's surface, including its CO2 deposits.
Future missions
A Uranus Orbiter and Probe, the top flagship recommendation of the 2023–2032 planetary decadal survey, could map Umbriel's unseen north and probe its interior.
The moons of Uranus
Surprising facts about Umbriel
- Umbriel and Ariel are almost the same size, yet Ariel reflects roughly twice as much light, making them a striking light-and-dark pair.
- Wunda's bright ring is one of the most noticeable features in the whole Uranian system, and scientists still are not sure what it is made of.
- In Pope's poem, Umbriel is a 'dusky melancholy sprite' who travels to the Cave of Spleen — a fitting namesake for such a dark moon.
- Umbriel's surface is among the oldest in the Uranus system, preserving a crater record that may stretch back around four billion years.
- Even this apparently dead moon may not be entirely frozen inside: models suggest a thin ocean is possible, though far from proven.
Keep exploring
- Ariel, moon of Uranus: uranus' bright, canyon-carved moon
- Oberon, moon of Uranus: the ancient king of the outer moons
- Callisto, moon of Jupiter: a battered relic of the early solar system
- Uranus: the planet guide
- Watch Umbriel orbit Uranus in the interactive 3D solar system
- All the major moons of the Solar System