Uranus' bright, canyon-carved moon
Ariel
Moon of Uranus · discovered 1851 by William Lassell
Ariel is the fourth-largest of Uranus' moons and the brightest of them all, reflecting more sunlight than any of its siblings. About 1,160 kilometres across, it is criss-crossed by long fault valleys whose floors appear to have been flooded by icy material from within.
Its surface looks surprisingly young for a small, frozen world far from the Sun. That freshness, along with intriguing deposits of carbon dioxide ice, makes Ariel one of the best candidates in the Uranus system for recent geological activity — and perhaps even a hidden ocean.
Key facts
- Diameter
- 1,158 km33% of Earth's Moon
- Mass
- 1.25 × 10²¹ kg
- Surface gravity
- 0.269 m/s²0.027 g
- Density
- 1,592 kg/m³
- Distance from Uranus
- 191,020 km≈ 7.5 Uranus radii
- Orbital period
- 2.52 daysSame as its rotation (tidally locked)
- Discovered
- 1851William Lassell
- Surface
- Water ice with carbon dioxide ice and darker material
- Atmosphere
- None detected
- Surface temperature
- Roughly −190 °C to −210 °C
- Rotation
- Synchronous — always shows the same face to Uranus
- Named after
- A sylph 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
Ariel's most striking features are its chasmata — immense fault-bounded valleys, or grabens, that wind across the moon for hundreds of kilometres. The largest, Kachina Chasmata, stretches for more than 600 kilometres. These canyons formed as the crust stretched and cracked, probably when the interior expanded as it froze.
Many canyon floors are smooth and sometimes bowed upwards, with central ridges or grooves. Scientists widely interpret this as the work of cryovolcanism: viscous, icy material welling up and partly filling the valleys, much as lava floods low ground on rocky worlds.
Ariel has fewer large craters than Umbriel, Titania or Oberon, which suggests much of its surface was resurfaced relatively late in the moon's history. Its craters are named after benevolent spirits and bright sprites from world folklore, such as Yangoor and Abans.
Interior and composition
Ariel is thought to be roughly half water ice and half rock, probably separated into a rocky core beneath an icy mantle. In 2023, NASA-led modelling suggested that Ariel, along with Umbriel, Titania and Oberon, could retain a thin layer of liquid water deep inside, helped by antifreeze-like salts and ammonia — a possibility that remains unconfirmed.
Telescopes have found carbon dioxide ice concentrated on Ariel's trailing hemisphere. Observations with the James Webb Space Telescope reported in 2024 detected unusually strong CO2 signatures and hints of carbon monoxide and possibly carbonate minerals. Some researchers think at least part of this material may come from inside the moon, though radiation chemistry at the surface is another candidate source.
Orbit and rotation
Ariel takes about two and a half days to orbit Uranus and, like the other major moons, keeps one face permanently turned towards the planet. Its orbit lies almost exactly in Uranus' equatorial plane and is nearly circular.
Because Uranus is tipped by about 98 degrees, Ariel experiences extreme seasons: each pole basks in roughly 42 years of continuous daylight, followed by an equally long night. Ariel also passes through Uranus' magnetosphere, where charged particles strike its trailing side — one reason that hemisphere looks different.
Origin
Ariel most likely formed from a disc of material around the young Uranus, possibly debris from the giant impact many scientists think tipped the planet on its side. Its later activity may have been powered by tidal heating during a past orbital resonance with neighbouring moons, which could have warmed the interior enough to drive the cracking and flooding seen today.
Exploration
Voyager 2 is the only spacecraft to have visited Ariel. Because the southern hemisphere was facing the Sun during the 1986 flyby, only about a third of the moon was imaged at useful resolution.
Discovery (1851)
William Lassell discovered Ariel on 24 October 1851, on the same night he found Umbriel, using a telescope he built himself near Liverpool. John Herschel later proposed the names.
Voyager 2 (1986)
Voyager 2 flew through the Uranus system in January 1986, returning the only close-up images of Ariel's canyons and smooth-floored valleys to date.
Telescopes today
Ground-based infrared spectroscopy and the James Webb Space Telescope now track Ariel's ices, revealing CO2 deposits that no one expected on a moon this far from the Sun.
Future missions
The proposed Uranus Orbiter and Probe, the top flagship priority of the 2023–2032 planetary science decadal survey, would map Ariel and test whether it hides an ocean.
The moons of Uranus
Surprising facts about Ariel
- Ariel is the most reflective of Uranus' major moons, bouncing back noticeably more sunlight than its near-twin in size, the dark Umbriel.
- Carbon dioxide ice should slowly escape or be destroyed at Ariel's distance from the Sun, so its persistence suggests a source is replenishing it — possibly from within.
- Kachina Chasmata runs for more than 600 kilometres — roughly the driving distance from London to Edinburgh — on a moon only about 1,160 kilometres across.
- The name Ariel belongs to two literary spirits: a sylph in Pope's The Rape of the Lock and Prospero's servant in Shakespeare's The Tempest.
- Ariel and Umbriel were both discovered on the same night in 1851 by William Lassell, a wealthy brewer turned astronomer.