Ariel is the fourth-largest moon of Uranus. Ariel orbits and rotates in Uranus's equatorial plane, which is almost perpendicular to the planet's orbit, giving the moon an extreme seasonal cycle.
It was discovered on 24 October 1851 by William Lassell and named for a character in two different pieces of literature. As of 2019, much of the detailed knowledge of Ariel derives from a single flyby of Uranus performed by the space probe Voyager 2 in 1986, which managed to image around 35% of the moon's surface. There are no active plans at present to return to study the moon in more detail, although various concepts such as a Uranus Orbiter and Probe have been proposed.
After Miranda, Ariel is the second-closest of Uranus's five major rounded satellites. Among the smallest of the Solar System's 19 known spherical moons (it ranks 14th among them in diameter), it is believed to be composed of roughly equal parts ice and rocky material. Its mass is approximately equal in magnitude to Earth's hydrosphere.
Like all of Uranus's moons, Ariel probably formed from an accretion disc that surrounded the planet shortly after its formation, and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Ariel has a complex surface consisting of extensive cratered terrain cross-cut by a system of scarps, canyons, grabens and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating.
Discovery and name Both Ariel and the slightly larger Uranian satellite Umbriel were discovered by William Lassell on 24 October 1851. Although William Herschel, who discovered Uranus's two largest moons Titania and Oberon in 1787, claimed to have observed four additional moons, this was never confirmed and those four objects are now thought to be spurious.
All of Uranus's moons are named after characters from the works of William Shakespeare or Alexander Pope's The Rape of the Lock. The names of all four satellites of Uranus then known were suggested by John Herschel in 1852 at the request of Lassell, though it is uncertain if Herschel devised the names, or if Lassell did so and then sought Herschel's permission. Ariel is named after the leading sylph in The Rape of the Lock. It is also the name of the spirit who serves Prospero in Shakespeare's The Tempest. The moon is also designated Uranus I.
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed an A (the initial of Ariel) combined with the low globe of Jérôme Lalande's Uranus symbol as the symbol of Ariel (). This symbol is not widely used.
Orbit Among Uranus's five major moons, Ariel is the second closest to the planet, orbiting at the distance of about 190,000 km. Its orbit has a small eccentricity and is inclined very little relative to the equator of Uranus. Its orbital period is around 2.5 Earth days, coincident with its rotational period. This means that one side of the moon always faces the planet; a condition known as tidal lock. Ariel's orbit lies completely inside the Uranian magnetosphere. The trailing hemispheres (those facing away from their directions of orbit) of airless satellites orbiting inside a magnetosphere like Ariel are struck by magnetospheric plasma co-rotating with the planet. This bombardment may lead to the darkening of the trailing hemispheres observed for all Uranian moons except Oberon (see below). Ariel also captures magnetospheric charged particles, producing a pronounced dip in energetic particle count near the moon's orbit observed by Voyager 2 in 1986.
Because Ariel, like Uranus, orbits the Sun almost on its side relative to its rotation, its northern and southern hemispheres face either directly towards or directly away from the Sun at the solstices. This means it is subject to an extreme seasonal cycle; just as Earth's poles see permanent night or daylight around the solstices, Ariel's poles see permanent night or daylight for half a Uranian year (42 Earth years), with the Sun rising close to the zenith over one of the poles at each solstice. The Voyager 2 flyby coincided with the 1986 southern summer solstice, when nearly the entire northern hemisphere was dark. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. A number of such events occurred in 2007–2008, including an occultation of Ariel by Umbriel on 19 August 2007.
Currently Ariel is not involved in any orbital resonance with other Uranian satellites. In the past, however, it may have been in a 5:3 resonance with Miranda, which could have been partially responsible for the heating of that moon (although the maximum heating attributable to a former 1:3 resonance of Umbriel with Miranda was likely about three times greater). Ariel may have once been locked in the 4:1 resonance with Titania, from which it later escaped. Escape from a mean motion resonance is much easier for the moons of Uranus than for those of Jupiter or Saturn, due to Uranus's lesser degree of oblateness. This resonance, which was likely encountered about 3.8 billion years ago, would have increased Ariel's orbital eccentricity, resulting in tidal friction due to time-varying tidal forces from Uranus. This would have caused warming of the moon's interior by as much as 20 K.
Composition and internal structure
Ariel is the fourth-largest of the Uranian moons by size and mass. It is also the 14th-largest moon in the Solar System. The moon's density is 1.54 g/cm3, which indicates that it consists of roughly equal parts water ice and a dense non-ice component. The latter could consist of rock and carbonaceous material including heavy organic compounds known as tholins. The presence of water ice is supported by infrared spectroscopic observations, which have revealed crystalline water ice on the surface of the moon, which is porous and thus transmits little solar heat to layers below. Water ice absorption bands are stronger on Ariel's leading hemisphere than on its trailing hemisphere. The cause of this asymmetry is not known, but it may be related to bombardment by charged particles from Uranus's magnetosphere, which is stronger on the trailing hemisphere (due to the plasma's co-rotation). The energetic particles tend to sputter water ice, decompose methane trapped in ice as clathrate hydrate and darken other organics, leaving a dark, carbon-rich residue behind.
Except for water, two other compounds have been identified on the surface of Ariel by infrared spectroscopy. The first is carbon dioxide (CO2), which is concentrated mainly on its trailing hemisphere. Ariel shows the strongest spectroscopic evidence for CO2 of any Uranian satellite, and was the first Uranian satellite on which this compound was discovered. The origin of the carbon dioxide is not completely clear. It might be produced locally from carbonates or organic materials under the influence of the energetic charged particles coming from Uranus's magnetosphere or solar ultraviolet radiation. This hypothesis would explain the asymmetry in its distribution, as the trailing hemisphere is subject to a more intense magnetospheric influence than the leading hemisphere. Another possible source is the outgassing of primordial CO2 trapped by water ice in Ariel's interior. The escape of CO2 from the interior may be related to past geological activity on this moon.
The second compound identified by its feature at wavelength of 2.2 μm on Ariel is ammonia, which is distributed more or less homogeneously over the surface. The presence of ammonia may indicate that Ariel was geologically active in recent past.
Given its size, rock/ice composition and the possible presence of salt or ammonia in solution to lower the freezing point of water, Ariel's interior may be differentiated into a rocky core surrounded by an icy mantle. If this is the case, the radius of the core (372 km) is about 64% of the radius of the moon, and its mass is around 56% of the moon's mass—the parameters are dictated by the moon's composition. The pressure in the center of Ariel is about 0.3 GPa (3 kbar). The current state of the icy mantle is unclear. The existence of a subsurface ocean is currently considered possible,Overlooked Ocean Worlds Fill the Outer Solar System. John Wenz, Scientific American. 4 October 2017. though a 2006 study suggests that radiogenic heating alone would not be enough to allow for one. More scientific research concluded that an active underwater ocean is possible for the 4 largest moons of Uranus.
In April 2025, a published study attempted to constrain the actual depth of Ariel's possible subsurface ocean using clues gleaned from all the visible fractures and grabens on Ariel's surface, as well as clues from Ariel's inferred past orbital eccentricity. The study claimed that in the past, Ariel's orbital eccentricity could have been as high as 0.04, causing extreme tidal stresses that provided the heat necessary to maintain Ariel's subsurface ocean in a liquid state. The study also pointed out the fact that volatile materials, particularly ammonia, were tentatively detected on Ariel. Since these compounds dissipate rapidly in space and are easily broken down by interaction with Uranus' magnetic field, they consider it as an indication that something is replenishing them on Ariel, suggesting that the moon's subsurface ocean was still active in the recent past. The study concluded that Ariel's subsurface ocean might have been as deep as .
Surface
Albedo and color Ariel is the most reflective of Uranus's moons. Its surface shows an opposition surge: the reflectivity decreases from 53% at a phase angle of 0° (geometrical albedo) to 35% at an angle of about 1°. The Bond albedo of Ariel is about 23%—the highest among Uranian satellites. The surface of Ariel is generally neutral in color. There may be an asymmetry between the leading and trailing hemispheres; the latter appears to be redder than the former by 2%. Ariel's surface generally does not demonstrate any correlation between albedo and geology on one hand and color on the other hand. For instance, canyons have the same color as the cratered terrain. However, bright impact deposits around some fresh craters are slightly bluer in color. There are also some slightly blue spots, which do not correspond to any known surface features.
Possible geological activity In February 2025, planetary scientists at Johns Hopkins University published their findings on the possibility that Ariel might be geologically active using images taken by Voyager 2 back in 1986. According to the study, the numerous grooves and grabens running across Ariel's surface might be the result of liquid water oozing out from the moon's possible subsurface ocean. As the liquid water flows out, it pushes aside the solid icy surface of the moon, splitting apart Ariel's surface similar to how lava flowing out of the Earth's mantle pushes apart the oceanic crust on Earth's Mid-Atlantic Ridge; but instead of lava, it is liquid water for Ariel. The freshly ejected water then freezes once it reaches the surface, creating a new icy crust and the cycle of splitting apart the crust and generating fresh crust continues. If this study's conclusions are correct, then the fresh materials at the center of the grooves and gravens might be exposed samples of what a subsurface ocean might contain, making them attractive places to study and even get samples from to learn more about the nature of subsurface oceans. The study suggests that the grooves are good sources of carbon oxides samples.
Surface features