Ariel is the fourth largest of the Uranus moons, and the 14th largest moon in the Solar System.
The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating.
Discovery
Ariel, along with another Uranian satellite, Umbriel, was discovered by William Lassell on October 24, 1851.
Naming
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.
Stats
Diameter: 1,158 km
Semi-major axis: 191,020 km
Orbital Period: 2.52 days
Formation
Ariel is thought to have formed from an accretion disc or subnebula; a disc of gas and dust that either existed around Uranus for some time after its formation or was created by the giant impact that most likely gave Uranus its large obliquity.
Orbit
Ariel's orbital period is around 2.52 Earth days, coincident with its rotational period. In other words, Ariel is a synchronous or tidally locked satellite, with one face always pointing toward its parent planet.
Ariel's orbit lies completely inside the Uranian magnetosphere. This is important, because the trailing hemispheres of airless satellites orbiting inside a magnetosphere (like Ariel) are struck by magnetospheric plasma, which co-rotates with the planet.
This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon.
Composition
Ariel consists of roughly equal parts water ice and a dense non-ice component. The latter could be made 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. Water ice absorption bands are stronger on Ariel's leading hemisphere than on the trailing hemisphere.
The cause of this asymmetry is not known, but it may be related to the bombardment by charged particles from the magnetosphere of Uranus, which is stronger on the trailing hemisphere. 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, the only other compound identified on the surface of Ariel by infrared spectroscopy is carbon dioxide, 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.
Life
Ariel is likely differentiated, with an inner core of rock surrounded by a mantle of ice. The current state of Ariel's icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.
Exploration Status
So far the only close-up images of Ariel have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986.
No other spacecraft has ever visited the Uranian system or Ariel, and no mission is planned in the foreseeable future.
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