Ariel is one of Uranus’s moons. It was discovered in 1851 by British astronomer William Lassell.

Diameter: 1.15 x 106 m
Mass: 1.35 x 1021 kg
Composition: Water-ice and rock
Orbit: 1.91 x 105 m from Uranus

Ariel is the fourth-largest of Uranus’s moons and the second closest of its satellites. Uranus has five major satellites and 22 minor satellites. The larger moons were discovered by direct observation from Earth, 11 others by the Voyager 2 spacecraft mission and the rest by sensitive charge-coupled-device (CCD) detectors from Earth.

Ariel is named after the spirit in Shakespeare’s “The Tempest” who is bound to the service of the magician, Prospero. Ariel previously had been the servant of Sycorax, a sorceress and had been trapped in a tree for 12 years until Prospero released him. The name derives from the Hebrew for “Lion of God” and appears in the Old Testament. Ariel is the brightest and the youngest of Uranus’s moons.

The composition of Ariel is mostly water-ice and rock, together with some tholins. These are carbonaceous compounds formed by the ultraviolet irradiation of simple hydrocarbons such as methane and ethane. Ariel’s interior consists of a rock inner core surrounded by a mantle of ice.

The surface of Ariel is extensively cratered and cut by scarps, canyons and ridges. Canyon widths range between 10 and 100 km, while their depths vary between 3 and 4 km. There are signs of tectonic activity such as cryovolcanism. This is an eruption of water ice, methane and ammonia rather than molten rock. It probably was caused by tidal heating and the compressional effects of these forces may have created the ridges on Ariel’s surface.

Infrared observations from Earth indicate that the hemisphere that faces forward along Ariel’s orbit has different characteristics to the one that faces backward. There is evidence of carbon dioxide on the trailing hemisphere. Ariel has no detectable atmosphere or magnetic field, but there are signs of erosion on its surface. There are smooth patches on its surface that indicate deposition processes. The flat patches may also be caused by shield volcanism. This is the outflow of volcanic material over a flat, wide surface. In Ariel’s case, the erupted material could be a super-cooled, viscous mixture of water and ammonia. Ariel’s surface contrasts with that of Umbriel, Uranus’ third-largest moon, which shows no signs of geological activity.

Ariel orbits synchronously around Jupiter over an orbital period of 2.52 days. This coincides with the rotational period of the moon on its own axis. The same hemisphere of Ariel always faces Uranus. Along with Uranus and its other satellites, Ariel takes 84 Earth years to orbit the sun.

The Voyager 2 spacecraft mission in 1986 photographed Uranus and its moons. Its closest approach to the moon was at a distance of 130,000 km and the photographs had a resolution of 2.4 km. There have been no other missions to Uranus and its moons.

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