High-resolution color image of Ariel, taken by Voyager 2
|Date of discovery||October 24, 1851|
|Name of discoverer||William Lassell|
|Name origin||A sylph in The Rape of the Lock by Alexander Pope; a sprite in The Tempest by William Shakespeare|
|Order from primary||15|
|Semi-major axis||191,020 km|
|Sidereal month||2.520379 da|
|Avg. orbital speed||5.52 km/s|
|Inclination||0.31° to Uranus's equator|
|Sidereal day||2.520379 da|
|Rotational speed||16.7 m/s|
|Mass||1.35 * 1021 kg|
|Mean radius||578.9 km|
|Surface gravity||0.27 m/s²|
|Escape speed||0.541 km/s|
|Surface area||4,211,310 km²|
|Mean temperature||58 K|
|Composition||Water ice and rock|
Discovery and naming
The name Ariel, suggested by Sir John Herschel at Lassell's request, is the name of a sylph in Alexander Pope's poem The Rape of the Lock. It is also the name of a sprite and servant to Prospero in William Shakespeare's play The Tempest.
Ariel is in a slightly eccentric orbit around Uranus at an average distance of 191,020 km. Its sidereal month is about 2.52 Earth days. The orbit of Ariel is only slightly inclined from the equator of Uranus but is severely inclined to the ecliptic.
Tittemore suggests that Ariel was once in a 4:1 orbital resonance with Titania. If it was, it is no longer in such a resonance. For this reason, some astronomers suggest that Ariel was much hotter in the past than it is today.
Transits of Uranus
Ariel is rarely able to transit the surface of Uranus in such a manner as to cast a shadow on it. This is because Ariel, like all the moons of Uranus, is in an orbit with an almost perpendicular inclination to the ecliptic. Such transits usually occur only in years close to Uranian equinoxes. One such transit occurred on July 26, 2006; the Hubble Space Telescope captured that transit on the image at the left. The white dot in the upper-right-hand quadrant of Uranus is Ariel itself; the black dot is its shadow.
Ariel is in tidal lock with Uranus.
The surface of Ariel has relatively few impact craters, and also a system of interconnecting valleys and rifts. Some of these valleys bear signs of smoothing by a fluid. Liquid water would not smooth the valley floors in this fashion, and for this reason astronomers suspect that the fluid might have been liquid ammonia or methane. Other valleys and canyons appear to have formed by faulting.
Most of the surface of Ariel appears relatively "young" by uniformitarian standards. In fact, some authorities say that Ariel's surface is the youngest such surface among the major moons of Uranus.
Problems for uniformitarian theories
Ariel poses the same problem for uniformitarian astronomy as do all the other moons of Uranus: its orbit is inclined severely to the ecliptic, though not to Uranus' own equator. How the Uranian system came to have such an inclination has never been explained.
Observation and Exploration
Ariel is visible from Earth with any telescope having an aperture of 30 cm or more.
Today the Hubble Space Telescope is the most powerful telescope to take images of Ariel.
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