 Click image for description | |
| Discovery | |
|---|---|
| Discovered by | William Lassell |
| Discovery date | 24 October 1851 |
| Designations | |
| Alternate name | Uranus I |
| Adjective | Arielian |
| Semi-major axis | 191,020 km |
| Mean orbit radius | 190,900 km |
| Eccentricity | 0.0012 |
| Orbital period | 2.520 d |
| Inclination | 0.260° (to Uranus' equator) |
| Satellite of | Uranus |
| Physical characteristics | |
| Dimensions | 1162.2 × 1155.8 × 1155.4 km |
| Mean radius | 578.9 km (0.0908 Earths) |
| Surface area | 4,211,300 km² |
| Volume | 812,600,000 km³ |
| Mass | 1.35 × 1021 kg (2.26 × 10-4 Earths) |
| Mean density | 1.67 g/cm³ |
| Equatorial surface gravity | 0.27 m/s2 |
| Escape velocity | 0.56 km/s |
| Rotation period | synchronous |
| Albedo | 0.39 |
| Temperature | ~58 K |
| Apparent magnitude | 14.16 [1] |
| | |
Ariel (pronounced /ˈɛəriəl/[2]) is a moon of Uranus discovered on 24 October 1851 by William Lassell. It was discovered at the same time as Umbriel.
Name
Ariel is named after the leading sylph in Alexander Pope's poem The Rape of the Lock. It is also the name of the spirit who serves Prospero in Shakespeare's The Tempest.
The name "Ariel" and the names of all four satellites of Uranus then known were suggested by John Herschel in 1852 at the request of Lassell.[3] Lassell had earlier endorsed Herschel's 1847 naming scheme for the seven then-known satellites of Saturn and had named his newly-discovered eighth satellite Hyperion in accordance with Herschel's naming scheme in 1848. The adjectival form of the name is Arielian.
It is also designated Uranus I.
Physical characteristics
The first and so far only close-up observations of Ariel were made by the Voyager 2 probe during its 1986 Uranus fly-by. Voyager 2 made its closest approach of Ariel on 24 January 1986 and passed within 127 000 km of the moon.[4] Because the moon's south pole was pointed towards the Sun, only the southern hemisphere was photographed.
Due to the current margin of error, it is not yet known for certain whether Ariel is more massive than the sister satellite Umbriel.[5] Ariel's composition is roughly 70% ices (water ice, carbon dioxide ice, and possibly methane ices) and 30% silicate rock, and it appears to have regions of fresh frost in places, particularly in the ejecta radiating from young impact craters. The oldest and most extensive geologic unit observed on Ariel by Voyager 2 was a vast area of cratered plains centered near Ariel's south pole. Analysis of craters seen on Ariel's cratered plains suggest most are younger than many of those seen on Titania, Oberon, and Umbriel.[6] The largest crater observed on Ariel is Yangoor, at only 78 km across,[7] and shows signs of deformation since its formation. Voyager 2 also observed a network of faults, canyons, and icy outflows running along Ariel's mid-southern latitudes, breaking up the cratered plains region. The canyons probably represent grabens formed by extensional faulting. Smooth material and grooves are often seen running down length of Ariel's valley networks, suggested that some canyon floors have been covered in warm ice extruded from Ariel's interior.[6]
Ariel's past geologic activity is believed to have been driven by tidal heating at a time when its orbit was more eccentric than currently. Early in its history, Ariel was apparently captured in a 4:1 orbital resonance with Titania, from which it subsequently escaped.[8] The resonance would have increased orbital eccentricity; resulting tidal friction due to time-varying tidal forces from Uranus would have caused warming of the moon's interior. In the Uranus system, due to the planet's lesser degree of oblateness, and the larger relative size of its satellites, escape from a mean motion resonance is much easier than for satellites of Jupiter or Saturn.
Official names have been given to the following geological feature types on Ariel:
Ariel may be like Saturn's Dione. They are very similar in size, density and mass with Ariel only having slightly higher numbers. Both appear to have had past geological activity.
Transits
- On 26 July 2006, the Hubble Space Telescope captured a rare transit made by Ariel across the face of Uranus, during which the satellite cast a shadow that could be seen on the Uranian cloudtops. Such events are rare and only occur around equinoxes, as the moon's orbital plane about Uranus is tilted 98° to Uranus's orbital plane about the Sun.[9]
- During Uranus' equinox (December 2007) Ariel was producing eclipses across the center of Uranus.[10]
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