 Click image for description | |
| Discovery | |
|---|---|
| Discovered by | William Lassell |
| Discovery date | October 24, 1851 |
| Designations | |
| Alternate name | Uranus II |
| Adjective | Umbrielian |
| Semi-major axis | 266 000 km |
| Eccentricity | 0.? |
| Orbital period | 4.144 d |
| Inclination | 0.205° (to Uranus' equator) |
| Satellite of | Uranus |
| Physical characteristics | |
| Mean radius | 584.7 km (0.092 Earths)[1] |
| Surface area | 4 296 000 km² (0.008 Earths) |
| Volume | 837 300 000 km³ (0.0008 Earths)[2] |
| Mass | (1.2 ± 0.1) × 1021 kg (2 × 10-4 Earths) |
| Mean density | 1.40 ± 0.16 g/cm³ |
| Equatorial surface gravity | 0.23 m/s² (~0.023 g) |
| Escape velocity | 0.52 km/s |
| Rotation period | presumed synchronous |
| Axial tilt | 0 |
| Albedo | 0.21 ± 0.02[1] |
| Temperature | ~61 K |
| Apparent magnitude | 14.81[3] |
| | |
Umbriel (pronounced /ˈʌmbriəl/[4]) is a moon of Uranus discovered on October 24, 1851, by William Lassell. It was discovered at the same time as Ariel.
Name
The name "Umbriel" and the names of all four satellites of Uranus then known were suggested by John Herschel in 1852 at the request of Lassell.[5] 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. Umbriel is the 'dusky melancholy sprite' in Alexander Pope's The Rape of the Lock, and the name suggests the Latin umbra, shadow. The adjectival form of the name is Umbrielian.
It is also designated Uranus II.
Physical characteristics
So far the only close-up images of Umbriel are from the Voyager 2 probe, which made observations of the moon during its Uranus flyby in January 1986. During the flyby the southern hemisphere of the moon was pointed towards the Sun so only it was studied.
Due to the current observational error, it is not yet known for certain whether Ariel is more massive than Umbriel.[1]
Umbriel's surface is the darkest of the Uranian moons, and reflects only about half as much light as Ariel, a sister satellite of similar size.[1] Umbriel's heavily cratered surface has probably been stable since the Late Heavy Bombardment. It has far more and larger craters than do Ariel and Titania and is also the least geologically active.[6] It is mostly composed of water ice, with the balance made up of silicate rock, and other ices such as methane. Methane can break down and form reddish-black organic compounds such as tholins when bombarded by high-energy particles.[7] Near-IR spectra of Ariel and Umbriel clearly show that water ice dominates the spectra of these objects.[8]
Umbriel's most prominent feature is Wunda, a large ring of bright material near Umbriel's equator (see picture; the viewpoint is nearly polar). Wunda is presumably some kind of crater, but its exact nature is mysterious. Nearby, seen along the terminator, is the crater Skynd, which lacks a bright rim but possesses a bright central peak.
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