Umbriel

Umbriel, taken by Voyager 2
Date of discovery |  October 24, 1851[1][2][3]
Name of discoverer |  William Lassell[1][2][3][4]
Name origin |  Evil spirit in Alexander Pope's poem The Rape of the Lock[1][5][6]
Orbital characteristics
Primary | Uranus
Order from primary |  16
Periuranion | 264,968 km[7]
Apuranion | 267,362 km[7]
Semi-major axis | 266,300 km[8]
Orbital eccentricity | 0.0050[8]
Sidereal month | 4.144177 da[8]
Avg. orbital speed | 4.67 km/s[4]
Inclination | 0.36°[8] to Uranus's equator
Rotational characteristics
Sidereal day |  4.144177 da[8][9]
Rotational speed |  0.08858 km/s[7]
Axial tilt |  0°[8]
Physical characteristics
Mass |  1.17 * 1021 kg[8]
Density |  1400 kg/m³[8][9]
Mean radius |  584.7 km[4][8]
Surface gravity |  0.2287 m/s²[7]
Escape speed |  0.517 km/s[7]
Surface area |  4,296,117 km²[7]
Mean temperature |  61 K
Composition |  Water ice and rock[6]
Color |  Dark gray
Albedo |  0.19[8]
Umbriel or Uranus II is the sixteenth moon of Uranus in order of distance and the fourth moon of Uranus to be discovered.

## Contents

* 1 Discovery and naming
* 2 Orbital characteristics
* 3 Rotational characteristics
* 4 Physical characteristics
* 4.1 Surface
* 5 Problems for uniformitarian theories posed by Umbriel
* 6 Observation and Exploration
* 7 References

## Discovery and naming[edit]

William Lassell discovered Umbriel and its next companion, Ariel, on October 24, 1851, and reported his findings after observing these two satellites on three more occasions.[2][3]

The name Umbriel, suggested by Sir John Herschel at Lassell's request, is the name of an evil spirit in Alexander Pope's poem The Rape of the Lock.[5][6] The name is especially appropriate. Umbriel derives from the Latin umbra shadow, and Umbriel happens to be the darkest of all of Uranus' major moons.

## Orbital characteristics[edit]

Umbriel is in a slightly eccentric orbit around Uranus at an average distance of 266,300 km. Its sidereal month is about 4.14 Earth days. The orbit of Umbriel is only slightly inclined from the equator of Uranus but is severely inclined to the ecliptic.

## Rotational characteristics[edit]

Umbriel is in tidal lock with Uranus.

## Physical characteristics[edit]

Umbriel is the fourth heaviest moon of Uranus, and slightly less dense than Ariel. Its density suggests that Umbriel is composed of water ice and rock.

### Surface[edit]

The surface of Umbriel is covered with impact craters, most of which measure 100 to 200 km in diameter. Most astronomers believe that these craters formed early in the history of the solar system, during a period of heavy bombardment of many solar system bodies. The distribution of craters is uniform.[4]

Remarkably, the surface has no features suggesting any tectonic activity.[6][9] Moreover, Umbriel is the darkest and least reflective of all the major moons of Uranus.[4][6][8][10] Many astronomers suspect that Umbriel is covered with a dark material similar to that which covers the Cassini Region of Iapetus. Some suspect that this material is methane.[9]

The most surprising feature of the surface of Umbriel is the bright ring, or Wunda ring, near the north pole. The nature and composition of that ring are unknown. The favorite theory is that it is the floor of a crater 40 km in diameter.[4][9]

## Problems for uniformitarian theories posed by Umbriel[edit]

The orbit of Umbriel is more eccentric than the orbit of Ariel. Yet Ariel shows signs of past tectonic activity, which some astronomers impute to tidal heating.[11] But Umbriel shows no such signs.

In addition, Umbriel 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[edit]

Voyager 2 approached to within 325,000 km of Umbriel on January 24, 1986, and took a small number of images.[12][13] No other detailed images are available.

## References[edit]

1. ↑ 1.0 1.1 1.2 "Gazetteer of Planetary Nomenclature: Planetary Body Names and Discoverers." US Geological Survey, Jennifer Blue, ed. March 31, 2008. Accessed April 17, 2008.
2. ↑ 2.0 2.1 2.2 Lassell, W. "Letter to the Editor re Discovery of Two Satellites of Uranus." Astron. J. 2:70, 1851. Accessed June 13, 2008.
3. ↑ 3.0 3.1 3.2 Lassell, William. "Entdeckung von 2 neuen Uranus Trabanten." Astronomische Nachrichten, 33:259-260, 1852. Accessed June 13, 2008
4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Hamilton, Calvin J. "Entry for Umbriel." Views of the Solar System, 2001. Accessed June 13, 2008.
5. ↑ 5.0 5.1 Lassell, William. "Beobachtungen der Uranus-Satelliten." Astronomische Nachrichten 34:325-328, 1852. Accessed June 12, 2008.
6. ↑ 6.0 6.1 6.2 6.3 6.4 Arnett, Bill. "Entry for Umbriel." The ~~Nine~~ 8 Planets, December 11, 2004. Accessed June 13, 2008.
7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 Calculated
8. ↑ 8.00 8.01 8.02 8.03 8.04 8.05 8.06 8.07 8.08 8.09 8.10 Williams, David R. "Uranian Satellite Fact Sheet." National Space Science Data Center, NASA, November 23, 2007. Accessed June 13, 2008.
9. ↑ 9.0 9.1 9.2 9.3 9.4 Ingersoll, Andrew P. "Umbriel." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 14 June 2008.
10. ↑ Overbye, Roger. "Voyager was on target again; in the latest unmanned triumph, Voyager 2 surveyed Uranus and sent back a real bull's-eye." Discover, April 1986. Accessed June 14, 2008.
11. ↑ Tittemore, William C. "Tidal Heating of Ariel." Icarus 87:110-139, September 1990. doi:10.1016/0019-1035(90)90024-4 Accessed June 13, 2008.
12. ↑ "Voyager Mission Description: Voyager 2 Uranus Encounter." February 19, 1997. Accessed June 13, 2008.
13. ↑ Smith, B.A., Soderblom, L.A., Beebe, R., et al. "Voyager 2 in the Uranian system - Imaging science results." Science 233:43-64, July 4, 1986. Accessed June 13, 2008.

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