Umbriel, taken by Voyager 2
|Date of discovery||October 24, 1851|
|Name of discoverer||William Lassell|
|Name origin||Evil spirit in Alexander Pope's poem The Rape of the Lock|
|Order from primary||16|
|Semi-major axis||266,300 km|
|Sidereal month||4.144177 da|
|Avg. orbital speed||4.67 km/s|
|Inclination||0.36° to Uranus's equator|
|Sidereal day||4.144177 da|
|Rotational speed||0.08858 km/s|
|Mass||1.17 * 1021 kg|
|Mean radius||584.7 km|
|Surface gravity||0.2287 m/s²|
|Escape speed||0.517 km/s|
|Surface area||4,296,117 km²|
|Mean temperature||61 K|
|Composition||Water ice and rock|
Discovery and naming
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. 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.
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.
Umbriel is in tidal lock with Uranus.
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.
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.
Remarkably, the surface has no features suggesting any tectonic activity. Moreover, Umbriel is the darkest and least reflective of all the major moons of Uranus. 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.
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.
Problems for uniformitarian theories posed by Umbriel
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
- "Gazetteer of Planetary Nomenclature: Planetary Body Names and Discoverers." US Geological Survey, Jennifer Blue, ed. March 31, 2008. Accessed April 17, 2008.
- Lassell, W. "Letter to the Editor re Discovery of Two Satellites of Uranus." Astron. J. 2:70, 1851. Accessed June 13, 2008.
- Lassell, William. "Entdeckung von 2 neuen Uranus Trabanten." Astronomische Nachrichten, 33:259-260, 1852. Accessed June 13, 2008
- Hamilton, Calvin J. "Entry for Umbriel." Views of the Solar System, 2001. Accessed June 13, 2008.
- Lassell, William. "Beobachtungen der Uranus-Satelliten." Astronomische Nachrichten 34:325-328, 1852. Accessed June 12, 2008.
- Arnett, Bill. "Entry for Umbriel." The
Nine8 Planets, December 11, 2004. Accessed June 13, 2008.
- Williams, David R. "Uranian Satellite Fact Sheet." National Space Science Data Center, NASA, November 23, 2007. Accessed June 13, 2008.
- Ingersoll, Andrew P. "Umbriel." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 14 June 2008.
- 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.
- 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.
- "Voyager Mission Description: Voyager 2 Uranus Encounter." February 19, 1997. Accessed June 13, 2008.
- 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.