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{{Planet
|image=Umbriel.jpg
|caption=Umbriel, taken by Voyager 2
|date=October 24, 1851<ref name=usgs>"[http://planetarynames.wr.usgs.gov/append7.html Gazetteer of Planetary Nomenclature: Planetary Body Names and Discoverers]." US Geological Survey, Jennifer Blue, ed. March 31, 2008. Accessed April 17, 2008.</ref><ref name=Lassell1>Lassell, W. "[http://adsabs.harvard.edu//full/seri/AJ.../0002//0000070.000.html Letter to the Editor re Discovery of Two Satellites of Uranus]." ''Astron. J.'' 2:70, 1851. Accessed June 13, 2008.</ref><ref name=Lassell2>Lassell, William. "[http://adsabs.harvard.edu//full/seri/AN.../0033//0000136.000.html Entdeckung von 2 neuen Uranus Trabanten]." ''Astronomische Nachrichten'', 33:259-260, 1852. Accessed June 13, 2008</ref>
|discname=William Lassell<ref name=usgs/><ref name=Lassell1/><ref name=Lassell2/><ref name=Hamilton>Hamilton, Calvin J. "[http://www.solarviews.com/eng/umbriel.htm Entry for Umbriel]." ''Views of the Solar System'', 2001. Accessed June 13, 2008.</ref>
|origname=Evil spirit in Alexander Pope's play ''The Rape of the Lock''<ref name=usgs/><ref name=Lassell3>Lassell, William. "[http://articles.adsabs.harvard.edu//full/1852AN.....34..325./0000169.000.html Beobachtungen der Uranus-Satelliten]." ''Astronomische Nachrichten'' 34:325-328, 1852. Accessed June 12, 2008.</ref><ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/umbriel.html Entry for Umbriel]." ''The <s>Nine</s> 8 Planets'', December 11, 2004. Accessed June 13, 2008.</ref>
|primary=Uranus
|order=16
|periapsis=264,968 km<ref name=calc>Calculated</ref>
|apoapsis=267,362 km<ref name=calc/>
|semimajor=266,300 km<ref name=uranussatfact>Williams, David R. "[http://nssdc.gsfc.nasa.gov/planetary/factsheet/uraniansatfact.html Uranian Satellite Fact Sheet]." National Space Science Data Center, [[National Aeronautics and Space Administration|NASA]], November 23, 2007. Accessed June 13, 2008.</ref>
|eccentricity=0.0050<ref name=uranussatfact/>
|sidereal=4.144177 da<ref name=uranussatfact/>
|orbitspeed=4.67 km/s<ref name=Hamilton/>
|inclination=0.36°<ref name=uranussatfact/>
|siderealday=4.144177 da<ref name=uranussatfact/><ref name=Britannica>Ingersoll, Andrew P. "[http://www.britannica.com/eb/article-9074205 Umbriel]." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 14 June 2008.</ref>
|rotatespeed=0.08858 km/s<ref name=calc/>
|axialtilt=0°<ref name=uranussatfact/>
|mass=1.17 * 10<sup>21</sup> kg<ref name=uranussatfact/>
|density=1400 kg/m³<ref name=uranussatfact/><ref name=Britannica/>
|meanradius=584.7 km<ref name=Hamilton/><ref name=uranussatfact/>
|surfacegrav=0.2287 m/s²<ref name=calc/>
|escapespeed=0.517 km/s<ref name=calc/>
|surfacearea=4,296,117 km²<ref name=calc/>
|meantemp=61 K
|composition=Water ice and rock<ref name=Arnett/>
|color=#666666
|albedo=0.19<ref name=uranussatfact/>
}}'''Umbriel''' or '''Uranus II''' is the sixteenth [[moon]] of [[Uranus]] in order of distance and the fourth moon of Uranus to be discovered.
== Discovery and naming ==
[[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.<ref name=Lassell1/><ref name=Lassell2/>
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''.<ref name=Lassell3/><ref name=Arnett/> 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 ==
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 ==
Umbriel is in [[tidal lock]] with Uranus.
== Physical characteristics ==
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 ===
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.<ref name=Hamilton/>
Remarkably, the surface has no features suggesting any tectonic activity.<ref name=Arnett/><ref name=Britannica/> Moreover, Umbriel is the darkest and least reflective of all the major moons of Uranus.<ref name=uranussatfact/><ref name=Hamilton/><ref name=Arnett/><ref name=Overbye>Overbye, Roger. "[http://findarticles.com/p/articles/mi_m1511/is_v7/ai_5083814/pg_1 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.</ref> Many astronomers suspect that Uranus is covered with a dark material similar to that which covers the Cassini Region of [[Iapetus]]. Some suspect that this material is [[methane]].<ref name=Britannica/>
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.<ref name=Hamilton/><ref name=Britannica/>
== Problems for uniformitarian theories posed by Umbriel ==
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]].<ref name=Tittemore>Tittemore, William C. "[http://adsabs.harvard.edu/abs/1990Icar...87..110T Tidal Heating of Ariel]." ''Icarus'' 87:110-139, September 1990. {{doi|10.1016/0019-1035(90)90024-4}} Accessed June 13, 2008.</ref> 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 ==
[[Voyager 2]] approached to within 325,000 km of Umbriel on January 24, 1986, and took a small number of images.<ref name=Voyager1>"[http://pds-rings.seti.org/voyager/mission/#v2uenc Voyager Mission Description: Voyager 2 Uranus Encounter]." February 19, 1997. Accessed June 13, 2008.</ref><ref name=Smith>Smith, B.A., Soderblom, L.A., Beebe, R., ''et al.'' "[http://adsabs.harvard.edu/abs/1986Sci...233...43S Voyager 2 in the Uranian system - Imaging science results]." ''Science'' 233:43-64, July 4, 1986. Accessed June 13, 2008.</ref> No other detailed images are available.
== References ==
{{reflist|2}}
{{Solarsystem}}
{{Solarsystem}}
[[Category:Astronomy]]
[[category:astronomy]]