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== Orbital characteristics ==

== 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.

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 ==

== Rotational characteristics ==

=== Surface ===

=== 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/>

The surface of Umbriel is covered with impact craters, most of which measure 100 to 200&nbsp;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 Umbriel is covered with a dark material similar to that which covers the Cassini Region of [[Iapetus (moon)|Iapetus]]. Some suspect that this material is [[methane]].<ref name=Britannica/>

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=Hamilton/><ref name=Arnett/><ref name=uranussatfact/><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 Umbriel is covered with a dark material similar to that which covers the Cassini Region of [[Iapetus (moon)|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/>

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&nbsp;km in diameter.<ref name=Hamilton/><ref name=Britannica/>

== Problems for uniformitarian theories posed by Umbriel ==

== Problems for uniformitarian theories posed by Umbriel ==

== Observation and Exploration ==

== 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.

[[Voyager 2]] approached to within 325,000&nbsp;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 ==

== References ==