Umbriel
Uranus II

Umbriel [UM-bree-ul] is the darkest satellite of Uranus. It is about the same size as Ariel and has about the same density. The surface appears to be old with large craters and does not change much from one location to another. Near the top is a puzzling bright ring called the fluorescent cheerio. It is probably the floor of a crater.

Umbriel Statistics
Discovered by | William Lassell
Date of discovery | 1851
Mass (kg) | 1.27e+21
Mass (Earth = 1) | 2.1252e-04
Equatorial radius (km) | 584.7
Equatorial radius (Earth = 1) | 9.1675e-02
Mean density (gm/cm^3) | 1.52
Mean distance from Uranus (km) | 265,970
Rotational period (days) | 4.144177
Orbital period (days) | 4.144177
Mean orbital velocity (km/sec) | 4.67
Orbital eccentricity | 0.0050
Orbital inclination (degrees) | 0.36
Escape velocity (km/sec) | 0.538
Visual geometric albedo | 0.18
Magnitude (Vo) | 14.81

Views of Umbriel

Umbriel
The surface of Umbriel appears to be old with large craters and does not change much from one location to another. Near the top is a puzzling bright ring called the fluorescent cheerio. It is probably the floor of a crater. (Credit: Calvin J. Hamilton)
Albedo Map of Umbriel
This image is an adaptation of a figure from an article by Paul Helfenstein, Peter C.Thomas and Joseph Veverka published in the March 23, 1989 issue of Nature. The color image is an albedo map of Umbriel after shading correction and contrast enhancement. Dark regions are represented as red and bright regions are represented as blue. The two white regions are bright ejecta from two fresh craters. In the article the authors suggested that the dark areas are the result of an internally driven resurfacing event early in Umbriel's history involving darker material. Similar ice volcanism is also seen on Miranda, Ariel, and possibly on Oberon. They also state that the limited data also hint that the darker regions are lower in elevation. (Courtesy A.Tayfun Oner)

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##### Copyright © 1997 by Calvin J. Hamilton. All rights reserved.