The Icy Satellites

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Uranus' Satellites

 The Icy Satellites Uranus' Satellites Umbriel

Umbriel :
Umbriel was officially discovered by William Lassel in 1851, at the same time as Ariel. Like Ariel, this satellite has a composition which is 40 to 50 % ice water, the rest is rock (silicates). The colour of the surface is grey but its weak albedo makes it the darkest of Uranus' satellites. Umbriel's surfaced is very cratered (like Oberon) and has not been resurfaced since the time for the last intense bombardment that the bodies of the solar system experienced, contrary to Ariel.
Image by Voyager 2(crédit: JPL/NASA)
Distance to Uranus265 980 km
Period of revolution4.14 earth days
Diameter1170 km
Mass (Earth=1)0.0002126
Density (water =1)1.51
CompositionIce water (40-50%), rocks
Temperature on the surface-188 °C
The image above is misleading. The bright icecap that can be seen at the top is not a polar icecap. Indeed, it is near the equator. It is the ring, 140 km in diameter, called Wunda and is the most reflective region on Umbriel. It could be a deposit of shiny material due to the impact of a meteorite, but this is far from certain. There is a second bright region by the edge of the image in the top right corner. This is the central spike of a crater called Vuver that is 110 km wide.

The weak albedo and the almost uniform colour of Umbriel could be explained in two ways. The surface could have been covered in dark material.
  • This material could have been ejected in the course of an impact with a large meteorite in the un-photographed hemisphere.

  • Or it could be due to volcanic activity, supposing Umbriel has a little tectonic activity. Some researchers suggested that Umbriel is more active than the cratered surface would lead to believe, basing their statement on the statistical analysis of craters and on the small variations of the albedo on the surface.

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