to have undergone a period of intense activity that lead to many fault valleys, and what appear to be extensive flows of icy material. Where many of the larger valleys intersect, their surfaces are smooth; this could indicate that the valley floors have been covered with younger icy flows. Umbriel is ancient and dark, appears to have undergone little geologic activity. Large craters pockmark its surface. The darkness of Umbriel's surface may be due to a coating of dust and small debris somehow created nearby and confined to the vicinity of that moon's orbit. The outermost of the moons discovered before Voyager, Oberon, also has an old, heavily cratered surface with little evidence of internal activity other than some unknown dark material apparently covering the floors of many craters.Miranda , innermost of the five large moons, is one of the strangest bodies yet observed in the solar system. Voyager images, which showed some areas of the moon at resolutions of a kilometer or less, consists of huge fault canyons as deep as 20 kilometers (12 miles), terraced layers and a mixture of old and young surfaces. The younger regions may have been produced by incomplete differentiation of the moon, a process in which upwelling of lighter material surfaced in limited areas. Alternatively, Miranda may be a conglomerate of material from an earlier time when the moon was fractured into pieces by a violent impact. Given Miranda's small size and low temperature (-335 degrees Fahrenheit or -187 Celsius), the degree and diversity of the tectonic activity on this moon have surprised scientists. It is believed that an additional heat source such as tidal heating caused by the gravitational tug of Uranus must have been involved. In addition, some means must have mobilized the flow of icy material at low temperatures. The Voyager 2 flyby mission has made plenty information available on the satellites, ring system, atmosphere and geology of Uranus. It...
