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From left, Miranda, Ariel, Umbriel, Titania, and Oberon
From left, in order of increasing distance from the planet, they are Miranda, Ariel, Umbriel, Titania and Oberon. (Click on the image for a larger view)


Voyager 2 obtained clear, high-resolution images of each of the five large moons of Uranus known before the encounter: Miranda, Ariel, Umbriel, Titania and Oberon. The two largest, Titania and Oberon, are about 1,600 kilometers (1,000 miles) in diameter, roughly half the size of Earth's Moon. The smallest, Miranda, is only 500 kilometers (300 miles) across, or just one-seventh the lunar size.

The 10 new moons discovered by Voyager bring the total number of known Uranian satellites to 15. The largest of the newly detected moons, named Puck, is about 150 kilometers (about 90 miles) in diameter, or larger than most asteroids.

Preliminary analysis shows that the five large moons are ice-rock conglomerates like the satellites of Saturn. The large Uranian moons appear, in fact, to be about 50 percent water ice, 20 percent carbon- and nitrogen-based materials, and 30 percent rock. Their surfaces, almost uniformly dark gray incolor, display varying degrees of geologic history. Very ancient, heavily cratered surfaces are apparent on some of the moons, while others show strong evidence of internal geologic activity.

Titania, for example, is marked by huge fault systems and canyons that indicate some degree of geologic activity in its history. These features may be the result of tectonic movement in its crust. Ariel has the brightest and possibly the geologically youngest surface in the Uranian moon system. It is largely devoid of craters greater than about 50 kilometers (30 miles) in diameter. This indicates that low-velocity material within the Uranian system itself peppered the surface, helping to obliterate larger, older craters. Ariel also appears to have undergone a period of even more intense activity leading 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, apparently having 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 near and confined to the vicinity of that moon's orbit.

The outermost of the pre-Voyager moons, 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 reaggregation 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 has 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.

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