Uranus has 27 known moons. Titania, the largest, is about half the size of our own moon. The moons are all made of ice and rock. Most of the moons, including Titania, Oberon, and Umbriel, seem to be "normal" moons without anything interesting; however, Miranda and Ariel seem to have had violent pasts. Most of the moons appear to be captured asteroids due to their size and orbits.
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.
Ariel has the brightest and possibly the geologically youngest surface in the Uranian moon system. Much of Ariel's surface is densely pitted with craters 5 to 10 km (3 to 6 mi) across. These craters are close to the threshold of detection in this picture. Numerous valleys and fault scarps crisscross the highly pitted terrain. Voyager scientists believe the valleys have formed over down-dropped fault blocks (graben); apparently, extensive faulting has occurred as a result of expansion and stretching of Ariel's crust. The largest fault valleys, near the terminator at right, as well as a smooth region near the center of this image, have been partly filled with deposits that are younger and less heavily cratered than the pitted terrain. Narrow, somewhat sinuous scarps and valleys have been formed, in turn, in these young deposits. It is not yet clear whether these sinuous features have been formed by faulting or by the flow of fluids.
The 10 new moons discovered by Voyager bring the total number of known Uranian satellites to 27. The largest of the newly detected moons, named Puck, is about 150 kilometers (about 90 miles) in diameter, or larger than most asteroids. Puck and the other small moons are very dark.
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.
Voyager 2 obtained this full-disk view of Uranus' moon Titania in the early morning hours of Jan. 24, 1986, from a distance of about 500,000 kilometers (300,000 miles). Many circular depressions -- probably impact craters -- are visible in this clear-filter image returned by the Voyager narrow-angle camera. Other bright spots are distinguished by radiating rays and are probably halo craters that mark relatively more recent impacts. Even more interesting are linear troughs (right) that are probably fault canyons. The troughs break the crust in two directions, an indication of some tectonic extension of Titania's crust. These features indicate that this icy satellite has a dynamic, active interior. Titania is about 1,600 km (1,000 mi) in diameter.
One theory of Titania's history is that it was once hot enough to be liquid. The surface probably cooled first; when the interior froze it expanded forcing the surface to crack and resulting in the valleys that we see today.
Umbriel is ancient and dark, apparently having undergone little geologic activity. It reflects only about half as much light as Ariel, Uranus' brightest satellite.
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.
Umbriel and Oberon appear quite similar though Oberon is 35% larger. All of Uranus' large moons are a mixture of about 40-50% water ice with the rest rock, a somewhat larger fraction of rock than Saturn's large moons such as Rhea.
Umbriel's heavily cratered surface has probably been stable since its formation. It has far more and larger craters than do Ariel and Titania.
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.
Oberon and Umbriel appear quite similar though Oberon is 35% larger. All of Uranus' large moons are a mixture of about 40-50% water ice with the rest rock, a somewhat larger fraction of rock than Saturn's large moons such as Rhea.
Oberon's heavily cratered surface has probably been stable since its formation. It has far more and larger craters than do Ariel and Titania. Some of the craters have rays of ejecta similar to those seen on Callisto.
Some of the crater floors are dark, perhaps covered with darker material (dirty water?) that upwelled into the crater.
Large faults are also seen across the entire southern hemisphere of Oberon. This indicates some geologic activity early in Oberon's history.
It is actually possible to see Uranus's 4 largest moons with an amateur telescope. But it takes a very dark sky and a telescope with at least a 12 inch (30 cm) aperture.
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.
Miranda is about half water ice and half rocky material. It's surface is all mixed up with heavily cratered terrain intermixed with weird grooves, valleys and cliffs.
At first, Voyager 2's images of Miranda were a mystery. Everyone had expected that Uranus' moons would show very little history of internal activity (like Callisto). Explaining the bizarre hitherto unknown terrain proved quite an embarrassment to those who had to do it on live TV. Their usual impressive and esoteric technical jargon gave out and they had to resort to using such terms as "chevron" (right), "race track", and "layer cake" to describe Miranda's unique features.
It was initially thought that Miranda had been completely shattered and reassembled several times in its history, each time burying some parts of the original surface and exposing some of the interior. Now, however, a more mundane explanation involving the upwelling of partially melted ices seems to be in favor.
It is actually possible to see Uranus's 4 largest moons with an amateur telescope, but Miranda is a real challenge. Perhaps with a very dark sky and a telescope with an 18 inch (50 cm) aperture or more it might be possible.
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.
Back to The Solar System