nd 18 such satellites, but only two were photographed. The satellites of Uranus form two distinct classes: the 10 small very dark inner ones discovered by Voyager 2 and the five large outer ones. They all have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic). 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, 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 in color, 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. Huge fault systems and canyons that indicate an active geologic history, for example, mark Titania. 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 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 ...
