Although Uranus is devoid of surface features, its many moons display a fascinating portrait of a violent history. The cracked and distorted surfaces of Uranus’ moons are believed to have been caused by water. As liquid water rose from the interior of the moons, it froze and expanded, causing the crust to buckle outward. Miranda, one of Uranus’ larger outer moons, has one of the most chaotic surface patterns of anybody in the Solar System. The moon, shown left, resembles a patchwork, with parts of its core now on the surface, and parts of the crust buried deep underground. Scientists believe that this is because the moon was at one time pulled apart, and has gradually reformed.
Uranus, the seventh planet of the Solar System, has 27 known moons, most of which are named after characters that appear in, or are mentioned in, the works of William Shakespeare and Aloxander Pope. Uranus’s moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons. The inner moons are small dark bodies that share common properties and origins with Uranus’s rings. The five major moons are ellipsoidal, indicating that they reached hydrostatic equilibrium at some point in their past (and may still be in equilibrium), and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces. The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, about one-twentieth the mass of the Earth’s Moon. The orbits of the regular moons are nearly coplanar with Uranus’s equator, which is tilted 97.77° to its orbit. Uranus’s irregular moons have elliptical and strongly inclined (mostly retrograde) orbits at large distances from the planet.
William Herschel discovered the first two moons, Titania and Oberon, in 1787. The other three ellipsoidal moons were discovered in 1851 by William Lassell (Arial and Umbriel) and in 1948 by Gerard Kuiper (Miranda). These five have planetary mass, and so would be considered (dwarf) planets if they were in direct orbit about the Sun. The remaining moons were discovered after 1985, either during the Voyager 2 flyby mission or with the aid of advanced Earth-based telescopes.