Moons of Uranus


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 Alexander Pope.[1] Uranus’ 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’ rings. The five major moons are ellipsoidal, indicating that they reached hydrostatic equilibrium at some point in their past, some may still be in equilibrium, and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces.[2] 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’ equator, which is tilted 97.77° to its orbit. Uranus’ irregular moons have elliptical and strongly inclined (mostly retrograde) orbits at large distances from the planet.[3]

William Herschel discovered the first two moons, Titania and Oberon, in 1787. The other three ellipsoidal moons were discovered in 1851 by William Lassell (Ariel and Umbriel) and in 1948 by Gerard Kuiper (Miranda).[1] 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.[2][3]


Miranda is the smallest and innermost of Uranus’s five round satellites. It was discovered by Gerard Kuiper on February 16th, 1948 at McDonald Observatory in Texas, and named after Miranda from William Shakespeare’s play The Tempest. Like the other large moons of Uranus, Miranda orbits close to its planet’s equatorial plane. Because Uranus orbits the Sun on its side, Miranda’s orbit is perpendicular to the ecliptic and shares Uranus’ extreme seasonal cycle. The only close-up images of Miranda are from the Voyager 2 probe, which made observations of Miranda during its Uranus flyby in January 1986. During the flyby, Miranda’s southern hemisphere pointed towards the Sun, so only that part was studied. Miranda orbits closest to it, at roughly 129,000 km from the surface; about a quarter again as far as its most distant ring. Its orbital period is 34 hours, and, like that of the Moon, is synchronous with its rotation period, which means it always shows the same face to Uranus, a condition known as tidal locking.


Aiel is the 4th largest of Uranus’ 27 moons, orbiting and orating in the equatorial plane of the planet. This makes Ariel almost perpendicular to Uranus’ orbit and has an extreme seasonal cycle. Of the 5 major rounded satellites of Uranus, Ariel is the 2nd smallest and has a mass equal magnitude to earth’s hydrosphere. Like all of the other moons around the planet, Ariel probably formed from an accretion disc that surrounded the planet shortly after its formation, and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Ariel has a complex surface consisting of extensive cratered terrain cross-cut by a system of scarps, canyons, and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating. It was discovered on October 24th, 1851 by William Lassell and was then named for a sky spirit in Alexander Pope’s The Rape of the Lock and Shakespeare’s The Tempest.


Just like the moon Ariel, the moon Umbriel was discovered on October 24th, 1851, by William Lassell and named after a character in Alexander Pope’s poem The Rape of the Lock. Umbriel consists mainly of ice with a substantial fraction of rock, and could be made up of a rocky core and an icy mantle. Compared to other Uranian moons, the surface or Umbrial is the darkest and appears to have been shaped primarily by impacts. However, the presence of canyons suggests early endogenic processes, and the moon may have undergone an early endogenically driven resurfacing event that obliterated its older surface. Umbriel is the 2nd most heavily cratered satellite of Uranus after the moon named Oberon. The most prominent surface feature is a ring of bright material on the floor of Wunda crater. When visited by the Voyager 2 spacecraft, several images of Umbriel were taken, which then allowed scientists to map about 40% of the surface.


Discovered by William Herschel in 1787, Titania is the largest moon of Uranus with an orbit that lies inside Uranus’s magnetosphere. It was named after the queen of the fairies in Shakespeare’s A Midsummer Night’s Dream.When visited by the Voyager 2 spacecraft, several images of Titania were taken, which then allowed scientists to map around 40% of the surface.

Titania consists of approximately equal amounts of ice and rock, and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the core–mantle boundary. The surface of Titania, which is relatively dark and slightly red in color, appears to have been shaped by both impacts and endogenic processes. It is covered with numerous impact craters reaching up to 326 kilometres (203 mi) in diameter, but is less heavily cratered than Oberon, outermost of the five large moons of Uranus. Titania probably underwent an early endogenic resurfacing event which obliterated its older, heavily cratered surface. Titania’s surface is cut by a system of enormous canyons and scarps, the result of the expansion of its interior during the later stages of its evolution. Like all major moons of Uranus, Titania probably formed from an accretion disk which surrounded the planet just after its formation.


Further Reading

Check out the Planetary Bodies Category for previous and upcoming articles on the solar system planets.


[1] =  “Planet and Satellite Names and Discoverers”Gazetteer of Planetary Nomenclature. USGS Astrogeology. July 21, 2006.
[2] = Smith, B. A.; Soderblom, L. A.; Beebe, A.; Bliss, D.; Boyce, J. M.; Brahic, A.; Briggs, G. A.; Brown, R. H.; Collins, S. A. (4 July 1986). “Voyager 2 in the Uranian System: Imaging Science Results”Science233 (4759): 43–64. Bibcode:1986Sci…233…43Sdoi:10.1126/science.233.4759.43PMID 17812889.
[3] = Sheppard, S. S.; Jewitt, D.; Kleyna, J. (2005). “An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness”. The Astronomical Journal129 (1): 518–525. arXiv:astro-ph/0410059Bibcode:2005AJ….129..518Sdoi:10.1086/426329.

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