Uranus's satellites

Uranus’s natural satellites form a miniature solar system with distinctive properties that teach us about the complexity and diversity of planetary and satellite formation. The peculiar surface features of some of the satellites and their unusual orbital characteristics suggest an earlier era in the solar system with violent collisions among its members.

Overview

Uranus holds its place in the solar system as a member of the subgroup of planets that are called Jovian, after the largest planet in the group, Jupiter. These planets are also referred to as gas giants in that their atmospheres comprise the greatest portion of their structures. The physical nature and orbital properties of these planets' satellites are among the properties that set apart one Jovian planet from another. Surveying the most interesting properties of a planetary system like Uranus's can provide insights into the origin and evolution of the solar system itself.

Uranus has twenty-seven satellites that have been identified. Their names follow a theme that is distinctive in the solar system in that the satellites are not named for mythological figures, like those of the other planets, but instead, take their names from characters in plays by William Shakespeare and poems by Alexander Pope. Oberon, Titania, and Puck, for example, were named for characters from Shakespeare’s A Midsummer Night’s Dream; Ariel, Umbriel, and Belinda are named for characters in Pope’s The Rape of the Lock. Oberon and Titania were first discovered by William Herschel in 1781. Ariel and Umbriel were discovered in 1851 by William Lassell. In 1948, Gerard Kuiper discovered the last moon of any significant size, Miranda.

The only spacecraft to visit Uranus to date has been Voyager 2, which flew by the planet in 1986. Despite the briefness of its visit, Voyager 2 discovered ten small satellites: Juliet, Puck, Cordelia, Ophelia, Bianca, Desdemona, Portia, Rosalind, Cressida, and Belinda. (Perdita was also imaged by Voyager 2, but its discovery was not confirmed until 1999.) The number of known satellites has swelled to twenty-seven through observation with both orbiting telescopes the (Hubble Space Telescope and the James T. Webb Space Telescope) and ground-based observatories.

A way to organize the Uranian systems of satellites is to think of them as distributed in three divisions. The first division consists of the inner thirteen, relatively small, circular satellites starting just outside the ring system. The second division comprises the five large satellites that were discovered prior to the Voyager 2 mission. Finally, the last division comprises nine irregular satellites discovered more recently.

None of the natural satellites of Uranus can be considered on the scale of the largest satellites of the other planets of the solar system, such as Saturn's Titan, Neptune's Triton, the four Galilean satellites of Jupiter (Io, Europa, Callisto, and Ganymede), or even Earth’s own moon. Nor does any of these satellites contain an atmosphere, as does as Titan, or show active volcanoes, as seen on Io. However, the five largest Uranian satellites—Miranda, Ariel, Umbriel, Titania, and Oberon (in order outward from the planet)—all have enough mass to be spherical. Hence, if they were not orbiting Uranus and were free from any debris, they would qualify as dwarf planets, like Pluto. The composition of these satellites is mostly ice, with mixtures of ammonia and methane. They all, like Earth's Moon, exhibit synchronous rotation, rotating at the same rate they revolve around Uranus, always showing the same side facing the planet.

The large satellites of Uranus all show extensive cratering. The consensus of scientific opinion is that the larger craters were formed from collisions with planet-sized objects during the formation of the solar system, and the smaller craters were produced afterward from impacts from comets and meteoroids. Ariel, Umbriel, and Miranda have unusual surface features. Ariel, the lightest-colored, has craters, valleys, and canyons. It has a diameter of about 1,300 kilometers with grooves and crevices that extend over its entire surface, suggesting recent lava flow that has cooled and solidified. Umbriel is the darkest-colored satellite; its surface is old and cratered. Although Umbriel (with a diameter of 1,110 kilometers) shows the most uniform cratering, with little evidence of geological activity, it has a large, bright ring at the top edge of its southern hemisphere; this ring’s origin is unknown, but it has unofficially been named the “fluorescent Cheerio.” Miranda, at 500 kilometers in diameter, the smallest of these five large satellites, has the most distinctive surface features. There are sharp grooves and ridges along its surface. One feature resembles a large chevron, and another looks like a carved racetrack. Titania and Oberon are both larger than the other satellites (with diameters of approximately 1,500 to 1,600 kilometers), and they are also several times farther away from Uranus. Both Titania and Oberon show large craters, but the presence of circular regions on Oberon suggests that it has experienced more geological activity than Titania. Oberon’s surface is frozen and has a mountain six kilometers high. Titania, the biggest satellite among those of Uranus, has impact basins, craters, and rifts.

Inside Miranda’s orbit, there are the thirteen inner satellites, all with enough mass to be spherical and with diameters ranging from about 25 to 170 kilometers. They are (in order out from the planet) Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, Puck, and Mab. Their composition appears to be about half water and half rocklike materials. Their orbital distribution presents a more crowded arrangement than that of the large satellites, and it appears that these thirteen bodies interact gravitationally with each other, crossing paths and periodically colliding with one another. Some of these inner satellites may serve as shepherds for Uranus’s narrow rings, which orbit closest to the planet.

Outside the orbit of Oberon are the small irregular satellites: Francisco, Caliban, Stephano, Trinculo, Sycroax, Margaret, Prospero, Setebos, and Ferdinand. They are considered irregular, though, because they orbit in odd directions far from Uranus. Their respective sizes and compositions have not been measured accurately. Scientists have estimated that their diameters range from 18 to 150 kilometers. They have very eccentric (elliptical) orbits, and all orbit retrograde, that is, in the direction opposite to the general revolution of other bodies in the solar system, which is counterclockwise around the north polar axis of the Sun.

Knowledge Gained

The study of Uranus’s system of natural satellites, including their orbital and physical properties, makes it possible to understand more fully the processes not only of planet formation but of the evolution of the solar system itself. Satellites can be formed at the time of the formation of the planet they orbit, but they can also be captured by the planet’s gravitational pull at a later stage. It appears that both processes are at work in the Uranian system. The group of large satellites (Miranda, Ariel, Umbriel, Titania and Oberon), as well as the thirteen inner satellites, most likely formed at the same time Uranus condensed. However, events have transpired to alter these satellites’ orbits and surface features. For example, Miranda has a scarred surface that is presumed to be the result of a collision with another object and was fractured into pieces. These pieces then came back together unevenly and showed the rough terrain and scarps that characterize the satellite. An alternative hypothesis is that Miranda, being too small an object to complete its internal mixing, froze midway through the process of separating its structure into layers.

Several of the inner moons are in such close orbits with each other that they conceivably collide and switch orbital positions around Uranus. In addition, Cordelia and Ophelia serve as shepherding moons for the Epsilon ring system.

The outer, irregular satellites appear to be satellites that were previously solar system bodies, such as comets and asteroids, that were captured by Uranus’s gravitational pull and orbit around the planet in elliptical and retrograde orbits.

This accumulated knowledge points to a solar system that evolved sequentially over vast periods of time with many diverse objects that are still changing their orbital shapes and constitutions.

Context

There were only five known satellites of Uranus until Voyager 2 visited this planetary system and made its closest approach on January 24, 1986. Voyager 2 was launched in 1977 and visited Jupiter in 1979, Saturn in 1981, and, after its flyby of Uranus, Neptune in 1989.

As Voyager 2 approached the Uranus system, its onboard computers were reprogrammed by scientists and engineers back on Earth at NASA's Jet Propulsion Lab to enable the cameras to produce high-quality photographs in the reduced light and at the high speeds at which the spacecraft would be traveling on its flyby. Most of the photographs were taken in a six-hour period in and around the time of closest approach (9:59 am. PST) on January 24, 1986. On the way to the rendezvous with Uranus, Voyager 2 obtained clear, high-resolution images of each of the five large Uranian satellites (Miranda, Ariel, Umbriel, Titania, and Oberon). It was its discovery of eleven new satellites that added to our knowledge base of this planet. During the processing of images of the outer ring (Epsilon) of Uranus, it was discovered that two small satellites, Cordelia and Ophelia, were shepherding or keeping this thin ring in orbit around the planet. Also sighted were Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, and Perdita, which belong to what has been called the Portia Group of satellites. These satellites have similar orbits and light-reflecting properties. The closeness of their respective orbits leads to the hypothesis that this group interacts with each other and may, at times, collide.

Until 1997, the Uranian system was distinct from the other Jovian planets in that there were no identified irregular satellites. However, the discovery on September 6, 1997, of two irregular satellites, Caliban and Sycroax, by Brett J. Gladman, Philip D. Nicholson, Joseph A. Burns, J. J. Kavelaars, Brian G. Marsden, Gareth V. Williams, and Warren B. Offutt, using the 200-inch Hale telescope (at Palomar Observatory in Southern California) removed that distinction. Subsequently, Stephano, Prospero, and Setebos were discovered by Matthew J. Holman, Kavelaars, Gladman, Jean-Marc Petit, and Hans Scholl on July 18, 1999. Trinculo, Margaret, and Ferdinand were discovered by Holman, Kavelaars, and Dan Milisavljevic on August 13, 2001. Discoveries continued to be made about the satellites of Uranus in the twenty-first century. For example, in radiation data provided by Voyager 2, scientists discovered two of Uranus’s moons, Ariel and Miranda, show active water supplies in the forms of oceans, which may be emitting material into space.

Bibliography

Bennett, Jeffery, Megan Donahue, Nicholas Schneider, and Mark Voit. The Cosmic Perspective. 3rd ed. San Francisco: Pearson, 2004.

Burgess, Eric. Uranus and Neptune: The Distant Giants. New York: Columbia UP, 1988.

Carroll, Bradley W., and Dale A. Ostlie. An Introduction to Modern Astrophysics. San Francisco: Pearson, 2007.

Elkins-Tanton, Linda T. Uranus, Neptune, Pluto, and the Outer Solar System. New York: Chelsea House, 2006.

Encrenaz, Thérèse, et al., eds. The Outer Planets and Their Moons: Comparative Studies of the Outer Planets Prior to the Exploration of the Saturn System by Cassini-Huygens. New York: Springer, 2005.

Hunt, Garry E., and Patrick Moore. Atlas of Uranus. New York: Cambridge UP, 1988.

Lea, Robert. “Two Moons of Uranus May Have Active Subsurface Oceans.” Space.com, 20 Mar. 2023, www.space.com/uranus-moons-ariel-miranda-active-subsurface-oceans. Accessed 17 Sept. 2023.

Loewen, Nancy. The Sideways Planet: Uranus. Mankato: Picture Window, 2008.

Miner, Ellis. Uranus: The Planet, Rings, and Satellites. New York: Horwood, 1990.

“NASA's Webb Scores Another Ringed World With New Image of Uranus.” NASA, 6 Apr. 2023, www.nasa.gov/feature/goddard/2023/nasa-s-webb-scores-another-ringed-world-with-new-image-of-uranus. Accessed 17 Sept. 2023.

Schmude, Richard W. Uranus, Neptune, and Pluto and How to Observe Them. New York: Springer, 2008.

Tocci, Salvadore. A Look at Uranus. New York: Watts, 2003.

"Uranus: Overview." National Aeronautics and Space Administration. NASA, 26 Apr. 2013.