Oberon (moon)

Oberon is the second-largest and outermost moon of the planet Uranus. The moon, one of twenty-seven circling the ice giant, has a heavily cratered surface and is nearly evenly composed of ice and rock. With a diameter of about 945 miles (1,520 kilometers), Oberon is just under half the size of Earth’s moon. Oberon was first discovered in 1787 by the astronomer William Herschel and named after a character in a play by William Shakespeare. Astronomers did not know much about Oberon until the spacecraft Voyager 2 flew by Uranus in 1986. Among the spacecraft’s findings was a canyon on Oberon that was more than 300 miles (483 kilometers) long and a mountain that reached about 3.7 miles (6 kilometers) high.

Background

Uranus is the seventh planet from the sun, orbiting at a distance of about 1.78 billion miles (2.86 billion kilometers). At that distance, Uranus takes about eighty-four years to make one trip around the sun. It is the third-largest planet in our solar system with a diameter of about 31,500 miles (50,700 kilometers), which is approximately four times larger than Earth. Uranus is unique among the planets in that it rotates at a 90-degree angle from its orbit, meaning that it rotates on its side. Astronomers believe this tilt may have been caused by a long-ago collision with a planetary body about the size of Earth.

Unlike the gas giants Jupiter and Saturn, which are mainly composed of hydrogen and helium gas, Uranus is an ice giant with a small, rocky core surrounded by a slush-like mixture of water, methane, and ammonia. The planet has a thin atmosphere consisting mostly of hydrogen and helium. Uranus has twenty-seven known moons. The largest, Titania, is about 980 miles (1,577 kilometers) wide, and the smallest, Cupid, is about 11 miles (17.7 kilometers) wide. According to one possible theory, the planet’s moons may have been created from debris thrown up from the collision that knocked Uranus on its side.

Overview

Uranus was first discovered in 1781 by German-born British astronomer William Herschel. Because Uranus is on the edge of being visible to the human eye, it was the first planet discovered with the aid of a telescope. Herschel originally wanted to name the planet after Britain’s King George III, but his fellow astronomers rejected that idea and named it Uranus after the Greek god of the sky.

Six years after Herschel discovered Uranus, he also found its two largest moons: Titania and Oberon. At the suggestion of Herschel’s son John, the moons were named after characters from William Shakespeare’s play A Midsummer Night’s Dream. In that play, Oberon was the king of the fairies and Titania was his queen. Uranus’s other moons were all named after characters from Shakespeare’s works and those of fellow British writer Alexander Pope.

At 945 miles (1,521 kilometers) wide, Oberon is the tenth-largest moon in the solar system. It is about 43 percent the size of Earth’s moon and about 8.5 times smaller than Earth itself. Oberon is Uranus’s outermost moon, orbiting the planet in about 13.5 days at a distance of about 363,000 miles (584,191 kilometers). It takes about 13.5 days to both circle Uranus and make one rotation on its axis—a common occurrence among planetary moons known as tidal locking or synchronous rotation. Oberon does not have an atmosphere and has no magnetic field.

For almost two centuries after Oberon’s discovery, astronomers had only viewed the moon through telescopes and knew very little about its geology. In January 1986, the spacecraft Voyager 2 flew by Uranus and made the first detailed observations of the ice giant and its moons. Before Voyager’s arrival, astronomers only knew of five moons circling the planet. The spacecraft discovered another ten moons during its flyby; an additional twelve moons were discovered by later observations.

It was learned that Oberon was made up of roughly equal amounts of water ice and rock, with what scientists believe to be an icy mantle covering a rocky core. The moon’s surface is dark in color with a slightly reddish tint. Oberon is pockmarked by numerous large craters, many of which are covered with a dark substance that scientists have yet to identify. They speculate that it could have been liquid water that gushed up into the crater when a space object impacted the moon. The water later froze and became “dirty ice.” If conditions within the moon are right, scientists speculate that an ocean of liquid water could exist at the boundary between the core and the outer icy mantle. However, chances that life exists in this ocean are slim.

Oberon’s surface is the most heavily cratered of all the planet’s moons, suggesting that it is also the oldest natural satellite circling Uranus. Parts of the observed southern hemisphere of Oberon are criss-crossed by large faults or cracks in the surface. These features point to the possibility that the moon was subject to some geological activity early in its history. Oberon does not currently appear to be geologically active.

Voyager 2 had a chance to map about 40 percent of the moon’s southern hemisphere before the spacecraft sped away from Uranus. Among the geological features it discovered was an almost 4-mile-high mountain and a canyon that is about 334 miles (537 kilometers) long. In 1988, the canyon was named Mommur Chasma after the legendary forest home of Oberon from French folklore. The geological features on the moon were all named from Shakespearean characters. For example, the moon’s largest-known crater—with a diameter of 128 miles (206 kilometers)—was named Hamlet after the title character in Shakespeare’s famous play.

Bibliography

Howell, Elizabeth. “Moons of Uranus: Facts About the Tilted Planet’s Satellites.” Space.com, 28 Feb. 2018, www.space.com/22201-uranus-moons.html. Accessed 28 Aug. 2020.

Moore, Patrick, and Robin Rees. “Uranus.” Patrick Moore’s Data Book of Astronomy. Cambridge UP, 2011, pp. 224–234.

“Oberon.” National Aeronautics and Space Administration, 19 Dec. 2019, solarsystem.nasa.gov/moons/uranus-moons/oberon/in-depth/. Accessed 28 Aug. 2020.

“Oberon.” USGS Astrogeology Science Center, planetarynames.wr.usgs.gov/SearchResults?target=OBERON. Accessed 29 Aug. 2020.

“Oberon: Uranus’ Moon.” National Oceanic and Atmospheric Administration, sos.noaa.gov/datasets/oberon-uranus-moon/. Accessed 28 Aug. 2020.

“Today in Science: Voyager 2 Met Uranus.” EarthSky, 24 Jan. 2020, earthsky.org/space/voyager-2-meets-uranus-jan-24-1986. Accessed 28 Aug. 2020.

“Uranus.” National Aeronautics and Space Administration, 22 Nov. 2019, solarsystem.nasa.gov/planets/uranus/overview/. Accessed 28 Aug. 2020.

Williams, Matt. “Uranus’s Moon Oberon.” Universe Today, 20 June 2015, www.universetoday.com/55518/oberon/. Accessed 28 Aug. 2020.