Oberon (moon)

Oberon is the outermost and second-largest major moon of the planet Uranus. It was discovered by William Herschel in 1787 and is named after a mythical king of the fairies from Shakespeare's play A Midsummer Night's Dream. Oberon is the second-most massive of the Uranian moons and the tenth-largest moon in the entire Solar System.

Scientists believe Oberon formed from a disk of material that surrounded Uranus when the planet was born. The moon is made up of about equal parts ice and rock, with a rocky core and an icy mantle. There may even be a layer of liquid water where the mantle meets the core. The surface of Oberon is dark and reddish, shaped mainly by impacts from asteroids and comets. It is covered with many large craters, some as wide as 210 kilometers.

We have only seen Oberon up close once, when the spacecraft Voyager 2 flew by in January 1986. During this visit, Voyager 2 took pictures that allowed scientists to map about 40% of Oberon's surface.

Discovery and naming

Oberon was discovered by William Herschel on January 11, 1787, the same day he found Uranus's largest moon, Titania. For many years, only Herschel could see these moons, though today they can be observed with a good amateur telescope.

All of Uranus's moons are named after characters from the works of William Shakespeare or Alexander Pope. Oberon’s name comes from the King of the Fairies in A Midsummer Night's Dream. The names were suggested by Herschel’s son John in 1852, after William Lassell discovered two more moons, Ariel and Umbriel.

Orbit

Oberon orbits Uranus at a distance of about 584,000 km, making it the farthest of its five major moons. Its orbit has a small orbital eccentricity and inclination and takes around 13.5 days to complete. Oberon is tidally locked, meaning one face always points toward Uranus. Because Uranus spins on its side, Oberon and the other moons go through extreme seasons, with each pole experiencing 42 years of darkness followed by 42 years of sunlight.

Composition and internal structure

Oberon is the second-largest moon of Uranus and has a density that suggests it is made of about equal parts water ice and rocky material. Observations show that the surface of Oberon has water ice, especially on one side more than the other, which is different from other moons of Uranus.

Scientists think Oberon may have a rocky core surrounded by an icy layer. If there is enough ammonia in the ice, Oberon might have a thin layer of liquid water deep inside. However, much about Oberon's interior remains unknown.

Surface features and geology

Oberon has a dark, red surface that looks different depending on where you look. Some parts are redder than others because of tiny space particles hitting it over time. Its surface is covered with many craters from old impacts, showing it is one of the oldest moons around Uranus. The biggest crater, named Hamlet, is about 206 kilometers wide.

Besides craters, Oberon also has long, deep valleys called canyons. These valleys are thought to be cracks that formed when the moon’s icy surface expanded a little. Scientists are still unsure what makes some darker spots on its surface.

Origin and evolution

Oberon likely formed from a disc of gas and dust around the planet Uranus, either after Uranus was formed or from a big collision. This disc may have had less water and more rocky material than the discs around other planets, which explains why Oberon is denser.

During its formation, impacts heated Oberon’s outer layer, and later, radioactive elements inside it kept the interior warm. This heat may have melted ice inside Oberon, possibly creating a layer of liquid water rich in ammonia near its center. Over time, as the heat faded, the interior cooled and contracted, which might have caused cracks and canyons on its surface. Scientists are still learning about Oberon’s history and structure.

Exploration

Main article: Exploration of Uranus

The only close-up pictures of Oberon came from the Voyager 2 probe during its flyby of Uranus in January 1986. The best images show details about 6 kilometers across, but they only cover 40% of the moon's surface. At that time, Oberon's southern hemisphere faced the Sun, so its dark northern side could not be seen. No other spacecraft has visited the moons of Uranus.