Oberon (moon)

Oberon is the outer and second biggest major moon of the planet Uranus. It was found by William Herschel in 1787. It is named after a fairy king in Shakespeare’s play A Midsummer Night's Dream. Oberon is the second heaviest of the Uranian moons and the tenth biggest moon in the Solar System.

Scientists think Oberon formed from a ring of material around Uranus when the planet was first made. It is made of about half ice and half rock, with a rocky middle and an icy outer layer. There might also be a layer of liquid water where the ice and rock meet. Oberon’s surface is dark and reddish, and it has many big craters from collisions with asteroids and comets.

The only time we saw Oberon close up was in January 1986, when the spacecraft Voyager 2 flew by and took pictures. These pictures helped scientists 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 with his telescope. Today, they can be seen with a good amateur telescope from Earth.

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

Orbit

Oberon travels around Uranus at a distance of about 584,000 km. It is the farthest of the five main moons. Its path around Uranus is almost circular and stays close to the planet's equator. Oberon takes about 13.5 days to complete one orbit and always shows the same face to Uranus.

Because Uranus spins on its side, its moons have long periods of darkness and sunlight. Each pole of Uranus and its moons can stay in complete darkness for 42 years, then in continuous sunlight for another 42 years. In 2007, Oberon passed in front of another moon, helping scientists learn more about it.

Composition and internal structure

Oberon is the second-largest moon of the planet Uranus and one of the biggest moons in the Solar System. It is made of water ice and denser materials like rock and organic compounds. The surface of Oberon has water ice, more on one side than the other.

Scientists think Oberon may have a rocky core with an icy layer around it. If there is enough ammonia or other substances in the ice, Oberon might have a thin layer of liquid water deep inside. However, we still do not know enough about Oberon’s history to be sure about its internal structure.

Surface features and geology

Oberon is one of the darkest large moons of Uranus. Its surface looks different depending on the angle from which we see it. It is usually red, but some areas where impacts have recently hit look more neutral or slightly blue. The surface of Oberon is covered with many craters, showing it is very old. The largest crater, called Hamlet, is about 206 kilometers across. Bright lines of ice surround many of these craters.

Besides craters, Oberon also has deep canyons called chasmata. These canyons are huge cracks in the ice crust of the moon, formed when the surface expanded slightly. The most noticeable canyon is named Mommur Chasma. Scientists are still unsure what causes the dark patches on Oberon's surface.

Origin and evolution

Oberon probably formed from a disc of gas and dust around the planet Uranus. This disc may have had less water than the moons of Saturn, but it might have had carbon and nitrogen in forms like carbon monoxide and N₂. This could explain why Oberon and other moons of Uranus are denser than Saturn’s moons.

As Oberon formed, impacts made heat in its outer layers. After it formed, its inside kept heating up because some elements in its rocks were breaking down. This heating and cooling made stress on Oberon’s surface. This stress may have caused cracks, which could have made the canyons we see today. Scientists think there may have once been a layer of liquid water deep inside Oberon, but we still know very little about its past. Recent studies suggest that bigger moons of Uranus might still have oceans under their surfaces.

Exploration

Main article: Exploration of Uranus

The only close-up pictures of Oberon come from the Voyager 2 probe. It flew by Uranus in January 1986 and took photos from about 470,600 kilometers away. These pictures show details as small as 6 kilometers across. They cover about 40% of Oberon’s surface, but only 25% was clear enough to study. During this visit, the moon’s southern side faced the Sun, so we could not see its dark northern side. No other spacecraft has ever gone near Uranus or its moons.