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 ring of material that surrounded Uranus when the planet was just being created. It is made up of about equal parts ice and rock, with a rocky center and an icy outer layer. There may even be a layer of liquid water where the ice and rock meet. The surface of Oberon is dark and reddish, and it is covered with many large craters made by collisions with asteroids and comets.

The only time we have seen Oberon up close was in January 1986, when the spacecraft Voyager 2 flew by and took pictures. These images 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 after this, no one else could see these moons except with Herschel's telescope, although today they can be spotted 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, making it 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 experience 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. During a special event in 2007, Oberon passed in front of another moon, allowing scientists to learn more about it.

Composition and internal structure

Oberon is the second-largest moon of the planet Uranus and the ninth-most massive moon in the Solar System. It is made up of about equal parts water ice and denser materials like rock and organic compounds. 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 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 certain about its internal structure.

Surface features and geology

Oberon is one of the darkest large moons of Uranus. Its surface changes in brightness depending on the angle from which we look at it. It is usually red, but some areas where impacts have recently hit appear 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, with ideas ranging from ancient volcanic activity to material brought up by impacts.

Origin and evolution

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

As Oberon formed, impacts created heat in its outer layers. After formation, the interior continued to heat up due to the decay of certain elements in its rocks. This heating and cooling process caused stress in Oberon’s surface, leading to cracks that may have formed the canyons we see today. Scientists think there may have been a layer of liquid water deep inside Oberon, but today we know very little about its past. Recent studies suggest that larger moons of Uranus might still have oceans beneath their surfaces.

Exploration

Main article: Exploration of Uranus

The only close-up pictures we have of Oberon come from the Voyager 2 probe. It flew by Uranus in January 1986 and took photos when it was about 470,600 kilometers away. These pictures show details as small as 6 kilometers across. They cover around 40% of Oberon’s surface, but only 25% was clear enough to study its features. 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.