Oberon (moon)

Oberon is the outermost and second-largest major moon of the planet Uranus. It is also the second-most massive of the Uranian moons. In the context of our entire Solar System, it ranks as the tenth-largest moon.

This moon is quite distant from its parent planet. It orbits Uranus at a distance of approximately 584,000 km. This makes it the farthest of the five major moons in the Uranian system. Because of its distance, its orbit lies partially outside the magnetosphere of Uranus. A magnetosphere is a magnetic bubble that surrounds a planet.

Scientists believe Oberon formed from an accretion disk. This was a disk of gas and dust that surrounded Uranus shortly after the planet formed. The moon's composition appears to be roughly equal parts water ice and rock. This density of 1.68 g/cm³ suggests it is more substantial than many moons of Saturn. It is likely differentiated into distinct layers. This means different materials have separated into layers based on their weight. The moon may have a rocky core with a radius of about 480 km. This core likely contains about 54% of the moon's total mass. Surrounding this core is an icy mantle.

There is a possibility of a liquid ocean inside the moon. This ocean might exist at the boundary between the rocky core and the icy mantle. If it exists, the ocean could be up to 40 km thick. The temperature there might be around 180 K. Scientists think substances like ammonia might act as an antifreeze to keep the water liquid. The internal structure of Oberon depends on its thermal history, which is still being studied. Some researchers believe that large moons like Uranus's may have active subterranean oceans. This would mean there is liquid water hidden deep beneath the icy surface.

Oberon's surface is dark and has a slightly red color. It is actually the reddest of the major Uranian moons. The surface is covered in many impact craters from asteroids and comets. These craters reach diameters of up to 210 km. The largest known crater is named Hamlet, and it is 206 km wide.

Because it has so many craters, scientists believe Oberon has the most ancient surface among the moons of Uranus. The surface density of craters is so high that it approaches saturation. This happens when new impacts destroy older ones at a similar rate. Many large craters, like Othello and Macbeth, have floors made of very dark material.

In addition to craters, the surface features deep canyons called chasmata. These are long, steep-sided depressions. The most prominent canyon is named Mommur Chasma. These canyons were formed by crustal extension. This means the moon's outer crust stretched and cracked. This happened because the interior of the moon expanded during its early evolution. The expansion was about 0.5% of the moon's size. This geological activity was an endogenic process, meaning it came from within the moon. These cracks helped shape the moon's appearance alongside the external impacts.

We learned much about Oberon thanks to the Voyager 2 spacecraft. It flew past the moon in January 1986. This was the closest approach to the moon. The spacecraft took many images that allowed scientists to map 40% of the surface.

Before this mission, the Uranian system had not been studied up close. Oberon is also tidally locked to Uranus. This means its rotational period matches its orbital period of about 13.5 days. Consequently, one face of the moon always points toward the planet.

William Herschel discovered Oberon on January 11, 1787. On that same day, he also discovered Titania, the largest moon of Uranus. The names for the moons were suggested later in 1852. John Herschel, William's son, suggested names from the works of William Shakespeare. Oberon is named after the King of the Fairies in the play *A Midsummer Night's Dream*. Most surface features follow this theme. For example, craters are named after characters like Romeo, Lear, and Falstaff. This connects the study of space to the history of literature.