2nd Largest Moon of Uranus - Oberon (18th Moon outwards from Uranus)

Oberon is the second largest of the Uranus moons and the tenth largest moon in the Solar System.

Discovery

Oberon was spotted by Sir William Herschel on January 11, 1787, six years after he had discovered the planet itself, on the same day he discovered Uranus's largest moon, Titania.

Naming

Oberon is named after the mythical king of the fairies who appears as a character in Shakespeare's A Midsummer Night's Dream.

Stats

Diameter: 1,522 km

Semi-major axis: 583,520 km

Orbital Period: 13.46 days

Orbit

Oberon orbits Uranus at a distance of about 583,520 km, being the farthest from the planet among its five major moons. Its orbital period is around 13.5 days, coincident with its rotational period. In other words, Oberon is a synchronous satellite, tidally locked, with one face always pointing toward the planet.

Oberon spends a significant part of its orbit outside the Uranian magnetosphere. As a result, its surface is directly struck by the solar wind.

Formation

Oberon probably formed from an accretion disk that surrounded the planet just after its formation.

The initial accretional heating together with continued decay of radioactive elements were probably strong enough to melt the ice if some antifreeze like ammonia (in the form of ammonia hydrate) or some salt was present.

Further melting may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle. A layer of liquid water ('ocean') rich in dissolved ammonia may have formed at the core–mantle boundary.

The eutectic temperature of this mixture is 176 K. If the temperature dropped below this value the ocean would have frozen by now. Freezing of the water would have led to expansion of the interior, which may have also contributed to the formation of canyon-like graben. Still, present knowledge of the evolution of Oberon is very limited.

Exploration Status

So far the only close-up images of Oberon have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986.

No other spacecraft has ever visited the Uranian system or Oberon, and no mission is planned in the foreseeable future.