Umbriel is the third largest of the Uranus moons, and the 13th largest moon in the Solar System.
Discovery
Umbriel, along with another Uranian satellite, Ariel, was discovered by William Lassell on October 24, 1851.
Naming
Umbriel is named after the 'dusky melancholy sprite' in Alexander Pope's poem The Rape of the Lock, and the name suggests the Latin umbra, meaning shadow.
Stats
Diameter: 1,169 km
Semi-major axis: 266,300 km
Orbital Period: 4.14 days
Formation
Umbriel is thought to have formed from an accretion disc or subnebula; a disc of gas and dust that either existed around Uranus for some time after its formation or was created by the giant impact that most likely gave Uranus its large obliquity.
Orbit
Umbriel's orbital period is around 4.1 Earth days, coincident with its rotational period. In other words, Umbriel is a synchronous or tidally locked satellite, with one face always pointing toward its parent planet.
Umbriel's orbit lies completely inside the Uranian magnetosphere. This is important, because the trailing hemispheres of airless satellites orbiting inside a magnetosphere (like Umbriel) are struck by magnetospheric plasma, which co-rotates with the planet.
This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon.
Composition
Umbriel consists of water ice, while a dense non-ice component constitutes around 40% of its mass. The latter could be made of rock and carbonaceous material including heavy organic compounds known as tholins.
The presence of water ice is supported by infrared spectroscopic observations, which have revealed crystalline water ice on the surface of the moon. Water ice absorption bands are stronger on Umbriel's leading hemisphere than on the trailing hemisphere.
The cause of this asymmetry is not known, but it may be related to the bombardment by charged particles from the magnetosphere of Uranus, which is stronger on the trailing hemisphere. The energetic particles tend to sputter water ice, decompose methane trapped in ice as clathrate hydrate and darken other organics, leaving a dark, carbon-rich residue behind.
Life
The current state of Umbriel's icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.
Exploration Status
So far the only close-up images of Umbriel 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 Umbriel, and no mission is planned in the foreseeable future.
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