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Umbriel is one of Uranus’ moons discovered by the astronomer William Lassell on October 24, 1851. Lassell discovered Ariel, a fellow moon of Uranus, at the same time. All of Uranus’ 27 moons are named after characters from Alexander Pope’s The Rape of the Lock and plays by William Shakespeare. These names were suggested by John Herschel, the son of William Herschel – a famous astronomer who discovered Titania and Oberon. Umbriel is the name of a sprite in The Rape of the Lock.
Umbriel is one of Uranus’ five major moons; the others are Titania, Ariel, Oberon, and Miranda. Ariel and Umbriel are nearly the same size with diameters of 1,158 kilometers and 1,170 kilometers respectively. Umbriel’s diameter is about one-third the size of the Earth’s diameter. Umbriel is the third largest moon of Uranus, but it is only the fourth largest in terms of mass. Scientists believe that most of the planet is water ice with about 40% of it composed of some dense material, such as rock.
Umbriel has the darkest surface of any of Uranus’ moons. The satellite’s bond albedo – which is basically the amount of electromagnetic radiation, or light, that gets reflected back from the surface – is very low. Scientists have only discovered water and carbon dioxide on the satellite’s surface as of yet.
It takes the moon approximately 4.1 days to orbit Uranus. Since it is in a synchronous orbit with the planet it also takes that long for it to complete one rotation. The satellite is an average of 266,000 kilometers from its planet. Uranus is the only planet in the Solar System to rotate on its side, causing severe seasonal changes. Because they are in the planet’s equatorial plane, Uranus’ satellites also experience these changes. Umbriel’s north and south poles spend 42 years in light and then 42 years in darkness before repeating the cycle.
The only surface features that scientists have recognized on the satellite are craters. Oberon is the only one of Uranus’ moons with more craters than Umbriel. The largest known crater on Umbriel, Wokolo, has a diameter of 210 kilometers. Wunda, a crater with a diameter of about 131 kilometers, is the most noticeable surface feature. It has a ring of bright material, which scientists think are from the impact, on the floor of the crater. Other craters include Fin, Peri, and Zlyden. Umbriel’s surface features are named after dark sprites from different cultures’ mythology.
Astronomy Cast has an episode on Uranus that you should check out.