Umbriel

Umbriel is the third-largest moon of Uranus. It was discovered on October 24, 1851, by William Lassell at the same time as its neighboring moon Ariel. Umbriel was named after a character in Alexander Pope's 1712 poem The Rape of the Lock.

This moon is made mostly of ice mixed with a lot of rock. Scientists think Umbriel may have a rocky core surrounded by an icy mantle. Its surface is the darkest of all Uranus's moons and is covered with many impact craters, some as wide as 210 kilometers. The most noticeable feature is a bright ring of material inside a crater called Wunda.

Umbriel, like the other moons of Uranus, likely formed from a ring of material that surrounded the planet when it was young. The only time we got a close look at Umbriel was in January 1986, when the spacecraft Voyager 2 flew by and took pictures. These pictures helped scientists map about 40% of Umbriel's surface.

Discovery and name

Umbriel, along with another moon called Ariel, was discovered by William Lassell on October 24, 1851. All of Uranus’s moons are named after characters from stories by William Shakespeare or Alexander Pope. Umbriel is named after a character in Alexander Pope’s poem The Rape of the Lock. The name connects to the Latin word umbra, which means 'shadow'. The moon is also called Uranus II.

Orbit

Umbriel orbits Uranus at a distance of about 266,000 km (165,000 mi), making it the third farthest of its five major moons. Its orbit has a small stretch and tilts very little compared to Uranus's equator. Umbriel takes about 4.1 Earth days to go around Uranus, and it always shows the same face to the planet because its rotation matches its orbit.

Because Uranus spins on its side, its moons, including Umbriel, go through big changes in light. Each pole can be dark for 42 years and then get constant sunlight for another 42 years. When Voyager 2 flew by in 1986, it was summer in the southern part of Umbriel, and the north was mostly dark.

Composition and internal structure

Umbriel is the third-largest moon of Uranus. It is made mostly of water ice, with about 40% of its mass being rocky and carbonaceous material, including heavy organic compounds. Observations using infrared light show that the surface has crystalline water ice, which appears more strongly on one side of the moon than the other. Scientists think this difference might be due to particles from Uranus’s magnetic field affecting the moon’s surface.

Besides water ice, small amounts of carbon dioxide have been found, mostly on one side of Umbriel. This carbon dioxide might come from materials on the moon’s surface or from gases that escaped from inside the moon in the past. Umbriel might have a rocky core surrounded by an icy layer, but the exact state of its interior is still unknown.

Surface features

Umbriel, one of the moons of Uranus, has a very dark surface that reflects only a little light. It is darker than its neighbor Ariel and has fewer bright spots. The surface is mostly covered with many craters, which are deep round pits caused by impacts from space rocks. The largest known crater on Umbriel is called Wokolo, and it is huge—about 210 kilometers across! Another big crater named Wunda has a bright spot in the middle, which might be made of frozen carbon dioxide.

Besides craters, Umbriel also has some long, deep valleys or canyons that run across its surface. These features show that Umbriel has not changed much in a very long time. Scientists think its surface might be covered by a thin layer of dark material, which makes it look so uniform and dark.

Origin and evolution

Umbriel, one of the moons of Uranus, is believed to have formed from a disc of gas and dust around Uranus. This disc may have been created after a big impact changed Uranus’s tilt. The moons of Uranus, including Umbriel, are denser than those of Saturn, suggesting they formed in a space with less water and more rock.

When Umbriel was forming, impacts created heat in its outer layers, reaching about 180 K about 3 kilometers deep. After formation, the surface cooled while the inside stayed warm due to natural processes in its rocks. This caused stretching in the moon’s crust, possibly leading to cracks. Any activity inside Umbriel likely stopped billions of years ago.

Exploration

Further information: Exploration of Uranus

The only close-up pictures we have of Umbriel come from the Voyager 2 probe. It took these photos when it flew by Uranus in January 1986. The closest Voyager 2 got to Umbriel was about 325,000 kilometers, so the pictures show details as small as about 5.2 kilometers across. These images cover around 40% of Umbriel's surface, but only about 20% was clear enough to study in detail. During this flyby, the moon's southern side faced the Sun, so we could not see its northern, darker side.