Umbriel

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

This moon is made mostly of ice mixed with rock. Scientists think Umbriel may have a rocky core surrounded by an icy mantle. Its surface is very dark and has many impact craters, some as wide as 210 kilometers. The most noticeable feature is a bright ring inside a crater called Wunda.

Umbriel, like the other moons of Uranus, likely formed from a ring of material around the planet when it was young. The only time we saw Umbriel up close was in January 1986, when the spacecraft Voyager 2 flew by and took pictures. These pictures helped scientists map part of Umbriel's surface.

Discovery and name

Umbriel was found by William Lassell on October 24, 1851, along with another moon called Ariel. 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 comes from the Latin word umbra, meaning 'shadow'. The moon is also called Uranus II.

Orbit

Umbriel orbits Uranus at a distance of about 266,000 km (165,000 mi). It is the third farthest of Uranus's 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. 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 might be due to particles from Uranus’s magnetic field.

Besides water ice, small amounts of carbon dioxide have been found on one side of Umbriel. This 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 we do not yet know its exact interior.

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 craters, which are deep round pits caused by space rocks hitting it. The largest known crater on Umbriel is called Wokolo, and it is very big—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 is one of the moons of Uranus. It is thought to have formed from a disc of gas and dust around Uranus. This disc may have appeared after a big impact changed Uranus’s tilt. The moons of Uranus, including Umbriel, are denser than those of Saturn. This suggests they formed in a place with less water and more rock.

When Umbriel was forming, impacts created heat in its outer layers. After it formed, the surface cooled but the inside stayed warm due to natural processes in its rocks. This may have caused stretching in the moon’s crust and possibly cracks. Any activity inside Umbriel likely stopped billions of years ago.

Exploration

Further information: Exploration of Uranus

The only close-up pictures of Umbriel come from the Voyager 2 probe. It took these pictures when it flew by Uranus in January 1986. Voyager 2 came within about 325,000 kilometers of Umbriel. 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 well. During this flyby, the moon's southern side faced the Sun. So we could not see its northern, darker side.