Umbriel

Umbriel (/ˈʌmbriəl/) 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. The moon was named after a character in Alexander Pope's poem The Rape of the Lock. It is made mostly of ice mixed with rock, and may have a rocky core surrounded by an icy mantle.

Its surface is very dark and has many large impact craters, some as wide as 210 kilometers (130 miles). One well-known feature is a bright ring of material inside a crater called Wunda. Scientists think Umbriel formed from a ring of material around Uranus when the planet was young, in a process called an accretion disk.

The only time scientists saw Umbriel up close was in January 1986, when the Voyager 2 spacecraft flew by and took pictures. These pictures helped scientists map about 40% of the moon's surface. Even though Umbriel looks quiet and old now, some deep valleys hint that it may have been active in the past.

Discovery and name

Umbriel was found by William Lassell on October 24, 1851, along with another moon called Ariel. The moons of Uranus have names from the stories of William Shakespeare and Alexander Pope. Umbriel is named after a character in Pope's poem The Rape of the Lock. The name comes from the Latin word umbra, which means 'shadow'.

Orbit

Umbriel orbits Uranus at a distance of about 266,000 km. It is the third farthest of the five major moons from the planet. Its orbit has a small eccentricity and is only slightly inclined to the equator of Uranus. Umbriel takes about 4.1 Earth days to complete one orbit, which is the same as its rotational period. This means one side of Umbriel always faces Uranus.

Umbriel is inside the Uranian magnetosphere, which affects its surface. The side of Umbriel that trails behind as it orbits is bombarded by charged particles from the magnetosphere, which may darken that part of the moon. Because Uranus spins on its side, its moons experience extreme seasons, with each pole spending 42 years in darkness and then 42 years in continuous sunlight.

Composition and internal structure

Umbriel is the third-largest moon of Uranus. It is mostly made of water ice with some rock and carbonaceous material. Scientists can see this water ice on the moon’s surface with special tools.

Besides water, the only other thing found on Umbriel’s surface is carbon dioxide. It is mostly on the side of the moon that faces away from Uranus. Scientists think it may come from deep inside the moon or from radiation from Uranus. Umbriel might have a rocky core surrounded by an icy mantle, but we do not yet know its exact structure.

Surface features

Umbriel is one of Uranus's moons. It has a very dark surface that reflects only a little light. It is darker than its neighbor Ariel. Scientists have found craters on Umbriel. The largest crater, Wunda, is about 210 kilometers across. Umbriel does not show much geological activity.

The surface also has long, narrow valleys and dark shapes called polygons. These might have formed long ago when the moon was more active inside. There are bright spots inside craters like Wunda. These spots might be made of frozen carbon dioxide. Why Umbriel looks so dark and uniform is still a mystery.

Origin and evolution

Umbriel is the third-largest moon of Uranus. It probably formed from a disc of gas and dust around Uranus after the planet was created. This disc may have had little water but included elements like nitrogen and carbon. Over many years, impacts and the decay of radioactive elements shaped Umbriel's structure. It may have a rocky core and an icy mantle.

Early heating might have caused melting inside, creating a layer of liquid water. But this ocean likely froze a long time ago. Umbriel changed less than other moons of Uranus, showing how old and unchanged it is today.

Exploration

Further information: Exploration of Uranus

The only close-up pictures of Umbriel were taken by the Voyager 2 probe when it flew by Uranus in January 1986. Voyager 2 came within 325,000 km of Umbriel. This gave us images that show details about 5.2 km across. These images covered about 40% of Umbriel's surface. However, only 20% of the surface was clear enough to study well. At that time, Umbriel's southern hemisphere was facing the Sun. This meant the northern, darker hemisphere could not be seen.