Umbriel

Umbriel (/ˈʌmbriəl/) 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. The moon was named after a character in Alexander Pope's 1712 poem The Rape of the Lock. Made mostly of ice mixed with rock, Umbriel may have a rocky core surrounded by an icy mantle.

Its surface is one of the darkest among Uranus's moons and is covered with many large impact craters, some as wide as 210 kilometers (130 miles). One famous feature is a bright ring of material inside a crater called Wunda. Scientists believe Umbriel formed from a ring of material that surrounded Uranus when the planet was young, a process called an accretion disk.

The only time scientists got a close look at Umbriel was in January 1986, when the spacecraft Voyager 2 flew by and took pictures. These images helped scientists map about 40% of the moon's surface. Even though Umbriel looks quiet and old today, some deep valleys suggest it may have had an active past.

Discovery and name

Umbriel, along with another moon called Ariel, was discovered by William Lassell on October 24, 1851. The moons of Uranus are named after characters from the works of William Shakespeare and Alexander Pope. Umbriel is named after a character in Pope's poem The Rape of the Lock. The name connects to the Latin word umbra, meaning '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 made mostly of water ice with a dense non-ice part that could be rock and carbonaceous material. Scientists have seen this water ice on the moon’s surface using special tools.

Besides water, the only other thing found on Umbriel’s surface is carbon dioxide. This is mostly on the side of the moon that faces away from Uranus. Scientists think it might come from materials deep inside the moon or from radiation from Uranus. Umbriel might have a rocky core surrounded by an icy mantle, but its exact structure is still a mystery.

Surface features

Umbriel, one of Uranus's moons, has a very dark surface that reflects only a little light. It is darker than its neighbor Ariel and shows less brightness. Scientists have found mostly craters on Umbriel, with the largest one, Wunda, being about 210 kilometers across. Unlike some other moons, Umbriel does not show much geological activity.

The surface of Umbriel also features some long, narrow valleys and dark shapes called polygons. These might have formed a long time ago when the moon was more active inside. Despite its dark appearance, there are bright spots inside craters like Wunda, possibly made of frozen carbon dioxide. The exact reason why Umbriel looks so dark and uniform remains a mystery.

Origin and evolution

Umbriel, the third-largest moon of Uranus, likely formed from a disc of gas and dust that surrounded Uranus after its creation. This disc may have been low in water but contained elements like nitrogen and carbon. Over thousands of years, impacts and the decay of radioactive elements shaped Umbriel's structure, possibly leading to a rocky core and an icy mantle.

The moon's early heating might have caused melting inside, forming a layer of liquid water, but this ocean probably froze a long time ago. Umbriel experienced less reshaping than other moons of Uranus, showing how ancient and unchanged it remains today.

Exploration

Further information: Exploration of Uranus

The only close-up images of Umbriel were taken by the Voyager 2 probe during its flyby of Uranus in January 1986. The closest distance between Voyager 2 and Umbriel was 325,000 km, resulting in images with a resolution of about 5.2 km. These images covered around 40% of Umbriel's surface, though only 20% was clear enough for detailed study. During this time, the southern hemisphere of Umbriel was facing the Sun, so the northern, darker hemisphere could not be examined.