Umbriel

What is Umbriel?

Moons· Diameter 1,169 km. The darkest of Uranus's moons — an old, crater-blanketed surface with one puzzling bright ring, Wunda crater, standing out. It is Ariel's twin in size and its opposite in character. Find its orbit between Ariel and Titania in the orbit view.

The orbit of Umbriel at this moment: https://nowsky.org/solar/orbit?fw=moon:uranus:umbriel&sp=0.006 — replayed live, no login.

Orbit and size

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Parent planet and sibling moons

Frequently asked

How big is Umbriel?

1,169 km.

How far is Umbriel from Earth?

266,000 km.

What is notable about Umbriel?

bright ring in Wunda crater.

Which planet does Umbriel orbit?

Uranus.

What darkened Umbriel

Umbriel reflects only 16 percent of its sunlight, the darkest of the Uranian moons and half as bright as its equal-sized neighbour Ariel. The surface material is the same water ice; the difference is put down to the amount of dark impurity mixed in — carbonaceous dust or organics altered by radiation. With no geological activity to expose fresh ice, four billion years of accumulated darkening has stayed on the surface.

Its one bright feature, Wunda, is a 140 km bright ring on the floor of a 131 km crater. Bright ice exposed around the crater's central peak, or frost thrown out by the impact and piled in a ring, are the proposed explanations, but with a single low-resolution Voyager image nothing is settled. Umbriel's northern hemisphere, too, has never been observed.

The moon in depth

Umbriel was discovered by William Lassell in 1851 and named for the dusky sprite in Alexander Pope's The Rape of the Lock. True to its name it is the darkest of the Uranian moons, reflecting only 16% of sunlight. At 1,169 km it is nearly the same size as Ariel, but it orbits 266,000 km from Uranus every 4.14 days.

Voyager 2 found Umbriel to be the opposite of Ariel: an old surface crowded with large craters and almost no sign of geological activity, suggesting its interior cooled early. Its great mystery is Wunda, a 131-km crater near the equator whose floor holds a bright ring of material. It may be fresh ice exposed by the impact or a deposit of carbon dioxide frost, but no one knows for sure.

Track Uranus in the orbit view to find Umbriel circling outside Ariel and inside Titania. Why two moons of equal size differ so sharply — Ariel young and bright, Umbriel old and dark — is best explained by tidal heating: Ariel once passed through an orbital resonance that warmed it, while Umbriel never did.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.