Umbriel is one of the major moons of Uranus and the darkest of the planet’s large satellites, discovered in 1851 by the astronomer William Lassell. It has a diameter of about 1,170 kilometers and is composed mainly of ice mixed with rock, giving it a dense, ancient surface that has remained largely unchanged for billions of years. Umbriel’s surface is heavily cratered and extremely dark, reflecting very little sunlight due to a coating of carbon-rich material or dust that may have settled from Uranus’s outer moons. One of its most distinctive features is a bright, mysterious ring inside the crater Wunda, which stands out sharply against the moon’s otherwise blackened landscape. Umbriel orbits Uranus at a distance of about 266,000 kilometers and is tidally locked, meaning the same side always faces the planet. Because only the Voyager 2 spacecraft has ever flown past Uranus—and only briefly in 1986—Umbriel remains poorly understood, with many of its geological features, origins of its dark surface, and internal structure still unknown. Its name, like many of Uranus’s moons, comes from literature: Umbriel is a gloomy spirit from Alexander Pope’s poem The Rape of the Lock, fitting its shadowy appearance.