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What are the particles in Uranus's rings made of?

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Images obtained by Voyager 2. — Uranus Two views of the southern hemisphere of Uranus, produced from images obtained by Voyager 2 on Jan. 17, 1986. In colours visible to the unaided human eye, Uranus is a bland, nearly featureless sphere (left). In a colour-enhanced view processed to bring out low-contrast details, Uranus shows the banded cloud structure common to the four giant planets (right). From the polar perspective of Voyager at the time, the bands appear concentric around the planet's rotational axis, which is pointing nearly toward the Sun. Small ring-shaped features in the right image are artifacts arising from dust in the spacecraft's camera.

The rings of Uranus are composed primarily of large particles, with objects measuring greater than 140 cm (4.6 feet) across. In addition to these larger components, there are smaller dust particles in the micrometer size range, though these are found in only small amounts within the main rings and are mostly distributed in the spaces between them.1

The particles have a nearly flat reflectance spectrum, appearing gray in color, and reflect less than 5 percent of incident sunlight. Their composition is not definitively known, but they are often compared to ordinary soot, which is mostly carbon. Scientists have hypothesized that this carbon may be intrinsic to the particles or result from the darkening of methane caused by particle bombardment. Because the rings are losing mass through collisions and are subject to atmospheric drag, the material is thought to be short-lived, requiring rapid and continual replenishment.1