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Why do the major moons of Uranus have dark, dirty ice surfaces?

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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 surfaces of the major moons of Uranus—Miranda, Ariel, Umbriel, Titania, and Oberon—are composed of "dirty" water ice. While water ice is present in their surface spectra, their low reflectivity compared to pure ice indicates the presence of a dark, non-ice component.1

The exact nature of this dark material remains unknown, but it is characterized by a neutral gray color, which rules out iron-bearing minerals that would otherwise produce a reddish tinge. One leading theory suggests the dark substance is carbon. This carbon may originate from within the moons themselves or from Uranus’s ring system. In this process, methane gas could be released and subsequently decomposed into solid carbon when bombarded by solar ultraviolet light and charged particles.1