What are the surfaces of Uranus's major moons 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 surfaces of Uranus's five major moons—Miranda, Ariel, Umbriel, Titania, and Oberon—are composed of "dirty water ice."1

Because the reflectivity of these moons is lower than that of pure ice, it is inferred that the ice is mixed with a dark, neutral-gray material. While the exact composition of this dark component remains unknown, it is not thought to be iron-bearing minerals, as those would produce a reddish tinge. One possibility is that the material is carbon derived from the moons' interiors or from Uranus's rings, potentially produced when methane gas is decomposed by charged particles and solar ultraviolet light.1

Additionally, observations indicate that these surfaces are porous and highly insulating, characterized by loosely stacked particles. The surfaces also feature volcanic deposits, which are believed to have originated from a fluid mixture of water and ammonia, as well as riftlike canyons resulting from crustal expansion.1