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What evidence suggests that volcanic fluids once flowed on Uranus's major moons?

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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.

Evidence suggesting that volcanic fluids once flowed on the major moons of Uranus includes the presence of flat deposits characterized by lobed edges and surface ripples, which are indicative of fluid flow. Given the extremely low temperatures of the outer solar system, scientists believe these erupting fluids were likely a mixture of water and ammonia, which possesses a lower melting point than pure water ice.1

Additionally, observations of Ariel reveal long, valleylike formations that appear partially filled with material that may have upwelled from the moon's interior due to past tectonic activity. In some instances, this material seems to have spread across broad plains, resembling glacier flows on Earth. These features, combined with a relative lack of large impact craters, suggest that Ariel possesses the youngest surface among the major Uranian moons.2