Comparative global albedo and color maps of the Uranian satellites

The large Uranian satellites comprise a unique class of dark, grayish objects. They exhibit important differences in albedo and color among each other and on their individual surfaces. Except for Umbriel, albedo variegations of up to a factor of 2 occur on their surfaces, and in the case of Ariel, Titania, and Oberon these changes are correlated with high albedo impact features. The bright regions of each satellite differ in the extent to which their color is different from the surrounding terrain: these areas of Miranda, Titania, and Oberon are clearly bluer, but morphologically similar areas on Ariel show no clear differences in color. This result suggests compositional differences on the satellites' surfaces. Specifically, Ariel's upper mantle contains a larger fraction of slightly redder material. Although Umbriel has albedo variegations of ≤10%, its color variegations—which are ≥20%—are equal to or greater than those of the other four bodies. The satellites tend to become redder as the distance from Uranus increases. The largest internal color differences are exhibited by a leading/trailing dichotomy: the leading side of at least the outer four satellites is redder by 2-23% than the trailing side. The magnitude of this dichotomy also increases with the distance from the primary. The color measurements are not consistent with a surficial alteration process involving magnetospheric interactions. Rather it appears that the optical properties of these satellites are affected by the accretion of low albedo reddish meteoritic dust which may be common in the outer Solar System. The specific source of this material in the Uranian system may be undiscovered retrograde satellites orbiting outside the region of Oberon.