Probing the regoliths of the classical Uranian satellites: Are their surfaces mantled by a layer of tiny H₂O ice grains?

We investigate whether the surfaces of the classical moons of Uranus are compositionally stratified, with a thin veneer of mostly tiny H₂O ice grains (≤2 μm diameters) mantling a lower layer composed of larger grains of H₂O ice, dark material, and CO₂ ice (~10–50 μm diameters). Near-infrared observations (~1–2.5 μm) have determined that the H₂O ice-rich surfaces of these moons are overprinted by concentrated deposits of CO₂ ice, found almost exclusively on their trailing hemispheres. However, best fit spectral models of longer wavelength datasets (~3–5 μm) indicate that the spectral signature of CO₂ ice is largely absent, and instead, the exposed surfaces of these moons are composed primarily of tiny H₂O ice grains. To investigate possible compositional layering of these moons, we have collected new data using the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope (~3–5 μm). Spectral modeling of these new data is consistent with prior analyses, suggesting that the exposed surfaces of the Uranian moons are primarily composed of tiny H₂O ice grains. Furthermore, analysis of these new data reveal that the trailing hemispheres of these moons are brighter than their leading hemispheres over the 3 to 5 μm wavelength range, except for Miranda, which displays no hemispherical asymmetries in its IRAC albedos. Our analyses also reveal that the surface of Ariel displays five distinct, regional-scale albedo zones, possibly consistent with the spatial distribution of CO₂ ice on this moon. We discuss possible processes that could be enhancing the observed leading/trailing albedo asymmetries exhibited by these moons, as well as processes that could be driving the apparent compositional stratification of their near surfaces.