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Near-infrared spectra of Umbriel and the other classical Uranian moons exhibit 2.2 μm absorption bands that could result from ammonia (NH3) bearing species, possibly exposed in the geologically recent past. However, Umbriel has an ancient surface with minimal evidence for recent endogenic activity, raising the possibility that more refractory species are present, and/or that NH3 is retained over long timescales. We analyzed 33 spectra of Umbriel to investigate its 2.2 μm band, along with three other absorption features we identified near 2.14, 2.22, and 2.24 μm. We assessed the subobserver longitudinal distributions of these four bands, finding that they are present across Umbriel and may be spatially associated with geologic features such as craters and large basins. We compared the bands to 15 candidate constituents. We found that Umbriel’s 2.14 μm and 2.22 μm bands are most consistent with the spectral signature of organics, its 2.24 μm band is best matched by NH3 ice, and its 2.2 μm band is consistent with the signatures of NH3–H2O mixtures, aluminum-bearing phyllosilicates, and sodium-bearing carbonates. However, some of these candidate constituents do not match Umbriel’s spectral properties in other wavelength regions, highlighting the gaps in our understanding of the Uranian moons’ surface compositions. Umbriel’s 2.14 μm band may alternatively result from a 2 ν 3 overtone mode of CO2 ice. If present on Umbriel, these candidate constituents could have formed in contact with an internal ocean and were subsequently exposed during Umbriel’s early history. Alternatively, these constituents might have originated in an undifferentiated crust or were delivered by impactors.

Uranus’s moon Miranda has a complex surface reflecting multiple episodes of activity. We estimated the heat flux near Inverness Corona, the youngest terrain on Miranda ( 100 − 100 + 400 Ma), to gain insight into recent endogenic resurfacing. We modeled flexure associated with Argier Rupes, which separates Inverness from the Cratered Terrain. Our results indicate an elastic thickness of 2.2–3.1 km and a heat flux of 35–140 mW m−2 in this region, assuming Miranda’s lithosphere is composed of pure H2O ice without porosity. Because the formation of the bounding flexure that we analyzed followed the formation of Argier Rupes, our results indicate that Miranda experienced high heat flux in the past 100 − 100 + 400 Ma. These results are also consistent with heating from a past resonance, possibly an Ariel–Umbriel 5:3 mean motion resonance, estimated to have generated heat fluxes >100 mW m−2 on Miranda. However, if Miranda instead has a porous lithosphere, our heat flux estimates are lower: 34–135 mW m−2 for 5% porosity, 29–114 mW m−2 for 15% porosity, and 20–81 mW m−2 for 25% porosity. Alternatively, if Miranda’s lithosphere includes NH3-hydrates, then our are estimates are even lower, 7–56 mW m−2 without porosity. These estimates decrease further when assuming NH3-hydrates and porosity: 7–54 mW m−2 for 5% porosity, 6–46 mW m−2 for 15% porosity, and 4–32 mW m−2 for 25% porosity. Better constraints on Miranda’s heat fluxes require more information on its ice shell properties.

CO2 ice is present on the trailing hemisphere of Ariel but is mostly absent from its leading hemisphere. The leading/trailing hemispherical asymmetry in the distribution of CO2 ice is consistent with radiolytic production of CO2, formed by charged particle bombardment of H2O ice and carbonaceous material in Ariel’s regolith. This longitudinal distribution of CO2 on Ariel was previously characterized using 13 near-infrared reflectance spectra collected at “low” sub-observer latitudes between 30°S and 30°N. Here we investigated the distribution of CO2 ice on Ariel using 18 new spectra: 2 collected over low sub-observer latitudes, 5 collected at “mid” sub-observer latitudes (31°N–44°N), and 11 collected over “high” sub-observer latitudes (45°N–51°N). Analysis of these data indicates that CO2 ice is primarily concentrated on Ariel’s trailing hemisphere. However, CO2 ice band strengths are diminished in the spectra collected over mid and high sub-observer latitudes. This sub-observer latitudinal trend may result from radiolytic production of CO2 molecules at high latitudes and subsequent migration of this constituent to low-latitude cold traps. We detected a subtle feature near 2.13 μm in two spectra collected over high sub-observer latitudes, which might result from a “forbidden” transition mode of CO2 ice that is substantially stronger in well-mixed substrates composed of CO2 and H2O ice, consistent with regolith-mixed CO2 ice grains formed by radiolysis. Additionally, we detected a 2.35 μm feature in some low sub-observer latitude spectra, which might result from CO formed as part of a CO2 radiolytic production cycle.