Evidence for Nitrogen-Bearing Species on Umbriel: Sourced from a Subsurface Ocean, Undifferentiated Crust, or Impactors?

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 by endogenic processes 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 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 sub-observer longitudinal distributions of these four bands, finding that they are present across Umbriel with only weak evidence for leading/trailing hemispherical trends. These results suggest that the four bands may be spatially associated with 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 matches the spectral signature of NH3 ice, and its 2.2-μm band is consistent with the signatures of NH3-H2O mixtures, aluminum-bearing phyllosilicates, and perhaps sodium-bearing carbonates. Umbriel’s 2.14-μm band may alternatively result from a 2ν3 overtone mode of crystalline CO2 ice. The possible presence of these constituents is consistent with materials formed in contact with an internal ocean that were subsequently exposed on Umbriel’s surface in its early history. Alternatively, these constituents may have originated in an undifferentiated crust or could have been delivered by impactors.