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

Abstract

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 2ν 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.

Are NH3 and CO2 Ice Present on Miranda?

“…All five satellites show evidence of a weak absorption feature near 2.2 μm (Bauer et al 2002;Cartwright et al 2018Cartwright et al , 2020aCartwright et al , 2023. This 2.2 μm absorption feature appears qualitatively similar to the 2.21 μm band on Charon, which has been attributed to NH-bearing species, such as NH 3 hydrates and NH 4 Cl (e.g., Cook et al 2007Cook et al , 2018Cook et al , 2023.…”

“…Further research has also presented evidence of similar weak absorption features in the 2.2 μm region on Umbriel (Cartwright et al 2023). However, the surface of Umbriel is ancient and heavily cratered (within the resolution limits of Voyager 2 imaging), lacking evidence of resurfacing driven by endogenic activity.…”

“…cycle (Cartwright et al 2022(Cartwright et al , 2023. Our band parameter measurements for the 2.13 μm band are shown in Table 4 and Figures 4 and 5.…”

“…Synthetic Hapke-Mie spectra of intimate mixtures, including some of the same materials as in Cartwright et al (2023), are plotted in Figure 6 and tabulated in Table 8. We compared these synthetic spectra to a spectrum of Miranda's leading hemisphere obtained with GNIRS (spectrum UT201008).…”

Tethys’s Heat Fluxes Varied with Time in the Ithaca Chasma and Telemus Basin Region

“…However, these spectral features are quite weak (Cartwright et al 2021) and may include contamination from a telluric methane band centered near 2.2 μm (Figure 3). Other species, including hydrated phyllosilicates, carbonates, and organics, exhibit absorption features in this wavelength range (Cartwright et al 2023). Determining whether NH 3 is present on Tethys using the 2.2 μm band is therefore challenging.…”