The Uranian Moon Ariel, a Carbon Dioxide Wonderland Observed by JWST

The surface of Ariel exhibits a variety of tectonic and possibly cryovolcanic features that formed in the geologically-recent past. Prior ground-based observations had indicated that Ariel's surface composition includes a large fraction of crystalline CO₂ ice, with hints of NH-bearing species and CO ice that have yet to be confirmed. However, the origin of these volatiles is not well understood. To investigate Ariel's surface composition and the processes that modify it, we analyzed reflectance spectra collected over its leading and trailing hemispheres by the NIRSpec spectrograph on the James Webb Space Telescope (G395M, 2.9 - 5.1 µm). These data reveal Ariel's surface composition over the 4 to 5.1 µm wavelength range for the first time.

The NIRSpec spectra show a double-lobed scattering peak centered near 4.20 and 4.25 µm, flanked by a 4.27 µm absorption band, which are all associated with the ν₃ stretching mode of ¹²CO₂ ice. The 4.25 µm lobe could represent the largest CO₂ ice Fresnel peak yet observed in the Solar System. Numerous other CO₂ ice features are present, including prominent 4.38 µm ¹³CO₂ and 4.90 µm ¹²CO₂ bands. The JWST spectra confirm the presence of ¹²CO ice via its 4.67 µm ν₃ mode (and possibly the 4.78 µm ¹³CO ν₃ mode). They also reveal a broad 4 µm band, potentially resulting from carbonate minerals, as well as other subtle features between 4.4 and 4.6 µm that could result from carbon suboxide and nitriles. We see no reliable evidence for NH-bearing species, hydrogen peroxide, or hydrocarbons in these data. Comparison to radiative transfer models suggests that some CO₂ ice deposits could be more than 10 mm thick and segregated from H₂O ice.

The possible presence of carbonates and concentrated CO₂ ice deposits on both hemispheres suggests that some carbon oxides could originate in Ariel's interior. Carbonates are particularly interesting as they often form over long timescales in aqueous environments, supporting the presence of a subsurface ocean at Ariel, either now or in the past. The surface distribution of these species could be shaped by seasonal migration of CO₂ and CO and perhaps via interactions with Uranus' magnetosphere, albeit the lack of H₂O₂ points to a somewhat quiescent charged particle environment. The non-detection of ammonia and hydrocarbons indicates that they are efficiently oxidized and removed once exposed on Ariel's surface, and/or their spectral signatures are obscured by strong H₂O ice absorption between 2.9 and 3.5 µm.