JWST Reveals CO Ice, Concentrated CO2 Deposits, and Evidence for Carbonates Potentially Sourced from Ariel's Interior

The Uranian moon Ariel exhibits a diversity of geologically young landforms, with a surface composition rich in CO₂ ice. The origin of CO₂ and other species, however, remains uncertain. We report observations of Ariel's leading and trailing hemispheres, collected with NIRSpec (2.87–5.10 μm) on the James Webb Space Telescope. These data shed new light on Ariel's spectral properties, revealing a double-lobed CO₂ ice scattering peak centered near 4.20 and 4.25 μm, with the 4.25 μm lobe possibly representing the largest CO₂ Fresnel peak yet observed in the solar system. A prominent 4.38 μm ¹³CO₂ ice feature is also present, as is a 4.90 μm band that results from ¹²CO₂ ice. The spectra reveal a 4.67 μm ¹²CO ice band and a broad 4.02 μm band that might result from carbonate minerals. The data confirm that features associated with CO₂ and CO are notably stronger on Ariel's trailing hemisphere compared to its leading hemisphere. We compared the detected CO₂ features to synthetic spectra of CO₂ ice and mixtures of CO₂ with CO, H₂O, and amorphous carbon, finding that CO₂ could be concentrated in deposits thicker than ∼10 mm on Ariel's trailing hemisphere. Comparison to laboratory data indicates that CO is likely mixed with CO₂. The evidence for thick CO₂ ice deposits and the possible presence of carbonates on both hemispheres suggests that some carbon oxides could be sourced from Ariel's interior, with their surface distributions modified by charged particle bombardment, sublimation, and seasonal migration of CO and CO₂ from high to low latitudes.