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The Uranian moon Ariel exhibits a diversity of geologically young landforms, with a surface composition rich in CO _2 ice. The origin of CO _2 and other species, however, remains uncertain. We report observations of Ariel’s leading and trailing hemispheres, collected with NIRSpec (2.87–5.10 μ m) on the James Webb Space Telescope. These data shed new light on Ariel's spectral properties, revealing a double-lobed CO _2 ice scattering peak centered near 4.20 and 4.25 μ m, with the 4.25 μ m lobe possibly representing the largest CO _2 Fresnel peak yet observed in the solar system. A prominent 4.38 μ m ^13 CO _2 ice feature is also present, as is a 4.90 μ m band that results from ^12 CO _2 ice. The spectra reveal a 4.67 μ m ^12 CO ice band and a broad 4.02 μ m band that might result from carbonate minerals. The data confirm that features associated with CO _2 and CO are notably stronger on Ariel’s trailing hemisphere compared to its leading hemisphere. We compared the detected CO _2 features to synthetic spectra of CO _2 ice and mixtures of CO _2 with CO, H _2 O, and amorphous carbon, finding that CO _2 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 CO _2 . The evidence for thick CO _2 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 CO _2 from high to low latitudes.

Published near-IR spectra of the four largest classical Uranian satellites display the presence of discrete deposits of CO _2 ice, along with subtle absorption features around 2.2 μ m. The two innermost satellites, Miranda and Ariel, also possess surfaces heavily modified by past endogenic activity. Previous observations of the smallest satellite, Miranda, have not detected the presence of CO _2 ice, and a report of an absorption feature at 2.2 μ m has not been confirmed. An absorption feature at 2.2 μ m could result from exposed or emplaced NH _3 - or NH _4 -bearing species, which have a limited lifetime on Miranda’s surface, and therefore may imply that Miranda’s internal activity was relatively recent. In this work, we analyzed near-IR spectra of Miranda to determine whether CO _2 ice and the 2.2 μ m feature are present. We measured the band area and depth of the CO _2 ice triplet (1.966, 2.012, and 2.070 μ m), a weak 2.13 μ m band attributed to CO _2 ice mixed with H _2 O ice, and the 2.2 μ m band. We confirmed a prior detection of a 2.2 μ m band on Miranda, but we found no evidence for CO _2 ice, either as discrete deposits or mixed with H _2 O ice. We compared a high signal-to-noise-ratio spectrum of Miranda to synthetic and laboratory spectra of various candidate compounds to shed light on what species may be responsible for the 2.2 μ m band. We conclude that the 2.2 μ m absorption is best matched by a combination of NH _3 ice with NH _3 hydrates or NH _3 –H _2 O mixtures. NH _4 -bearing salts like NH _4 Cl are also promising candidates that warrant further investigation.

Near-infrared spectra of Umbriel and the other classical Uranian moons exhibit 2.2 μ m absorption bands that could result from ammonia (NH _3 ) 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 NH _3 is retained over long timescales. We analyzed 33 spectra of Umbriel to investigate its 2.2 μ m band, along with three other absorption features we identified near 2.14, 2.22, and 2.24 μ m. 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 μ m and 2.22 μ m bands are most consistent with the spectral signature of organics, its 2.24 μ m band is best matched by NH _3 ice, and its 2.2 μ m band is consistent with the signatures of NH _3 –H _2 O 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 μ m band may alternatively result from a 2 _ν _3 overtone mode of CO _2 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.

Many tidally locked icy satellites in the outer solar system show leading/trailing hemispherical asymmetries in the strength of near-infrared (NIR) H _2 O ice absorption bands, in which the absorption bands are stronger on the leading hemisphere. This is often attributed to a combination of magnetospheric irradiation effects and impact gardening, which can modify grain size, expose fresh ice, and produce dark contaminating compounds that reduce the strength of absorption features. Previous research identified this leading/trailing asymmetry on the four largest classical Uranian satellites but did not find a clear leading/trailing asymmetry on Miranda, the smallest and innermost classical moon. We undertook an extensive observational campaign to investigate variations of the NIR spectral signature of H _2 O ice with longitude on Miranda’s northern hemisphere. We acquired 22 new spectra with the TripleSpec spectrograph on the ARC 3.5 m telescope and four new spectra with GNIRS on Gemini North. Our analysis also includes three unpublished and seven previously published spectra taken with SpeX on the 3 m IRTF. We confirm that Miranda has no substantial leading/trailing hemispherical asymmetry in the strength of its H _2 O ice absorption features. We additionally find evidence for an anti-Uranus/sub-Uranus asymmetry in the strength of the 1.5 μ m H _2 O ice band that is not seen on the other Uranian satellites, suggesting that additional endogenic or exogenic processes influence the longitudinal distribution of H _2 O ice band strengths on Miranda.

CO _2 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 CO _2 ice is consistent with radiolytic production of CO _2 , formed by charged particle bombardment of H _2 O ice and carbonaceous material in Ariel’s regolith. This longitudinal distribution of CO _2 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 CO _2 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 CO _2 ice is primarily concentrated on Ariel’s trailing hemisphere. However, CO _2 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 CO _2 molecules at high latitudes and subsequent migration of this constituent to low-latitude cold traps. We detected a subtle feature near 2.13 μ m in two spectra collected over high sub-observer latitudes, which might result from a “forbidden” transition mode of CO _2 ice that is substantially stronger in well-mixed substrates composed of CO _2 and H _2 O ice, consistent with regolith-mixed CO _2 ice grains formed by radiolysis. Additionally, we detected a 2.35 μ m feature in some low sub-observer latitude spectra, which might result from CO formed as part of a CO _2 radiolytic production cycle.