The surfaces of the large Uranian satellites are characterized by a mixture of H2O ice and a dark, potentially carbon-rich, constituent, along with CO2 ice. At the mean heliocentric distance of the Uranian system, native CO2 ice should be removed on timescales shorter than the age of the Solar System. Consequently, the detected CO2 ice might be actively produced. Analogous to irradiation of icy moons in the Jupiter and Saturn systems, we hypothesize that charged particles caught in Uranus' magnetic field bombard the surfaces of the Uranian satellites, driving a radiolytic CO2 production cycle. To test this hypothesis, we investigated the distribution of CO2 ice by analyzing near-infrared (NIR) spectra of these moons, gathered using the SpeX spectrograph at NASA's Infrared Telescope Facility (IRTF) (2000 -2013). Additionally, we made spectrophotometric measurements using images gathered by the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope (2003 -2005). We find that the detected CO2 ice is primarily on the trailing hemispheres of the satellites closest to Uranus, consistent with other observations of these moons. Our band parameter analysis indicates that the detected CO2 ice is pure and segregated from other constituents. Our spectrophotometric analysis indicates that IRAC is not sensitive to the CO2 ice detected by SpeX, potentially because CO2 is retained beneath a thin surface layer dominated by H2O ice that is opaque to photons over IRAC wavelengths. Thus, our combined SpeX and IRAC analyses suggest that the near-surfaces (i.e., top few 100 microns) of the Uranian satellites are compositionally stratified. We briefly compare the spectral characteristics of the CO2 ice detected on the Uranian moons to icy satellites elsewhere, and we also consider the most likely drivers of the observed distribution of CO2 ice.

The large and tidally-locked "classical" moons of Uranus display longitudinal and planetocentric trends in their surface compositions. Spectrally red material has been detected primarily on the leading hemispheres of the outer moons, Titania and Oberon. Furthermore, detected H2O ice bands are stronger on the leading hemispheres of the classical satellites, and the leading/trailing asymmetry in H2O ice band strengths decreases with distance from Uranus. We hypothesize that the observed distribution of red material and trends in H2O ice band strengths results from infalling dust from Uranus' irregular satellites. These dust particles migrate inward on slowly decaying orbits, eventually reaching the classical satellite zone, where they collide primarily with the outer moons. The latitudinal distribution of dust swept up by these moons should be fairly even across their southern and northern hemispheres. However, red material has only been detected over the southern hemispheres of these moons, during the Voyager 2 flyby of the Uranian system (subsolar latitude ~81ºS). Consequently, to test whether irregular satellite dust impacts drive the observed enhancement in reddening, we have gathered new ground-based data of the now observable northern hemispheres of these moons (sub-observer latitudes ~17 -35ºN). Our results and analyses indicate that longitudinal and planetocentric trends in reddening and H2O ice band strengths are broadly consistent across both southern and northern latitudes of these moons, thereby supporting our hypothesis. Utilizing a suite of numerical best fit models, we investigate the composition of the reddening agent detected on these moons, finding that both complex organics and amorphous pyroxene match the spectral slopes of our data. We also present spectra that span L/L' bands (~2.9 -4.1 µm), a previously unexplored wavelength range in terms of spectroscopy for the Uranian moons, and we compare the shape and albedo of the spectral continua in these L/L' band data to other icy moons in the Jovian and Saturnian systems. Additionally, we discuss possible localized enhancement of reddening on Titania, subtle differences in H2O ice band strengths between the southern and northern hemispheres of the classical satellites, the distribution of constituents on Miranda, and the possible presence of NH3-hydrates on these moons. In closing, we explore potential directions for future observational and numerical modeling work in the Uranian system. !

We investigated whether ammonia-rich constituents are present on the surface of the Uranian moon Ariel by analyzing 32 near-infrared reflectance spectra collected over a wide range of sub-observer longitudes and latitudes. We measured the band areas and depths of a 2.2 μm feature in these spectra, which has been attributed to ammonia-bearing species on other icy bodies. Ten spectra display prominent 2.2 μm features with band areas and depths >2σ. We determined the longitudinal distribution of the 2.2 μm band, finding no statistically meaningful differences between Ariel’s leading and trailing hemispheres, indicating that this band is distributed across Ariel’s surface. We compared the band centers and shapes of the five Ariel spectra displaying the strongest 2.2 μm bands to laboratory spectra of various ammonia-bearing and ammonium-bearing species, finding that the spectral signatures of the Ariel spectra are best matched by ammonia-hydrates and flash frozen ammonia-water solutions. Our analysis also revealed that four Ariel spectra display 2.24 μm bands (>2σ band areas and depths), with band centers and shapes that are best matched by ammonia ice. Because ammonia should be efficiently removed over short timescales by ultraviolet photons, cosmic rays, and charged particles trapped in Uranus’ magnetosphere, the possible presence of this constituent supports geologic activity in the recent past, such as emplacement of ammonia-rich cryolavas and exposure of ammonia-rich deposits by tectonism, impact events, and mass wasting.

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