Distribution of CO₂ ice on the large moons of Uranus and evidence for compositional stratification of their near-surfaces

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 100μm) 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.