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

Abstract The surfaces of the large uranian satellites are characterized by a mixture of H 2 O ice and a dark, potentially carbon-rich, constituent, along with CO 2 ice. At the mean heliocentric distance of the uranian system, native CO 2 ice should be removed on timescales shorter than the age of the Solar System. Consequently, the detected CO 2 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 CO 2 production cycle. To test this hypothesis, we investigated the distribution of CO 2 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 CO 2 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 CO 2 ice is pure and segregated from other constituents. Our spectrophotometric analysis indicates that IRAC is not sensitive to the CO 2 ice detected by SpeX, potentially because CO 2 is retained beneath a thin surface layer dominated by H 2 O 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 μm) of the uranian satellites are compositionally stratified. We briefly compare the spectral characteristics of the CO 2 ice detected on the uranian moons to icy satellites elsewhere, and we also consider the most likely drivers of the observed distribution of CO 2 ice.