A CO2 Cycle on Ariel? Radiolytic Production and Migration to Low-latitude Cold Traps

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.