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We use the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment surface rock abundance data set to investigate how rock content changes with surface age in the maria. We find that surface rock abundance decreases with unit age as expected for a thickening regolith, but age alone cannot explain most of the observed regional variability in rock abundance. We propose that this additional variability is primarily caused by differences in the initial mare basalt properties such that some flows break down more readily than others. We calculate the best-fit relationship between the surface rock abundance and unit age and investigate the regional variability in the residual errors of this fit. Relative to this best-fit relationship, we find that Mare Humorum and parts of Oceanus Procellarum are anomalously rocky, and Mare Australe is rock-poor for its age. These regional differences, which cannot be explained by unit age, are likely due to variability in the initial properties of mare basalts.

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.