Investigating the Distribution of Lunar Polar Volatiles Using Mini-RF Surface Roughness Data

Questions surrounding the presence, distribution, and nature of volatiles, such as water, at the lunar poles have driven much of lunar science over the past two decades. A multitude of datasets have suggested the presence of water both at volumetric depth, and as surficial frost; and data from the LCROSS impactor directly attests to the presence of water at the Moons south pole. However, despite highly suggestive data acquired from a number of different instruments, results from radar exploration of the Moon have remained ambiguous. Recent studies combining Diviner-derived ice stability zones and LOLA surface roughness has suggested that there is a correlation between regions of ice stability and regions of decreased surface roughness. Motivated by this, we undertook an investigation of lunar polar surface roughness using a controlled Mini-RF monostatic circular polarization ration (CPR) south polar mosaic. We investigated 42 south polar craters and 7 background regions, and divided the craters by floor illumination into fully PSR (permanently shaded region), no PSR, and mixed. Our results showed that non-PRS craters had consistently higher CPR than fully PSR and mixed PSR craters. Amongst the mixed and fully PSR craters, a smaller subset of craters emerged with significantly lower CPRs than standard low-roughness lunar regions. The lowest CPR craters include Sverdrup, Laveran, an unnamed crater at 168.4 E, 88.7 S, both the PSR and non PSR portions of the floors of Cabeus and Cabeus B, and a non-crater floor background region near Sverdrup. The magnitude of CPR difference between these exceptionally smooth polar craters and standard low-roughness lunar regions is similar in magnitude to the difference between standard low-roughness lunar regions and the roughest average lunar regions. This suggests that the CPR deviations observed in these craters are both highly unusual, and significant. We hypothesize that these crater floor regions that include the floor of Cabeus crater sampled during the LCROSS missionare the result of an enhanced volatile presence in the regolith. Continued analysis will help to determine if this signature is the result of a low-porosity intimate mixture of water ice and regolith, near surface smoothing resulting from deeply buried ice lenses, or another previously unrecognized process.