Earth and Planetary Science Letters 541, 116274 Link to Article [https://doi.org/10.1016/j.epsl.2020.116274]
1University of Southern California, Ming Hsieh Department of Electrical and Computer Engineering, 3737 Watt Way, Los Angeles, CA 90089, USA
2Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
3Department of Earth and Planetary Sciences, The University of Tennessee Knoxville, 1621 Cumberland Avenue, Knoxville, TN 37996, USA
4The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
Identifying polarimetric radar signatures of ice in smooth regolith fines on the floors of permanently shadowed lunar craters is hindered by uncertainties in their dielectric properties. We address this deficiency through polarimetric radar analysis of surface backscatter to derive the dielectric constant () of smooth, rock-free regolith fines covering brecciated crater floors observed by Mini-RF, which offer ideal locations for unambiguous retrieval of surface from linear polarimetric scattering models and CPR analysis for volatile identification. Specifically, we select fines covering crater fills in north polar and equatorial regions to constrain the range of variability of as a function of latitude and crater diameter, where we hypothesize that the latter is indicative of the excavation depth of these fines. Our observations suggest that there is measurable variability in the dielectric properties of fines on lunar crater floors as a function of crater size and potentially with impact excavation depth, suggesting that small craters <5-km in diameter have ranging from 2.3-to-3, and large ones >5-km have higher values of that range from 3-to-3.8. We find that the most plausible explanation for the observed variability of of regolith fines on crater floors is mineralogical differences, suggesting an increase in metal abundance in the original excavated substrate with depth, i.e., in the uppermost kilometer of the lunar crust. Finally, we suggest that regolith fines on the floors of permanently shadowed craters <5 km in diameter are optimal targets for the unambiguous detection of water-ice enrichment using S-band radar observations.