Unravelling the mystery of lunar anomalous craters using radar and infrared observations

Abstract

In Miniature Radio Frequency (Mini‐RF) radar images, anomalous craters are those having a high circular polarization ratio (CPR) in their interior but not exterior to their rims. Previous studies found that most CPR‐anomalous craters contain permanently shadowed regions (PSRs) and that their population is overabundant in the polar regions. However, there is considerable controversy in the interpretation of these signals: both water ice deposits and rocks/surface roughness have been proposed as the source of the elevated CPR values. To resolve this controversy, we have systematically analyzed >4000 impact craters with diameters between 2.5 and 24 km in the Mini‐RF radar image and Diviner rock abundance (RA) map. We first constructed two controlled orthorectified global mosaics using 6818 tracks of Mini‐RF raw data, and then analyzed the correlations between radar CPR and surface slope, rock abundance, and depth/diameter ratios of impact craters. Our results show that CPR‐anomalous craters are distributed relatively uniformly across the lunar surface, with no apparent difference in CPR between the polar, potentially icy, and non‐polar, not icy, craters. Most CPR‐anomalous craters are relatively young with a large depth/diameter ratio, and they actually represent an intermediate stage of crater evolution. Comparison with a two‐component radar scattering model suggests that rocks and surface roughness are major contributors to the observed CPR values. Using craters of 4.7–22 km in diameter with known ages, we find that craters spend up to 120 Ma with a high exterior RA, and ∼3 Ga in the CPR‐anomalous phase.