Boulder distribution, circular polarization, and optical maturity: A survey of example lunar polar terrains for future landing sites

Abstract

Boulder occurrence at the southern polar regions of the Moon was analysed to see connections between optical, radio, spectral, and topography-related roughness datasets. On optical images, boulders tend to concentrate in groups (the groups are 0.9–1.8 km diameter and cover about 9% of the surveyed regions; while boulders inside these groups have up to 6 boulder/100 m2, e.g., 60,000 boulder/km2 (54% of areal coverage), while outside these groups around 7 boulder/km2 (0.06% areal coverage). The correlation between optical boulder density and Circular Polarization Ratio: CPR (0.26) and Optical Maturity: OMAT (0.38) values are weak, while CPR and OMAT distribution patterns are elevated around fresh craters with elevated boulder density. While fresh craters with boulders coincide with elevated CPR values, but in some cases, elevated CPR is without elevated surface boulder occurrence, indicating shallow subsurface boulders. A good correlation between CPR values and surface roughness (0.68) indicates boulder exhumation at topographic differences of craters. OMAT and CPR values of fresh craters (with elevated and sharp rims) show elevated values above the adjacent regions. OMAT is elevated up to 1–3 times the crater diameter, while CPR is elevated at a smaller area than OMAT values. The CPR might partly come from boulders that could not be ejected as far as smaller regolith grains indicated by OMAT-related freshness. Boulder occurrence at fresh impact craters indicates a thickness of the loose top porous regolith layer of 6–11 m, not far from those gained by another author (5–18 m) for terrain areas.