Lunar polar craters – Icy, rough or just sloping?

• Published in: Icarus (New York, N.Y. 1962) Vol. 241; pp. 66 - 78
• Main Authors: Eke, Vincent R., Bartram, Sarah A., Lane, David A., Smith, David, Teodoro, Luis F.A.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.10.2014
• Subjects: Angle of incidence; Craters; Exteriors; Ices; Incidence angle; Indian spacecraft; Moon, surface; Mosaics; Parallax; Radar observations; PN, Arctic, North Pole; Moon, surface; Ices; Radar observations
• DOI: 10.1016/j.icarus.2014.06.021
• ISSN: 0019-1035

•We consider the variation of CPR with position within anomalous polar craters.•The increase of CPR with incidence angle is quantified.•CPR in the centres of anomalous craters is indistinguishable from that outside.•High CPR is located on crater walls and does not correlate with temperature.•We introduce a crater-finding algorithm and show anomalous craters are of intermediate age. Circular Polarisation Ratio (CPR) mosaics from Mini-SAR on Chandrayaan-1 and Mini-RF on LRO are used to study craters near to the lunar north pole. The look direction of the detectors strongly affects the appearance of the crater CPR maps. Rectifying the mosaics to account for parallax also significantly changes the CPR maps of the crater interiors. It is shown that the CPRs of crater interiors in unrectified maps are biased to larger values than crater exteriors, because of a combination of the effects of parallax and incidence angle. Using the LOLA Digital Elevation Map (DEM), the variation of CPR with angle of incidence has been studied. For fresh craters, CPR ∼0.7 with only a weak dependence on angle of incidence or position interior or just exterior to the crater, consistent with dihedral scattering from blocky surface roughness. For anomalous craters, the CPR interior to the crater increases with both incidence angle and distance from the crater centre. Central crater CPRs are similar to those in the crater exteriors. CPR does not appear to correlate with temperature within craters. Furthermore, the anomalous polar craters have diameter-to-depth ratios that are lower than those of typical polar craters. These results strongly suggest that the high CPR values in anomalous polar craters are not providing evidence of significant volumes of water ice. Rather, anomalous craters are of intermediate age, and maintain sufficiently steep sides that sufficient regolith does not cover all rough surfaces.