Using complementary remote sensing techniques to assess the presence of volatiles at the lunar north pole

Abstract

Permanently shadowed regions (PSRs) at the poles of the Moon are potential reservoirs of trapped volatile species, including water ice. Knowledge of the distribution and abundance of water ice at the poles provides key scientific background for understanding the evolution of volatiles in the Earth-Moon system and for human exploration efforts. The Lunar Reconnaissance Orbiter Camera (LROC) acquired images of the terrain within PSRs to search for indications of water ice. In addition, the LRO Miniature Radio-Frequency (Mini-RF) instrument acquired S-band radar observations to further characterize these regions. Specifically, the m-chi decomposition was used to assess the distribution of materials within and around PSRs based on the type of backscatter. Double-bounce backscatter is indicative of water ice, but could also be produced by randomly distributed blocks at the wavelength scale. To ascertain whether these signatures are due to water ice or blocks, we quantified the abundance of detectable blocks in areas with double-bounce backscatter using the LROC Narrow Angle Camera (NAC). Block populations were measured for a suite of craters with different ages, sizes, and radar characteristics. For fresh craters, a correlation between block size, block density and double-bounce backscatter was found. Within PSRs exhibiting double-bounce backscatter, no blocks were found. Additionally, no albedo variations were observed at PSRs, in contrast to observations of PSRs on Mercury. While the possibility of water ice in some lunar craters still exists, these results indicate that they are likely small-scale, and that the observed radar anomalies at PSR-bearing craters are most likely due to the presence of wavelength-scale blocks.