Integration of lunar polar remote‐sensing data sets: Evidence for ice at the lunar south pole

Abstract

Abstract. In order to investigate the feasibility of ice deposits at the lunar south pole, we have integrated all relevant lunar polar data sets. These include illumination data, Arecibo ground-based monostatic radar data, newly processed Clementine bistatic radar data, and Lunar Prospector neutron spectrometer measurements. The possibility that the lunar poles harbor ice deposits has important implications not only as a natural resource for future human lunar activity but also as a record of inner solar system volatiles (e.g., comets and asteroids) over the past billion years or more. We find that the epithermal neutron flux anomalies, measured by Lunar Prospector, are coincident with permanently shadowed regions at the lunar south pole, particularly those associated with Shackleton crater. Furthermore, these areas also correlate with the • = 0 circular polarization ratio (CPR) enhancements revealed by new processing of Clementine bistatic radar echoes, which in turn are colocated with areas of anomalous high CPR observed by Arecibo Observatory on the lower, Sun-shadowed wall of Shackleton crater. Estimates of the extent of high CPR from Arecibo Observatory and Clementine bistatic radar data independently suggest that •10 km 2 of ice may be present on the inner Earth-facing wall of Shackleton crater. None of the experiments that obtained the data presented here were ideally suited for definitively identifying ice in lunar polar regions. By assessing the relative merits of all available data, we find that it is plausible that ice does occur in cold traps at the lunar south pole and that future missions with instruments specifically designed to investigate these anomalies are worthy. IntroductionThe lunar poles have long been theorized to harbor ice deposits in permanently shadowed regions because these regions can act to cold trap volatile compounds, including water introduced into the lunar environment [Watson et al., 1961]. This is a fascinating possibility both because such deposits would serve as a natural resource for future human lunar activity and because the plausible sources of lunar water (e.g., comets and asteroids) are of inherent interest. In fact, modeling the temperatures of shadowed craters near the poles [Ingersoll et al., 1992;Salvail and Fanale, 1994; Vasavada, 1998] shows temperatures low enough to cold trap materials substantially more volatile than water ice. Studies of the transport and retention of water ice and other volatiles also support the possibility of water ice being present at the pole [Butler, 1997; Morgan and Shemansky, 1991 ]. The latter work suggested that sputtering was rapid enough to destroy slow continuous deposits of water ice, for example, from micrometeorite water, or water produced from reduction of lunar surface materials to produce water from solar wind hydrogen but that thick deposits of ice introduced by comets or large "wet" asteroids might survive by sequestering of ice by regolith overturn.•Naval Research Laboratory, Washington, D.C. No conclusive evidence of w...

Evidence for water ice on the Moon: Results for anomalous polar craters from the LRO Mini‐RF imaging radar

“…[19] High CPR on the Moon may be caused by one or both of two principal factors (1) large degrees of wavelength-scale surface roughness [e.g., Campbell, 2002Campbell, , 2012 or (2) the presence of water ice [e.g., Nozette et al, 1996Nozette et al, , 2001]. In the case of the former, geologically fresh surfaces (such as ejecta from young craters) create high degrees of surface roughness at wavelength-scales.…”

“…This quantity is defined to be the magnitude of the same sense (i.e., the left or right sense of the transmitted circular polarization) divided by the opposite sense polarized signals that are received. Ice deposits are known to have high CPR [e.g., Ostro, 2002;Campbell, 2002] caused by both volumetric backscatter and the coherent backscatter opposition effect (CBOE) [Mishchenko, 1992;Peters, 1992;Black et al, 2001;Nozette et al, 2001], an interferometeric enhancement of backscatter seen at very low beta (phase) angles. Surface scattering from dry, fine-grained planetary regolith typically has CPR less than unity [Campbell, 2002;Heggy et al, 2007], with increasing CPR seen in geologic units that have high degrees of surface roughness at scales similar to the wavelength of the imaging radar [Campbell, 2002[Campbell, , 2012.…”

“…Yet another study of reprocessed Clementine bistatic data failed to find an enhancement in the south polar CPR data [Simpson and Tyler, 1999]. However, the Clementine team reexamined all of the then-available data (Arecibo Earthbased radar, Clementine bistatic results, and Lunar Prospector (LP) neutron data) and concluded that although no single piece of information was definitive, the preponderance of evidence indicated that water ice had probably been detected near the south pole of the Moon [Nozette et al, 2001].…”

“…To obtain volumetric coherent opposition backscatter (CBOE) [see Nozette et al, 2001] from these ices, the thickness of the deposit must be on the order of 100 wavelengths or more [Thompson et al, 2011] in order to have enough internal reflections to reverse the polarization. Thus, at S-band frequencies, the observed ice must be on the order of 10-20 m thick, as inclusion of silicate contaminants would increase attenuation in the ice and thus decrease these thicknesses by a factor of 2 to 3.…”

Regolith properties in the south polar region of the Moon from 70-cm radar polarimetry

“…This bistatic phase angle will lead to some reduction in the circular polarization ratio for a target characterized by coherent backscatter (e.g., thick ice), but phase angles >1-2 • are required to entirely suppress such an effect (e.g., Fig. 1 in Nozette et al, 2001). High circular polarization ratios in areas with numerous surface and near-surface blocks on the scale of the radar wavelength will not be affected.…”