The Changing Polar Moon

Abstract:

The Lunar Reconnaissance Orbiter (LRO) and LCROSS have provided an avalanche of new data regarding the lunar poles: LCROSS directly detected water vapor and other volatiles in its impact plume; LRO LAMP has detected surface frost using UV ratios; LEND has refined understanding of the distribution of hydrogen; LOLA and LAMP have showed that the spectral properties of permanently shadowed regions (PSR) are anomalous and may be due to unusual surface texture or altered space weathering; Diviner shows both where the coldest portions of the poles exist, and its quantitative results show where temperatures are low enough to preserve water ice at depth, well outside the PSRs. Yet while we are data rich, our understanding of the lunar poles is maddeningly poor. Our poverty of understanding is made even more baffling by the MESSENGER results from Mercury. At Mercury's poles the distribution of volatiles is dictated by temperature: where subsurface temperatures inferred from topography are consistent with long term preservation of water ice, radar anomalies indicating thick ice are present; where surface temperatures are consistent with preservation of surface frost, high reflectance anomalies indicating surface frost are revealed by laser reflectance. The distribution of water ice on Mercury is well understood.

In contrast, temperature is only a weak indicator of the presence of volatiles at the lunar poles; there is little ability to predict the location and abundance of hydrogen or water. The difference may in the age of the volatile deposits on the two planets. Turn the clock forward a few billion years on Mercury and the deposits may appear more lunar. Surface lag deposits may have long ago succumbed to impact gardening, as has much of the shallow buried ice. Ice retained could be patchy, and confined to the coldest places that may tend to preserve it more effectively, even when finely comminuted. Lunar polar volatiles, a possible relic of an ancient, Mercury-like deposit, may retain evidence of that state, modified by several billion years of impact gardening and space weathering.