WATER ON THE MOON: QUANTITATIVE SPATIAL AND TEMPORAL VARIATIONS AS SEEN BY THE MOON MINERALOGY MAPPER

Numerous recent studies have provided evidence that the Moon hosts a more significant inventory of volatiles than previously recognized. Specifically, laboratory studies of returned samples reveal that the lunar interior contains appreciable OH/H2O, and orbital reflectance spectroscopy has shown that the lunar surface is also hydrated. However, quantitative estimates of surficial OH and/or H2O from near-infrared data such as those acquired by the Moon Mineralogy Mapper (M3) are hampered by the combined effects of reflected and thermally emitted radiation at wavelengths beyond 2 um. Here we present a new method for thermal correction of M3 data which, when integrated with empirical relationships between absorption strength and water content derived from laboratory experiments, allows for quantitative mapping of lunar surface hydration. We find that equatorial regions generally lack or exhibit only weak hydration features and that hydration at these latitudes varies over the course of a lunar day, reaching a minimum near local noon. In contrast, latitudes >40-50 degrees exhibit significantly higher ‘water’ contents (500-2000 ppm), and latitudes poleward of these regions remain hydrated throughout the course of a lunar day. For such latitudes, there is a positive relationship between optical maturity and hydration level, suggesting OH/H2O content of lunar regolith is linked to exposure age. In addition, numerous (but not all) pyroclastic deposits mapped in previous studies exhibit increased hydration signatures regardless of their latitude and local time of day. In one location we have observed that up to 1000 ppm water is present even at local noon, whereas surrounding (non-pyroclastic) regions exhibit no hydration features at this time of day. These results indicate that lunar surface hydration is spatially and temporally complex, and some observed hydration features may result from both exogenous (solar wind) and endogenous (volcanic) processes. The presence of stable hydration signatures associated with pyroclastic deposits and the observed variations between pyroclastic deposits suggests that detailed analysis of M3 data may reveal new insights about interior volatile inventories and deep-seated magmatic processes.