SEARCHING FOR BOUND HYDROXYL/WATER ON THE MOON

In 2009, surficial hydroxyl (OH) and potentially water (H2O) were detected on the lunar surface by near-infrared (NIR) spectrometers on the Chandrayaan-1, EPOXI and Cassini spacecrafts. Since that time, the distribution, abundance, and diurnal nature of the hydration has been avidly debated. The Moon Mineralogy Mapper (M³) provided the best spatially-resolved (0.54-2.98 μm; 70-280 m/pixel) coverage. However, because the majority of minerals in lunar returned samples are anhydrous, and water on the illuminated lunar surface is unstable, M³ was not optimized to rigorously quantify water abundance.

Over the last decade, different thermal corrections of the M³ data have been attempted to enable better understanding of spatial and temporal distribution of lunar surface water on sunlit terrains to understand what the origin of the OH/H2O might be. Nevertheless, the 3-micron cutoff wavelength for M³ results in ambiguities in the strength, shape, and position of the absorption band. Thermal emission further complicates the data, as the true temperature of the measured surface needs to be subtracted from the signal to quantify the water band depth. Discrepancies exist between results from different techniques using M3, coupled with other datasets or modeling assumptions, leading to conflicting results about whether the species is H2O or OH, its concentration and distribution, and whether it migrates over a lunar day. However, there are clear anomalies associated with geological features and may be detections of native lunar water.

NASA recently selected the SIMPLEx smallsat Lunar Trailblazer, which will carry the High-resolution Volatiles and Minerals Moon Mapper (HVM³; 0.6-3.6 μm, similar to M³ but explicitly designed to focus on water measurements), as well as the Lunar Thermal Mapper, a thermal infrared multispectral imager to directly and simultaneously measure the temperature and rock type within each HVM³ pixel. With >1000 targeted images per instrument, Lunar Trailblazer will directly test hypotheses for the distribution, abundance and form of OH/H2O on the sunlit lunar surface. We will review the current understanding of candidate locations for bound OH/H2O on the Moon, and describe how Lunar Trailblazer will test the hypothesis that this OH/H2O is bound, rather than adsorbed.