Temporal and Spatial Variability of Lunar Hydration as Observed by the Deep Impact, Part Ii: the South Pole

Introduction: The discovery of hydroxyl [OH] and/or water [H2O] widely distributed on the lunar surface [1,2,3], outside of the permanently shadowed polar regions, adds significant evidence that the Moon is not completely devolatilized as had previously been assumed [4,5]. Lunar surface hydration is most likely an exogenic process, resulting from solar wind protons interacting with oxygen-rich minerals in the regolith [6]. We present a quantitative analysis of the lunar hydration using recalibrated Deep Impact (DI) flyby observations of the Moon, including those over the South Pole that have not previously been analyzed, in order to investigate the mechanisms responsible for the formation, retention, and disruption of OH and/or H2O across the lunar surface. Deep Impact lunar data: On Nov 4, 2010, the DI flyby spacecraft [7] successfully encountered comet 103P/Hartley 2 [8], employing two multi-spectral imager and a 1.04 – 4.83 μm infrared spectrometer (HRIIR). During the cruise phase to Hartley 2, various instrument calibrations were conducted using observations of the Moon obtained during a series of Earth gravitational assists. The analysis of the DI HRI-IR lunar data turned out to be fundamental, not only for calibration purposes, but also to undeniably confirm the presence of hydroxyl [OH] and/or water [H2O] on the surface of the Moon [2], complementing the results by Pieters et al. [1] and Clark [3]. Previous work: Sunshine et al. [2], through analysis of DI data covering the equatorial and the northern polar regions of the Moon, demonstrated that lunar surface hydration varies with local time of day, with the entire lunar surface hydrated during at least part of the lunar day, and that hydration is mainly controlled by instantaneous temperature. Evidence of changes in the shape of the overall 3-μm absorption with time of day were also seen, suggesting differential loss of H2O vs. OH and thus multiple rates and possibly multiple mechanisms for diurnal hydration cycling. However, unambiguous detection of H2O was not possible given the signal-to-noise ratio (SNR) of the data. New calibration: Recognizing the importance of the lunar data and the desire to deconvolve OH and H2O, the DI team acquired a series of >6500 observations to collect spectra of calibration stars specifically to support an improved absolute calibration of the DI lunar data. The new calibration [9] not only improves the absolute flux, but also the shape of the spectral energy distribution and thus the shape of the absorption bands. This results in better temperature and OH/H2O content determination. Moreover, the updated calibration enhances the SNR, which helps to uniquely identify the presence of absorption bands (e.g., OH vs H2O). This improved calibration has now been applied to all DI lunar data, including those presented in the previous work by Sunshine et al. [2]. South pole DI data: DI observations of the Lunar South Pole were acquired after the study done by Sunshine et al. [2] and are presented here for the first time. These observations include spatially resolved scans separated by one-quarter of a lunar day (Fig. 1). Specifically, three sets of data were acquired at seven-day intervals, during which the Moon rotated by ~90°. These spatially resolved scans of the southern polar regions allow us to study the variability of lunar hydration with respect to local time. It is therefore possible to compare the same location on the Moon at different local times and thus under different insolation conditions. Furthermore, it is possible to investigate whether both mare and highland terrains present the same diurnal hydration cycle or not.