Variability of Hydration Across the Southern Hemisphere of the Moon as Observed by Deep Impact

Abstract

Measurements of the 3 μm absorption feature, associated with the presence of hydroxyl and potentially molecular water, were first observed in 2009 by three separate spacecraft’ observations. Subsequent observations have revealed widespread but variable hydration over the sunlit regions of the Moon. The variability can help to disentangle the individual contributions of OH and H2O to the 3 μm absorption feature and provide insight into the mechanism of production and loss of OH/H2O on the lunar surface. We investigate the spatial and diurnal variations of hydration on the southern hemisphere of the Moon as observed by the Deep Impact spacecraft during the lunar flybys in 2009 at spatial scales of 30–70 km/pixel. For a subset of observations of across the lunar south polar region (∼2% of the lunar surface), repeat coverage includes three different times spanning half a lunar day, allowing for exploration of diurnal variability. We determine that OH/H2O is widespread but variable across the lunar south pole. At all but the lowest temperatures observed, highland regions have stronger hydration absorption features than the maria. Changes in band strength demonstrate variable loss rates controlled by surface temperatures with H2O lost quicker at higher temperatures. Observed variability in the band shape strongly suggests higher H2O abundance at low temperatures. These observations are strong evidence that the unique shape of the 3 μm band is due to both OH and H2O. The rapid diurnal evolution of the absorption feature implies that migration of these constituents occurs locally over short distances driven by temperature changes.

Lunar South Polar Water Cycle and Water Resources: Diurnal and Spatial Variations in Surficial Hydration From Repeated Moon Mineralogy Mapper Observations

“…Specifically, the OMAT of SPA is also comparatively higher (Figure 2; Lemelin et al., 2022). SPA exhibits a lower hydration band depth than surrounding highland areas, consistent with previous observations that highland areas typically contain more hydration than maria (Honniball et al., 2020; Laferriere et al., 2022; McCord et al., 2011; Pieters et al., 2009; Wӧhler et al., 2017). It is currently unclear and under debate which factor, mineral abundance or OMAT, dominates the hydration content.…”

“…However, comparison of repeat In‐Magnetotail and Out‐Magnetotail mosaics showed that there is no obvious hydration content variation between the Moon is in and outside the magnetotail. This observation seems to support the migration mechanism (Hendrix et al., 2019; Laferriere et al., 2022). However, Wang et al.…”

“…The hydration band depth changes efficiently with local time, suggesting that OH/H 2 O is continuously removed and replenished. There are two hypotheses that could explain the variation in hydration band depth with temperature: (a) Lunar surficial hydration is migrating along temperature gradients, for example, migrating to colder higher latitudes and/or colder regions with earlier or later local times (Hendrix et al., 2019; Laferriere et al., 2022; Sunshine et al., 2009). (b) The surficial hydration is under the dynamic equilibrium processes between solar wind implantation and diffusion (Y. C. Xu et al., 2022).…”

“…Stronger hydration absorptions were observed to occur at higher latitudes of the Moon (Clark, 2009;Honniball et al, 2020;Li & Milliken, 2017;McCord et al, 2011;Pieters et al, 2009;Sunshine et al, 2009;W ö hler et al, 2017;Wohlfarth et al, 2023), while Bandfield et al (2018) demonstrated that prominent 3 μm spectral features can be observed across a range of latitudes, local times, and surface types. Li and Milliken (2017) suggested that surficial hydration is not significantly related to the mineralogy composition at a global scale, but rather to regolith maturity, while several observations showed that the felsic highland terrains retained hydration more efficiently than did the mafic mare terrains (Honniball et al, 2020;Laferriere et al, 2022;McCord et al, 2011;Pieters et al, 2009;Sunshine et al, 2009;W ö hler et al, 2017). A recent study by Wohlfarth et al (2023) revealed that highlands exhibit stronger absorption bands than mare regions during lunar midday, whereas the reverse is observed during lunar morning and evening.…”

The Distribution of Molecular Water in the Lunar South Polar Region Based upon 6 μm Spectroscopic Imaging

“…The presence of localized enhancements in water abundance may support the hypothesis that at least part of the water produced by solar wind and meteoroid bombardment on the lunar surface may subsequently migrate toward the poles (Schorghofer et al 2017;Hendrix et al 2019), that localized topography and roughness may play a role in water retention on the lunar surface (Davidsson & Hosseini 2021), and that colder portions of the polar regions may retain water even if not permanently shadowed (Kloos et al 2019;Hayne et al 2021). Both the observations from Deep Impact (Sunshine et al 2009;Laferriere et al 2022) and SOFIA show higher H 2 O abundances where the surface is locally colder, and where migrating water may collect in cold traps and be reduced at high temperature on a diurnal cycle. The observational results presented in this paper pertain to the distribution of water traced by the 6.1 μm emissivity feature, which arise from water molecules very near the surface but could trace a more pervasive source.…”