Regional Map of Molecular Water at High Southern Latitudes on the Moon Using 6 μm Data From the Stratospheric Observatory for Infrared Astronomy

Abstract

A map of surface molecular water was derived from long slit spectroscopy of the south polar region of the Moon using the Faint Object infraRed CAmera for the SOFIA Telescope spectrometer on the Stratospheric Observatory for Infrared Astronomy. Mean water abundances detected are about 250 μg/g over that of a mare reference surface at Mare Fecunditatis. Water abundances are locally anticorrelated with temperature. The distribution of water is consistent with derivation of water from pre‐existing hydroxyl subsequently trapped in impact glass, provided hydroxyl increases with latitude as some models and measurements suggest. The detected water cannot be in equilibrium with the exosphere because insufficient water is present in the exosphere to maintain the surface abundance. The data are consistent with a high latitude water‐bearing mineral host that may be a precursor to recently detected high latitude hematite.

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

“…A 6.1 μm spectral feature uniquely detects H 2 O (as opposed to just the O-H bond) because it arises from an H-O-H bending mode that requires all three atoms of the molecule. This feature was detected at a sunlit region near the Clavius crater (Honniball et al 2021) and has been confirmed over a larger area with more recent measurements (Honniball et al 2022). In this work, we present the results for part of the southern polar region, in order to determine whether the 6.1 μm water feature correlates with other localized structures.…”

“…Water molecules were recently detected even on the sunlit surface of the Moon (Honniball et al 2021(Honniball et al , 2022, and this paper extends those results to present the first well-sampled, wide-area images of localized variations in water abundance. A 6.1 μm spectral feature uniquely detects H 2 O (as opposed to just the O-H bond) because it arises from an H-O-H bending mode that requires all three atoms of the molecule.…”

“…The different latitude trends could be due to local differences due to different parts of the lunar surface that were sampled, or due to the different lunar times. The subsolar longitude was −12°for the observations presented in this paper, compared to +21°for the prior regional map (Honniball et al 2022); at the Moretus crater, that means the lunar time of day was just 0.4 hr after noon for the new observations and 1.8 hr before noon for the prior regional map. It is possible that the lunar time of day plays a significant role in the mobility of water at high latitudes.…”

“…Honniball et al (2021) showed the water feature was stronger (W ∼ 0.4) at latitude −75°and decreased to a plateau (W ∼ 0.2) at latitude −55°; the range of values is similar to what is seen in the map reported here, but the trend versus latitude is different. Honniball et al (2022) showed in a larger regional map that the water feature was stronger (W ∼ 0.3) for the higher-flux surface at latitudes around −60°a nd decreased (to W ∼ 0.1) toward the lower-flux surface at The shades of blue indicate the emissivity, relative to the continuum fit, from 0.997 (no color) to 1.019 (dark blue). The thick, horizontal dark band is the 6.1 μm water feature.…”

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

“…Honniball et al. (2022) later detected surface abundance from the 6 μm feature that exceeds the amount measured for the lunar exosphere, suggesting that the feature is caused by molecular water trapped in an impact glass. In situ observations by the Chang’E−5 lander provide evidence for hydration, but cannot conclusively state whether the feature is provided by hydroxyl or molecular water (Lin et al., 2022).…”