Molecular water detected on the sunlit Moon by SOFIA

Abstract

Widespread hydration was detected on the lunar surface through observations of a characteristic absorption feature at 3 µm by three independent spacecraft1,2,3. Whether the hydration is molecular water (H2O) or other hydroxyl (OH) compounds is unknown and there are no established methods to distinguish the two using the 3 µm band4. However, a fundamental vibration of molecular water produces a spectral signature at 6 µm that is not shared by other hydroxyl compounds5. Here, we present observations of the Moon at 6 µm using the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). Observations reveal a 6 µm emission feature at high lunar latitudes due to the presence of molecular water on the lunar surface. On the basis of the strength of the 6 µm band, we estimate abundances of about 100 to 400 µg g−1 H2O. We find that the distribution of water over the small latitude range is a result of local geology and is probably not a global phenomenon. Lastly, we suggest that a majority of the water we detect must be stored within glasses or in voids between grains sheltered from the harsh lunar environment, allowing the water to remain on the lunar surface.

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Fig. 1: Lunar 6 µm emission bands.
Fig. 2: 6 µm band centre frequency distribution.
Fig. 3: Latitude variations.

Data availability

The data that support the plots within this paper and other findings of this study are publicly available from the SOFIA Data Cycle System at https://dcs.sofia.usra.edu and the Infrared Science Archive hosted by the Infrared Processing & Analysis Center (IPAC)) or from the corresponding author on reasonable request.

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Acknowledgements

Observations were made using the NASA/DLR SOFIA. SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA) under NASA contract number NNA17BF53C and the Deutsches SOFIA Insitut (DSI) under DLR contract number 50 OK 0901 to the University of Stuttgart.

Author information

Affiliations

Authors

Contributions

C.I.H. and P.G.L. carried out observations and data analysis. Data reduction to physical units of flux was done by S.S. S.L. provided the calibration of absorption to abundance of water and M3 data. C.A.H. provided laboratory meteorite data for 6 µm band comparisons and provided guidance regarding lunar material contributions at 6 µm. T.M.O. and W.M.F. advised C.I.H and P.G.L. on the activation energies, monolayer coverage and surface area estimates. D.M.H. provided the interpretation of these results for the lunar exosphere.

Corresponding author

Correspondence to C. I. Honniball.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Matthew A. Siegler and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Location map.

LRO WAC Image of the Moon with the slit location of the mare reference and Clavius overlaid.

Extended Data Fig. 2 Z score.

Z score histogram density for all Clavius observations.

Extended Data Fig. 3 Water calibration.

Water bearing glass beads showing a 3 and 6 µm absorption (a) used to derive a relationship between band depth (b) and H2O content (c).

Extended Data Fig. 4 Brightness temperature.

Brightness temperature from the Clavius region. The variation in brightness temperature corresponds to locations of more or less illumination.

Extended Data Fig. 5 Abundance histogram.

Distribution of H2O abundance for all data acquired in the Clavius region.

Extended Data Fig. 6 Zoomed Clavius map.

Left: Image of Clavius crater from quickmap that show the visible image with the SOFIA slit areas show by the white box. Right: The same image of Clavius crater but with the Clemintine UVVIS color ratio overlaid to highlight compositional variations. The SOFIA slit intersects Tycho crater ejecta at lower latitudes.

Supplementary information

Supplementary Information

Supplementary Figs. 1–13 and Tables 1 and 2.

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Honniball, C.I., Lucey, P.G., Li, S. et al. Molecular water detected on the sunlit Moon by SOFIA. Nat Astron (2020). https://doi.org/10.1038/s41550-020-01222-x

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