Untangling the formation and liberation of water in the lunar regolith

Crandall, Parker B.; Gillis-Davis, J. J.; Ishii, H. A.; Bradley, J. P.; Corley, L. M.; Zhu, Cheng

doi:10.1073/pnas.1819600116

Cited by 53 publications

(44 citation statements)

References 57 publications

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“…Our results are more consistent with the existence of a mechanism that produces water by impact from pre-existing lunar material, perhaps the mechanism of Zhu et al 17 , than impact-delivered water. Analysis of data taken near the equator by the Lunar Atmosphere and Dust Environment Explorer (LADEE) suggests that smaller impacts that probe the upper few centimetres of the regolith do not result in water released to the exosphere and that the uppermost few centimetres of soils are desiccated 15 .…”

supporting

confidence: 90%

“…Water can be formed in situ on grain surfaces from pre-existing hydroxyl that undergoes recombinative desorption at high lunar noontime temperatures, particularly at the equator, also releasing this water into the exosphere for later loss or trapping 16 . Water can also be formed in situ from pre-existing hydroxyl during micrometeorite impact, when high temperatures promote the reaction, as has been recently demonstrated in the laboratory 17 . In experiments, the water was detected as vapour, but presumably a portion of that water can also be sequestered into impact glasses.…”

mentioning

confidence: 75%

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Molecular water detected on the sunlit Moon by SOFIA

et al. 2020

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Widespread hydration was detected on the lunar surface through observations of a characteristic absorption feature at 3 µm by three independent spacecraft 1-3. Whether the hydration is molecular water (H 2 O) or other hydroxyl (OH) compounds is unknown and there are no established methods to distinguish the two using the 3 µm band 4. However, a fundamental vibration of molecular water produces a spectral signature at 6 µm that is not shared by other hydroxyl compounds 5. 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 H 2 O. 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. Using SOFIA and the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST) instrument, we conducted observations of the lunar surface at 6 µm on 31 August 2018 in a search for molecular water. FORCAST is well suited to look for 6 µm lunar water due to its wavelength coverage from 5 to 8 µm, spectral resolution of R = 200 and high signal-to-noise ratios. The FORCAST entrance slit that defines the portion of the Moon observed is 2.4 × 191 arcsec sampled with 248 pixels. At the lunar centre of disk the slit has a spatial extent of 4.8 × 1.5 km 2 (the spatial resolution near the limb is lower due to foreshortening). During the observations, the Moon was at a phase angle of 57.5°. We observed a region at high southern latitudes near Clavius crater and a low-latitude portion of Mare Serenitatis (Extended Data Fig. 1). Details regarding observations, site selection and data reduction can be found in the Methods. Data from SOFIA reveal a strong 6 µm emission band at Clavius crater and the surrounding terrain (Fig. 1) relative to the control location near the lunar equator, which shows low hydration in some analyses (see Methods). All spectra from the Clavius region exhibit this 6 µm emission feature. The majority of these emission peaks (98%) exceed 2σ significance relative to the background noise, and about 20% exceed 4σ significance (Extended Data Fig. 2). To determine whether the spectral properties of the lunar 6 µm band are consistent with spectra of particulate water-bearing materials, we examined other planetary materials that show a 6 µm

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confidence: 90%

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confidence: 75%

Molecular water detected on the sunlit Moon by SOFIA

et al. 2020

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“…Furthermore, the implantation of solar wind can be responsible for modifications of the regolith. Protons for example are expected to be responsible for hydroxyl and water formation in minerals on Moon and Mercury [5]. The retention of He in silicate minerals has been investigated by Lord [6].…”

Section: Introductionmentioning

confidence: 99%

Solar wind Helium ion interaction with Mg and Fe rich pyroxene as Mercury surface analogue

Biber

Szabo

Jäggi

et al. 2020

Nuclear Instruments and Methods in Physics Research Section B:

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The surface of Mercury is continuously exposed to impinging solar wind ions. To improve the understanding of space weathering and exosphere formation, a detailed investigation of the ion-surface interaction is necessary. Magnesium and iron rich pyroxene (Ca,Mg,Fe) 2 [Si 2 O 6 ] samples were used as analogues for Mercury's surface and irradiated with He + ions at solar wind energies of 4 keV. Several regimes of implantation and sputtering were observed there. The total estimated mass of implanted He coincides with the mass decrease due to He outgassing during subsequent Thermal Desorption Spectroscopy measurements. Comparison to established modeling efforts and SDTrimSP simulations show that a He saturation concentration of 10 at.% has to be assumed. A complete removal of He is observed by heating to 530 K. On the surface of Mercury, temperatures between about 100 K and 700 K are expected. This temperature will therefore influence the implantation and release of He into Mercury's exosphere.

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“…Thus, other forms of energy or catalyst will be required to form water. Micrometeoroid impact has been considered as a major catalyst for transforming hydroxyl to water on lunar surface because the heat released by impact can provide sufficient energy to start the reactions (Zhu et al., 2019).…”

Section: Introductionmentioning

confidence: 99%

Molecular Dynamics Simulations of Dielectric Breakdown of Lunar Regolith: Implications for Water Ice Formation on Lunar Surface

Huang

Nomura

Nakano

et al. 2021

Geophysical Research Letters

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Molecular dynamics simulations are carried out to investigate dielectric breakdown of lunar regolith induced by space weather events and its potential effects on water ice formation on lunar surface. We find that dielectric breakdown can trigger the water formation process by breaking the chemical bonds of regolith grains and exposing the oxygen atoms to react with the hydrogen implanted by solar wind. In the permanently shadowed region, the water molecules formed become attached to regolith grains in the molecular structure of ice after the event. Thus, dielectric breakdown can also enable the preservation of water molecules by changing the hydrophobicity of regolith grains.

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Untangling the formation and liberation of water in the lunar regolith

Cited by 53 publications

References 57 publications

Molecular water detected on the sunlit Moon by SOFIA

Molecular water detected on the sunlit Moon by SOFIA

Solar wind Helium ion interaction with Mg and Fe rich pyroxene as Mercury surface analogue

Molecular Dynamics Simulations of Dielectric Breakdown of Lunar Regolith: Implications for Water Ice Formation on Lunar Surface

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