Hydroxylation of Apollo 17 Soil Sample 78421 by Solar Wind Protons

J. L. McLain, M. J. Loeffler, W. M. Farrell, C. I. Honniball, J. W. Keller, R. Hudson

Research output: Contribution to journalArticlepeer-review

4 Scopus citations

Abstract

Hydroxylation by solar wind protons has been simulated in our laboratory on Apollo 17 lunar sample 78421, a very mature regolith sample that is rich with agglutinates (68%). The goal of this study was to determine the rate of hydroxyl formation and their thermal stability by monitoring changes in the SiOH (hydroxyl) stretching band near 3 μm using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). A 2 keV H2+ ion beam was used to simulate proton implantation on 78421 and on a crushed fused silica sample. We find that the OH band does not change unless the samples have been annealed in vacuum prior to irradiation. Qualitatively, the OH bands for the fused silica and 78421 are very different. The OH band for fused silica is centered at 2.74 μm and is relatively sharp ranging from 2.67 to 3.1 μm at full width at half maximum (FWHM), while the OH band for 78421 is centered at 3.0 μm and ranges from 2.74 to 3.37 μm at FWHM. The increase in wavelength and broadened nature of the OH band in 78421 may be associated with the OH’s proximity to surface defects and/or lattice vacancies. The lack of the H2O bending mode at 6.1 μm indicates that any adsorbed terrestrial H2O is below our detection limit, and therefore the H2O stretching mode at 2.9 μm is not significantly contributing to the broad 3 μm OH band and implies that proton implantation by itself does not lead to water formation. To simulate the maximum dayside temperature on the lunar surface, the lunar sample was heated after proton irradiation. The proton induced OH concentration was reduced by as much as 25% after heating to 400 K (127°C).

Original languageEnglish (US)
Article numbere2021JE006845
JournalJournal of Geophysical Research: Planets
Volume126
Issue number5
DOIs
StatePublished - May 2021

Keywords

  • Apollo 17
  • hydroxylation
  • lunar regolith
  • proton irradiation
  • solar wind

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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