Hydroxylation of Apollo 17 Soil Sample 78421 by Solar Wind Protons

Abstract

Solar wind induced hydroxylation on the Moon and other airless bodies is critically important to understand how space weathering produces and distributes potential hydrogen-bearing resources such as water. Our understanding of these hydrogen-bearing resources was significantly advanced in 2009 and 2010 when hydroxyls in the mid-latitude and cool terminator regolith was observed via infrared spectroscopy (Clark, 2009;Pieters et al., 2009;Sunshine et al., 2009), and when water-ice within permanently shadowed polar craters was confirmed (Colaprete et al., 2010). The few percent depth of this hydroxyl 2.8 μm absorption band is consistent with concentrations of OH/H 2 O at 10-1,000 ppm. It was suggested that solar wind proton implantation was a possible source for the OH absorption feature (McCord et al., 2011;Pieters et al., 2009). In this case, solar wind protons at 1 keV would implant into the oxygen-bearing regolith, and

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

“…The desorption rate of adsorbed H 2 O as described in Poston et al (2015) is likely the controlling factor for the change in band minima. However, McLain et al (2021) found that the 3 μm band of the Apollo sample 78,421 was shifted to longer wavelengths with increased proton irradiation fluence. The spectra from that work lack the 6.1 μm water feature.…”

“…However, McLain et al. (2021) found that the 3 μm band of the Apollo sample 78,421 was shifted to longer wavelengths with increased proton irradiation fluence. The spectra from that work lack the 6.1 μm water feature.…”

Chang’E-5 samples reveal high water content in lunar minerals

“…Previous studies have suggested that ~20–50% of solar wind protons can be bonded with oxygen to form OH/H 2 O 49 . Based on this, we calculated the surface density of hydrogen ( ρ H ) from the following equation 50 : where the p is the fraction of hydrogen bonded to oxygen, and the value of 20–50% was taken for our calculations. D is the penetration depth for solar wind protons, N A is the Avogadro’s constant (6.02 × 10 23 ), and ρ m is the density of minerals, which is 3.35 g/cm 3 for olivine, 2.73 g/cm 3 for plagioclase, and 3.21 g/cm 3 for pyroxene.…”

“…Combined with the proton flux of solar wind (4 × 10 15 H/cm −2 /yr), we calculated the accumulated implantation time of solar wind for lunar minerals to be ~270–4212 years (Supplementary Table 5 ). However, there may be considerable uncertainty in these values, such as the conversion of proton flux to OH/H 2 O, for which values can vary considerably 15 , 50 , the complex factors influencing the formation processes of solar wind-derived water, the escape process of solar wind-derived water by temperature and ultraviolet radiation, the diffusion of solar wind-derived water. Estimation of the exposure age of solar wind implantation for lunar soil still requires further investigation.…”

Simulated Lunar Surface Hydration Measurements Using Multispectral Lidar at 3 µm

“…The Supplementary Material contains spectral comparisons and accompanying discussion of the hydrated MORB glasses used here to the 3 μm band shape from lunar remote sensing data from the InfraRed Telescope Facility (IRTF, Figure S1 in Supporting Information S1) and from M 3 (Figure S2 in Supporting Information S1). The shape of the MORB 3 μm band can also be compared to the hydroxyl feature in laboratory spectra of proton-irradiated Apollo soils (Ichimura et al, 2012;McLain et al, 2021). Ubiquitous (but unquantified) adsorbed surface water is present in the laboratory reference spectra of Apollo samples we used (Figure S3 in Supporting Information S1) due to the high reactivity of lunar soil samples, with similar band shape to the hydrated MORB glasses.…”