Hydrogen implantation in silicates: The role of solar wind in SiOH bond formation on the surfaces of airless bodies in space

Abstract

Hydroxyl on the lunar surface revealed by remote measurements has been thought to originate from solar wind hydrogen implantation in the regolith. The hypothesis is tested here through experimental studies of the rate and mechanisms of OH bond formation due to H + implantation of amorphous SiO 2 and olivine in ultrahigh vacuum. . The initial conversion rate of implanted H + into hydroxyl species was found to be~90% and decreased exponentially with fluence. There was no evidence for molecular water formation due to proton irradiation. Translating the laboratory measurements in thin plate samples to the granular lunar regolith, it is estimated that the measurements can account for a maximum of 17% relative OH absorption in reflectance spectroscopy of mature soils, consistent with spacecraft observations in the infrared of the Moon.

Radiation Effects on Airless Bodies in Space: Radiolysis, Sputtering, and Sintering in Regoliths

“…Although clear spectral changes were seen after H + irradiation of amorphous SiO 2 and olivine thin sections [Schaible and Baragiola, 2014], no radiation induced spectral changes could be clearly discerned from the data for LS15058. The samples used in the two experiments were very different.…”

“…Changes in the optical features of the surface can be studied observationally [Clark et al, 2014] and in laboratory simulations [Bennett et al, 2013;Savin et al, 2012;Allodi et al, 2013]. For example, the solar wind has been shown to cause reddening of lunar-type silicates [Pieters et al, 1993;Hapke, 2001], and experiments using analog silicate materials suggest that solar wind H + bombardment is sufficient to explain an observed OH absorption signal present at the lunar equator [Ichimura et al, 2012;Schaible and Baragiola, 2014]. Optical spectra of solar system bodies are typically determined through reflectance spectroscopy [Brown et al, 2004;Pieters et al, 2009, e.g.…”

“…Optical spectra of solar system bodies are typically determined through reflectance spectroscopy [Brown et al, 2004;Pieters et al, 2009, e.g. ], although studying returned materials, meteorites, and analog samples in the laboratory [Pieters et al, 1993;Schaible and Baragiola, 2014] greatly aids interpretation. Additionally, computational methods can be used to simulate the weathering processes to explain various observed features [Hurley et al, 2012;Schaible et al, 2016], or more basic modeling can be carried out to predict what chemical and physical environments might be expected in early solar systems similar to our own [Hincelin et al, 2015;Drozdovskaya et al, 2016].…”

“…Although the bulk of the work presented in this section was completed as part of an M.S. thesis presented at UVA in Feb, 2014 [Schaible, 2014;Schaible and Baragiola, 2014], experimental data taken for 5 − 15 keV H + irradiation of lunar soil sample 15058 will be presented here for the first time.…”

“…The irradiated soils show clear spectral changes as a result of ion radiation. However, due to the opaqueness of the lunar soil and the weak transmitted IR signal through the sample, small changes in the 2.8 µm absorption band expected due to combination of incident H + ions with structural O [Schaible and Baragiola, 2014] could not be distinguished.…”