Solar wind implantation into lunar regolith: Hydrogen retention in a surface with defects

Abstract

a b s t r a c t Solar wind protons are implanted directly into the top 100 nm of the lunar near-surface region, but can either quickly diffuse out of the surface or be retained, depending upon surface temperature and the activation energy, U, associated with the implantation site. In this work, we explore the distribution of activation energies upon implantation and the associated hydrogen-retention times; this for comparison with recent observation of OH on the lunar surface. We apply a Monte Carlo approach: for simulated solar wind protons at a given local time, we assume a distribution of U values with a central peak, U c and width, U w , and derive the fraction retained for long periods in the near-surface. We find that surfaces characterized by a distribution with predominantly large values of U (>1 eV) like that expected at defect sites will retain implanted H (to likely form OH). Surfaces with the distribution predominantly at small values of U (<0.2 eV) will quickly diffuse away implanted H. However, surfaces with a large portion of activation energies between 0.3 eV < U < 0.9 eV will tend to be H-retentive in cool conditions but transform into H-emissive surfaces when warmed (as when the surface rotates into local noon). These mid-range activation energies give rise to a diurnal effect with diffusive loss of H at noontime. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Solar Wind Implantation Into the Lunar Regolith: Monte Carlo Simulations of H Retention in a Surface With Defects and the H2 Exosphere

“…Emissive surfaces degassed their hydrogen content in less than a lunation, whereas retentive surfaces possessed more high activation energy trapping sites retaining atoms longer than a lunation period. For the intermediate energy range Farrell et al () found that the surface concentration could vary diurnally. However, these previous studies considered the dynamic buildup of the hydrogen surface concentration on timescales much smaller than a lunation.…”

“…Estimates of the diffusional lifetime for atoms trapped physically and chemically are given in Table . The lifetimes were evaluated using the τ 1 with D 0 = 10 −6 m 2 /sand h = 100 nm from Farrell et al (). Within crystalline SiO 2 , the energy barrier to vacancy diffusion and H bound in OH groups is ~1–3 eV.…”

The statistical mechanics of solar wind hydroxylation at the Moon, within lunar magnetic anomalies, and at Phobos

“…It was previously found [ Farrell et al ., ] that Gaussian‐shaped distributions in activation energy with a progressively lower number of trapped H atoms with increasing activation energy above about 0.4 eV could account for the observations, including modeling the minimum in H retention at noontime locations where solar wind incidence is the greatest. Specifically, at cool temperatures, a large portion of F ( U , T ) remains trapped, but as temperature increases to 400 K, only those few H atoms occupying the high‐energy tail of the distribution on sites with U > 0.5 eV remain, giving rise to an H retention minimum at local noon.…”

“…Past modeling of the H retention in the top tens of nanometers of lunar regolith [ Farrell et al ., ] was designed to find a distribution of activation energy F ( U o , Δ U ) that was consistent with the Chandrayaan‐1 Moon Mineralogy Mapper observations and the complementary IR data sets from Cassini and EPOXI [ Pieters et al ., ; Sunshine et al ., ; Clark , ; McCord et al ., ]. Specifically, these new observations, reported on an OH feature that was of relatively low concentration (mild retention) at 10–1000 ppm [ Clark , ], tended to become more pronounced at large solar zenith angles (near the cooler terminator regions) [ McCord et al ., ], possibly having a diurnal effect [ Sunshine et al ., ] but also possessing a mineralogical component associated with feldpathic mineralogy in highland soils [ McCord et al ., ].…”

“…As shown in Figure 2 of Farrell et al . [], these sites become highly H retentive below about 200 K but H emitting above 200 K and could be responsible for a diurnal effect where the surface appears to “breath” H atoms.…”