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Solar Wind Implantation into Lunar Regolith II: Monte Carlo Simulations of Hydrogen Retention in a Surface with Defects and the Hydrogen (H, H2) Exosphere
| Content Provider | Semantic Scholar |
|---|---|
| Author | Tucker, O. J. Farrell, William M. Killen, Rosemary Margaret Hurley, Dana M. |
| Copyright Year | 2018 |
| Abstract | Introduction: Recently, the near-infrared observations of the OH veneer on the lunar surface by the Moon Mineralogy Mapper (M) have been refined to constrain the OH content to 500 – 750 parts per million (ppm) [1]. The observations indicate diurnal variations in OH up to 200 ppm possibly linked to warmer surface temperatures at low latitude. We examine the M observations using a statistical mechanics approach to model the diffusion of implanted H in the lunar regolith [2, 3]. We present results from Monte Carlo simulations of the diffusion of implanted solar wind H atoms and the subsequently derived H and H2 exospheres. Hydrogen Retention: The hydrogen retention model is taken from Farrell et al. (2015, 2017) [2, 3]. The solar wind (SW) flow is composed of density, nsw = 5 × 10 H/m, with velocity vsw = 400 km/s. The source rate is defined with nswvswcos(Z) where Z is the solar zenith angle. Farrell et al. (2015) [2] demonstrated that the outgassing of hydrogen atoms implanted by the solar wind is more accurately described by considering a distribution of trapped energy values as opposed to a singular value for binding sites within the surface. In this approach, each implanted H atom is given a binding energy U from a Monte Carlo selection of a Gaussian distribution, e. g. F(U0, UW) ~ exp(-(U – U0)/UW), defined by the peak energy U0 and peak width UW [2, 3]. The random binding energy sites and local surface temperature are used to define the diffusion time of H to the surface. Farrell et al. (2017) [3] found that SW hydrogen could be retained in the implantation layer ~22 nm if there are binding energy sites with energy U > ~0.5 eV consistent with values derived for irradiated silica and in mature lunar samples. They determined that a Gaussian distribution used to characterize surficial sites with U0 = 0.5 eV and UW = 0.1 eV could produce a diurnal modulation of surface concentrations with lower concentrations in warm regions and the highest concentrations in terminator/polar regions [3]. We refer to this as the nominal case. We build upon the Farrell et al. studies by performing Monte Carlo simulations of hydrogen in the lunar environment to make quantitative comparisons to the M observations. For example, in Figure 1 we show results of simulated surface concentrations for an emissive, nominal and retentive surface each defined by U0 = 0.3, 0.5, 0.7 eV, respectively. For the nominal case, we obtained a quasi-steady state after 2 lunations. At low latitudes the influx is balanced by outgassing, however there is continual buildup of densities near the poles that resulted in a total mass of ~ 2 × 10 g over 9 lunations. |
| Starting Page | 2549 |
| Ending Page | 2549 |
| Page Count | 1 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20180002105.pdf |
| Alternate Webpage(s) | https://www.hou.usra.edu/meetings/lpsc2018/pdf/2549.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
