The Statistical Mechanics of Solar Wind Hydroxylation at the Moon, within Lunar Magnetic Anomalies, and at Phobos

1W. M. Farrell, 2D. M. Hurley, 3V. J. Esposito, 4J. L. McLain, 2M. I. Zimmerman
Journal of Geophysical Research (Planets) Link to Article [DOI: 10.1002/2016JE005168]
1NASA/Goddard Space Flight Center, Greenbelt, MD, USA
2Johns Hopkins University/Applied Physics Laboratory, Laurel, MD, USA
3NASA Goddard Summer Intern Program, NASA/Goddard Space Flight Center, Greenbelt, MD, USA
4University of Maryland, College Park, MD, USA
Published by arrangement with John Wiley & Sons

We present a new formalism to describe the outgassing of hydrogen initially implanted by the solar wind protons into exposed soils on airless bodies. The formalism applies a statistical mechanics approach similar to that applied recently to molecular adsorption onto activated surfaces. The key element enabling this formalism is the recognition that the inter-atomic potential between the implanted H and regolith-residing oxides is not of singular value, but possess a distribution of trapped energy values at a given temperature, F(U, T). All subsequent derivations of the outward diffusion and H retention rely on the specific properties of this distribution. We find that solar wind hydrogen can be retained if there are sites in the implantation layer with activation energy values exceeding 0.5 eV. We especially examine the dependence of H retention applying characteristic energy values found previously for irradiated silica and mature lunar samples. We also apply the formalism to two cases that differ from the typical solar wind implantation at the Moon. First, we test for a case of implantation in magnetic anomaly regions where significantly lower energy ions of solar wind origin are expected to be incident with the surface. In magnetic anomalies, H retention is found to be reduced due to the reduced ion flux and shallower depth of implantation. Second, we also apply the model to Phobos where the surface temperature range is not as extreme as the Moon. We find the H atom retention in this second case is higher than the lunar case due to the reduced thermal extremes (that reduces outgassing).

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