Polar regions of the moon as a potential repository of solar-wind-implanted gases

Abstract

Possible global distributions of solar-wind-implanted elements in the lunar regolith are considered. Arguments are given that concentrations of such elements are controlled by the rate of their desorption from the implanted layers on the regolith particles. This is confirmed by a large difference in concentration ratios of solar-wind-implanted elements in the regolith and the solar wind. Desorption rate of the implanted atoms is proportional to exp(− U/ kT), where k is Boltzmann constant and U is activation energy, so it sharply decreases with decreasing temperature. This favors accumulation of the implanted elements in polar regions of the Moon. The implanted layers on the regolith particles damaged by radiation contain a variety of atomic configurations, hence a wide range of trapping energies (characterized by the activation energy U) should be expected for the implanted atoms. High values of U (>∼1 eV) typical of vacancies and broken chemical bonds account for high (by many orders greater than the equilibrium solubility) concentrations of the implanted gases found in the regolith of the equatorial regions. Lower trapping energies U are not effective at low latitudes, but contribute to long trapping of the implanted gases in polar regions. For hydrogen, whose molecule consists of two atoms, inhibited molecule formation at the surfaces of regolith particles can be an additional mechanism of retention of the implanted atoms. Inhibited degassing overcompensates the lack of solar wind flux at high latitudes. This is consistent with the enrichment of the lunar polar regions in hydrogen observed by Lunar Prospector. A similar global distribution can be expected for the other solar-wind-implanted elements, in particular, for 3He that is a potential thermonuclear fuel. Thus, polar regions may be the most promising sites for extraction of 3He, as well as the other gases.