Monte Carlo Simulation of the Global Migration of Lunar Hydroxyl From a Magnetic-Shielded Solar Wind Source

Abstract

Solar wind has been regarded as an important source for the surficial water on the Moon. Here we investigate the global migration of solar wind-derived hydroxyl (OH) molecules with a Monte Carlo model, in which the shielding effects of both the Earth's magnetosphere and the lunar magnetic anomalies on the production rate of OH are considered. It is found that the OH surface concentration depends on the latitude, the local time, and the lunar phase. Specifically, the surface concentration can increase from nearly zero ppm near the subsolar point to about 1,000 ppm near the polar regions, and there is a local peak near the morning terminator that can result in a diurnal variation of about 100 ppm. Moreover, the OH abundances can be reduced by about 15% in the magnetotail when at low and mid latitudes. But when in the polar region, the OH abundance can keep increasing, resulting in an average deposition rate of $1.0\times {10}^{12}{m}^{-2}{s}^{-1}$. In addition, a south-north asymmetry can be caused by the magnetic anomalies, in which the OH abundance in the southern hemisphere is generally lower than that in the northern hemisphere, with maximum difference of about 31% near the 75° latitude. In the exosphere, the OH number density shows a dawn-dusk asymmetry, with relatively higher number densities on the dawnside, and the maximum density can reach $2.8\times {10}^9{m}^{-3}$ near the subsolar region. These results can greatly improve our understanding on the migration and distribution of OH molecules on the Moon.