Hydrogen migration to the lunar poles by solar wind bombardment of the moon

Abstract

We investigate the deposition of hydrogen at the lunar poles from the incident solar wind proton flux. We follow an incident solar wind proton as it interacts with the lunar surface, is emitted into the atmosphere, and migrate through a series of ballistic hops across the surface of the Moon. We trace the path of the particle until it is removed from the system by photo-processes such as ionization or dissociation, by thermal escape, by surface chemistry, or by reaching a cold trap. Iterating with various compounds formed through surface chemistry, we follow all hydrogen atoms input into the system, regardless of their molecular composition. Accumulating statistics on the outcomes for various input particles, we determine that .6 of the hydrogen forms H 2 through chemical sputtering. Most of the remaining H thermally desorbs as atomic in a fraction of .27. Although the amount converted to OH is small (10%), most hydrogen reaches the pole through OH migration, .008 OH/H +. Finally, we calculate the amount of time required for the solar wind to supply the amount of hydrogen detected at the poles by the Lunar Prospector Neutron Spectrometer to be 7 Myr. If the poles only retain water due to the low freezing points of the other molecules, the time required is 100 Myr.