Helium and hydrogen in the lunar atmosphere

Of the gas species supplied by the solar wind, only helium and hydrogen are light enough to be lost from the moon by Jeans' thermal escape mechanism. To study the behavior of helium and hydrogen, a Monte Carlo technique has been used, in which random ballistic trajectories of individual molecules are traced over a spherical moon. In the computation, a particle is 'created' on the sunlit surface, and the locations of its subsequent encounters with the surface are recorded until it escapes. Global distributions of helium and hydrogen concentrations have been computed, based on the hypothesis that the release of neutral gases from the lunar surface is confined to daytime and is correlated with the solar wind influx. The resulting helium model is in good agreement with the measurements from the Apollo 17 lunar surface mass spectrometer.