THE ATOMIC SODIUM EXOSPHERE/COMA OF THE MOON

A Monte Carlo model is used to study the random walk process of sodium atoms created by meteoroid impacts on the lunar surface. The collisional surface interaction is parameterized by a thermal accomodation coefficient of 0.5. Sodium atoms launched with initial velocities well below the surface escape velocity of 2.38 km s-1 will be recycled and contribute to the formation of an exosphere with a temperature of about 500 K on the sunlit side. For large launch velocities (> 2 km s-1) there will be systematic transport of the escaping sodium atoms to the antisolar direction as a result of the acceleration effect of the solar radiation pressure. An extended coma with a significant elongation in the antisolar direction can form this way. Recent spectroscopic and imaging observations are consistent with this picture. The present model calculations suggest that meteoroid impact effect (and solar wind sputtering) should be the major supplier of the lunar sodium exosphere; the possible existence of an efficient loss mechanism of the sodium atoms during surface interaction is also indicated.