A DIFFUSION SOURCE FOR SODIUM AND POTASSIUM IN THE ATMOSPHERES OF MERCURY AND THE MOON

This paper demonstrates that grain boundary diffusion at depth and regolith diffusion through a well-fractured crust and regolith explains the observed Na to K ratios in the lunar and Mercurian atmospheres. In addition, it explains the large enhancement in the Na abundance seen at Mercury over the Moon (∼250-600). The higher temperatures at Mercury and the strong dependence of diffusion coefficient on temperature are responsible for the ability of the diffusion mechanism to reproduce the observed abundances in contrast to other proposed source mechanisms. The establishment of diffusion as a viable source mechanisms for the Na and K atmospheres at Mercury and the Moon allows it to be compared with other proposed sources for Na and K. Diffusion will result in a thermalized source population. Impact volatilization results in a hot source population which is mostly thermalized on the timescale of the free fall time for ballistic orbits of atoms at the volatilization temperature. It is suggested that the source population at the Moon is dominated by the hot source populations of impact volatilization and the release mechanism of surface sputtering. Diffusion provides a new supply at the surface along with gardening by meteoritic bombardment. At Mercury it is predicted that diffusion is the dominant source. A hot component of Na and K at Mercury is smaller in proportion to the total abundances of these two constituents than at the Moon. The hot component is commensurate with a population of meteoritic material similar to that at the Moon and a surface composition which is no more depleted in K than that of the Moon. © 1990.