MAINTAINING THE NA ATMOSPHERE OF MERCURY

We have calculated the global supply rates of Na to the atmosphere of Mercury either directly from depth as gaseous Na or through resupply of fresh regolith materials to the surface for subsequent placement into the atmosphere by surface-active processes. To match the supply of Na to the atmosphere of Mercury with the production of photoions (1-2 × 107 ions cm-2 sec-1), assumed to be the dominant loss process, it is necessary to assume either that the abundance of Na in the crust of Mercury is 1-2% (i.e., terrestrial), that a large fraction (80%) of the ions are recycled, or that the atmosphere is only temporary. Of these, the most likely appears to be the first. The possible rates of diffusion of gaseous Na to the surface range from 100 to 105 atoms cm-2 sec-1, with the most likely rate less than 3 × 102 atoms cm-2 sec-1, five orders of magnitude less than that required to maintain the present exosphere. This is in sharp contrast to an earlier estimate in the literature (A. L. Sprague, 1990, Icarus 84, 93-105). We also show that a system of deep, well-connected vents or fractures cannot provide any significant supply of gaseous Na to the exosphere in contrast with the suggestion by A. Sprague, R. W. H. Kozlowski, and D. M. Hunten (1991, Science 249, 1140-1143). Under the assumption that the abundance of Na in the upper crust is 0.004 by mass (lunar Na abundance), the most likely rate of supply of Na to the surface by creation of fresh regolith is 3.5 × 106 atoms cm-2 sec-1. Therefore, surface-acting processes can maintain the supply of Na to the atmosphere if the abundance of Na in the upper crust is at least 0.01 by mass. © 1993 by Academic Press, Inc.