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R.M. Killen (Southwest Research Institute)
40Ar in the atmospheres of the planets is a measure of potassium abundance in the interiors since 40Ar is a product of radiogenic decay of 40K by electron capture with the subsequent emission of a 1.46 eV g-ray. Although the 40Ar in the earth's atmosphere is expected to have accumulated since the late bombardment, 40Ar in the atmospheres of Mercury and the Moon is eroded quickly by photoionization and electron impact ionization. Thus, the argon content in the exospheres of the Moon and Mercury is representative of current effusion rather than accumulation over the lifetime of the planet. We reconsider the source and loss processes for the argon atmospheres of Mercury and the Moon in order to investigate what these atmospheres tell us about the structure and composition of the megaregolith. Argon is especially important because it does not engage in chemistry. In order to interpret measurements of argon in Mercury's atmosphere, we must have a model including sources from the interior and loss from the atmosphere. Our model was trained on the moon, where in-situ measurements are available from instruments left on the surface by the Apollo 17 astronauts. First we consider production of 40Ar from K, and its diffusion to the surface. We assume fractal distributions of distances to a connected pore space. We first solve the diffusion equations for radiogenic argon and train the solutions to predict the average lunar atmosphere. Then we use these solutions with the appropriate boundary conditions for Mercury. In order to determine what "rock" size distribution is required to produce a given atmosphere by diffusion, we used the equations for diffusion of argon through uniform spheres of radius, a, with diffusion coefficient, D, for a time, t (Mussett, 1969). The effective rock size may be smaller than the actual rock size since the effective radius may be the minimum distance to a rock defect or dislocation. Once the argon has diffused out of the crystalline rock and into a pore space, the effusion to the surface is by Knudsen flow. We conclude that the argon atmospheres of Mercury and the Moon can be supplied by current effusion from the upper few kilometers but the models are not unique. Either the grain sizes are small, similar to those at the surface, and pore space remains open to great depth, or contributing rock sizes are large. We consider the second scenario the more likely.