Source processes for the alkali metals in the atmosphere of Mercury

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Abstract
A large (fivefold) increase in Mercury's potassium(K) column abundance on 14 October 1987, has been reported by Sprague et al.[199O](SKH), who attributed the enhancement to diffusion through the surface in the Caloris Basin, from depths of order 10 km. The postulated source rate is much larger than any previously estimated diffusion rate, and if true certainly affects consideration of the origin of other atmospheric species. However, Killen et al.[1991] have pointed out that the claim is not supported by the published observations of K or sodium(Na) as a whole. Sprague et al.[1991] have responded by further hypothesizing the existence of several other sources of gas diffusing out of the regolith, all of which are time variable. In any case the SKH data indicate large variations in abundance, and it is important to understand the cause. With this issue in mind we have examined the available abundance estimates for correlation with possible controlling physical parameters. We have found a significant correlation between the average zenith K column abundance and indices of solar activity, although we are left with the intrinsic uncertainty of a small data set.