DIFFERENTIAL ABLATION OF COSMIC DUST AND IMPLICATIONS FOR THE RELATIVE ABUNDANCES OF ATMOSPHERIC METALS

Although it is generally accepted that extraterrestrial material is th e source of metals in the upper atmosphere, the relative abundances of mesospheric metal atoms and ions present us with a conundrum. Lidar o bservations have consistently shown that the abundances of neutral met als in the atmosphere and the abundances of these metals in the meteor ic material that falls to Earth are significantly disproportionate. Th e column density of neutral sodium is perhaps 2 orders of magnitude la rger than that of calcium, while the abundances in meteorites are appr oximately equal. By contrast, ion mass spectroscopy has shown that the abundances of the meteoric ions match reasonably well those in the me teorites. We present here a model that attempts to address these discr epancies. At the heart of the model is the concept of differential abl ation, which suggests that more volatile metals sublimate earlier in t he descent of a cosmic dust particle than do the less volatile compone nts. We model three different meteoric metals: sodium, magnesium, and calcium. Results suggest that sodium ablates to a greater extent than does calcium and that it ablates at a substantially higher altitude. D eposition at lower altitudes leads to more rapid conversion of the ato mic calcium into complexes through three-body reactions. Thus the depl etion of calcium arises from both a decrease in deposition and an incr ease in the rate of removal of that which is deposited. We examine the behavior of the model in several respects, comparing predicted result s with measurements and finding reasonable agreement. We argue that th e success of this model indicates that differential ablation is a key factor in the determination of the relative abundances of meteoric met als in the mesosphere.