THE martian atmosphere has a high 129Xe/132Xe ratio compared with any on Earth and most meteorites. The 129Xe/132Xe ratio in the martian atmosphere is also high relative to the martian mantle1. In contrast, Earth's upper mantle has a higher 129Xe/132Xe ratio than its atmosphere2. As 129Xe is the daughter product of the extinct nuclide 129I, a means of fractionating iodine from xenon early in martian history appears necessary to account for the 129Xe/132 ratios of its known reservoirs. Crystal/melt partitioning will fractionate iodine from xenon in the right sense, but the fractionation is probably inadequate in magnitude; differences in the silicate melt solubilities of iodine and xenon would cause fractionation in the wrong direction. Here we present a model to account for the martian xenon data which relies on the very different solubilities of the two elements in water to fractionate them after outgassing. Atmospheric xenon is lost by impact erosion during heavy bombardment, followed by release of 129Xe produced from 129I decay in the crust.
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