EARLY OUTGASSING OF MARS SUPPORTED BY DIFFERENTIAL WATER SOLUBILITY OF IODINE AND XENON

THE martian atmosphere has a high Xe-129/Xe-132 ratio compared with any on Earth and most meteorites. The Xe-129/Xe-132 ratio in the martian atmosphere is also high relative to the martian mantle 1. In contrast, Earth's upper mantle has a higher Xe-129/Xe-132 ratio than its atmosphere 2. As Xe-129 is the daughter product of the extinct nuclide I-129, a means of fractionating iodine from xenon early in martian history appears necessary to account for the Xe-129/Xe-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 X-129 produced from I-129 decay in the crust.