Early outgassing of Mars supported by differential water solubility of iodine and xenon

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.