THE XENON ISOTOPIC COMPOSITION OF THE PRIMORDIAL MARTIAN ATMOSPHERE -CONTRIBUTIONS FROM SOLAR AND FISSION COMPONENTS

Previous models of the evolution of the isotopic composition of Xe in the Martian atmosphere have little room for any Xe produced by fission of Pu-244, although there is a contribution from the decay of shorter -lived I-129. We find that the previously calculated ratios of I-deriv ed to Pu-derived Xe can only be matched if Mars accreted very early an d outgassing of I-derived Xe after atmospheric loss was much stronger than outgassing of Pu-derived Xe. Alternatively, we find that if we as sume that primordial Xe on Mars had the isotopic composition of the so lar wind, rather than the previously assumed chondritic Xe, a larger c ontribution from Pu-244 is allowed, and it is much easier to generate models of outgassing and atmospheric loss that match the present atmos phere. Furthermore, the isotopic composition of solar wind Xe matches that of the Chassigny meteorite (widely considered to represent a samp le of Martian interior Xe), so it is not necessary to postulate separa te sources of Xe for the interior and atmosphere of Mars.