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 244Pu, although there is a contribution from the decay of shorter lived 129I. We find that the previously calculated ratios of I-derived to Pu-derived Xe can only be matched if Mars accreted very early and outgassing of I-derived Xe after atmospheric loss was much stronger than outgassing of Pu-derived Xe. Alternatively, we find that if we assume that primordial Xe on Mars had the isotopic composition of the solar wind, rather than the previously assumed chondritic Xe, a larger contribution from 244Pu is allowed, and it is much easier to generate models of outgassing and atmospheric loss that match the present atmosphere. Furthermore, the isotopic composition of solar wind Xe matches that of the Chassigny meteorite (widely considered to represent a sample of Martian interior Xe), so it is not necessary to postulate separate sources of Xe for the interior and atmosphere of Mars.¿ 1997 American Geophysical Union