EVOLUTION OF MARTIAN ATMOSPHERIC ARGON - IMPLICATIONS FOR SOURCES OF VOLATILES

We have examined processes affecting isotopes of argon (Ar-36, Ar-38, Ar-40) in order to determine important atmospheric sources and sinks. Our simple model for argon evolution incorporates production of radiog enic argon in the mantle, outgassing of all argon species by extrusive and intrusive volcanism, and loss to space by knock-on sputtering abo ve the exobase. Sputtering has been shown previously to be an importan t loss process for atmospheric species, especially isotopes of noble g ases, which have few other mechanisms of escape. The integrated evolut ion of argon (Ar-36, Ar-38, and Ar-40, respectively) is modeled in ter ms of these variables: (1) the planetary concentration of potassium, ( 2) the fraction of juvenile argon released catastrophically during the first 600 Myr., (3) potential variation in the time-history of sputte ring loss from that suggested by Luhmann et al. [1992], and (4) the vo lume of total outgassing to the surface as compared to outgassing cont ributed by volcanic release. Our results indicate that Mars has lost b etween 85-95% of Ar-36 and 70-88% of outgassed Ar-40. Due to this subs tantial loss, the planet must have outgassed the equivalent of between 10 and 100 times the total volume of gases released by extrusive and intrusive volcanics. This indicates that volcanic outgassing, alone, i s insufficient to explain the present-day abundances of Ar-36 and Ar-4 0 in the Martian atmosphere. Similar calculations for Ne-20 suggest ou tgassed volumes of between 100 and 1800 times in excess of that due to volcanism. This results in a distinct Ne/Ar elemental fractionation, with a preference for outgassing argon, of the order of 10 to 17. Alth ough the results must be evaluated within the model uncertainties, the results are compelling in that they unequivocally show the existence of additional sources of atmospheric volatiles and helps define a mean s to identify them.