Ks. Hutchins et Bm. Jakosky, EVOLUTION OF MARTIAN ATMOSPHERIC ARGON - IMPLICATIONS FOR SOURCES OF VOLATILES, J GEO R-PLA, 101(E6), 1996, pp. 14933-14949
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.