A reappraisal of the Martian Ar-36/Ar-38 ratio

The currently used value of 4.1 +/- 0.2 for the Ar-36/Ar-38 isotopic ratio in the Martian atmosphere is reappraised, and a significantly lower ratio is suggested. Previous analyses of noble gases in impact glass from some Martian meteorites demonstrate that large quantities of Martian atmospheric gases were shock-emplaced into these samples. However, several observations indicate that this trapped Martian gas is composed of two components, one atmospheric and one probably from the Martian mantle. These observations include large variations in Ar-36/Ar-38 during stepwise gas release, variations in the Ar/Kr/Xe elemental ratios among Martian meteorites, and variations in the cosmogenic-corrected, trapped Ar-36/Ar-38 ratios over 3.5-4.3 among shock glass samples. Uncertainties in applied corrections for cosmogenic Ar cannot explain these variations. By assuming a range of reasonable values for Martian atmospheric and mantle Ar-40/Ar-36, the observed Ar-40/Ar-36 ratios in shock glass are used to correct for the mantle component and to derive a more precise ratio for the atmospheric component. Using this procedure, one obtains an upper limit for Martian atmospheric Ar-36/Ar-38 of 3.9, and a probable, but poorly defined range for Martian atmospheric Ar-36/Ar-38 of similar to 3.0-3.6. These lower ratios imply that the degree of mass fractionation during Ar loss from Mars' upper atmosphere is considerably larger than that previously calculated.