Figure 3. Comparison of Martian meteorite data with mixing lines between Martian and terrestrial atmospheres.
Data are from ref. 27. Each point corresponds to one burst of volatiles produced by an increment of crushing. Gases of interest are normalized to CO2. Ar/O2 plot (a) implies a primary Martian signature with limited contamination from the terrestrial atmosphere. CH4/O2 plot (b) implies substantial enrichment in CH4 by a non-atmospheric source, most likely in the Martian crust. The Ar/O2 meteorite data lie close to the composition of terrestrial soil gas, but the CH4 enrichment excludes soil gas as a major contaminant. Together the plots show that contamination from terrestrial atmosphere was minimal, and that the gas signature is crustal rather than atmospheric. Errors are obtained from molar volumes of individual species±one weighted s.d. (see Methods).