Martian atmospheric carbon dioxide and weathering products in SNC meteorites

SNC meteorites—four shergottites, three nakhlites and Chassigny—are postulated to have originated on Mars1. Their late crystallization ages (<1,300 Myr compared with 4,600 Myr for other igneous meteorites) and the presence, in shock-produced glass in EETA79001, of noble gas2and nitrogen3components resembling the martian atmosphere provide evidence for such a provenance. If this interpretation is correct then carbon dioxide, by far the most abundant constituent of the martian atmosphere4, should also be present. The stepped combustions described here show that most of the carbon present in the samples can be ascribed to terrestrial contamination. EETA79001, however, contains 4.6 p.p.m. of an isotopically distinct component enriched in 13C (δ13C = +36‰), whereas high-temperature carbon of inferred igneous origin in this meteorite and other SNCs has a δ13C value of about −30‰. The 12C/13C ratio of the isotopically heavy component is within the error limits of Viking measurements4in the martian atmosphere and, thus, strengthens the case for a planetary origin. Another carbon-containing species, believed to be carbonate, has been found in Nakhla (δ13C = between +12 and +24‰) and may be a product of atmospheric weathering on Mars.