Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover

Mars' Atmosphere from Curiosity The Sample Analysis at Mars (SAM) instrument on the Curiosity rover that landed on Mars in August last year is designed to study the chemical and isotopic composition of the martian atmosphere. Mahaffy et al. (p. 263) present volume-mixing ratios of Mars' five major atmospheric constituents (CO2, Ar, N2, O2, and CO) and isotope measurements of 40Ar/36Ar and C and O in CO2, based on data from one of SAM's instruments, obtained between 31 August and 21 November 2012. Webster et al. (p. 260) used data from another of SAM's instruments obtained around the same period to determine isotope ratios of H, C, and O in atmospheric CO2 and H2O. Agreement between the isotopic ratios measured by SAM with those of martian meteorites, measured in laboratories on Earth, confirms the origin of these meteorites and implies that the current atmospheric reservoirs of CO2 and H2O were largely established after the period of early atmospheric loss some 4 billion years ago. Data from the Curiosity rover provide a detailed account of the chemical and isotopic composition of Mars’ atmosphere. Volume mixing and isotope ratios secured with repeated atmospheric measurements taken with the Sample Analysis at Mars instrument suite on the Curiosity rover are: carbon dioxide (CO2), 0.960(±0.007); argon-40 (40Ar), 0.0193(±0.0001); nitrogen (N2), 0.0189(±0.0003); oxygen, 1.45(±0.09) × 10−3; carbon monoxide, < 1.0 × 10−3; and 40Ar/36Ar, 1.9(±0.3) × 103. The 40Ar/N2 ratio is 1.7 times greater and the 40Ar/36Ar ratio 1.6 times lower than values reported by the Viking Lander mass spectrometer in 1976, whereas other values are generally consistent with Viking and remote sensing observations. The 40Ar/36Ar ratio is consistent with martian meteoritic values, which provides additional strong support for a martian origin of these rocks. The isotopic signature δ13C from CO2 of ~45 per mil is independently measured with two instruments. This heavy isotope enrichment in carbon supports the hypothesis of substantial atmospheric loss.

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