Mars Atmospheric Argon Isotopes Measured by the SAM Instrument Suite on MSL

The Sample Analysis at Mars (SAM) Suite (Mahaffy et al. 2012, SSR 170) is one of the science packages on the Mars Science Laboratory (MSL) “Curiosity” rover. The SAM is comprised of a Quadrupole Mass Spectrometer (QMS), a Tunable Laser Spectrometer (TLS) and a Gas Chromatograph (GC). Together these instruments, along with the solid sample manipulation and gas processing system, are able to measure directly the composition of the Mars atmosphere as well as gases thermally evolved from the rock samples delivered to SAM. During the first 200 sols of the mission, the QMS carried out three nighttime in situ atmospheric measurements, providing an initial determination of the chemical and isotopic composition of the Mars atmosphere. These direct atmospheric experiments determine the atmospheric 40Ar/36Ar ratio to be 1900 ± 300 (Mahaffy et al. 2013, Science, in press). A subsequent experiment on sol 232 examined an enriched sample of Mars atmosphere, with CO2 and H2O removed, increasing the signal-to-background ratio at m/z 36 by a factor of ~5. The SAM enrichment experiment confirms that MSL measures a Mars atmospheric ratio that is much lower than the 3000 ± 500 determined by the Viking landers (Owen et al. 1977, JGR 82). This finding agrees with the analyses of martian meteorites (made between the times of the Viking and MSL landings) suggesting that the Viking 40Ar/36Ar ratio was overestimated (e.g., Bogard and Garrison 1999, Met. & Pl. Sci. 34). The MSL 40Ar/36Ar measurement (as well as other isotopic ratios) constrains atmospheric escape and evolution, crustal degassing, and interpretation of martian meteorite composition. Recent experiments with the SAM QMS in the sol 200-360 range include daytime atmospheric measurements and a gas enrichment experiment in which the abundance and isotopic composition of Kr, Xe and 38Ar/36Ar was measured. Results of these experiments will be presented.