Reevaluated martian atmospheric mixing ratios from the mass spectrometer on the Curiosity rover

•The Sample Analysis at Mars (SAM) instrument measures the composition of the martian atmosphere.•Calibration constants that were derived pre-flight and adjusted shortly after landing on Mars have been refined based on new data.•New constants add corrections for observed ionization effects, spectral interferences, and change in instrument background.•Refined constants allow improved accuracy in Mars atmospheric mixing ratios and precise calculation of CO abundance.

The Sample Analysis at Mars (SAM) instrument suite of the Mars Science Laboratory (MSL) Curiosity rover is a miniature geochemical laboratory designed to analyze martian atmospheric gases as well as volatiles released by pyrolysis of solid surface materials (Mahaffy et al., 2012). SAM began sampling the martian atmosphere to measure its chemical and isotopic composition shortly after Curiosity landed in Mars׳ Gale Crater in August 2012 (Mahaffy et al., 2013). Analytical methods and constants required for atmospheric measurements with SAM׳s quadrupole mass spectrometer (QMS) were provided in a previous contribution (Franz et al., 2014). Review of results obtained through application of these constants to repeated analyses over a full martian year and supporting studies with laboratory instruments offer new insights into QMS performance that allow refinement of the calibration constants and critical reassessment of their estimated uncertainties. This report describes the findings of these studies, provides updated calibration constants for atmospheric analyses with the SAM QMS, and compares volume mixing ratios for the martian atmosphere retrieved with the revised constants to those initially reported (Mahaffy et al., 2013). Sufficient confidence is enabled by the extended data set to support calculation of precise abundances for CO rather than an upper limit. Reanalysis of data acquired on mission sols 45 and 77 (at solar longitudes of 175° and 193°, respectively) with the revised constants leads to the following average volume mixing ratios: CO2 0.957(±0.016), N2 0.0203(±0.0003), Ar 0.0207(±0.0002), O2 1.73(±0.06)×10−3, CO 7.49(±0.026)×10−4.