Change in the 40Ar/N of the Mars Atmosphere from Viking to MSL: A possible indication of climate change on Mars

Compositional measurements of the Mars atmosphere conducted by the Sample Analysis at Mars (SAM) suite on Mars Science Laboratory (MSL) have included precise measurements of Argon and N₂. As first reported in Mahaffy et al. (Science, 341:263, 2013), and supported by repeated measurements over the course of the first Earth year of the mission, the ⁴⁰Ar/N ratio measured by SAM is ~ 0.5 - a significantly higher ratio than measured by Viking (~0.33) in the 1970s. Given the expected stability of both Ar and N₂ in the Mars atmosphere, the apparent change in their ratio is a surprising result. We will show the Mars data from the SAM Quadrupole Mass Spectrometer (QMS), with a focus on the analysis and ground calibration that yield the current ⁴⁰Ar/N ratio. We will show the robustness of this ratio against the changing environmental conditions in Gale Crater, such as season, pressure, and CO₂ mixing ratio. Measurements of the ¹⁵N/¹⁴N of N₂ have been made, concurrently with the ⁴⁰Ar/N and in separate enrichment experiments, and these will also be presented to allow for comparisons to the meteorite mixing line (eg. Becker and Pepin, EPSL, 69:225, 1984). Finally, we will explore possible mechanisms by which a significant change in ⁴⁰Ar/N over ~ 30 years may be reconciled, with implications for the dynamism of the Mars atmosphere and exchange of gases with the regolith.