MSL/SAM Measurements of Nitrogen Isotopes in the Mars Atmosphere

The Sample Analysis at Mars (SAM) instrument suite on the Mars Science Laboratory (MSL) measured the Mars atmospheric δ₁₅N value on sol 232 of the mission. This value falls within the uncertainty of the Viking aeroshell mass spectrometer measurement, which was 619‰ ± 182‰ (Neir and McElroy 1977). The SAM measurement achieved about a factor of two smaller uncertainty than the Viking value, and about a factor of two higher signal to background ratio (S/BG). The MSL/SAM value was based on Quadrupole Mass Spectrometer measurements of an enriched sample of Mars atmosphere, with CO² and H²O removed. We used the m/z 14/14.5 ratio, from doubly-ionized nitrogen, because it had higher S/BG than other relevant count ratios in the experiment. The m/z 28/29 ratio confirms the isotopic ratio with slightly lower precision. The enrichment experiment, and several direct atmospheric experiments, gave a range of δ₁₅N values from 813‰ to 528‰, where the measurements with the highest S/BG gave values most consistent with the enrichment experiment. The direct measurements also used data at m/z 14 and 14.5, to avoid confusion from CO ions at m/z 28 and 29. We will report the SAM δ₁₅N value at the meeting, pending confirmation by an additional enrichment experiment on Mars to be completed just days after the abstract submission deadline. Trapped gases in martian meteorite glasses have previously been interpreted as mixtures of two components, based on ratios of ₁₅N/₁₄N and ₄₀Ar/₁₄N (e.g., Becker and Pepin 1984). One component is thought to have martian atmospheric composition, as measured by Viking. Recent MSL/SAM measurements (Mahaffy et al. 2013) of the ₄₀Ar/₁₄N ratio 0.5) are incompatible with the Viking ratio 0.3), complicating the interpretation of the meteorite gases. The slope of the meteorite mixing line is less consistent with Viking atmospheric composition if alternative assumptions are used for meteorite cosmic ray exposure ages, cosmogenic ₁₅N production rates, and trapped gas evolution temperatures, yet these assumptions still cannot place the meteorite data on a mixing line with the martian atmosphere as measured by SAM. The SAM argon and nitrogen results may lead to new understandings of martian meteorites.