Isotopes of nitrogen on Mars: Atmospheric measurements by Curiosity's mass spectrometer

Abstract

[1] The Sample Analysis at Mars (SAM) instrument suite on the Mars Science Laboratory (MSL) measured a Mars atmospheric¹⁴N/¹⁵N ratio of 173 ± 11 on sol 341 of the mission, agreeing with Viking's measurement of 168 ± 17. The MSL/SAM value was based on Quadrupole Mass Spectrometer measurements of an enriched atmospheric sample, with CO₂ and H₂O removed. Doubly ionized nitrogen data at m/z 14 and 14.5 had the highest signal/background ratio, with results confirmed by m/z 28 and 29 data. Gases in SNC meteorite glasses have been interpreted as mixtures containing a Martian atmospheric component, based partly on distinctive¹⁴N/¹⁵N and⁴⁰Ar/¹⁴N ratios. Recent MSL/SAM measurements of the⁴⁰Ar/¹⁴N ratio (0.51 ± 0.01) are incompatible with the Viking ratio (0.35 ± 0.08). The meteorite mixing line is more consistent with the atmospheric composition measured by Viking than by MSL.

Figure 1

High-resolution mass spectrum from the enrichment experiment on sol 232. Sample spectrum is an average of data from enrichment cycle 9 (see Figure 2). Nitrogen ions are present at m/z 14, 14.5, 28, and 29. The singly ionized N₂ “parent ion” appears in the Mars data at m/z 28 (for¹⁴N¹⁴N) and at m/z 29 (for¹⁵N¹⁴N). A third parent ion,¹⁵N¹⁵N, is so rare as to produce only a small signal below the background level (difference between red and blue curves). For a comparison of the m/z 14–15 range in the direct atmospheric experiment, see Wong et al. [2013].

Figure 2

Visual narrative of the sol 232 enrichment experiment. Parent and daughter nitrogen ions are shown as colored points. Background measurements (shaded) are used to characterize the background signal that must be subtracted from the data before calculating count ratios. Detector corrections have been applied to the data (see supporting information). In each of 10 enrichment cycles, an atmospheric sample was introduced, and CO₂ and H₂O were removed by chemical scrubbers [Mahaffy et al., 2012]. The enriched sample from one cycle was then combined with a new injection of Mars atmosphere and scrubbed to produce the next sample. Signal at m/z 28 decreases in enrichment cycle 2 and later due to detector saturation at high source pressures.

Figure 3

Pseudo-three-isotope plots at two different magnifications (comparing¹⁴N,¹⁵N, and⁴⁰Ar) showing Mars meteorite and atmosphere measurements and linear fits. The meteorite data suggest mixing between at least two components: Martian atmosphere and terrestrial atmosphere. Black meteorite data include only selected temperature steps in stepped heating gas release experiments. Red data points include the total gas released at all temperatures. Meteorite data are from Becker and Pepin [, EETA 79001A samples C1 and C2], Wiens et al. [, sample C3], Marti et al. [, Zagami], Chennaoui Aoudjehane et al. [, Tissint], and Avice [, Tissint]. Viking Ar/N ratio is from Oyama and Berdahl [1977], and Viking δ¹⁵N is from Nier and McElroy [1977]. MSL values are from SAM/QMS measurements, with δ¹⁵N described in this work, and Ar/N is from Mahaffy et al. [2013]. Linear fits included uncertainties in both Ar/N and δ¹⁵N, following the method of York et al. [2004], assuming uncorrelated errors.