Evidence for indigenous nitrogen in sedimentary and aeolian deposits from the Curiosity rover investigations at Gale crater, Mars

Abstract

The Sample Analysis at Mars (SAM) investigation on the Mars Science Laboratory (MSL) Curiosity rover has detected oxidized nitrogen-bearing compounds during pyrolysis of scooped aeolian sediments and drilled sedimentary deposits within Gale crater. Total N concentrations ranged from 20 to 250 nmol N per sample. After subtraction of known N sources in SAM, our results support the equivalent of 110-300 ppm of nitrate in the Rocknest (RN) aeolian samples, and 70-260 and 330-1,100 ppm nitrate in John Klein (JK) and Cumberland (CB) mudstone deposits, respectively. Discovery of indigenous martian nitrogen in Mars surface materials has important implications for habitability and, specifically, for the potential evolution of a nitrogen cycle at some point in martian history. The detection of nitrate in both wind-drifted fines (RN) and in mudstone (JK, CB) is likely a result of N2 fixation to nitrate generated by thermal shock from impact or volcanic plume lightning on ancient Mars. Fixed nitrogen could have facilitated the development of a primitive nitrogen cycle on the surface of ancient Mars, potentially providing a biochemically accessible source of nitrogen.

Keywords: Curiosity; Mars; astrobiology; nitrates; nitrogen.

Fig. 1.

Representative EGA pyrograms from RN, JK, and CB. Highest abundance species NO (blue) and HCN (orange) plotted as m/z 30 and 27, respectively, on left axis; low-abundance species ClCN (purple) and TFMA (green) plotted as m/z 61 and 127 on the right axis. Note the change in scale on the left axis for CB3, as NO was significantly more abundant in CB samples than at either RN or JK.

Fig. 2.

GC chromatograms of CB-5 sample with m/z corresponding to the base or diagnostic peak of each of the N-bearing compounds identified. 1, NO (m/z 30); 2, TFA (m/z 69); 3, HCN (m/z 27); 4, ClCN (m/z 61); 5, CH₃CN (m/z 41); 6, TFMA (m/z 127). Division factors were applied to the most abundant compounds for scaling: factor 10 for m/z 27 and factor 50 for m/z 30. None of the chosen masses contribute to another N-bearing compound identified in each chromatogram. Because of partial coelutions between the first four GC peaks, two separate chromatograms were reconstructed (dark green and light green plain lines). The corresponding dotted lines are the reconstructed chromatograms of the reheated CB-6–residue sample, which has the same GC temperature cut as CB-5 and can thus be considered as a blank. Although acetonitrile (CH₃CN) (m/z 41) is detected in the GC, its detection in EGA is complicated by methylpropene, which constitutes the majority of the signal at m/z 41.

Fig. 3.

The total nanomoles of N in the sum of all N species detected (NO, HCN, ClCN, TFA, TFMA; Table 1) in blue, compared with the worst-case scenario prediction of nanomoles of N contributed by MTBSTFA (Table 1 and SI Text) in light blue. In blanks, except for CB-Blank2, the estimate of MTBSTFA-derived N is greater than actual N detected. CB samples had the largest abundance of detected N species, and the lowest percentage N contribution from MTBSTFA (Table 1).

Fig. 4.

The m/z 30 trace for EGA of three nitrate species, calcium nitrate tetrahydrate [Ca(NO₃)₂•4H₂O], iron(III) nitrate nonahydrate [Fe(NO₃)₃•9H₂O], and magnesium nitrate hexahydrate [Mg(NO₃)₂•6H₂O] run on a laboratory SAM-like system compared with RN, JK, and CB SAM experiments. The release profile at RN-4 is most consistent with the two-step decomposition of Fe(NO₃)₃, suggesting that Fe-nitrate may be a component of the RN sample.