Abiotic Input of Fixed Nitrogen by Bolide Impacts to Gale Crater During the Hesperian: Insights From the Mars Science Laboratory

Abiotic Input of Fixed Nitrogen by Bolide Impacts to Gale Crater During the Hesperian: Insights From the Mars Science Laboratory Rafael Navarro‐González 1 , Karina F. Navarro 1 , Patrice Coll 2 , Christopher P. McKay 3 , Jennifer C. Stern 4 , Brad Sutter 5 , P. Douglas Archer Jr 5 , Arnaud Buch 6 , Michel Cabane 7 , Pamela G. Conrad 4 , Jennifer L. Eigenbrode 4 , Heather B. Franz 4 , Caroline Freissinet 7 , Daniel P. Glavin 4 , Joanna V. Hogancamp 5 , Amy C. McAdam 4 , Charles A. Malespin 4 , F. Javier Martín‐Torres 8,9 , Douglas W. Ming 10 , Richard V. Morris 10 , Benny Prats 11 , François Raulin 2 , José Antonio Rodríguez‐Manfredi 12 , Cyril Szopa 7,13 , María‐Paz Zorzano‐Mier 8,12 , Paul R. Mahaffy 4 , Sushil Atreya 14 , Melissa G. Trainer 4 , and Ashwin R. Vasavada 15 1 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico, 2 Laboratoire Interuniversitaire des Systèmes Atmosphériques, CNRS UMR 7583, Université Paris‐Est Créteil, Université Paris Diderot, Créteil, France, 3 NASA Ames Research Center, Moffett Field, CA, USA, 4 NASA Goddard Space Flight Center, Greenbelt, MD, USA, 5 Jacobs, NASA Johnson Space Center, Houston, TX, USA, 6 Ecole Centrale Paris, Châtenay‐Malabry, France, 7 Laboratoire Atmosphère, Milieux, Observations Spatiales, UMR CNRS 8190, Université Versailles Saint‐Quentin en Yvelines, UPMC Université Paris 06, Guyancourt, France, 8 Department of Computer Science, Electrical and Space Engineering, Luleå University of Technology, Luleå, Sweden, 9 Instituto Andaluz de Ciencias de la Tierra (CSIC‐UGR), Granada, Spain, 10 NASA Johnson Space Center, Houston, TX, USA, 11 NASA/eINFORMe, Inc., Goddard Space Flight Center, Planetary Environments Laboratory, Greenbelt, MD, USA, 12 Centro de Astrobiología (INTA‐CSIC), Madrid, Spain, 13 Institut Universitaire de France, Paris, France, 14 Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI, USA, 15 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Abstract Molecular hydrogen (H 2 ) from volcanic emissions is suggested to warm the Martian surface when carbon dioxide (CO 2 ) levels dropped from the Noachian (4100 to 3700 Myr) to the Hesperian (3700 to 3000 Myr). Its presence is expected to shift the conversion of molecular nitrogen (N 2 ) into different forms of fixed nitrogen (N). Here we present experimental data and theoretical calculations that investigate the efficiency of nitrogen fixation by bolide impacts in CO 2 ‐N 2 atmospheres with or without H 2 . Surprisingly, nitric oxide (NO) was produced more efficiently in 20% H 2 in spite of being a reducing agent and not likely to increase the rate of nitrogen oxidation. Nevertheless, its presence led to a faster cooling of the shock wave raising the freeze‐out temperature of NO resulting in an enhanced yield. We estimate that the nitrogen fixation rate by bolide impacts varied from 7 × 10 −4 to 2 × 10 −3 g N·Myr −1 ·cm −2 and could imply fluvial concentration to explain the nitrogen (1.4 ± 0.7 g N·Myr −1 ·cm −2 ) detected as nitrite (NO 2 − ) and nitrate (NO 3 − ) by Curiosity at Yellowknife Bay. One possible explanation is that the nitrogen detected in the lacustrine sediments at Gale was deposited entirely on the crater's surface and was subsequently dissolved and transported by superficial and ground waters to the lake during favorable wet climatic conditions. The nitrogen content sharply decreases in younger sediments of the Murray formation suggesting a decline of H 2 in the atmosphere and the rise of oxidizing conditions causing a shortage in the supply to putative microbial life. Plain Language Summary Climate models are able to warm early Mars when CO 2 sources were strong but fail at later times when liquid water still flowed on the surface. A possible solution for the climate puzzle is the presence of abundant H 2 arising from volcanic emissions that could have kept the planet from freezing. H 2 could have also played a key role in the chemistry of the atmosphere. Curiosity discovered the presence of nitrites and nitrates, forms of fixed nitrogen that are required for the origin and sustainability of life in sediments in Gale crater. Here we present theoretical and experimental data that quantify the conversion of molecular nitrogen into fixed nitrogen in the presence and absence of H 2 by the entry shocks of asteroids in the Martian atmosphere and surface. Fixed nitrogen was originally deposited on the surface of Gale crater and then transported to the lake during favorable wet climatic conditions. We found that H 2 is required to yield sufficient fixed nitrogen to explain its detection. The levels of fixed nitrogen sharply RESEARCH ARTICLE 10.1029/2018JE005852 Key Points: •A hydrogen‐rich atmosphere is required to explain the levels of fixed nitrogen that are found in sediments encountered in Gale crater •Fixed nitrogen was deposited on the surface of the crater and then transported to the lake during favorable wet climatic conditions •The levels of fixed nitrogen sharply decreased in younger sediments causing a shortage in the supply to putative microbial communities Supporting Information: •Supporting Information S1 Correspondence to: R. Navarro‐González, [email protected]nam.mx Citation: Navarro‐González, R., Navarro, K. F., Coll, P., McKay, C. P., Stern, J. C., Sutter, B., et al. (2019). Abiotic input of fixed nitrogen by bolide impacts to Gale crater during the Hesperian: Insights from the Mars Science Laboratory. Journal of Geophysical Research: Planets,124,94–113. https://doi.org/ 10.1029/2018JE005852 Received 15 OCT 2018 Accepted 15 DEC 2018 Accepted article online 19 DEC 2018 Published online 15 JAN 2019 Corrected 28 JAN 2019 This article was corrected on 28 JAN 2019. See the end of the full text for details. ©2018. The Authors. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made. NAVARRO‐GONZÁLEZ ET AL. 94 dropped in younger sediments suggesting a decline of H 2 in the atmosphere and the rise of oxidizing conditions causing a nitrogen crisis to putative microbial communities. 1. Introduction The presence of fluvial landforms on the Martian surface provides indirect evidence that an active hydrological cycle took place at the early stage of evolution of the planet (Luo et al., 2016). This implies that the atmosphere was denser and contained greenhouse gases that allowed a wet and warmer climate. The chemical composition of the early Martian atmosphere is uncertain. Carbon dioxide (CO 2 ) is generally believed to be the principal gas from the Pre‐Noachian (4500 to 4100 Myr) to the Noachian when the young Sun was fainter by ~30% (Gough, 1981). It is estimated that between 0.1 and 3 bars of CO 2 were outgassed during planetary accretion (Kahn, 1985). Climate models required 1–5 bars of CO 2 to keep Mars from freezing (Pollack et al., 1987). Nevertheless, CO 2 escaped to space by photochemistry (Hu et al., 2015), sputtering (Johnson & Liu, 1998), and impact erosion (Melosh & Vickery, 1989; Pham & Karatekin, 2016) as well as sequestered as carbonates (CO 32− ) by surface weathering (Tomkinson et al., 2013). A dense atmosphere (0.5–1 bar) could have persisted from the Pre‐Noachian to the Noachian only if atmospheric CO 2 was being continuously resupplied by recycling of CO 32− by volcanism (Grott et al., 2011; Pollack et al., 1987) and impact degassing (Carr, 1989). As the CO 2 levels dropped below 1 bar, climate models that consider only CO 2 and water are incapable to heat up the Noachian and Hesperian periods in the proximity of the freezing point of water (Wordsworth, 2016). Furthermore, geochemical evidence from sedimentary rocks in Gale crater reveals aqueous alteration but a lack of CO 32− minerals, suggesting very low levels of CO 2 (0.01–0.1 bar) at the time of deposition, around 3500 Myr ago (Bristow et al., 2017). A possible solution for the Martian climate puzzle is the presence of 10–20% molecular hydrogen (H 2 ) from volcanic emissions that would have warmed the atmosphere episodically by collision‐induced absorption with CO 2 (Ramirez et al., 2014; Sagan, 1977; Wordsworth et al., 2017) or N 2 (Wordsworth & Pierrehumbert, 2013). The total inventory of N 2 in the Martian atmosphere is estimated to range from 0.03 to 0.3 bar (McKay & Stoker, 1989). If N 2 found in the atmosphere of Venus is scaled to Mars, the total inventory of N 2 increases to 0.5–0.6 (von Paris et al., 2013). Such levels of N 2 would have also provided additional warming of the atmosphere by pressure broadening and collision‐induced N 2 ‐N 2 absorption (von Paris et al., 2013). Using the size distribution of ancient craters as a proxy for paleopressure, it is inferred that Mars had a total pressure of 0.9–1.9 bars around 3600 Myr ago (Kite et al., 2014). In addition to its greenhouse effect, H 2 may have had a role in N fixation, for example, the conversion of N 2 into fixed forms of N, such as NO 3 − and hydrogen cyanide (HCN). The key parameters determining the type of N species formed and their rates of fixation are the ratios of carbon (C), oxygen (O), and hydrogen (H) atoms in the atmosphere (Chameides & Walker, 1981; Navarro‐González, McKay, & Nna Mvondo, 2001; Stribling & Miller, 1987): for instance, a methane‐and H 2 ‐rich atmosphere (high C and H) produces reduced forms of fixed nitrogen, such as HCN, while neutral (CO 2 ) or oxidized atmospheres (O 2 ) generate oxidized forms of fixed nitrogen, such as NO. N is a necessary element for life and is frequently inaccessible to microbial communities as it is present in the kinetically inert state, as N 2 in the atmosphere (Postgate, 1996). Because the energy required to break the triple bond in N 2 is high, N fixation, is kinetically limited in spite of being thermodynamically favorable (Howard & Rees, 1996). Therefore, processes that transform N 2 into biologically accessible chemical forms, such as NO 3 − , restrict the supply of N to microorganisms. The N fixation was probably triggered by volcanic lightning, ultraviolet light, and bolide impacts on early Mars (Manning et al., 2009; Segura & Navarro‐González, 2005), but the role of H 2 has not yet been explored. So far the Sample Analysis at Mars (SAM) instrument suite of the NASA Mars Science Laboratory (MSL) has discovered the presence of NO 3 − in Hesperian sediments along a stratigraphic transect investigated by the Curiosity rover at Gale crater (Ming et al., 2014; Navarro‐González et al., 2013; J. C. Stern et al., 2015, 2017, 2018; Sutter et al., 2017); the origin of NO 3 − is thought to be from the shocks of bolide impacts (J. C. Stern et al., 2015). In this study we present experimental data and theoretical calculations that investigate the efficiency of shocks from bolide impacts for N fixation in atmospheres (1 bar) containing different CO 2 /(CO 2 +N 2 ) ratios with or without H 2 . These values are used to derive the lower and upper boundaries of the N fixation rate by 10.1029/2018JE005852 Journal of Geophysical Research: Planets NAVARRO‐GONZÁLEZ ET AL. 95 bolide impacts. In addition we have reexamined the SAM data to search for the presence of NO 2 − and NO 3 − , determine the amount of fixed nitrogen present in the stratigraphic sequence investigated by the Curiosity rover up to date, and estimate the N deposition rate. The rate of N fixation by bolide impacts and other forms of energy are examined to account for the supply of the observed N deposition at Gale crater. 2. Materials and Methods 2.1. Theoretical Estimate of NO The equilibrium concentrations of N, O, NO, and N 2 as a function of temperature were calculated using a computer program that models chemical speciation at thermodynamic equilibrium (Bale et al., 2016). The program predicts the chemical species and their mixing ratios that are in thermochemical equilibrium at a given temperature and pressure (1 bar) based on the chemical composition of the initial gas mixture. The rates of reactions as a function of temperature were obtained from the National Institute of Standards and Technology Chemical Kinetics Database (Manion et al., 2015). 2.2. Preparation of Simulated Atmospheres Simulated atmospheres of different composition containing CO 2 (99.8% purity), N 2 (99.998% purity), and H 2 (99.999% purity) were prepared using a computerized gas‐blending system equipped with eight gas lines regulated by high accuracy and fast response mass flow controllers that operate with a maximum rate of 20 cm 3 /min at 4 bars. Each gas tank is connected to a two‐stage regulator, a particle filter (2 μm), and stainless steel tubing. At the end of the line there is a pneumatic switch valve connected to the mass flow control and a check valve which merges into a 4‐L stainless steel container for filling up the simulated atmosphere and which restricts the backflow of the gases preventing contamination of the gas lines. The gas‐blending system was connected into a manifold gas line with vacuum, pressure, and temperature meters. The gas‐ blending system and the manifold were evacuated to 8.6 × 10 −3 mbar before opening the gas cylinder valves. Finally, the gas‐blending system was filled to 4 bars (value restricted by the operation of the mass flow control modules) with the desired atmosphere in about 8 hr. Once the simulated atmosphere was ready for use, it was transferred into previously vacuum‐evacuated round borosilicate (Pyrex) reactors of 1‐L capacity equipped with high vacuum stopcocks and filled to 1 bar at room temperature (21 °C). 2.3. Simulation of Bolide Impacts The effect of bolide impacts were simulated in the laboratory by shocks created under a controlled atmosphere by concentrating a pulsed Nd:YAG laser beam of 1.06 μm photons inside a closed Pyrex reactor of 1‐L capacity at 1 bar using a planoconvex optical glass lens with antireflecting coating with a positive focal length of 10 cm and a focal aberration of ~10 μm. Laser‐induced plasmas have been extensively used to study the effect of shock waves in planetary atmospheres (Managadze et al., 2003; McKay & Borucki, 1997; Scattergood et al., 1989). When the electric field of the infrared laser radiation becomes greater than that of the binding electrons to their nuclei near the focus point of the lens, it triggers breakdown of the gas molecules. This electric breakdown causes a cascade effect because the ionized gas becomes very absorbent to the laser light so that more of the energy is absorbed (Panarella, 1974). The plasma generated in our facility using air under similar experimental conditions was found to reach a temperature near 17000 K and creates a shock wave with initial velocity of >60 km/s at 20 ns, as determined by interferometry and shadowgraphy techniques (Sobral et al., 2000). Essentially this method is equivalent to a piston‐free shock tube with the advantage of conducting the experiment on a tabletop setup with a good controllability of a small explosion in a variety of confined atmospheres (Sasoh, 2016). The laser beam had an energy of 250 mJ per pulse in 5–7 ns operating at 10 Hz. The laser beam is not absorbed completely by the gas mixture in producing the plasma. It was found that between 20% and 30% of the energy was transmitted out of the reactor. The energy absorbed in the production of the plasma was calculated by eliminating the energy transmitted by the plasma in the direction opposite of the incoming laser beam and was measured with an optical power system (Labmaster Ultima, Coherent) using an optical sensor (LM‐P10). The reactors were exposed from 0 to 30 min at intervals of 5 min. 2.4. Gas Chromatography Coupled to Mass Spectrometry Analysis NO was analyzed by gas chromatography (GC)‐mass spectrometry (MS) using electron impact ionization. The irradiated atmosphere was introduced into the injection port of an Agilent Technologies 7890A GC 10.1029/2018JE005852 Journal of Geophysical Research: Planets NAVARRO‐GONZÁLEZ ET AL. 96 system held at 250 °C by an automatic six‐port gas‐sampling valve connected to a gas manifold with a vacuum line, a gas sampling port, and a stainless steel loop of 5 ml capacity for sample injection. A styrene‐divinylbenzene‐based porous polymer column was used (CP‐Porabond Q fused‐silica) of 50 m × 0.32 mm I.D. with a 5‐μm polymer thickness coating. The chromatographic separation was carried out using a program temperature that was initially kept at 50 °C for 5 min and then increased at rate of 10 °C/min until a final temperature of 240 °C, which was held for 6 min. Helium was used as the carrier gas with a flow of 1.2 ml/min. The sample split ratio was 1:100. The GC was interfaced at 250 °C with a mass detector (Agilent Technologies 5975C inert XL EL/CI MSD with Triple Axis detector). The mass spectrometer operated in scan mode from 10 to 150 m/zwith a mass resolution of 0.1 amu using electron impact ionization mode at 70 eV. The temperature zones of the ion source and the quadrupole were kept at 230 and 150 °C, respectively. NO was identified by its retention time and its characteristic fragmentation pattern in MS: NO + (100%), N + (7.5%), O + (1.5%), and NO 2+ (2.4%). Nitrous oxide (N 2 O) was detected in low yield, representing ≤0.06% of the NO signal, and was therefore not surveyed. Nitrogen dioxide (NO 2 ) was not observed in the experiments. Reduced forms of nitrogen were not detected in the experiments, such as ammonia (NH 3 ), HCN, acetonitrile (CH 3 CN), and cyanoacetylene (HC ≡C‐CN). These oxidized or reduced forms of nitrogen are resolved chromatographically into individual peaks (Do & Raulin, 1989; Nna Mvondo et al., 2001) and have sensitivities similar to or slightly higher than NO taking into account their ionization cross sections. 2.5. NO Calibration A calibration curve of NO was constructed from the analysis of 10 gas mixtures of NO (18 to 4,036 ppm in N 2 ) that were prepared using the computerized gas‐blending system described above using two NO calibration standards (390 and 4,036 ppm in N 2 ). 2.6. SAM‐Like Laboratory Experiments Several mixtures of NO 2 − or NO 3 − salts (10%) were mixed with different oxychlorine species (90%) in the form of chlorates (ClO 3 − ) or perchlorate (ClO 4 − ) salts. The chemicals used were reagent grade: NaNO 3 (Sigma Aldrich, >99.99%), KNO 3 (Química Meyer, >99.0%), Mg (NO 3 ) 2 (Fluka, >99.0%), Ca (NO 3 ) 2 (Sigma Aldrich, >99.0%), Fe (NO 3 ) 3 (Sigma Aldrich, >98.0%), NaNO 2 (J. T. Baker, >98.6%), NaClO 4 (Sigma Aldrich, >98.0%), KClO 4 (Sigma Aldrich, >99.0%), Mg (ClO 4 ) 2 (Sigma Aldrich, 99.0%), Ca (ClO 4 ) 2 (Sigma Aldrich, 99.0%), Fe (ClO 4 ) 2 (Sigma Aldrich, >98.0%), Fe (ClO 4 ) 3 (Sigma Aldrich, <0.1 chloride content), and synthesized Mg (ClO 3 ) 2 and Ca (ClO 3 ) 2 . Mg (ClO 3 ) 2 was not commercially available and was synthesized by mixing stoichiometric ratios of magnesium sulfate (MgSO 4 anhydrous, Sigma‐Aldrich, 99.5%) and barium chlorate (Ba (ClO 3 )·2H 2 O, Sigma‐Aldrich, 98.0%) according to the method used by Hanley et al. (2012). Ba (ClO 3 ) 2 was dissolved in bidistilled water on a stirplate, and then MgSO 4 was slowly added. Since BaSO 4 is highly insoluble, it quickly precipitated out of solution as a white solid, leaving behind Mg 2+ and ClO 3 − in solution. The clear solution was decanted and filtrated through a 20‐to 25‐μmfilter paper. Then it was centrifuged twice to separate the residual BaSO 4 , and finally it was freeze‐dried obtaining a solid powder. Ca (ClO 3 ) 2 was not commercially available, too, and was prepared using the same method by replacing magnesium sulfate for calcium sulfate (CaSO 4 anhydrous, Sigma‐ Aldrich, 99.0%). The purity of the synthesized Mg (ClO 3 ) 2 or Ca (ClO 3 ) 2 was confirmed by thermal analyses carried out by simultaneous measurements of thermogravimetric analysis and differential scanning calorimetry (DSC) coupled to evolved gas analysis by MS in the temperature range from 30 to 1450 °C. The mixtures of NO 2 − or NO 3 − salts mixed with different oxychlorine species (ClO 3 − or ClO 4 − ) were analyzed in the laboratory under SAM‐like conditions in order to facilitate the interpretation of the Mars data. Thermal analyses were carried out by simultaneous measurements by thermogravimetric analysis‐DSC‐MS that was configured to operate under SAM‐like conditions (Mahaffy et al., 2012). The instrument used was a Netzsch STA 449 F1 Jupiter thermobalance (TG‐DSC/DTA Apparatus) utilizing two furnaces made of steel or silicon carbide operating in the temperature range from −150 to 1550 °C which was interfaced to a Netzsch mass spectrometer QMS 403 C Aëolos. The sample was ground and sieved to <75 μm using an agate pestle and mortar set, and a portion (~15 mg) was introduced into alumina (Al 2 O 3 ) crucibles. An identical empty alumina crucible was used as reference material. The thermal analysis was carried out using the silicon carbide furnace that was heated from 30 to 850 °C at a rate of 35 °C/min. A nitrogen flow of 2 cm 3 /min was 10.1029/2018JE005852 Journal of Geophysical Research: Planets NAVARRO‐GONZÁLEZ ET AL. 97 used to transfer the evolved gases out of the thermobalance using an oven pressure of 35 mbar. The evolved gases were scanned by MS from 14 to 120 m/zusing electron impact ionization mode operated at 70 eV. 2.7. Martian Samples Curiosity has drilled 12 (1.6‐cm diameter, 6 cm deep) lacustrine mudstones during its traverse from the landing site at Bradbury at the lowest stratigraphic layers encountered on Aeolis Palus to the upper strata of the base of Aeolis Mons (see Figure 4): John Klein (Sol 182, 8 February 2013) and Cumberland (Sol 279, 19 May 2013) from the Yellowknife Bay formation, and Confidence Hills (Sol 759, 24 September 2014), Mojave (Sol 882, 29 January 2015), Telegraph Peak (Sol 908, 24 February 2015), Buckskin (Sol 1060, 30 July 2015), Oudam (Sol 1361, 4 June 2016), Marimba (Sol 1422, 6 August 2016), Quela (Sol 1464, 18 September 2016), Sebina (Sol 1495, 20 October 2016), Duluth (Sol 2057, 20 May 2018), and Stoer (Sol 2136, 8 August 2018) from the Murray formation. In addition, it has also drilled a sandstone in the Kimberley formation known as Windjana (Sol 621, 5 May 2014) composed primarily of fluvial conglomerate and deltaic sandstone (see Figure 4). Finally, it has also drilled four eolian sandstones of the Stimson formation that unconformably lie above the Murray formation (see Figure 4, main text): Big Sky (Sol 1119, 29 September 2015), Greenhorn (Sol 1137, 18 October 2015), Lubango (Sol 1320, 23 April 2016), and Okoruso (Sol 1332, 5 May 2016). The samples Lubango, Okoruso, and Sebina were not analyzed by SAM. The sampling sites have been described in detail elsewhere (Hogancamp et al., 2018; J. C. Stern et al., 2017). 2.8. SAM Measurements The SAM instrument suite has been described in detail previously (Mahaffy et al., 2012). SAM is positioned in the front body of the rover and receives samples that have been drilled and then processed on the end of Curiosity's robotic arm (Anderson et al., 2012). The analyses were carried out under the so‐called “nominal solid‐sample analysis mode.”Prior to a sample run, a single‐quartz cup was preheated to >800 °C under He flow with active pumping using SAM's wide‐range pumps to eliminate volatiles and potential contaminants that were previously absorbed. Then the cup was rotated to be positioned underneath the SAM solid sample inlet tube to acquire the sample from the Collection and Handling for In‐Situ Martian Rock Analysis (CHIMRA) device. The rock powder was sieved (<150 μm) and delivered in single (~76 mm 3 ), triple, or quadruple aliquots into one of SAM's cups. Once the sample was received, the cup was hermetically sealed and moved inside the SAM pyrolysis oven. The sample cup contains a porous quartz frit on the bottom where a stream of helium (∼0.8 cm 3 /min) flows vertically through the sample for efficient transport of evolved gases out of the oven during the heating process. The sample was heated from Mars ambient temperature to ∼870 °C at heating rate of 35 °C/min, maintaining an oven pressure of ∼25 mbar during the analysis. The evolved gases were continuously analyzed by a quadrupole mass spectrometer operating with electron impact ionization mode at 70 eV. The ion with a mass to charge ratio (m/z) of 30 was selected to monitor and quantify NO, which is the major product of thermal decomposition of nitrate. NO evolved at temperatures that was characteristic for the thermal decomposition of NO 3 − or mixtures of NO 3 − and ClO 4 − .NO 2 − also thermally decomposes releasing NO, but no studies were previously available to determine their presence in the Martian samples. Other plausible interferences on m/z30 include an isotopologue of CO, 12 C 18 O arising from the electron impact ionization of CO and CO 2 ; however, the m/z30 signal represents 0.2%, and 0.04% of the m/z28 signal arising from the electron impact ionization of CO and CO 2 , respectively. Even if CO 2 was released in large quantities in the Martian samples, the contribution of 12 C 18 O to the m/z30 signal is negligible (<1%). N‐methyl‐ N‐(tert‐butyldimethylsilyl) trifluoroacetamide, abbreviated as MTBSTFA, is a derivatization agent that was brought to Mars in sealed cups for wet chemistry SAM‐GCMS analysis. Nonetheless, one cup was found to leak and reacted with the samples during nominal solid‐sample analysis mode leading to the detection of hydrocarbons and NO (Freissinet et al., 2015; Glavin et al., 2013; J. C. Stern et al., 2015; Sutter et al., 2017). The interference of MTBSTFA in the NO measurement by SAM was calculated by the background correction method by J. C. Stern et al. (2015). Since each molecule of MTBSTFA has one N atom that can potentially decompose into NO, its contribution can be subtracted from the total amount of NO measured in the SAM experiments. This method was used to calculate the nitrogen content in the drilled samples from John Klein up to Greenhorn (J. C. Stern et al., 2015, 2017). In order to decipher if nitrites are present in the Martian samples, it is required to know the thermal evolution profiles of NO from nitrites and nitrates. In this scenario, it is not possible to use the background correction method. Instead, it is required to 10.1029/2018JE005852 Journal of Geophysical Research: Planets NAVARRO‐GONZÁLEZ ET AL. 98 eliminate the m/z30 signal of MTBSTFA products as they thermally evolve during the analysis. Laboratory experiments of MTBSTFA degradation products under SAM‐like conditions indicate that the major interferences to the m/z30 signal are formaldehyde (HCHO, m/z29 [100%] and m/z30 [~60%]) and ethane (C 2 H 6 ,m/z29 [~20%] and m/z30 [~20%]). Therefore, it is possible to eliminate their contribution according to the following empirical formula: NO signal ¼m=z30−bm=z29–m=z43ðÞ where bis a constant that varies from 0.6 (contribution from HCHO) to 0.8 (contributions from both HCHO and C 2 H 6 ) and even to 1.0 (contributions from HCHO, C 2 H 6 , and unidentified species) depending on the run. The m/z43 signal is attributed to ketones, alkyladehydes, and/or hydrocarbons. Supporting information Figures S1 through S14 show the signals for m/z29, 30, and 43 and the resultant NO profiles for all the samples investigated. These plots were dead time and background corrected. The nitrogen content in the Martian samples was calculated from the NO signal that was corrected taking into account its ionization cross section at 70 eV relative to the response of CO 2 in the sample, for which a calibration curve exists (Archer et al., 2014). The error reported for a single run includes the error in the determination of the area of NO and the uncertainty in the mass of the sample delivered to SAM. For multiple sample analysis the error reported was the mean and standard deviation (1σ) of the measurements. 3. Results and Discussion 3.1. Theoretical and Experimental Production of NO by Shock Waves The theoretical estimate for the production of NO takes into account that NO is formed at high temperature as the air is suddenly heated by the shock wave. The concentration of NO rapidly reaches thermochemical equilibrium with the temperature of the surrounding gas. As the shocked air expands and cools, a point in time is reached when thermochemical equilibrium is no longer kept during the rapid cooling process and a net amount of NO is “frozen”at a given temperature (Chameides et al., 1977). The formation of NO in shock‐ heated CO 2 /(CO 2 +N 2 ) atmospheres occurs at temperatures in excess of 2000 K (Navarro‐González, McKay, & Nna Mvondo, 2001) and is initiated by the reaction of atomic oxygen (O), arising from the dissociation of CO 2 , with N 2 via reaction (R1): OþN2⇆NO þN:(R1) This reaction is part of the Zel'dovitch mechanism for the oxidation of N 2 in air (Zel'dovitch & Raizer, 1966). As the shock wave expands and the gas cools, the NO equilibrium mixing ratio (f NO ) changes with temperature (Figure 1); f NO is locked at a characteristic temperature, usually referred to as the freeze‐out temperature (T F ), when the relaxation time of NO (τ NO ) becomes equal to or greater than the cooling time of the heated gas (Chameides et al., 1977). f NO and the equilibrium concentrations of N, O, NO, and N 2 were calculated as a function of temperature; τ NO was determined using the rate constants (k) for the forward (f) and reverse (r) pathways of reaction (R1). The relaxation time of NO (τ NO ) to establish equilibrium conditions to a drop in temperature as the air expands and cools is given in equation (1): τNO ¼1 kfO½þN2 ½ðÞþkrNO½þN½ðÞ ;(1) where [O], [N], [NO], and [N 2 ] are the equilibrium concentrations before the temperature change, and k f and k r are the rates of reactions at the jump down temperature. The rates of reactions as a function of temperature are kf¼3:0×−10cm3·molecule−1·s−1e−ð318:0 kJmole−1=RTÞand kr¼7:1×−11cm3·molecule−1·s−1 e−ð6:6 kJmole−1=RTÞ(Manion et al., 2015). Figure 1. The NO equilibrium mixing ratio (f NO ) and the relaxation time for NO (τ NO ) as a function of temperature. f NO is expressed as the number of moles of NO divided by the total number of moles of all gas constituents in a mixture composed of 50% CO 2 and 50% N 2 without H 2 at 1 bar. 10.1029/2018JE005852 Journal of Geophysical Research: Planets NAVARRO‐GONZÁLEZ ET AL. 99 Figure 1 also shows how the relaxation time varies with temperature. T F can vary depending on the instantaneous energy input delivered to produce the shock wave; for example, more energy results in faster cooling time of the heated gas (Chameides, 1979) and the chemical composition of the gas mixture (Chameides & Walker, 1981). The energy yield for the production of NO produced by bolide impacts was estimated from a revised version of the model of Chameides et al. (1977), in which about 50% of the energy of the impact was dissipated by the shock wave in the form of heat, and the Cp for the gases in the mixture were taken into account, according to the following equation (equation (2)): ENO ¼0:5×NA×fNO TF ðÞ TF×CO 2 ½CpCO2þN2 ½CpN2þH2 ½CpH2  ;(2) where E NO refers to the number of NO molecules that formed per joule absorbed; N A is the Avogadro constant; f NO (T F )is the predicted nitric oxide equilibrium mixing ratio at T F ; [gas] is the mole fraction of each gas in the mixture; and Cp is the specific heat at constant pressure of each gas. T F and f NO were not actually measured experimentally, but T F was left as a free parameter to adjust the experimental value of E NO with that computed using equation (2). Shock waves in the laboratory were generated by focusing a pulse laser inside a reactor with simulated atmospheres (CO 2 ,N 2 , and H 2 ) at 1 bar (Navarro‐González, Villagrán‐Muniz, et al., 2001; Sobral et al., 2000). The presence of H 2 was expected to lead to the fixation of reduced forms of N, such as HCN and HC ≡C‐CN. Surprisingly, none of these species were detected. The net amount of NO produced in the experiments was determined by GC coupled to MS, and the energy deposited was determined optically. E NO was derived from the slope of a linear plot of the number of molecules formed as a function of the energy absorbed in the experiments (Navarro‐González, McKay, & Nna Mvondo, 2001). E NO has been found to vary linearly with pressure (Rahman & Cooray, 2008). Tables S1 and S2 and Figure 2 show how the experimental and predicted energy yields for the production of NO by shock waves vary with the CO 2 /(CO 2 +N 2 ) ratio in different simulated primitive Martian atmospheres in the absence and presence of 10% and 20% H 2 . It also includes data from previous experiments (Levine et al., 1982; Navarro, 2014; Navarro‐González, McKay, & Nna Mvondo, 2001) in the absence of H 2 . In particular, the experimental data at 0% H 2 indicate that the formation of NO increases from ~1.9 × 10 15 molecules/J at CO 2 /(CO 2 +N 2 ) = 0.98 to ~1.3 × 10 16 molecules/J at CO 2 /(CO 2 +N 2 ) from 0.8 to 0.5 and then drastically drops to ~4.9 × 10 13 molecules/J at CO 2 /(CO 2 +N 2 ) = 0.01. The expected T F for NO in shock‐heated air (N 2 /O 2 ) is 2300 K (Navarro‐González, Villagrán‐Muniz, et al., 2001). The predicted trend using this value is in good agreement with experiments at CO 2 /(CO 2 +N 2 )≥0.5. At lower CO 2 levels T F for NO is probably much lower resulting in a diminishing in the NO yield. Surprisingly there is a 1.7‐fold and 2.6‐fold increase in the experimental NO energy yield when 10% and 20% H 2 are included in the system, respectively. However, if T F for NO were to remain constant at 2300 K, the expected NO energy yield would drop off at 69% and 88% in 10% and 20% H 2 , respectively. Consequently the discrepancy between experimental data and theoretical trends shown in Figure 2 is due to the appropriate T F values used in the computations. In order to explain the sudden increase in the NO energy yield when H 2 is included, the T F for NO must change to 2650 and 3000 K in 10% and 20% H 2 , successively. This means that the shock wave cools off much faster in the presence of H 2 when f NO has a higher value resulting in an enhanced amount of NO frozen in the heated gas by the shock wave. This is counterintuitive because H 2 is a reducing agent and was not expect to lead to an enhancement in the rate of nitrogen oxidation. This finding has important implications for the N fixation rate of the Martian atmosphere under reducing conditions. 3.2. The Nitrogen Fixation Rate by Bolide Impacts The N fixation rate (N F ) was estimated assuming that NO was quantitatively converted to NO 2 − and/or NO 3 − and no losses occurred in either the atmosphere (Mancinelli & Banin, 2003; Summers & Khare, Figure 2. Variation of E NO as a function of the CO 2 /(CO 2 +N 2 ) mole ratio in shock waves simulating different possible compositions of the primitive Martian atmosphere at 1 bar. Symbols are experimental data, and lines are predictions. Ref. 1 = Navarro‐González, McKay, and Nna Mvondo (2001); Ref. 2 = Levine et al. (1982); Ref. 3 = Navarro (2014). 10.1029/2018JE005852 Journal of Geophysical Research: Planets NAVARRO‐GONZÁLEZ ET AL. 100 2007) or surface due to N 2 O (Samarkin et al., 2010). The impactor flux on Mars was used to calculate the N fixation rate through time. This was calculated by using the Hartmann and Neukum (2001) to derive the cumulative number (N) of projectiles producing craters with diameters ≥1 km in an area of 1 km 2 over the entire history of Mars. The analytical description of the model is given by equation (3): NF¼2:68×10−14 e6:93tðÞ –1  þ4:13×10−4t;(3) where, tdenotes time in billion years. The nature of the planetary objects that impacted early Mars is not known. Analysis of crater size distributions indicates two populations of projectiles (Strom et al., 2005). Population 1 has a radius centered at 2 km (Strom et al., 2005) and was responsible for the late heavy bombardment (LHB): from ~4200 to ~3500 Myr (Bottke & Norman, 2017). The sources of these objects were likely asteroids (Strom et al., 2005) and to a lesser extent comets (Gomes et al., 2005) that were dynamically ejected by orbital migration of the giant planets. Population 2 has a radius centered at 0.5 km, similar in size to the near‐Earth asteroids, and was responsible for the impacts (Strom et al., 2005) after the LHB. The mass (m) of projectiles from populations 1 and 2 was calculated using a density of 3 g/cm 3 , typical of a basaltic asteroid (Kring & Cohen, 2002). The average impact velocity (v) estimated for Mars is 9.8 km/s (Ivanov, 2001) and was assumed to be similar for both populations of projectiles. The energy deposited into the atmosphere by these two populations of impactors was calculated as mv 2 /2 (Melosh & Vickery, 1989). The estimated upper and lower boundaries for the N fixation rate by bolide impacts are given in Table S3, and their trends are shown in Figure 3 from the Pre‐Noachian to the Hesperian considering the presence or absence of H 2 in the atmosphere. The abrupt change in the slopes at 3500 Myr is due to the different size of projectiles during and after LHB. The upper boundary ([CO 2 /(CO 2 +N 2 )] = 0.5, and 20% H 2 ) has a maximum rate of 10.8 g N·Myr −1 ·cm −2 at the start of the LHB in the Pre‐ Noachian, and then it rapidly decreased to 0.2 g N·Myr −1 ·cm −2 at the end of the LHB at the early Hesperian (Figure 3). During this time the total accumulated mass of nitrogen in the surface is predicted to be 22.4 g N/cm 2 , equivalent to a global deposit of 60 cm of purely solid sodium nitrate (NaNO 3 ). In contrast, the lower boundary ([CO 2 /(CO 2 +N 2 )] = 0.5, and 0% H 2 ) has a maximum rate of 3.7 g N·Myr −1 ·cm −2 at 4200 Myr ago, decreasing steadily to 0.07 g N·Myr −1 ·cm −2 at the end of the LHB (Figure 3). The total accumulated mass of nitrogen on the Martian surface is predicted to be 7.7 g N/cm 2 , equivalent to a global deposit of 20 cm of purely solid NaNO 3 . In both scenarios it is possible that a significant fraction of the N fixed may have been destroyed (Manning et al., 2008) or buried (Hartmann et al., 2001) due to resurfacing or gardening during the LHB. After the LHB, the rate of N fixation slowly dropped from 3.3 × 10 −3 to 1.8 × 10 −3 g N·Myr −1 ·cm −2 and from 1.1 × 10 −3 to 6.2 × 10 −4 g N·Myr −1 ·cm −2 from the early to the end of the Hesperian for the upper and lower boundaries, respectively (Figure 3). It is predicted that a mass of nitrogen ranging from 4.4 to 12.8 mg N/cm 2 would have been globally deposited on the surface during the Hesperian, equivalent to a deposit of NaNO 3 ranging from 12 to 34 mm for the lower and upper boundaries, correspondingly. 3.3. Fixed Nitrogen Products Detected by MSL The Phoenix mission landed in the northern plains of Mars on 25 May 2008. It has been the only spacecraft designed to search for soil NO 3 − but was unable to detect any by the Wet Chemistry Laboratory and Thermal and Evolved Gas Analyzer (Hecht et al., 2009). The Wet Chemistry Laboratory was equipped with an ion selective electrode for measuring nitrate from an aqueous extract of Martian soil, but with the unexpected Figure 3. The nitrogen fixation (N F ) and the nitrogen deposition (N D ) or the nitrogen accumulation (N A ) rates as a function of time in the Martian atmosphere are shown by the red and blue lines and symbols, respectively. The dash and solid lines constrain the lower (0% H 2 ) and upper (20% H 2 ) boundaries in CO 2 /(CO 2 +N 2 ) atmospheres by bolide impacts, consecutively. The abrupt change in slopes at 3500 Myr is due to different radius of projectiles during (population 1 = 2 km) and after (population 2 = 0.5 km) the late heavy bombardment process. The stars show the rate by ultraviolet light (after Yung et al., 1977; Smith et al., 2014). The triangles show the rate by volcanic lightning (after Navarro‐González et al., 1998; Segura & Navarro‐González, 2001, 2005). The square and diamond symbols show N D rates determined for the Cumberland and John Klein at Yellowknife Bay, in Gale crater assuming a sediment deposition rate of 20,000 cm/Myr (see section 3.5). The N A rates for bolide impacts, ultraviolet light, and volcanic lightning were calculated assuming that NO 2 − and NO 3 − deposited entirely on the Gale's crater were ultimately transported to the bottom of the lake (see sections 3.5 and 3.6). 10.1029/2018JE005852 Journal of Geophysical Research: Planets NAVARRO‐GONZÁLEZ ET AL. 101 presence of perchlorate in the soil, the response of the electrode resulted to be stronger for perchlorate than nitrate by a factor of 1,000. The Thermal and Evolved Gas Analyzer was unable to detect any evolved NO from the thermal treatment of the arctic Martian soil at concentrations below the natural background levels of 15 N≡ 15 N present in the N 2 (Yeung et al., 2017), which was used as the carrier gas to transfer the evolved gases from the oven to the mass spectrometer. The first detection of NO 3 − in soils and sediments was carried out by the SAM instrument suite of the MSL Curiosity rover, after landing on Bradbury at Gale crater on 6 August 2012 (Archer et al., 2014; Ming et al., 2014; Navarro‐González et al., 2013; J. C. Stern et al., 2015, 2017, 2018; Sutter et al., 2017). NO 3 − thermally decomposes releasing NO which has been used to quantify it in the Martian surface. Curiosity has traversed a total distance of 19.809 km up until Sol 2221 (5 November 2018), and during this time it has drilled 17 sedimentary rocks (see Figure 4). Fourteen of these rock samples have been analyzed by SAM from the lowest stratigraphic unit Sheepbed Figure 4. Rocks studied by NASA's Curiosity rover during its three‐Martian‐year traverse on Gale crater until Sol 2136 (8 August 8 2018). The inset on the right of the base map is a composite image showing the drilled holes performed by Curiosity. The base map shows on the left the rover traverse with the locations of the rocks surveyed (red dots). Upon landing on Aeolis Palus in August of 2012, Curiosity traversed east from the Bradbury landing site to Yellowknife Bay, and then southwest toward Aeolis Mons reaching the base of the mountain on Sol 746. The total driven distance was 19.641 km until Sol 2156 (30 August 2018). North is toward the upper left corner. The scale bar represents 2 km (1.2 miles). The base map is from the High Resolution Imaging Science Experiment camera on NASA's Mars Reconnaissance Orbiter. Drilled hole images were taken with the Mars Hand Lens Imager (MAHLI) camera on the end of the arm from a distance of about 5 cm. The drill holes are ~1.6 cm wide. 10.1029/2018JE005852 Journal of Geophysical Research: Planets NAVARRO‐GONZÁLEZ ET AL. 102 Episodic explosive volcanic eruptions probably occurred in Tharsis and Elysium volcanic provinces, lasting from the Hesperian to the Amazonian (3000 Myr to present) (Xiao et al., 2012). Such eruptions were probably accompanied by copious lightning discharges causing the conversion of N 2 into HCN and/or NO 3 − at high temperatures depending on the nature of the gases emitted by volcanoes (Navarro‐González et al., 1998; Segura & Navarro‐González, 2001, 2005). The energy flux delivered by volcanic lightning has been calculated during the Hesperian period (Segura & Navarro‐González, 2001) considering a global magma production of 5 km 3 /year (Xiao et al., 2012). Assuming that all compounds containing fixed N were finally converted into NO 3 − ,N F by volcanic lightning is estimated to be 5 × 10 −8 g N·Myr −1 ·cm −2 (Segura & Navarro‐González, 2005). If all NO 3 − deposited over the entire surface of Gale crater was transported into the lake, the N A value would increase to 3 × 10 −5 g N·Myr −1 ·cm −2 . These values are below the lower boundary limit set up by bolide impacts by 2 orders of magnitude and therefore are inadequate to account for the NO 3 − deposition rate derived for the Cumberland sample (see Figure 3). Other energy sources such as cosmic rays, corona and lightning discharges from thunderstorms, and heat from volcanoes had a minor role in N fixation, contributing to a N F value of <2 × 10 −9 g N·Myr −1 ·cm −2 (Segura & Navarro‐González, 2005). Recently, it has been argued that coronal mass ejection events from the young Sun, referred to as superflares, generated energetic particles that initiated reactions converting molecular nitrogen, methane, and carbon dioxide into HCN, NO, and N 2 O in the early Earth (Airapetian et al., 2016). This is a well‐known mechanism where HCN is produced photochemically from N sourced from the ionosphere in a pathway that depends on CH 4 (Tian et al., 2011; Zahnle, 1986). This process was potentially relevant to early Mars, particularly because methane was probably another greenhouse gas present in the atmosphere (Wordsworth et al., 2017), but the type of products fixed and their yields have not been evaluated yet. An additional source of fixed N arises from the exogenous delivery of organics by comets and interplanetary particles to the Martian surface, but its contribution was negligible, for example, <10 −9 g N·Myr −1 ·cm −2 (Segura & Navarro‐González, 2005). 4. Conclusions and Implications A sharp decline on the amount of fixed nitrogen was found of about 2 orders of magnitude in the rocks sampled by the Curiosity rover during its traverse from the lowest stratigraphic layers encountered on Aeolis Palus to the upper strata of the base of Aeolis Mons. Two possible scenarios were discussed to account for the decline of fixed nitrogen in the stratigraphic record in Gale crater: (1) diagenesis and leaching of fixed nitrogen in the sedimentary rocks or (2) change in the rate of nitrogen deposition. The decline of fixed nitrogen found in Gale crater seems to resemble the leaching of nitrate from the upper stratigraphic layers to the lowest one, as seen in the driest region of the Atacama with three distinct nitrogen layers centered at −4,436, −4,447, and −4,520 m with the lowest exhibiting the highest concentration. However, the evidences against extensive leaching of nitrite and nitrates in the sedimentary rocks at Gale crater are the following: (1) The concentration of NO 2 − and NO 3 − in the sediments should be higher by 10 3 to 10 4 orders than the highest value observed considering the Atacama nitrate deposits; (2) the existence of a reduced layer of fixed nitrogen (nitrite) below an oxidized layer of nitrate indicates different deposition conditions; (3) the release of NO in the nitrate layer occurs at various temperatures in the different strata investigated indicating the presence of several nitrate/oxychlorine salts and the existence of a variety of sedimentary environments; (4) there are water‐soluble as well as less soluble salts in almost all of the layers investigated; and (5) the Sheepbed formation experienced postdepositional aqueous alteration but with a low water/rock interaction. Therefore, diagenesis and leaching of fixed nitrogen in the sedimentary rocks is considered less likely but still possible. On the other hand, the abrupt decline of the amount of fixed nitrogen could alternatively imply that the chemistry of the Martian atmosphere drastically changed if it is assumed that the energy sources responsible for its formation remained constant during the period when these lacustrine and eolian deposits formed. Our experimental results suggest that a high N fixation rate was possible in the presence of H 2 in the atmosphere. The presence of H 2 from volcanic emissions could have resulted in an enhanced yield in the formation of NO 2 − and NO 3 − by collisions of asteroids into the Martian atmosphere and surface. This enhancement was due to a faster cooling rate of the shock wave freezing NO when its concentration was higher in the heated gas. The impactor flux was used to calculate the nitrogen fixation rate and was found to vary from 10.1029/2018JE005852 Journal of Geophysical Research: Planets NAVARRO‐GONZÁLEZ ET AL. 109 7×10 −4 to 2 × 10 −3 g N·Myr −1 ·cm −2 around 3250 Myr ago in the absence or presence of H 2 , respectively. In contrast, the nitrogen deposition rate derived from the SAM data at the lowest stratigraphic unit encountered by Curiosity was estimated to vary from 0.3(±0.1) to 1.4(±0.7) g N·Myr −1 ·cm −2 for John Klein and Cumberland, respectively. This value was too high to explain atmospheric NO 2 − and NO 3 − deposition directly on the surface of the lake at Gale crater. It was inferred that fixed nitrogen found at the Sheepbed unit formed in a H 2 ‐rich atmosphere (up to 20%) and was deposited on the entire crater's surface. Upon favorable wet climatic conditions, it was dissolved, transported, and concentrated into the lake by superficial and ground waters. The nitrogen accumulation rate by bolide impacts increases to 0.4 and 1.2 g N·Myr −1 ·cm −2 for the lower and upper boundaries, correspondingly. The value obtained from the Cumberland sample is greater than the maximum predicted value for bolide impacts. Possible explanations for this variation include (1) The nitrogen that was dissolved and transported to the lake extended the confines of Gale crater; (2) the sediment deposition rate was not constant along the stratigraphic column investigated by Curiosity; and (3) some degree of diagenesis and leaching of fixed nitrogen took place. The nitrogen accumulation rate by ultraviolet light was found to be similar to the lowest nitrogen deposition values derived from the SAM data. Other sources of fixed nitrogen were found to be too small compared to that supplied by bolide impacts to Gale crater. The caveats of this scenario are that other atmospheric compositions are possible for Mars, such as CO 2 ‐CH 4 ‐ N 2 and CO 2 ‐CO‐N 2 ‐H 2 . Such atmospheres could potentially have an impact on the rate of N fixation and a shift in the type of N species formed as they evolved into CO 2 ‐N 2 . Therefore, further laboratory studies are required to determine the rate of N fixation by bolide impacts in these atmospheres. If the decline of amount of fixed nitrogen found in Gale crater was due to a change in the rate of N deposition, this caused a shortage in the accessibility of fixed nitrogen that could have led to a crisis to microbial life at Gale crater and could have triggered the development of biological nitrogen fixation. A similar nitrogen crisis was inferred for early Earth based on a sharp decline in the NO rate by lightning during the conversion of the atmosphere from mostly CO 2 to primarily N 2 (Navarro‐González, McKay, & Nna Mvondo, 2001). The study of the N and O isotopes present in NO 2 − and NO 3 − from the sediments investigated by Curiosity and other future missions could provide clues on its origin. 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