The Mars Science Laboratory (MSL) recently discovered nitrates in Gale Crater (e.g., Stern et al., 2015; Sutter et al., 2017). One possible mechanism for ancient nitrate deposition on Mars is through HNOx formation and rain out in the atmosphere, for which lightning-induced NO is likely the fundamental source. This study investigates nitrogen (N₂) fixation in early Mars' atmosphere, with implications for early Mars' habitability. We consider a 1 bar atmosphere of background CO₂, with abundance of N₂, hydrogen, and methane varied from 1% to 10% to explore a swath of potential early Mars climates. We derive lightning-induced thermochemical equilibrium fluxes of NO and HCN by coupling the lightning-rate parametrization from the study of Romps et al. (2014) with chemical equilibrium with applications, and we use a Geant4 simulation platform to estimate the effect of solar energetic particle events. These fluxes are used as input into KINETICS, the Caltech/JPL coupled photochemistry and transport code, which models the chemistry of 50 species linked by 495 reactions to derive rain-out fluxes of HNOx and HCN. We compute equilibrium concentrations of cyanide and nitrate in a putative northern ocean at early Mars, assuming hydrothermal vent circulation and photoreduction act as the dominant loss mechanisms. We find average oceanic concentrations of ∼0.1–2 nM nitrate and ∼0.01–2 mM cyanide. HCN is critical for protein synthesis at concentrations >0.01 M (e.g., Holm and Neubeck, 2009), and our result is astrobiologically significant if secondary local concentration mechanisms occurred. Nitrates may act as high-potential electron acceptors for early metabolisms, although the minimum concentration required is unknown. Our study derives concentrations that will be useful for future laboratory studies to investigate the habitability at early Mars. The aqueous nitrate concentrations correspond to surface nitrate precipitates of ∼1–8 × 10⁻⁴ wt % that may have formed after the evaporation of surface waters, and these values roughly agree with recent MSL measurements.

中文翻译：

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早期火星的固氮

火星科学实验室 (MSL) 最近在盖尔陨石坑中发现了硝酸盐（例如，Stern等人，2015 年；Sutter等人，2017 年）。火星上古代硝酸盐沉积的一种可能机制是通过 HNOx 的形成和大气中的降雨，其中闪电引起的 NO 可能是基本来源。这项研究调查了早期火星大气中的氮 (N ₂ ) 固定情况，对早期火星的宜居性有影响。我们考虑 1 bar 的背景 CO ₂大气，具有丰富的 N ₂、氢气和甲烷从 1% 到 10% 不等，以探索一系列潜在的早期火星气候。我们通过耦合 Romps等人的研究中的闪电率参数化来推导出 NO 和 HCN 的闪电诱导热化学平衡通量。(2014) 与应用的化学平衡，我们使用 Geant4 模拟平台来估计太阳高能粒子事件的影响。这些通量被用作 KINETICS（加州理工学院/JPL 耦合光化学和传输代码）的输入，该代码模拟了 50 个物种的化学反应，由 495 个反应连接，以推导出 HNOx 和 HCN 的雨出通量。我们计算了火星早期假定的北部海洋中氰化物和硝酸盐的平衡浓度，假设热液喷口环流和光还原是主要的损失机制。我们发现平均海洋浓度为~0.1-2 nM 硝酸盐和~0.01-2 mM 氰化物。HCN 对于浓度 >0.01 M 的蛋白质合成至关重要（例如, Holm and Neubeck, 2009)，如果发生二次局部浓度机制，我们的结果在天体生物学上具有重要意义。硝酸盐可以作为早期代谢的高电位电子受体，尽管所需的最低浓度未知。我们的研究得出的浓度将有助于未来的实验室研究，以调查早期火星的宜居性。硝酸盐水溶液的浓度对应于可能在地表水蒸发后形成的约 1–8 × 10 ^-4 wt % 的表面硝酸盐沉淀物，这些值与最近的 MSL 测量结果大致一致。