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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.

We quantify the amount of nitrogen oxides (NOx) produced through lightning and photochemical processes in the Hadean atmosphere to be available in the Hadean ocean for the emergence of life. Atmospherically generated nitrate (NO_3−) and nitrite (NO_2−) are the most attractive high-potential electron acceptors for pulling and enabling crucial redox reactions of autotrophic metabolic pathways at submarine alkaline hydrothermal vents. The Hadean atmosphere, dominated by CO_2 and N_2, will produce nitric oxide (NO) when shocked by lightning. Photochemical reactions involving NO and H_2O vapor will then produce acids such as HNO, HNO_2, HNO_3, and HO_2NO_2 that rain into the ocean. There, they dissociate into or react to form nitrate and nitrite. We present new calculations based on a novel combination of early-Earth global climate model and photochemical modeling, and we predict the flux of NOx to the Hadean ocean. In our 0.1-, 1-, and 10-bar pCO_2 models, we calculate the NOx delivery to be 2.4 × 10^5, 6.5 × 10^8, and 1.9 × 10^8 molecules cm^(−2) s^(−1). After only tens of thousands to tens of millions of years, these NOx fluxes are expected to produce sufficient (micromolar) ocean concentrations of high-potential electron acceptors for the emergence of life.