Nitrogen Oxides in Early Earth's Atmosphere as Electron Acceptors for Life's Emergence

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 (NO₃^-) and nitrite (NO₂^-) 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 CO₂ and N₂, will produce nitric oxide (NO) when shocked by lightning. Photochemical reactions involving NO and H₂O vapor will then produce acids such as HNO, HNO₂, HNO₃, and HO₂NO₂ 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 pCO₂ models, we calculate the NOx delivery to be 2.4 × 10⁵, 6.5 × 10⁸, and 1.9 × 10⁸ 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. Key Words: Nitrogen oxides-Nitrate-Nitrite-Photochemistry-Lightning-Emergence of life. Astrobiology 17, 975-983.