New Estimates of Nitrogen Oxide and Hydrogen Cyanide

Fixed nitrogen species generated by the early Earth’s atmosphere are thought to be critical to the emergence of life and the sustenance of early metabolisms. Previously, Wong et al. (2017) estimated nitrogen fixation in the Hadean Earth’s N2/CO2-dominated atmosphere, but that study only considered a limited chemical network that produces NOx species (i.e., no HCN formation) via the thermochemical dissociation of N2 and CO2 in lightning flashes, followed by photochemistry. Here, we present an updated model of nitrogen fixation on the Hadean Earth. We use CEA (Chemical Equilibrium with Applications) and a Geant4 simulation to estimate lightning-induced NO and HCN formation. We used KINETICS (the 1-D Caltech/JPL photochemical model) to assess the photochemical production of fixed nitrogen species that rain out into the Earth’s early ocean and our updated photochemical model contains hydrocarbon and nitrile chemistry. We study the impact of a novel pathway for HCN production via HCN2, inspired by the experimental results of Trainer et al. (2012), which suggest that reactions with CH radicals (from CH4 photolysis) may facilitate the incorporation of N into the molecular structure of aerosols. Results show that this new reaction pathway results in a four-fold increase in the total HCN rainout rate. Finally we calculate the equilibrium concentration of fixed nitrogen species under kinetic steady state in the Hadean ocean, considering loss of NOx to photoreduction and loss of HCN to hydrolysis. Results with an assumed surface methane mixing ratio of 3% atmospheric concentration of methane show that HCN concentrations exceed the ~0.01 M of HCN required for protein synthesis, possibly relevant to the emergence and sustenance of early life (eg., Holm & Neubeck, 2009).