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A critical early stage for the origin of life on Earth may have involved theproduction of hydrogen cyanide (HCN) in a reducing, predominantly H$_2$atmosphere. HCN is crucial for the origin of life as it is a possible precursorto several biomolecules that make up RNA and proteins including nucleobases,nucleotides, amino acids, and ribose. In this work, we perform an in depthexperimental and theoretical investigation of HCN production in reducingatmospheric conditions (89-95% H$_2$) possibly representing the earliest stagesof the Hadean eon, ~4.5-4.3 billion years ago. We make use of cold plasmadischarges - a laboratory analog to shortwave UV radiation - to simulate HCNproduction in the upper layers of the atmosphere for CH$_4$ abundances rangingfrom 0.1-6.5%. We then combine experimental mass spectrum measurements with ourtheoretical plasma models to estimate the HCN concentrations produced in ourexperiments. We find that upper atmospheric HCN production scales linearly withCH$_4$ abundance with the relation [HCN] = 0.13 $\pm$ 0.01[CH$_4$].Concentrations of HCN near the surface of the Hadean Earth are expected to beabout 2-3 orders of magnitude lower. The addition of 1% water to ourexperiments results in a ~50% reduction in HCN production. We find that fourreactions are primarily responsible for HCN production in our experiments: (i)$^4$N + CH$_3$ -> H$_2$CN + H -> HCN + H$_2$, (ii) $^4$N + CH -> CN + Hfollowed by CN + CH$_4$ -> HCN + CH$_3$, (iii) C$_2$H$_4$ + $^4$N -> HCN +CH$_3$, and (iv) $^4$N + $^3$CH$_2$ -> HCN + H. The most prebioticallyfavorable Hadean atmosphere would have been very rich in CH$_4$ (> 5%), and asa result of greenhouse effects the surface would be likely very hot. In such aprebiotic scenario, it may have been important to incorporate HCN into organichazes that could later release biomolecules and precursors into the firstponds.

An experimental and theoretical investigation of HCN production in the Hadean Earth atmosphere