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

Abstract

A critical early stage for the origin of life on Earth may have involved the production of hydrogen cyanide (HCN) in a reducing, predominantly H${}_2$ atmosphere. HCN is crucial for the origin of life as it is a possible precursor to several biomolecules that make up RNA and proteins including nucleobases, nucleotides, amino acids, and ribose. In this work, we perform an in depth experimental and theoretical investigation of HCN production in reducing atmospheric conditions (89-95% H${}_2$) possibly representing the earliest stages of the Hadean eon, ~4.5-4.3 billion years ago. We make use of cold plasma discharges - a laboratory analog to shortwave UV radiation - to simulate HCN production in the upper layers of the atmosphere for CH${}_4$ abundances ranging from 0.1-6.5%. We then combine experimental mass spectrum measurements with our theoretical plasma models to estimate the HCN concentrations produced in our experiments. We find that upper atmospheric HCN production scales linearly with CH${}_4$ 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 be about 2-3 orders of magnitude lower. The addition of 1% water to our experiments results in a ~50% reduction in HCN production. We find that four reactions 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 + H followed 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 prebiotically favorable Hadean atmosphere would have been very rich in CH${}_4$ (> 5%), and as a result of greenhouse effects the surface would be likely very hot. In such a prebiotic scenario, it may have been important to incorporate HCN into organic hazes that could later release biomolecules and precursors into the first ponds.

Accepted for publication in ACS Earth and Space Chemistry, 24 pages, 7 figures, 5 tables, Supporting info

References in corpus (11)

• The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth
• Hydrogen Cyanide in Nitrogen-Rich Atmospheres of Rocky Exoplanets
• New chemical scheme for studying carbon-rich exoplanet atmospheres
• Sulfur-driven Haze Formation in Warm CO2-rich Exoplanet Atmospheres
• Abundant atmospheric methane from volcanism on terrestrial planets is unlikely and strengthens the case for methane as a biosignature
• Chemistry of Temperate Super-Earth and Mini-Neptune Atmospheric Hazes from Laboratory Experiments
• Towards RNA life on Early Earth: From atmospheric HCN to biomolecule production in warm little ponds
• A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan Atmospheres: Quantum Calculations of Reaction Rate Coefficients
• HCN production in Titan's Atmosphere: Coupling quantum chemistry and disequilibrium atmospheric modeling
• Neutral dissociation of methane by electron impact and a complete and consistent cross section set
• CRAHCN-O: A Consistent Reduced Atmospheric Hybrid Chemical Network Oxygen Extension for Hydrogen Cyanide and Formaldehyde Chemistry in CO2-, N2-, H2O-, CH4-, and H2-Dominated Atmospheres

Cited by in corpus (4)

• Organic hazes as a source of life's building blocks to warm little ponds on the Hadean Earth
• Low temperature formation of pyridine and (iso)quinoline via neutral neutral reactions
• Towards Prebiotic Chemistry on Titan: Impact experiments on organic haze particles
• Hydrocarbon complexity and photochemical shielding of prebiotic feedstock molecules in exoplanet atmospheres