A critical early stage for the origin of life on Earth may have involved the production of hydrogen cyanide (HCN) in a reducing, predominantly H2 atmosphere. HCN is critical to 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 study of HCN production under reducing atmospheric conditions (89–95% H2), possibly representing the earliest stages of the Hadean Age, around 4.5–4.3 billion years ago. We use cold plasma discharges – a laboratory analogue of shortwave UV radiation – to simulate HCN production in the upper atmosphere for CH4 abundances between 0.1 and 6.5%.
We then combine experimental mass spectrum measurements with our theoretical plasma models to estimate the HCN concentrations generated in our experiments. We find that HCN production in the upper atmosphere scales linearly with CH4 abundance with the relationship [HCN] = 0.13 ± 0.01[CH4].
Concentrations of HCN are expected to be about 2-3 orders of magnitude lower near the surface of the Hadean Earth. The addition of 1% water to our experiments resulted in a ~50% reduction in HCN production. We find that four reactions are mainly responsible for HCN production in our experiments: (i) 4N + CH3 -> H2CN + H -> HCN + H2, (ii) 4N + CH -> CN + H followed by CN + CH4 -> HCN + CH3, (iii) C2H4 + 4N -> HCN + CH3 and (iv) 4N + 3CH2 -> HCN + H.
The prebiotically most favorable Hada atmosphere would have been very rich in CH4 (> 5%) and the surface would likely be very hot as a result of greenhouse effects. In such a prebiotic scenario, it might have been important to incorporate HCN into organic turbidities that could later release biomolecules and precursors into the first ponds.
Ben KD Pearce, Chao He, Sarah M. Hörst
Comments: Accepted for publication in ACS Earth and Space Chemistry, 24 pages, 7 figures, 5 tables, Supporting Information
Subjects: Astrophysics of the Earth and Planets (astro-ph.EP); Atmospheric and Ocean Physics (physics.ao-ph); Chemical physics (physics.chem-ph)
Cite as: arXiv:2209.09257 [astro-ph.EP] (or arXiv:2209.09257v1 [astro-ph.EP] for this version)
By: Ben KD Pearce PhD
[v1] Mon, September 19, 2022 18:00:01 UTC (3,449 KB)