Astrochemistry

Atmospheric Supply Of HCN Is Not The Rate Limiting Step For Prebiotic Chemistry Across Rocky Exoplanets

By Keith Cowing

Status Report

astro-ph.EP

March 22, 2026

Filed under Archean, astro-ph.EP, atmosphere, biochemistry, exoplanet, genomics, hydrogen cyanide, meteorites, origin of life, RNA world, rocky exoplanet

Atmospheric Supply Of HCN Is Not The Rate Limiting Step For Prebiotic Chemistry Across Rocky Exoplanets — HCN rainout rate over the conjoint parameter space of stellar effective temperature (Teff ) and effective solar flux in current Earth values (Seff ) and C/O ratio. a) Baseline Archean Earth C/O ratio of 0.514 with the black box on the plot and gray horizontal line on the colourbar representing the Archean Earth model and estimated maximum HCN delivery rate by meteorites, respectively; b) Histogram of all simulated HCN rainout rates with gray, black, and pink vertical dashed lines representing the estimated maximum exogenous HCN delivery rate, the Archean Earth model, and the median HCN rainout rate, respectively. — astro-ph.EP

Hydrogen cyanide (HCN) is crucial for the RNA World hypothesis, forming biomolecules essential for early life.

Life likely emerged around 4 billion years ago during the early Archean Eon, a period on Earth with a fainter sun, frequent impacts, and a weakly reducing atmosphere. Warm little ponds (WLPs) are hypothetical protective aqueous environments that help explain the emergence and evolution of fragile prebiotic chemistry in such a hostile environment. WLPs need to undergo cycles of evaporation and rehydration, concentrating prebiotic molecules that increase the likelihood of (de-)polymerisation and forming early RNA molecules.

We use a 1-D model of atmospheric chemistry to compare atmospheric HCN delivery to WLPs with exogenous sources. Using early Archean Earth as our baseline, we examine the sensitivity of atmospheric HCN delivery to the atmospheric C/O ratio, semi-major axis, assumed stellar host type, and methane budget, exploring conditions across rocky exoplanets.

We find that atmospheric HCN delivery is sensitive to these parameters but its values generally exceed that of meteoritic delivery and our baseline Archean Earth. Planetary atmospheres with higher C/O ratios within the habitable zones of G stars and those closely orbiting M-dwarfs deliver the most atmospheric HCN.

We find that atmospheric HCN delivery is remarkably robust, so this molecule is likely not the rate limiting step for the emergence of prebiotic chemistry on rocky exoplanets. This finding, with important caveats, potentially increases the probability of life emerging on other worlds.

Gergely Friss, Paul I. Palmer, Marrick Braam, Ken Rice

Comments: 26 pages, 15 figures (8 in main text, 7 in appendix), 2 tables, submitted to and accepted by the Astrophysical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2603.18769 [astro-ph.EP] (or arXiv:2603.18769v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2603.18769
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Related DOI:
https://doi.org/10.3847/1538-4357/ae505d
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Submission history
From: Gergely Friss
[v1] Thu, 19 Mar 2026 11:23:04 UTC (1,328 KB)
https://arxiv.org/abs/2603.18769
Astrobiology, Astrochemistry,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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