Atmospheric Supply of Hydrogen Cyanide Is Not the Rate-limiting Step for Prebiotic Chemistry across Rocky Exoplanets

TL;DR

This study models atmospheric hydrogen cyanide (HCN) delivery to early Earth-like planets, finding it is generally abundant and not the limiting factor for prebiotic chemistry, thus increasing prospects for life elsewhere.

Contribution

It demonstrates that atmospheric HCN supply is robust across various conditions and unlikely to limit prebiotic chemistry on rocky exoplanets.

Findings

Atmospheric HCN delivery exceeds meteoritic sources in baseline conditions.

Higher C/O ratios and closer orbiting M-dwarfs increase atmospheric HCN delivery.

HCN delivery is not the rate-limiting step for prebiotic chemistry on rocky exoplanets.

Abstract

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.