Atmospheric supply of HCN is not the rate limiting step for prebiotic chemistry across 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.

Figures (15)

• Figure 1: Temperature-pressure and Eddy diffusion profile in our base Archean simulation. The Eddy-diffusion profile is Earth-like and fixed throughout all our models.
• Figure 2: All 13 stellar spectra used in our study scaled to the respective stellar surface.
• Figure 3: HCN rainout rates ( first column), vertical profiles of HCN and water condensates ( second and third column) and TP profiles ( last column, calculated using HELIOS helios_1helios_2helios_3helios_4) for four explored physical parameters (C/O, semi-major axis, type for host star and initial CH$_4$ mixing ratios, respectively from top to bottom row). HCN rainout rate from our baseline Archean simulation is marked with a black square, and the gray dashed line represent our upper estimate for exogenous HCN delivery.
• Figure 4: Important reactions and their relative contribution to HCN production (first row) and destruction (second row), along with the net production and destruction rates and their difference, the total reaction rate of HCN (third row) for chosen C/O ratios.
• Figure 5: Vertical structure of various species contributing to HCN production or destruction for chosen C/O ratios.