Hydrocarbon Complexity And Photochemical Shielding Of Prebiotic Feedstock Molecules In Exoplanet Atmospheres

The potential of prebiotic chemistry to propagate on an exoplanet fundamentally depends on whether the atmospheric conditions can facilitate the production of prebiotic feedstock molecules.

Photochemical simulations of exoplanet atmospheres can be used to explore this potential atmospheric synthesis, but require a comprehensive chemical network. We present the implementation of the CRAHCN-O network, constructed to simulate the formation of feedstock molecules such as HCN, H₂CO, and simple hydrocarbons, into the VULCAN photochemical kinetics code.

We investigate the production of feedstock molecules driven by M-star radiation and compare these to predictions by the N-C-H-O network in VULCAN, for N₂-dominated atmospheres with C/O ratios between 0.5-1.5. Predicted abundances are similar for C/O=0.5. Once CH₄ is included (i.e., for C/O>0.5), the abundance profiles diverge in the photochemical regions.

By analysing the attenuation of UV radiation, we find that hydrocarbon photochemical shielding causes the diverging profiles. CRAHCN-O accumulates C₂H₆, while N-C-H-O accumulates C₄H₃ and C₃H₄. Importantly, C₂H₆ is photochemically active whereas C₄H₃ and C₃H₄ are assumed inactive.

With mixing ratios up to a few percent in CRAHCN-O, C₂H₆ shields CH₄ and CO₂ from photodissociation and weakens the destruction of HCN and H₂CO. Maximum HCN mixing ratios reach 1000 ppm with CRAHCN-O compared to only 3 ppm with N-C-H-O. Other feedstock molecules like HC₃N and C₂H₂ form more efficiently in N-C-H-O. The shielding mechanism and its impact on feedstock molecules persist for radiation from distinct M-star types.

These results demonstrate the crucial role of chemical kinetics in understanding prebiotic processes in exoplanet atmospheres, including important considerations for the construction and applicability of chemical networks.

Graph networks comparing the chemical networks used in this study: N-C-H-O (top) and CRAHCN-O (bottom), for a reference pressure of 1 × 10−3 bar. Chemical species are represented as nodes, whereas reactions between species are shown as edges with their length corresponding to the inverse reaction rate. The clustering and position of species give a relative measure of their connectivity. The colour of the species relates to their degree, or the number of reaction connections to other species. The size of nodes represents the eigenvector centrality, which measures the importance of a species by taking into account the number of reactions it is involved with, the rates of these reactions, and the connections to reactive species. CRAHCN-O has an additional slow pathway from C₂H₄ to C₂H₃ , which was omitted from the plot for readability. — astro-ph.EP

Marrick Braam, Ellery Gopaoco, Shang-Min Tsai, Gergely Friss, Paul I. Palmer, Paul B. Rimmer, Skyla B. White

Comments: 36 pages, 20 figures, accepted for publication in Icarus Special issue entitled ‘Carbon in Planetary Environments: Sources and Evolution’
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2603.08172 [astro-ph.EP] (or arXiv:2603.08172v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2603.08172
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Related DOI:
https://doi.org/10.1016/j.icarus.2026.117032
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Submission history
From: Marrick Braam
[v1] Mon, 9 Mar 2026 09:52:42 UTC (1,289 KB)
https://arxiv.org/abs/2603.08172

Astrobiology, Astrochemistry,