Hydrocarbon complexity and photochemical shielding of prebiotic feedstock molecules in exoplanet atmospheres

The potential of prebiotic chemistry to initiate and propagate on an exoplanet fundamentally depends on whether the atmospheric physical and chemical 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 for the conditions considered. We present the implementation of the Consistent Reduced Atmospheric Hybrid Chemical Network with Oxygen extension (CRAHCN-O), constructed to simulate the formation of feedstock molecules such as hydrogen cyanide (HCN), formaldehyde (H 2 CO), and simple hydrocarbons, into the VULCAN photochemical kinetics code. We investigate the photochemical production of feedstock molecules driven by M-star radiation and compare these to predictions by the N-C-H-O network in VULCAN, for N 2 -dominated atmospheres with C/O ratios between 0.5–1.5 from CO 2 and CH 4 abundances. Predicted abundances are similar for C/O = 0.5 in the absence of CH 4 . As soon as CH 4 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 ethane (C 2 H 6 ), while sinks to higher-order hydrocarbons in N-C-H-O accumulate C 4 H 3 (the 1-butene-3-yne-1-yl radical) and C 3 H 4 (allene). Importantly, C 2 H 6 is photochemically active whereas C 4 H 3 and C 3 H 4 are assumed inactive. With mixing ratios up to a few percent in CRAHCN-O, C 2 H 6 shields CH 4 and CO 2 from photodissociation. Therefore, these species survive to lower pressures, in turn weakening the destruction of HCN and H 2 CO. Maximum HCN mixing ratios reach 1000 ppm for the highest CH 4 abundances with CRAHCN-O compared to only 3 ppm with N-C-H-O. Other feedstock molecules like cyanoacetylene (HC 3 N) and acetylene (C 2 H 2 ) 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. We present the key pathways to feedstock molecules, along with photochemical cycles and net reactions to hydrocarbons for each network. These findings can be used to prioritise experimental rate coefficient determination, to reconcile network differences, and to develop more complex climate-chemistry modeling. The results demonstrate the crucial role of chemical kinetics in understanding prebiotic chemistry in exoplanet atmospheres, including important considerations for the construction and applicability of chemical networks. • The production of prebiotic feedstock molecules varies with exoplanet atmospheric environments and chemical networks. • Hydrocarbon shielding significantly affects prebiotic chemical processes. • Key pathways identified to guide experimental rate measurements, chemical network reconciliation, and reduction strategies for complex climate–chemistry models. • Crucial role of photochemical network structure and kinetics in understanding prebiotic feedstock molecules in exoplanet atmospheres.