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

Abstract

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, H$_2$CO, 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 N$_2$-dominated atmospheres with C/O ratios between 0.5-1.5. Predicted abundances are similar for C/O${=}$0.5. Once CH$_4$ is included (i.e., for C/O${>}$0.5), the abundance profiles diverge in the photochemical regions. By analysing the attenuation of UV radiation, we find that hydrocarbon photochemical shielding causes the diverging profiles. CRAHCN-O accumulates C$_2$H$_6$, while N-C-H-O accumulates C$_4$H$_3$ and C$_3$H$_4$. 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 and weakens the destruction of HCN and H$_2$CO. Maximum HCN mixing ratios reach 1000 ppm with CRAHCN-O compared to only 3 ppm with N-C-H-O. Other feedstock molecules like HC$_3$N and 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. 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.