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.

Figures (20)

• Figure 1: Background stellar and planetary environments: (a) the spectral radiant flux density as received by Proxima Centauri b and planets around different M-type stars receiving the same irradiance, compared to the Solar flux at 1 AU. Panel (b) shows vertical profiles of the temperature from braam_lightning-induced_2022 (solid line) compared to Earth's temperature profile (dashed line) and eddy diffusion ($K_{zz}$), following tsai_comparative_2021 and massie1981stratospheric.
• Figure 2: Graph networks comparing the chemical networks used in this study: N-C-H-O (top) and CRAHCN-O (bottom), for a reference pressure of $1\times10^{-3}$ 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 C$_2$H$_4$ to C$_2$H$_3$, which was omitted from the plot for readability.
• Figure 3: Grid of selected mixing ratios versus atmospheric pressure for VULCAN/CRAHCN-O and VULCAN/N-C-H-O simulations. The horizontal rows correspond to C/O=0.5 (a-c), 1.0 (d-f), and 1.5 (g-i), respectively. Each column shows one of the background CO$_2$ cases: 400 ppm (left), 1% (middle), and 10% (right). Solid lines correspond to the VULCAN/CRAHCN-O and dashed lines to the VULCAN/N-C-H-O simulations. The selected chemical compounds are part of the initialisation (CO$_2$, CH$_4$, H$_2$O) and prebiotically relevant (HCN, H$_2$CO, C$_2$H$_2$, C$_2$H$_6$, C$_4$H$_3$).
• Figure 4: The UV photosphere, indicating the pressure level at which the atmosphere gets optically thick (optical depth $\tau=1$) to incoming radiation, along with the individual contributions by important absorbing species for these wavelength ranges and background composition. Shown are the simulations with the N-C-H-O (a, c, e) and CRAHCN-O (b, d, f) network for the three CO$_2$ scenarios at C/O=1.0.
• Figure 5: Vertically averaged mixing ratios from simulations with CRAHCN-O (squares) and N-C-H-O (circles), for 400 ppm CO$_2$ (top), 1% CO$_2$ (middle), and 10% CO$_2$ (bottom). For each species, the mixing ratios are shown as a function of C/O ratio as specified in Table \ref{['tab:merged_comp']}.