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Abiotic Oxygen on the Atmospheres of Venus-Like Exoplanets

Presentation #1155 in the session “Open Engagement Session B”.

Published onMar 17, 2021
Abiotic Oxygen on the Atmospheres of Venus-Like Exoplanets

Photolysis of CO2 in CO2-dominated atmospheres could potentially generate large amounts of abiotic O2 in exoplanetary contexts (e.g., Gao et al., 2015). Interestingly, ground-state O2 has never been observed on Venus, despite high CO2 photolysis rates in the upper atmosphere (Trauger & Lunine, 1983; Krasnopolsky, 2006). This lack of O2 has been attributed to catalytic cycles involving HOx, ClOx, SOx, and NOx molecules that can efficiently recombine photochemically generated CO and O into CO2 (DeMore & Yung, 1982; Mills et al., 2007; Yung & DeMore, 1999). Hitherto, it is unknown how these photochemical networks would be impacted by different stellar spectra. In this study, we model the photochemistry of Venus-like exoplanets around various stellar types. We use the Caltech/JPL 1-D photochemical model KINETICS, based upon the work of Zhang et al. (2012) and Bierson & Zhang (2020), to simulate 464 chemical reactions between 68 chemical species composed of H, C, O, N, S, and Cl. We consider an atmosphere primarily composed of CO2 (~90 bars) and N2 (~3 bars) with trace amounts of H2O, SO2, OCS, HCl, and the photochemical products thereof. These trace species contribute the HOx, ClOx, SOx, and NOx catalysts that control the steady-state abundance profiles of abiotic O2 in the atmosphere. Our model also simulates the condensation and evaporation of H2O and H2SO4 in the atmosphere’s cloud region. We assume that the surface mixing ratios of trace atmospheric species are controlled by surface mineralogical buffers relevant to Venus (Zolotov, 2018). We compare the effect of G- and M-dwarf spectral energy distributions on Venus-like worlds, placing the planets at orbital distances with the same total incident flux as Venus. Our preliminary results show that different spectral energy distributions result in different O2 buildup. In particular, the high FUV/NUV ratio of TRAPPIST-1 can cause a Venus-like planet to contain several percent O2 in the upper atmosphere. However, around a Sun-like star, where the NUV flux outweighs the FUV flux by orders of magnitude, the column mixing ratio of O2 is limited to ≪1 ppm.

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