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Investigating Chemical Disequilibrium of Surface-Sensitive Trace Species in H2-Dominated Atmospheres

Presentation #102.315 in the session Poster Session.

Published onJun 20, 2022
Investigating Chemical Disequilibrium of Surface-Sensitive Trace Species in H2-Dominated Atmospheres

The nature and classification of exoplanets of 1.25–4 REarth remain ambiguous as current data and observations do not enable the direct identification of surfaces and their depth below the visible atmosphere, if they exist. Yu et al. proposed a detection method for the existence of surfaces and their approximate surface pressure and temperature levels, through the disequilibrium chemical abundances of atmospheric trace species in the detectable H2 dominated atmospheres of sub-Neptunes [1]. Two follow-up studies have largely confirmed the surface sensitivities of these atmospheric trace species (in particular NH3) using two independent photochemical models [2, 3]. In principle, chemical disequilibrium depends on multiple factors, such as the existence of the surface, mixing-induced quenching, and photochemistry. To isolate these factors and evaluate their roles, here we use an open-source 1D photochemical model, VULCAN [4, 5], to further investigate the sensitivity of atmospheric species to different pressure, temperature, and atmospheric mixing conditions with a series of idealized simulations. Specifically, we ran a finer grid of surface pressure levels for K2-18b to find the threshold level that triggers species abundance variation compared to the no-surface case. We also examined how the threshold pressure and temperature levels of each species depend on their quench points to understand the fundamental chemistry and physics controlling the atmospheric evolution of sub-Neptunes.

[1] Yu, X., Moses, J.I., Fortney, J.J., & Zhang, X. 2021, ApJ, 914, 38.

[2] Hu, R., Damiano, M., Scheucher, M., et al. 2021, ApJL, 921, L8.

[3] Tsai, S.-M., Malik, M., Kitzmann, D., et al. 2021, ApJ, 923, 264.

[4] Tsai, S.-M., Lyons, J., Grosheintz, L., et al. 2017, ApJ Supplement Series, 228(2), 20.

[5] Tsai, S.-M., Innes, H., Lichtenberg, T., et al. 2021, ApJL, 922, L27.

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