Identify Surfaces on Sub-Neptunes in the JWST Era

Sub-Neptunes (Rp ~ 1.25-4 REarth) remain the most abundant type of detected exoplanets. However, it remains challenging for upcoming observations to directly identify the surface conditions (temperature Tsurf and pressure Psurf) on these intermediate-sized exoplanets. Yu et al. [1] proposed that the disequilibrium abundances of atmospheric species in the detectable hydrogen-dominated atmospheres of sub-Neptunes may indirectly be used as proxies to determine their surface conditions. [1] identified at least seven proxies to identify a surface on a temperate sub-Neptune, K2-18 b (equilibrium temperature Teq=255 K). 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 [2, 3], to further investigate the sensitivity of atmospheric species to different pressure, temperature, and atmospheric mixing conditions with a series of idealized simulations. In this work, we explored the threshold Tsurf and Psurf levels that triggered species abundance variation compared to the no-surface case. We found that the threshold Tsurf and Psurf levels of each species correspond to their individual quench points. We also explored the applicability of the surface identification framework towards cooler exoplanets (Teq < 200 K), where condensation is important, and hotter exoplanets (Teq > 300 K), where thermochemical equilibrium is promoted at the lower boundary. Given these results, we identified a range of JWST targets that allows us to conduct the surface identification framework.

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

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

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