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Arxiv: Probing Cold-to-Temperate Exoplanetary Atmospheres: The Role of Water Condensation on Surface Identification with JWST Published: 7/12/2024 8:58:28 AM Updated: 7/12/2024 8:58:28 AM

Paper abstract: Understanding the surface temperature and interior structure ofcold-to-temperate sub-Neptunes is critical for assessing their habitability,yet direct observations are challenging. In this study, we investigate theimpact of water condensation on the atmospheric compositions of sub-Neptunes,focusing on the implications for JWST spectroscopic observations. By modelingthe atmospheric photochemistry of two canonical sub-Neptunes, K2-18 b and LHS1140 b, both with and without water condensation and with and without thickatmospheres, we demonstrate that water condensation can significantly affectthe predicted atmospheric compositions. This effect is driven by oxygendepletion from the condensation of water vapor and primarily manifests as anincrease in the C/O ratio within the photochemically active regions of theatmosphere. This change in composition particularly affects planets with thinH_2-dominated atmospheres, leading to a transition in dominant nitrogen andcarbon carriers from N_2 and oxygen-rich species like CO/CO_2 towardsheavier hydrocarbons and nitriles. While our models do not fully account forthe loss mechanisms of these higher-order species, such molecules can go on toform more refractory molecules or hazes. Planets with thin H2-rich atmospheresundergoing significant water condensation are thus likely to exhibit very hazyatmospheres. The relatively flat JWST spectra observed for LHS 1140 b could beconsistent with such a scenario, suggesting a shallow surface with extensivewater condensation or a high atmospheric C/O ratio. Conversely, the JWSTobservations of K2-18 b are better aligned with a volatile-rich mini-Neptunewith a thick atmosphere.