Presentation #111.08 in the session “Extrasolar Planets: Atmospheric Models”.
The Kepler mission has detected a wealth of exoplanets without any resemblances in the Solar System: sub-Neptunes (Rp ~ 1.25–4 REarth). It is unclear if these intermediate-sized exoplanets are similar to the terrestrial planets, so-called “super-Earths”, where the gas-solid interfaces locate at a shallow pressure level (e.g., <100 bars), or if they like a scaled-down version of the ice giants, so-called “mini-Neptunes”, with a gas-solid/liquid interface (if there is any) located deeply at deep pressure levels (e.g., ≫1 kbar). Here we propose that the abundances of trace species in their atmospheres can be used as proxies for determining if these intermediate-sized exoplanets have shallow surfaces like the terrestrial planets. As an example, we used a state-of-the-art photochemical model  to simulate the atmospheric evolution of K2-18b and investigate the atmospheric trace compositions with surfaces located at different pressure and temperature levels (a 1-bar surface, a 10-bar surface, and deep surface). The location of the underlying solid surface has a significant impact on the atmospheric abundances of trace species primarily because the pressure-temperature condition at the lower boundary would determine whether the photochemically-formed complex organics can be recycled back to the atmosphere to replenish the reacted atmospheric species . For K2-18b, we found that with a cool shallow surface (1 bar, ~600 K), where complex hydrocarbons cannot be recycled back to methane, the photochemical process would convert much of the CH4 into stably-bonded CO and CO2, resulting a methane-poor atmosphere. If the exoplanet has a warmer, deeper surface (10 bar, ~1000 K), the photochemically-produced hydrocarbons would stay in equilibrium with CH4, but the CO and CO2 abundances are larger compared to the case with no surface (or a very deep surface), and the CO-H2O-CH4 system remains out of equilibrium. However, since CH4 can be efficiently recycled at the warm surface from the complex hydrocarbons, the mixing ratio of CH4 is similar to the model with no surface and much larger than the case with a shallow surface.  Moses, J. I., Line, M. R., Visscher, C., et al. 2013, ApJ, 777, 34  Strobel, D. F.1969, JAS, 26, 906