2021ApJ...914...38Y

Sub-Neptunes (R_p ∼ 1.25-4 R_Earth) remain the most commonly detected exoplanets to date. However, it remains difficult for observations to tell whether these intermediate-sized exoplanets have surfaces and where their surfaces are located. Here we propose that the abundances of trace species in the visible atmospheres of these sub-Neptunes can be used as proxies for determining the existence of surfaces and approximate surface conditions. As an example, we used a state-of-the-art photochemical model to simulate the atmospheric evolution of K2-18b and investigate its final steady-state composition with surfaces located at different pressures levels (P_surf). We find that the surface location has a significant impact on the atmospheric abundances of trace species, making them deviate significantly from their thermochemical equilibrium and "no-surface" conditions. This result arises primarily because the pressure-temperature conditions at the surface determine whether photochemically produced species can be recycled back to their favored thermochemical equilibrium forms and transported back to the upper atmosphere. For an assumed H₂-rich atmosphere for K2-18b, we identify seven chemical species that are most sensitive to the existence of surfaces: ammonia (NH₃), methane (CH₄), hydrogen cyanide (HCN), acetylene (C₂H₂), ethane (C₂H₆), carbon monoxide (CO), and carbon dioxide (CO₂). The ratio between the observed and the no-surface abundances of these species can help distinguish the existence of a shallow surface (P_surf < 10 bar), an intermediate surface (10 bar < P_surf < 100 bar), and a deep surface (P_surf > 100 bar). This framework can be applied together with future observations to other sub-Neptunes of interest.