How to Identify Exoplanet Surfaces Using Atmospheric Trace Species in Hydrogen-dominated Atmospheres

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