Photochemical model output data associated with: 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 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 (Psurf). We find 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 (Psurf < 10 bar), an intermediate surface (10
bar < Psurf < 100 bar), and a deep surface (Psurf >
100 bar). This framework can be applied together with future observations
to other sub-Neptunes of interest.