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.

These datafiles are output results using a one-dimensional (1D) thermochemical and photochemical kinetics and transport model, Caltech/JPL KINETICS code for a model exoplanet (K2-18b) without surfaces and with surfaces (model runs with surfaces located at 1, 10, 100 bars).

kin*.pun files: final concentrations of the different species
atm*.inp files: equilibrium concentrations of the different species

In file titles, "100x" means 100 times solar metallicity, "deep" is the model with no surface, "xxbar" has surface at xx bar, "hot" is the model with four times stellar flux, "t0" is the model with intrinsic temperature Tint=0 K and "t70" is the model with Tint=70 K, "Kzzdiv3" is the model with the nominal eddy diffusion (Kzz) profile divided by three and "Kzztimes3" is the model with the nominal eddy diffusion (Kzz) profile timed by three.

The atm*.inp and kin*.pun files have entries in row format:

altitude is in km (above 1 bar at altitude = 0)
pressure is in mbar
temperature is in kelvins
densitiy is in cm^-3
eddy diffusion coefficient (ignore in atm file) is in cm^2 s^-1
all species concentrations are in cm^-3 (divide by the density entry near the top of the file to get volume mixing ratio)