DOI: 10.7291/d1338m
title: Photochemical model output data associated with: How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres

description: 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 &lt; 10 bar), an intermediate surface (10
 bar &lt; Psurf &lt; 100 bar), and a deep surface (Psurf &gt;
 100 bar). This framework can be applied together with future observations
 to other sub-Neptunes of interest.
description: 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).
description: kin*.pun files: final concentrations of the different
 species<br> 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)<br> pressure is in mbar<br> temperature is in
 kelvins<br> densitiy is in cm^-3<br> eddy diffusion
 coefficient (ignore in atm file) is in cm^2 s^-1<br> all species
 concentrations are in cm^-3 (divide by the density entry near the top of
 the file to get volume mixing ratio)