How To Identify Exoplanet Surfaces Using Atmospheric Trace Species In Hydrogen-dominated Atmospheres


(a) The pressure-temperature profiles used in our model, based on the 1D radiative-convective model of Fortney et al. (2005, 2008). The blue lines show the nominal profile for K2-18b and the red lines are profiles for a hypothetical hotter sub-Neptune with four times the nominal stellar flux of K2-18b. The solid lines are P-T profiles with intrinsic temperatures Tint of 70 K and the dotted lines are for the Tint = 0 K cases. (b) The nominal eddy diffusion coefficient profiles used in our model for the nominal K2-18b and the hotter variant. Note that a detached convective zone is found around 1 bar in both cases.

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.

Xinting Yu, Julianne I. Moses, Jonathan J. Fortney, Xi Zhang

Comments: 30 pages, 12 figures, submitted to ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2104.09843 [astro-ph.EP] (or arXiv:2104.09843v1 [astro-ph.EP] for this version)
Submission history
From: Xinting Yu
[v1] Tue, 20 Apr 2021 09:12:48 UTC (5,191 KB)
https://arxiv.org/abs/2104.09843
Astrobiology, Astrochemistry,

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