Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021
EPSC Abstracts
Vol. 15, EPSC2021-184, 2021
https://doi.org/10.5194/epsc2021-184
European Planetary Science Congress 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres

Xinting Yu1, Julianne Moses2, Jonathan Fortney3, and Xi Zhang4
Xinting Yu et al.
  • 1University of California Santa Cruz, Department of Earth and Planetary Sciences, Santa Cruz, United States of America (xintingyu@ucsc.edu)
  • 2Space Science Institute, Colorado, United States of America (jmoses@spacescience.org)
  • 3University of California Santa Cruz, Department of Astronomy and Astrophysics, Santa Cruz, United States of America (jfortney@ucsc.edu)
  • 4University of California Santa Cruz, Department of Earth and Planetary Sciences, Santa Cruz, United States of America (xiz@ucsc.edu)

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.

Figure 1: Selected criteria and a flowchart of possible steps to identify the existence of the surface and the surface pressure for a hydrogen-dominated exoplanet with properties similar to K2-18b.

How to cite: Yu, X., Moses, J., Fortney, J., and Zhang, X.: How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres, European Planetary Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-184, https://doi.org/10.5194/epsc2021-184, 2021.

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