Thread by @JonesKuma

Paper officially on arxiv: arxiv.org/abs/2104.09843 -- How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres. A thread (1/11):

(2/11) In the work we want to see if we can use atmospheric characterization data to tell if an exoplanet has a surface or not, and where the surface could be located — without direct surface observations.

(3/11) I was inspired by two planets in the solar system, #Jupiter and my favorite planet #Titan and two trace species in their atmospheres: ammonia (NH3) and methane (CH4).

(4/11) Both NH3 and CH4 get destroyed photochemically to form N2 and the hydrocarbons in the upper atmospheres of the two bodies. But we still have tons of NH3 and CH4 Jupiter’s atmosphere, while almost none for Titan’s atmosphere (ignore the CH4 mystery for a second).

(5/11) This is because Jupiter has a very deep surface, and the photochemically formed N2 and hydrocarbons can be recycled to back to NH3 and CH4 in the deep, hot part of the atmosphere. In the end, Jupiter still have lots of NH3 and CH4.

(6/11) While for Titan, there is only a shallow, cold surface, and the formed N2 and hydrocarbons will just stay — since NH3 and CH4 are irreversibly destroyed — eventually Titan would be depleted in NH3 and CH4. The current amount of CH4 on Titan is likely due to outgassing.

(7/11) Okay—can we use the dichotomy between the abundances of CH4/NH3 to tell if an exoplanet has a surface or not? The answer is yes: we tested my idea using the KINETICS photochemical model thanks to the help of a great collaborator Julianne Moses.

(8/11) We find more than NH3 and CH4. Actually a range of species are all sensitive to the existence of surfaces: NH3, CH4, H2O, hydrocarbons such as C2H6 and C2H2, CO, and CO2.

(9/11) Another cool part is that they are sensitive to the different surface levels, because the sweet point where a species can be fully recycled is different for C and N species. NH3/HCN are sensitive to <100 bar surface and CH4//CO/CO2 are sensitive to <10 bar surface.

(10/11) Giving this result, we are able to come up with a flowchart—that uses atmosphere characterization to tell if an exoplanet has a surface or not.

(11/11) There are more than one parallel criteria in each step which is really nice — so if there is a contradict in one criteria caused by surface processes such as outgassing—we can still use other criteria to identify surfaces (it also hints surface process at the same time).

(12/11) Future: This work studies only K2-18b like exoplanet with Teq~ 250 K and 100x metallicity. We are looking forward to expand this framework to a large grid of exoplanets and come up with more of these kind of flowcharts to identify exoplanet surface/surface conditions!