Fingerprints of non-equilibrium chemistry on atmospheric spectra of exoplanets

Planetary atmospheres have been studied mostly by assuming thermochemical equilibrium. However, this assumption is likely to be not valid at all-pressures. In particular, it breaks down at mid/low-pressures where diffusion and photochemistry dominate. These pressures are also where we have access to the atmosphere through spectroscopy, and hence our interpretation of observations is susceptible to the effect of non-equilibrium chemistry (NEChem) on the atmospheric spectra of exoplanets.

We developed a fast 1D chemical kinetic model to study these effects over a wide range of planets. We employed our self-consistent radiative-convective model (petitCODE) to calculate temperature structure of these planets and we used our kinetic model to investigate how equilibrium temperature [400-2500k], surface gravity [log(g): 2.0-5.0], metallicity [Fe/H: -1.0 to 2.0], C/O [0.25-1.25], stellar type [M-F] and eddy diffusion coefficient [kzz: 106-1012 cm2/s] affect species abundances. We then calculated transmission and emission spectra of these planets within the JWST’s wavelength coverage. Altogether we studied spectra of more than 100,000 models.

We highlight the spectral regions where signatures of NEChem are more prominent. We present sensitivity of the spectra to the diffusion strength, what planets are the best candidates for the JWST to observe the spectral signatures due to NEChem, and where its effect is negligible. We also find transitional C/O values from water to methane dominated atmospheres for both equilibrium and NEChem, suggesting dependency of this value to the temperature, metallicity, log(g), stellar type and kzz, ranging from <0.1 to 2< values.

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