The case for detecting effects from non-LTE processes in the IR transmission and emission spectra from Venus-like planets orbiting around M-dwarf stars

The chemical evolution of an exoplanetary Venus-like atmosphere is dependent upon the ultraviolet to near ultraviolet (FUV-NUV) radiation ratio from the parent star, the balance between CO₂ photolysis and recombination via reactions that depend on the water abundance, and various catalytic chemical cycles via atmospheric composition, dynamics, energetics, and chemical behavior. A recent study has been completed using a 3-D GCM model simulating conditions for the state of a Venus-type exoplanet with a CO₂ atmosphere orbiting around the M-dwarf type star GJ436 by varying star/planet distances and considering the effects that the energy balance (heating/cooling) and dynamics have on photochemistry to determine abundances of O, O₂, O₃, CO, CO₂, SO, and SO₂ (Parkinson et al., 2022). The simulation includes the middle and upper atmosphere (<40 mbar). Overall, these model comparisons reveal that the impact of extreme ultraviolet to ultraviolet (EUV-UV) heating on the energy balance shows both radiative and dynamical processes are responsible for driving significant variations in zonal winds and global temperature profiles at < 10^-5 mbar. More specifically, CO₂ 15-μm cooling balances EUV/UV and Near InfraRed (NIR) heating at altitudes below 10^-7 mbar pressure with a strong maximum balance for pressures at ~10^-5 mbar, thus explaining the invariance of the temperature distribution at altitudes below 10^-5 mbar pressure for all cases. Our model comparisons also show that moderate changes in NIR heating result in relatively small changes in neutral temperature in the upper atmosphere, and virtually no change in the middle atmosphere. However, with larger changes in the NIR heating profile, much greater changes in neutral temperature occur in the entire upper and middle atmosphere studied. Using the retrieved volume mixing ratio (VMR) profiles for CO₂, CO, O₂ and temperature profiles from these simulations for the middle and upper atmosphere, we also model the IR transmission and emission spectrum of these key species of astrobiological significance, thus making the case for the potential to detect effects from non-LTE processes in the IR transmission and emission spectra from Venus-like planets orbiting around M-dwarf stars.