An Initial Exploration of the 3-D Structure of Terrestrial-like Exoplanetary Atmospheres Orbiting Around Different Parent Star Types

In recent years, many different types of exoplanets have been discovered and understanding the conditions and chemical cycles present on these diverse worlds will be enable by the spectroscopic characterization their atmosphere. Terrestrial-like exoplanets can contain many chemical species, yet only a few have the distinct spectral features and atmospheric abundance to be detectable, viz., CO₂, CO, CH₄, H₂O, O₂, and O₃. O₂ and O₃ can be produced abiotically by photolysis of CO₂ and biotically in the presence of life. Around Sun-like stars, CO₂ photolysis by FUV radiation is balanced by recombination reactions dependent on water abundance. Planets orbiting stars with more FUV radiation could be depleted in water due to prolonged high FUV exposure. For these exoplanets, a catalytic cycle relying on H₂O₂ photolysis can maintain a CO₂ atmosphere. This cycle breaks down for low atmospheric hydrogen mixing ratios, causing a significant fraction of the atmospheric CO₂ being converted to CO and O₂ on timescale of about1 Myr (Gao et al., 2015), especially on planets in the habitable zones of K- and M- type stars. The abundances of other species are determined by planetary atmospheric conditions during formation and subsequent evolution. This evolution is largely dependent upon the FUV-NUV radiation ratio from the parent star, the balance of CO₂ photolysis with recombination water abundance dependent reactions, and various catalytic chemical cycles characterized via atmospheric composition, dynamics, energetics, and chemical behavior. The 1-D photochemical models previously used to understand these processes are inconsistent, producing conflicting results under similar circumstances arising from their limited capabilities. We estimate conditions for the state and evolution of terrestrial-type exoplanets orbiting around GJ-436 with varying star/planet distances to determine abundances for key observable species by considering the effects of 3-dimensional GCM heating/cooling, dynamics, and HO_x and ClO_x photochemistry and catalytic cycles. Specifically, we: 1. Characterize terrestrial-like exoplanet dayside heating budgets of EUV heating/CO₂ 15-μm cooling in a 3-D GCM model allowing a self-consistent quantification of the associated photochemistry and climatological upper atmospheric circulation using existing validated published models for planets with terrestrial-type atmospheres GJ-436. 2. Assess the influence of HO_x and ClO_x, photochemistry and catalytic cycles, heating/cooling in 3-D GCM modeling to the determine distribution of key species abundances for terrestrial-type planetary atmospheres.