Venus as an Exoplanet: I. An Initial Exploration of the 3‐D Energy Balance for a CO ₂ ‐Rich Exoplanetary Atmosphere Around the M‐Dwarf Star GJ 436

Abstract The chemical state of a Venus‐like exoplanetary atmosphere depends on the far‐ultraviolet (100–200 nm) to near‐ultraviolet (NUV, 300–400 nm) flux ratio from the host star, the balance between CO 2 photolysis and recombination (which depends on water abundance), and catalytic chemical cycles. In this study, we use a three‐dimensional (3‐D) model to simulate conditions for a Venus‐like exoplanet orbiting the M‐dwarf star GJ436 by varying the ultraviolet (UV, <250 nm) and Near InfraRed (NIR, 0.7–5.0 μm) heating (Fels, 1977; National Research Council, 2000) to simulate a varying planet‐star distance and considering the resultant effects on heating/cooling and dynamics. The simulation includes the upper mesosphere and thermosphere (<40 mbar). These simulations reveal that UV heating affects the energy balance, with both radiative and dynamical processes driving significant variations in zonal winds and global temperature profiles at pressures <10 −5 mbar. More specifically, CO 2 15‐μm cooling balances UV and NIR heating at pressures >10 −7 mbar with a strong maximum balance for pressures at ∼10 −5 mbar, thus explaining the invariance of the temperature distribution at >10 −5 mbar for all cases. Our simulations also show that moderate changes in the standard NIR heating (0.7× and 1.3×) result in relatively small changes in neutral temperature for pressures less than ∼10 −5 mbar, and virtually no change for pressures greater than ∼10 −5 mbar. However, with larger changes in the NIR heating profile (2× and 4×), much greater changes in neutral temperature occur throughout the altitude range studied.