The Upper Atmospheres of Terrestrial Planets: Carbon Dioxide Cooling and the Earth's Thermospheric Evolution

Abstract: Context: The thermal and chemical structures of the upper atmospheres of planets crucially influence losses to space and must be understood to constrain the effects of losses on atmospheric evolution. Aims: We develop a 1D first-principles hydrodynamic atmosphere model that calculates atmospheric thermal and chemical structures for arbitrary planetary parameters, chemical compositions, and stellar inputs. We apply the model to study the reaction of the Earth's upper atmosphere to large changes in the CO$_2$ abundance and to changes in the input solar XUV field due to the Sun's activity evolution from 3~Gyr in the past to 2.5~Gyr in the future. Methods: For the thermal atmosphere structure, we consider heating from the absorption of stellar X-ray, UV, and IR radiation, heating from exothermic chemical reactions, electron heating from collisions with non-thermal photoelectrons, Joule heating, cooling from IR emission by several species, thermal conduction, and energy exchanges between the neutral, ion, and electron gases. For the chemical structure, we consider $\sim$500 chemical reactions, including 56 photoreactions, eddy and molecular diffusion, and advection. In addition, we calculate the atmospheric structure by solving the hydrodynamic equations. To solve the equations in our model, we develop the Kompot code and provide detailed descriptions of the numerical methods used in the appendices. Results: We verify our model by calculating the structures of the upper atmospheres of the modern Earth and Venus. By varying the CO$_2$ abundances at the lower boundary (65~km) of our Earth model, we show that the atmospheric thermal structure is significantly altered. [Abstract Truncated]