The physical and chemical structure of Titan’s upper atmosphere: energy balance, photochemistry and Jeans escape

Abstract Titan’s upper atmospheric layers provide a unique opportunity to study how photochemistry, photoionisation, haze microphysics and escape processes shape its atmospheric chemical structure, providing a unique benchmark for understanding these processes in exoplanets. Using a 1D first-principles approach, we extend the Kompot model to simulate Titan’s upper atmosphere and investigate its thermal and chemical structure, and Jeans escape. We performed self-consistent simulations of Titan’s upper atmosphere, above the dense haze layer, over a 1D radial grid by solving the coupled hydrodynamics, thermal and photochemical kinetic, and energy balance equations. We considered heating by stellar X-ray and ultraviolet (XUV) and infrared (IR) radiation, and cooling due to radiative emission of methane (CH4) and hydrogen cyanide (HCN). We also included additional nineteen photochemical and 117 thermal reactions to Kompot’s standard chemical network. Our modelled atmospheric temperature structures lie within the observed variability of Titan’s atmospheric temperature profile. Our simulations reproduce observed nitrogen (N2) and CH4 densities in the upper atmosphere, and abundance of minor chemical species also broadly agree with observationally derived abundances. Near the exobase, our CH4 mixing ratio overlap with Cassini-Huygens results, and our model H2 mixing ratios also exhibit broad agreement with observations. Our simulations suggest no significant thermal loss of CH4, even under the conditions of an early Sun’s XUV environment.