Self-consistent modelling of Titan's upper atmosphere: energy balance, photochemistry, and Jeans escape

Titan’s atmosphere provides a unique laboratory to study how photochemistry, photoionisation, radiative and escape processes shape the atmospheric properties of a nitrogen-rich atmosphere very different from Earth’s. We present a new extension of the 1D first principles upper atmospheric code Kompot, and benchmark it against Titan’s thermosphere. The code self-consistently calculates the thermal and chemical structure of Titan’s upper atmosphere by solving the coupled hydrodynamic, photochemical kinetic, and energy balance equations. The energy balance equation is primarily set by heating due to stellar X-ray and ultraviolet (XUV) and infrared radiation, chemical heating, radiative cooling by methane(CH₄) and hydrogen cyanide (HCN), and thermal conduction. We calculate XUV heating from first principles and do not use any efficiency factor. Our model results are in good agreement with Cassini-Huygens and ALMA observations of Titan. The simulated abundances of the key molecular species, including CH₄, also show strong agreement with Cassini-Huygens results. Molecular hydrogen has the strongest thermal Jeans escape in our model, where as the thermal escape of CH₄ is negligible. Our results suggest that present-day Titan’s CH4 abundance at the upper thermosphere can be explained by a self-consistent model without invoking strong atmospheric escape.