One dimension photochemical models in global mean conditions in question: Application to Titan

Abstract

Observational data of Titan's atmosphere composition, gathered in particular by the Cassini mission, allow accessing latitudinal, longitudinal and temporal variations of the altitudinal profiles of several species. While 1D models are a powerful tool to gain insight into the chemistry occurring in Titan's atmosphere, they cannot capture the four-dimensional nature of these processes at the same time. However, due to the large extent of the atmosphere and the complexity of chemistry, global 2D and 3D models with the chemical complexity of 1D models do not exist yet. On the way of developing a 2D photochemical model of Titan, including the very complex chemical schemes used in 1D models and also taking latitudinal variations into account, we developed an ultraviolet radiative transfer model to compute the actinic flux in a 2D geometry. We found that photolysis rates calculated in classical plane-parallel models to derive the rates in mean conditions give results notably different from a model that calculates the mean rates with a 2D geometry. We demonstrate that the introduction of the 2D geometry affects significantly the density profiles on both neutrals and ions, especially on nitriles. As a consequence, we advocate using radiative transfer model in 2D geometry from now on to interpret the wide diversity of observational data about Titan's atmosphere (pending the emergence of 2D models with complex chemical schemes).