Epistemic bimodality and kinetic hypersensitivity in photochemical models of Titan's atmosphere

We show that photochemical models of Titan's atmosphere can give rise to bimodal distributions in the abundances of some major compounds, like C2H2 and C2H4. Sensitivity analysis enabled us to identify the causes and conditions of this bimodality. We propose several methods to control this behavior in photochemical models. In particular, we point out the importance of two key reactions and the needs for a critical evaluation of the kinetic data. We also show that the abundances of some compounds are hypersensitive to the ratio [CH4]/[H], suggesting that a time-dependent variation of this ratio might lead to a real bistability in the high atmosphere of Titan.