The CH₄ structure in Titan's upper atmosphere revisited

In this study, we reanalyze the CH₄ structure in Titan's upper atmosphere combining the Cassini Ion Neutral Mass Spectrometer (INMS) data from 32 flybys and incorporating several updates in the data reduction algorithms. We argue that based on our current knowledge of eddy mixing and neutral temperature, strong CH₄ escape must occur on Titan. Ignoring ionospheric chemistry, the optimal CH₄ loss rate is ∼3×10²⁷s^-1 or 80kgs^-1 in a globally averaged sense, consistent with the early result of Yelle et al. (2008). The considerable variability in CH₄ structure among different flybys implies that CH₄ escape on Titan is more likely a sporadic rather than a steady process, with the CH₄ profiles from about half of the flybys showing evidence for strong escape and most of the other flybys consistent with diffusive equilibrium. CH₄ inflow is also occasionally required to interpret the data. Our analysis further reveals that strong CH₄ escape preferentially occurs on the nightside of Titan, in conflict with the expectations of any solar-driven model. In addition, there is an apparent tendency of elevated CH₄ escape with enhanced electron precipitation from the ambient plasma, but this is likely to be a coincidence as the time response of the CH₄ structure may not be fast enough to leave an observable effect during a Titan encounter.