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 similar to 3 x 10(27) s(-1) or 80 kg s(-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.