Species-dependent ion escape on Titan

摘要

Cassini observations over the past ten years have revealed that Titan possesses a chemically complex ionosphere.In this study,we investigate the relative contributions of different ion species to the total ion escape on Titan,by dividing all ion species probed by the Cassini Ion Neutral Mass Spectrometer(INMS)into six groups according to their mass-to-charge ratios(M/Z).For the three lightest ion groups,with characteristic M/Z of 22,41,and 52 daltons,the observed scale heights tend to be lower than the scale heights predicted by assuming diffusive equilibrium;for the three heavier groups,observed and predicted scale heights are in general agreement,implying that most ion escape from Titan is by relatively light species,with M/Z<60 daltons.A diffusion model is constructed to describe the density distribution of each ion group in regions where the effect of ionospheric chemistry could be neglected.The data model comparison predicts an optimal total ion escape rate of 3.1×10^24 s^–1,of which more than 99%is contributed by relatively light ions with M/Z<32 daltons.