Production of neutral species in Titan鈥檚 ionosphere through dissociative recombination of ions

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• 作者：Sylvain Plessis^a ; Nathalie Carrasco^c ; Michel Dobrijevic^d ; ^e ; Pascal Pernot^a ; ^b ; ^{pascal.pernot@u-psud.fr}
• 关键词：Ionospheres ; Atmospheres ; Chemistry ; Titan
• 刊名：Icarus
• 出版年：2012
• 期刊代码：90_00191035
• 类别：ph
• 出版时间：May, 2012
• 卷：219
• 期：1
• 页码：254-266
• 文件大小：1298 K

摘要

The production rates of neutral species by dissociative recombination (DR) of molecular ions with electrons in the ionosphere of Titan are quantified by a new model, including, for the first time, all the available kinetic data on this process. The calculation is based on the ion densities measured by the INMS instrument on Cassini orbiter during flyby T19 at 1100 km altitude. These production rates are compared with those predicted by photochemical models: we calculate that for many neutral species, DR has larger production rates than neutral chemistry. Concerning molecular growth in Titan鈥檚 ionosphere, DR is shown to have two antagonistic effects: (1) a global chemical lysis of ions through C-C and C-N bond breaking (missed by the 鈥淗-loss鈥?DR paradigm); and (2) an enhancement of the neutral chemistry by production of reactive radicals, such as C₂H or NH₂. Further exploration of this chemistry requires the development of ionospheric coupled models taking explicitly into account the richness of the DR process and the strong impact of ions on the budget of neutral species. This study emphasizes also the urgent need of additional experimental studies about DR of molecular ions, with two priorities: evaluation of the impact of the temperature of ions on the rates and fragmentation patterns, and the systematic study of the fragmentation patterns of ions with more than four heavy atoms (m/z > 60 u).