Negative ion chemistry in Titan’s upper atmosphere

The Electron Spectrometer (ELS), one of the sensors making up the Cassini Plasma Spectrometer (CAPS) revealed the existence of numerous negative ions in Titan’s upper atmosphere. The observations at closest approach ( 1000 km) show evidence for negatively charged ions up to 10,000 amu/q, as well as two distinct peaks at 2274 and 4478 amu/q, and maybe a third one at 82714 amu/q. We present the first ionospheric model of Titan including negative ion chemistry. We find that dissociative electron attachment to neutral molecules (mostly HCN) initiates the formation of negative ions. The negative charge is then transferred to more acidic molecules such as HC3N, HC5N or C4H2. Loss occurs through associative detachment with radicals (H and CH3). We attribute the three low mass peaks observed by ELS to CN , C3N /C4H and C5N . These species are the first intermediates in the formation of the even larger negative ions observed by ELS, which are most likely the precursors to the aerosols observed at