Negative ion chemistry in Titan's upper atmosphere

Abstract

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 22±4 and 44±8 amu/q, and maybe a third one at 82±14 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 HC ₃N, HC ₅N or C ₄H ₂. Loss occurs through associative detachment with radicals (H and CH ₃). We attribute the three low mass peaks observed by ELS to CN ^-, C ₃N ^-/C ₄H ^- and C ₅N ^-. 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 lower altitudes.