Carbon chain anions and the growth of complex organic molecules in Titan's ionosphere

Abstract

Cassini discovered a plethora of neutral and ionised molecules in Titan's ionosphere including, surprisingly, anions and negatively charged molecules extending up to 13,800 u/q. In this letter we forward model the Cassini electron spectrometer response function to this unexpected ionospheric component to achieve an increased mass resolving capability for negatively charged species observed at Titan altitudes of 950-1300 km. We report on detections consistently centered between 25.8-26.0 u/q and between 49.0-50.1 u/q which are identified as belonging to the carbon chain anions, CN${}^{-}$/C${}_3$N${}^{-}$ and/or C${}_2$H${}^{-}$/C${}_4$H${}^{-}$, in agreement with chemical model predictions. At higher ionospheric altitudes, detections at 73-74 u/q could be attributed to the further carbon chain anions C${}_5$N${}^{-}$/C${}_6$H${}^{-}$ but at lower altitudes and during further encounters, extend over a higher mass/charge range. This, as well as further intermediary anions detected at $>$100 u, provide the first evidence for efficient anion chemistry in space involving structures other than linear chains. Furthermore, at altitudes below $\sim$1100 km, the low mass anions ($<$150 u/q) were found to deplete at a rate proportional to the growth of the larger molecules, a correlation that indicates the anions are tightly coupled to the growth process. This study adds Titan to an increasing list of astrophysical environments where chain anions have been observed and shows that anion chemistry plays a role in the formation of complex organics within a planetary atmosphere as well as in the interstellar medium.

8 pages, 3 figures, 1 table, Astrophysical Journal Letter accepted June 01 2017

References in corpus (1)

• The Effects of Molecular Anions on the Chemistry of Dark Clouds

Cited by in corpus (10)

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• Positive Ion Chemistry in an N${}_2$-CH${}_4$ Plasma Discharge: Key Precursors to the Growth of Titan Tholins
• Heavy Positive Ion Groups in Titan's Ionosphere from Cassini Plasma Spectrometer IBS Observations
• Photodetachment and Test-Particle Simulation Constraints on Negative Ions in Solar System Plasmas
• Spatial variations of low mass negative ions in Titan's upper atmosphere
• Particle-In-Cell Simulations of the Cassini Spacecraft's Interaction with Saturn's Ionosphere during the Grand Finale
• Photochemical Haze Formation on Titan and Uranus: A Comparative Review
• Simulating secondary electron and ion emission from the Cassini spacecraft in Saturn's ionosphere