Nitrogen-containing Anions and Tholin Growth in Titan's Ionosphere: Implications for Cassini CAPS-ELS Observations

Abstract

The Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) instrument onboard Cassini revealed an unexpected abundance of negative ions above 950 km in Titan's ionosphere. \textit{In situ} measurements indicated the presence of negatively charged particles with mass-over-charge ratios up to 13,800 \textit{u/q}. At present, only a handful of anions have been characterized by photochemical models, consisting mainly of C${}_n$H${}^{-}$ carbon chain and C${}_{n-1}$N${}^{-}$ cyano compounds ($n=2-6$); their formation occurring essentially through proton abstraction from their parent neutral molecules. However, numerous other species have yet to be detected and identified. Considering the efficient anion growth leading to compounds of thousands of \textit{u/q}, it is necessary to better characterize the first light species. Here, we present new negative ion measurements with masses up to 200 \textit{u/q} obtained in an \ce{N2}:\ce{CH4} dusty plasma discharge reproducing analogous conditions to Titan's ionosphere. We perform a comparison with high altitude CAPS-ELS measurements near the top of Titan's ionosphere from the T18 encounter. The main observed peaks are in agreement with the observations. However, a number of other species (\textit{e.g.} \ce{CNN-}, \ce{CHNN-}) previously not considered suggests an abundance of N-bearing compounds, containing two or three nitrogen atoms, consistent with certain adjacent doubly-bonded nitrogen atoms found in tholins. These results suggest that an N-rich incorporation into tholins may follow mechanisms including anion chemistry, further highlighting the important role of negative ions in Titan's aerosol growth.

arXiv comment8 pages, 2 figures, accepted for publication in the Astrophysical Journal Letters

References in corpus (4)

• Carbon chain anions and the growth of complex organic molecules in Titan's ionosphere
• In situ Investigation of Neutrals Involved in the Formation of Titan Tholins
• Characterization of aromaticity in analogues of titan's atmospheric aerosols with two-step laser desorption ionization mass spectrometry
• Molecular Isomer Identification of Titan Tholins Organic Aerosols by Photoelectron/Photoion Coincidence Spectroscopy Coupled to VUV Synchrotron Radiation

Cited by in corpus (7)

• The Composition and Chemistry of Titan's Atmosphere
• Surface Energy of the Titan Aerosol Analog "Tholin"
• Positive Ion Chemistry in an N${}_2$-CH${}_4$ Plasma Discharge: Key Precursors to the Growth of Titan Tholins
• Low Pressure EUV Photochemical Experiments: Insight on the Ion-Chemistry Occurring in Titan's Atmosphere
• The Fundamental Vibrational Frequencies and Spectroscopic Constants of the Dicyanoamine Anion, NCNCN${}^{-}$ (C${}_2$N${}_3^{-}$): Quantum Chemical Analysis for Astrophysical and Planetary Environments
• Spatial variations of low mass negative ions in Titan's upper atmosphere
• Photochemical Haze Formation on Titan and Uranus: A Comparative Review