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

Dubois, D; Carrasco, N; Bourgalais, J; Vettier, L; Desai, RT; Wellbrock, A; Coates, AJ; (2019) Nitrogen-containing Anions and Tholin Growth in Titan's Ionosphere: Implications for Cassini CAPS-ELS Observations. Astrophysical Journal Letters , 872 (2) , Article L31. 10.3847/2041-8213/ab05e5. Green open access

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Abstract

The Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) instrument on board Cassini revealed an unexpected abundance of negative ions above 950 km in Titan's ionosphere. In situ measurements indicated the presence of negatively charged particles with mass-over-charge ratios up to 13,800 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−1N− 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 u/q, it is necessary to better characterize the first light species. Here, we present new negative ion measurements with masses up to 200 u/q obtained in an N2: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 (e.g., CNN−, 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.

Type: Article
Title: Nitrogen-containing Anions and Tholin Growth in Titan's Ionosphere: Implications for Cassini CAPS-ELS Observations
Open access status: An open access version is available from UCL Discovery
DOI: 10.3847/2041-8213/ab05e5
Publisher version: https://doi.org/10.3847/2041-8213/ab05e5
Language: English
Additional information: This version is the version of record . For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: Science & Technology, Physical Sciences, Astronomy & Astrophysics, astrochemistry, ISM: molecules, methods: laboratory: molecular, planets and satellites: atmospheres, planets and satellites: individual (Titan), CHEMISTRY, IDENTIFICATION, ATMOSPHERE, AEROSOLS, KINETICS, MODEL, IONS
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics
URI: https://discovery.ucl.ac.uk/id/eprint/10070152
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