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

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Titan

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.

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 CnH- carbon chain and Cn−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.

David Dubois, Nathalie Carrasco, Jérémy Bourgalais, Ludovic Vettier, Ravindra T. Desai, Anne Wellbrock, Andrew J. Coates
(Submitted on 14 Feb 2019)

Comments: 8 pages, 2 figures, accepted for publication in the Astrophysical Journal Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1902.05585 [astro-ph.EP] (or arXiv:1902.05585v1 [astro-ph.EP] for this version)
Submission history
From: David Dubois
[v1] Thu, 14 Feb 2019 20:08:05 UTC (3,115 KB)
https://arxiv.org/abs/1902.05585
Astrobiology. Astrochemistry

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