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. 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−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 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 N 2 :CH 4 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.

Cation, Anion, and Radical Isomers of C4H4N: Computational Characterization and Implications for Astrophysical and Planetary Environments

“…Accurate identification of new nitrogen-containing species in remote environments such as the atmosphere of Titan is experimentally difficult, and often relies on theoretical groundwork when existing molecular information is limited. Here, we explore the lower mass end of the INMS readings (<100 daltons), and specifically direct our investigation to the measured signal at m/z 66 [39][40][41][42] . There are several stoichiometries that could potentially contribute to this mass peak (e.g.…”

High‐resolution mass spectrometry for future space missions: Comparative analysis of complex organic matter with LAb‐CosmOrbitrap and laser desorption/ionization Fourier transform ion cyclotron resonance

“…C2HN3 is consistent with dicyanamide (NC-NH-CN). This molecule has been previously observed as deprotonated (C2N3at m/z 66) in the context of Titan's laboratory experiments: it appeared as a main fragment ion after MS/MS analysis of tholins ions in Carrasco et al, 2009 and as a major negative ion in the plasma discharge producing the tholins (Dubois et al, 2019). C2H4N4 has two major isomers: the linear cyanoguanidine HN=C(NH2)-NH-CN, or the cyclic aminotriazole NH2-C2N3H2.…”

On an EUV Atmospheric Simulation Chamber to Study the Photochemical Processes of Titan’s Atmosphere

“…) in a plasma of N 2 /CH 4 , suggesting that a growth in nitrogen chemistry would pass through the anions 73 . The isomers of CH 2 N 2 have been evoked as precursors to the formation of these anions via (dissociative) electronic attachment reactions.…”