Nitrogen-containing Anions and Tholin Growth in Titan’s Ionosphere: Implications for Cassini CAPS-ELS Observations. (arXiv:1902.05585v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Dubois_D/0/1/0/all/0/1">David Dubois</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Carrasco_N/0/1/0/all/0/1">Nathalie Carrasco</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bourgalais_J/0/1/0/all/0/1">Jérémy Bourgalais</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vettier_L/0/1/0/all/0/1">Ludovic Vettier</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Desai_R/0/1/0/all/0/1">Ravindra T. Desai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wellbrock_A/0/1/0/all/0/1">Anne Wellbrock</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Coates_A/0/1/0/all/0/1">Andrew J. Coates</a>
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.
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.
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