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Measuring Isotopic Ratios in Titan’s Atmospheric Nitriles

Presentation #204.05 in the session Titan II: Atmospheres (Poster)

Published onOct 23, 2023
Measuring Isotopic Ratios in Titan’s Atmospheric Nitriles

Titan, Saturn’s largest satellite, maintains a thick atmosphere that is composed primarily of molecular nitrogen (N2) at about 98% and methane (CH4) at about 1.5%. These molecules form the basis for a complex atmospheric chemical network. A diverse population of organic compounds is generated through high-altitude photochemistry, following dissociation by solar UV photons, or collisions by charged particles from Saturn’s magnetosphere, as well as galactic cosmic rays. Some of the yielded molecules include isotopologues of those organic compounds. Studying the isotopic ratios within an atmosphere can provide key insights into its origin, evolution, and the current physical and chemical processes acting on its constituent gases. Titan’s atmospheric nitriles (or cyanides, molecules that have a -CN group), have been found to contain substantially enhanced 15N abundances compared to the values found both on Earth, and in Titan’s dominant nitrogen (N2) reservoir. More detailed investigation of these isotopic ratios can provide a better understanding of the synthesis of nitrogen-bearing organics in planetary atmospheres and help constrain the origin of Titan’s surprisingly large nitrogen reservoir.

Among the various nitriles detected in Titan’s atmosphere, we chose three as the basis for our study: HNC, CH3CN, and HC3N. These molecules were observed at high signal-to-noise ratio with the Atacama Large Millimeter/submillimeter array (ALMA) using band 6 (211-275 GHz) over 3 dates in November and December 2019. From our disk-averaged data, we derived molecular abundances and carbon and nitrogen isotopic ratios as a function of altitude using a radiative transfer code designed for planetary atmospheres: Non-linear optimal Estimator for MultivariatE Spectral analySIS (NEMESIS), following methods that have previously proven successful for ALMA data. Our results show enhanced 14N/15N isotopic ratios in CH3CN as in other nitriles, whereas the carbon isotopic ratios (13C/12C) are more consistent with the bulk (Titan and terrestrial) values.

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