Isotopic Ratios in Titan’s Nitriles & Implications for Atmospheric Chemistry

Jonathon Nosowitz

Ph.D. Candidate
Physics Department
The Catholic University of America

Titan, Saturn's largest satellite, maintains a thick atmosphere composed primarily of molecular nitrogen (N₂) at about 98% and methane (CH₄) at about 2%. These molecules form the basis of
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 of 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 on Earth and in Titan's dominant nitrogen (N₂) reservoir. More detailed investigations 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 nitriles detected in Titan’s atmosphere, we observed CH₃CN using the Atacama Large Millimeter/submillimeter array (ALMA) band 6 receiver (211-275 GHz) over 3 dates in November and December 2019. From our high signal-to-noise, 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 the carbon isotopic ratios of ~89 and ~91 for CH₃CN/¹³CH₃CN and CH₃CN/CH₃ ¹³CN, respectively, are more consistent with the bulk (Titan
and terrestrial) values while the ¹⁴N/¹⁵N isotopic ratio is enhanced, ~69, in CH₃CN as in other nitriles, such as HCN and HC3N, when compared to Titan’s bulk atmospheric N₂, ¹⁴N/¹⁵N = 168. This elevated ¹⁵N fraction is evidence of isotope-selective photodissociation of N₂ in Titan’s upper atmosphere, followed by incorporation of ¹⁵N into the CN group, thus providing a new test for our understanding of nitrogen photochemistry in primitive planetary atmospheres.

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