Improved Carbon and Nitrogen Isotopic Ratios for CH$_3$CN in Titan's Atmosphere Using ALMA

Abstract

Titan, Saturn's largest satellite, maintains an atmosphere composed primarily of nitrogen (N$_2$) and methane (CH$_4$) that leads to a complex organic chemistry. Some of the nitriles (CN-bearing organics) on Titan are known to have substantially enhanced $^{15}$N abundances compared to Earth and to Titan's dominant nitrogen (N$_2$) reservoir. The $^{14}$N/$^{15}$N isotopic ratio in Titan's nitriles can provide better constraints on the synthesis of nitrogen-bearing organics in planetary atmospheres as well as insights into the origin of Titan's large nitrogen abundance. Using high signal-to-noise ratio ($>13$), disk-integrated observations obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 receiver (211-275 GHz), we measure the $^{14}$N/$^{15}$N and $^{12}$C/$^{13}$C isotopic ratios of acetonitrile (CH$_3$CN) in Titan's stratosphere. Using the Nonlinear optimal Estimator for MultivariatE spectral analySIS (NEMESIS), we derived the CH$_3$CN/$^{13}$CH$_3$CN ratio to be 89.2 $\pm$ 7.0 and the CH$_3$CN/CH$_3$$^{13}$CN ratio to be 91.2 $\pm$ 6.0, in agreement with the $^{12}$C/$^{13}$C ratio in Titan's methane, and other Solar System species. We found the $^{14}$N/$^{15}$N isotopic ratio to be 68.9 $\pm$ 4.2, consistent with previously derived values for HCN and HC$_3$N, confirming an enhanced $^{15}$N abundance in Titan's nitriles compared with the bulk atmospheric N$_2$ value of $^{14}$N/$^{15}$N = 168, in agreement with chemical models incorporating isotope-selective photodissociation of N$_2$ at high altitudes.