Improved Carbon and Nitrogen Isotopic Ratios for CH₃CN 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 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 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 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 NEMESIS, we derived the CH 3 CN/ 13 CH 3 CN ratio to be 89.2 ± 7.0 and the CH 3 CN/CH 3 13 CN ratio to be 91.2 ± 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 ± 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.