Improved Carbon and Nitrogen Isotopic Ratios for CH₃CN in Titan’s Atmosphere Using ALMA

Titan, Saturn's largest satellite, maintains an atmosphere composed primarily of nitrogen (N₂) and methane (CH₄) that leads to complex organic chemistry. Some of the nitriles (CN-bearing organics) on Titan are known to have substantially enhanced ¹⁵N abundances compared to Earth and Titan's dominant nitrogen (N₂) reservoir. The ¹⁴N/¹⁵N 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 ¹⁴N/¹⁵N and ¹²C/¹³C isotopic ratios of acetonitrile (CH₃CN) in Titan's stratosphere. Using the NEMESIS, we derived the CH₃CN/¹³CH₃CN ratio to be 89.2 ± 7.0 and the CH₃CN/CH₃¹³CN ratio to be 91.2 ± 6.0, in agreement with the ¹²C/¹³C ratio in Titan's methane and other solar system species. We found the ¹⁴N/¹⁵N isotopic ratio to be 68.9 ± 4.2, consistent with previously derived values for HCN and HC₃N, confirming an enhanced ¹⁵N abundance in Titan's nitriles compared with the bulk atmospheric N₂ value of ¹⁴N/¹⁵N = 168, in agreement with chemical models incorporating isotope-selective photodissociation of N₂ at high altitudes.