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
15N abundances compared to Earth and to
Titan's dominant nitrogen (N
2) 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 (ALMA) Band 6 receiver (211-275 GHz), we measure
the
14N/
15N and
12C/
13C isotopic ratios of acetonitrile
(CH
3CN) in Titan's stratosphere. Using the Nonlinear optimal Estimator for
MultivariatE spectral analySIS (NEMESIS), we derived the
CH
3CN/
13CH
3CN ratio to be 89.2
± 7.0 and the
CH
3CN/CH
313CN 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 HC
3N, confirming an enhanced
15N abundance in Titan's nitriles compared with the bulk atmospheric N
2
value of
14N/
15N = 168, in agreement with chemical models
incorporating isotope-selective photodissociation of N
2 at high altitudes.