Improved Carbon and Nitrogen Isotopic Ratios for CH 3 CN in Titan’s Atmosphere Using ALMA

Journal article

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.

Authors: Nosowitz, J; Cordiner, MA; Nixon, CA; Thelen, AE; Kisiel, Z

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: IOP Publishing
• Journal: The Planetary Science Journal
• Volume: 6
• Issue: 5
• Article number: 107
• Publication date: 2025-05-02
• Acceptance date: 2025-03-09
• DOI: 10.3847/psj/adc390
• EISSN: 2632-3338
• Language: English
• Keywords: Remote sensing; Submillimeter astronomy; Titan; Radio interferometry