Titan, Saturn’s largest satellite, maintains an atmosphere composed primarily of nitrogen (N2) and methane (CH4) 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 (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 (ALMA) Band 6 receiver (211-275 GHz), we measure the 14N/15N and 12CN/13CN isotopic ratios of acetonitrile (CH3CN) in Titan’s stratosphere.
Using the Nonlinear optimal Estimator for MultivariatE spectral analySIS (NEMESIS), we derived the CH3CN/13CNH3CN ratio to be 89.2 ± 7.0 and the CH3CN/CH313CN ratio to be 91.2 ± 6.0, in agreement with the 12CN/13CN 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.
Derived continuous CH3CN/CH3C 15N isotopic ratio profile with error envelope (red; shaded red region) plotted with photochemical model profiles from Vuitton et al. (2019) (blue) and Dobrijevic & Loison (2018) (green and gold). The black point is our measured scaling factor for the CH3CN/CH3C 15N isotopic ratio from this work, plotted at the weighted mean emission altitude of 230 km (with its corresponding error bars). — astro-ph.EP
J. Nosowitz, M. A. Cordiner, C. A. Nixon, A. E. Thelen, Z. Kisiel, N. A. Teanby, P. G. J. Irwin, S. B. Charnley, V. Vuitton
Comments: Accepted for publication in PSJ March 2025
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2503.09897 [astro-ph.EP] (or arXiv:2503.09897v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2503.09897
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Submission history
From: Jonathon Nosowitz
[v1] Wed, 12 Mar 2025 23:30:10 UTC (411 KB)
https://arxiv.org/abs/2503.09897
Astrobiology
