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 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