Improved Carbon and Nitrogen Isotopic Ratios for CH$_3$CN in Titan's Atmosphere Using ALMA
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| Auteurs principaux: | , , , , , , , , |
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| Format: | Preprint |
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2025
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| author | Nosowitz, J. Cordiner, M. A. Nixon, C. A. Thelen, A. E. Kisiel, Z. Teanby, N. A. Irwin, P. G. J. Charnley, S. B. Vuitton, V. |
| author_facet | Nosowitz, J. Cordiner, M. A. Nixon, C. A. Thelen, A. E. Kisiel, Z. Teanby, N. A. Irwin, P. G. J. Charnley, S. B. Vuitton, V. |
| contents | 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 $^{15}$N abundances compared to Earth and to Titan's dominant nitrogen (N$_2$) reservoir. The $^{14}$N/$^{15}$N 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 $^{14}$N/$^{15}$N and $^{12}$C/$^{13}$C isotopic ratios of acetonitrile (CH$_3$CN) in Titan's stratosphere. Using the Nonlinear optimal Estimator for MultivariatE spectral analySIS (NEMESIS), we derived the CH$_3$CN/$^{13}$CH$_3$CN ratio to be 89.2 $\pm$ 7.0 and the CH$_3$CN/CH$_3$$^{13}$CN ratio to be 91.2 $\pm$ 6.0, in agreement with the $^{12}$C/$^{13}$C ratio in Titan's methane, and other Solar System species. We found the $^{14}$N/$^{15}$N isotopic ratio to be 68.9 $\pm$ 4.2, consistent with previously derived values for HCN and HC$_3$N, confirming an enhanced $^{15}$N abundance in Titan's nitriles compared with the bulk atmospheric N$_2$ value of $^{14}$N/$^{15}$N = 168, in agreement with chemical models incorporating isotope-selective photodissociation of N$_2$ at high altitudes. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2503_09897 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Improved Carbon and Nitrogen Isotopic Ratios for CH$_3$CN in Titan's Atmosphere Using ALMA Nosowitz, J. Cordiner, M. A. Nixon, C. A. Thelen, A. E. Kisiel, Z. Teanby, N. A. Irwin, P. G. J. Charnley, S. B. Vuitton, V. Earth and Planetary Astrophysics Titan, Saturn's largest satellite, maintains an atmosphere composed primarily of nitrogen (N$_2$) and methane (CH$_4$) that leads to a complex organic chemistry. Some of the nitriles (CN-bearing organics) on Titan are known to have substantially enhanced $^{15}$N abundances compared to Earth and to Titan's dominant nitrogen (N$_2$) reservoir. The $^{14}$N/$^{15}$N isotopic ratio in Titan's nitriles can provide better constraints on the synthesis of nitrogen-bearing organics in planetary atmospheres as well as insights into the origin of Titan's large nitrogen abundance. Using high signal-to-noise ratio ($>13$), disk-integrated observations obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 receiver (211-275 GHz), we measure the $^{14}$N/$^{15}$N and $^{12}$C/$^{13}$C isotopic ratios of acetonitrile (CH$_3$CN) in Titan's stratosphere. Using the Nonlinear optimal Estimator for MultivariatE spectral analySIS (NEMESIS), we derived the CH$_3$CN/$^{13}$CH$_3$CN ratio to be 89.2 $\pm$ 7.0 and the CH$_3$CN/CH$_3$$^{13}$CN ratio to be 91.2 $\pm$ 6.0, in agreement with the $^{12}$C/$^{13}$C ratio in Titan's methane, and other Solar System species. We found the $^{14}$N/$^{15}$N isotopic ratio to be 68.9 $\pm$ 4.2, consistent with previously derived values for HCN and HC$_3$N, confirming an enhanced $^{15}$N abundance in Titan's nitriles compared with the bulk atmospheric N$_2$ value of $^{14}$N/$^{15}$N = 168, in agreement with chemical models incorporating isotope-selective photodissociation of N$_2$ at high altitudes. |
| title | Improved Carbon and Nitrogen Isotopic Ratios for CH$_3$CN in Titan's Atmosphere Using ALMA |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2503.09897 |