14N/15N isotopic ratio in CH3CN of Titan’s atmosphere measured with ALMA

Takahiro Iino, Hideo Sagawa, Takashi Tsukagoshi

(Submitted on 6 Jan 2020)

Each of the nitriles present in the atmosphere of Titan can be expected to exhibit different 14N/15N values depending on their production processes, primarily because of the various N2 dissociation processes induced by different sources such as ultraviolet radiation, magnetospheric electrons, and galactic cosmic rays. For CH3CN, one photochemical model predicted a 14N/15N value as 120–130 in the lower stratosphere. This is much higher than that for HCN and HC3N, ∼67–94. By analyzing archival data obtained by the Atacama Large Millimeter/submillimeter Array (ALMA), we successfully detected submillimeter rotational transitions of CH3C 15N (J = 19–18) locate at the 338 GHz band in Titan’s atmospheric spectra. By comparing those observations with the simultaneously observed CH3CN (J = 19–18) lines at the 349 GHz band, which probe from 160 to ∼400 km altitude, we then derived 14N/15N in CH3CN as 125+145 −44 . Although the range of the derived value shows insufficient accuracy due to data quality limitations, the best-fit value suggests that 14N/15N for CH3CN is higher than values that have been previously observed and theoretically predicted for HCN and HC3N. This may be explained by the different N2 dissociation sources according to the altitudes, as suggested by a recent photochemical model.

Comments: To be published in ApJ

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:2001.01484 [astro-ph.EP] (or arXiv:2001.01484v1 [astro-ph.EP] for this version)

Submission history

From: Takahiro Iino 

[v1] Mon, 6 Jan 2020 11:09:09 UTC (2,243 KB)

https://arxiv.org/abs/2001.01484

Astrobiology