14N/15N Isotopic Ratio in CH3CN of Titan's Atmosphere Measured with ALMA

The nitriles present in the atmosphere of Titan can be expected to exhibit different ${}^{14}{\rm{N}}/{}^{15}{\rm{N}}$ values depending on their production processes, primarily because of the various ${{\rm{N}}}_{2}$ dissociation processes induced by different sources such as ultraviolet radiation, magnetospheric electrons, and Galactic cosmic rays. For ${\mathrm{CH}}_{3}\mathrm{CN}$ , one photochemical model predicted a ¹⁴N/¹⁵N value of 120-130 in the lower stratosphere. This is much higher than that for HCN and ${\mathrm{HC}}_{3}{\rm{N}}$ , ∼67-94. By analyzing archival data obtained by the Atacama Large Millimeter/submillimeter Array, we successfully detected submillimeter rotational transitions of ${\mathrm{CH}}_{3}{{\rm{C}}}^{15}{\rm{N}}$ (J = 19-18) located in the 338 GHz band in Titan's atmospheric spectra. By comparing those observations with the simultaneously observed ${\mathrm{CH}}_{3}\mathrm{CN}$ (J = 19-18) lines in the 349 GHz band, which probe from 160 to ∼400 km altitude, we then derived ¹⁴N/¹⁵N in ${\mathrm{CH}}_{3}\mathrm{CN}$ as 125 ${}_{-44}^{+145}$ . Although the range of the derived value shows insufficient accuracy due to limitations on data quality, the best-fit value suggests that ¹⁴N/¹⁵N for ${\mathrm{CH}}_{3}\mathrm{CN}$ is higher than values that have previously been observed and theoretically predicted for HCN and ${\mathrm{HC}}_{3}{\rm{N}}$ . This may be explained by the different ${{\rm{N}}}_{2}$ dissociation sources according to altitude, as suggested by a recent photochemical model.