14 N/ 15 N Isotopic Ratio in CH 3 CN of Titan’s Atmosphere Measured with ALMA

Abstract

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