Ethyl cyanide on Titan: Spectroscopic detection and mapping using ALMA

We report the first spectroscopic detection of ethyl cyanide (C\(_2\)H\(_5\)CN) in Titan's atmosphere, obtained using spectrally and spatially resolved observations of multiple emission lines with the Atacama Large Millimeter/submillimeter array (ALMA). The presence of C\(_2\)H\(_5\)CN in Titan's ionosphere was previously inferred from Cassini ion mass spectrometry measurements of C\(_2\)H\(_5\)CNH\(^+\). Here we report the detection of 27 rotational lines from C\(_2\)H\(_5\)CN (in 19 separate emission features detected at \(>3\sigma\) confidence), in the frequency range 222-241 GHz. Simultaneous detections of multiple emission lines from HC\(_3\)N, CH\(_3\)CN and CH\(_3\)CCH were also obtained. In contrast to HC\(_3\)N, CH\(_3\)CN and CH\(_3\)CCH, which peak in Titan's northern (spring) hemisphere, the emission from C\(_2\)H\(_5\)CN is found to be concentrated in the southern (autumn) hemisphere, suggesting a distinctly different chemistry for this species, consistent with a relatively short chemical lifetime for C\(_2\)H\(_5\)CN. Radiative transfer models show that most of the C\(_2\)H\(_5\)CN is concentrated at altitudes 300-600 km, suggesting production predominantly in the mesosphere and above. Vertical column densities are found to be in the range (2-5)\(\times10^{14}\) cm\(^{-2}\).