Ethyl cyanide on Titan: Spectroscopic detection and mapping using ALMA

Abstract

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)$\times 10^{14}$ cm${}^{-2}$.

arXiv commentPublished in 2015, ApJL, 800, L14

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• HCN ice in Titan's high-altitude southern polar cloud
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