We report the first spectroscopic detection of ethyl cyanide (C2H5CN) 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 C2H5CN in Titan's ionosphere was previously inferred from Cassini ion mass spectrometry measurements of C2H5CNH+. Here we report the detection of 27 rotational lines from C2H5CN (in 19 separate emission features detected at >3σ confidence), in the frequency range 222-241 GHz. Simultaneous detections of multiple emission lines from HC3N, CH3CN and CH3CCH were also obtained. In contrast to HC3N, CH3CN and CH3CCH, which peak in Titan's northern (spring) hemisphere, the emission from C2H5CN 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 C2H5CN. Radiative transfer models show that most of the C2H5CN 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)×1014 cm−2.
@article{arxiv.1410.5325,
title = {Ethyl cyanide on Titan: Spectroscopic detection and mapping using ALMA},
author = {M. A. Cordiner and M. Y. Palmer and C. A. Nixon and P. G. J. Irwin and N. A. Teanby and S. B. Charnley and M. J. Mumma and Z. Kisiel and J. Serigano and Y. -J. Kuan and Y. -L. Chuang and K. -S. Wang},
journal= {arXiv preprint arXiv:1410.5325},
year = {2015}
}