An Improved Upper Limit of Cyanodiacetylene (HC ₅ N) in the Atmosphere of Titan

Abstract In the complex Titan atmospheric chemistry, photochemical reactions involving cyanodiacetylene (HC 5 N) may be important as reaction pathways that could lead to the production of amines and aromatic molecules. We carried out a new comprehensive search of HC 5 N in Titan’s atmosphere using Band 6 (215–275 GHz or 1.1–1.4 mm in wavelength) spectroscopic archival data from the Atacama Large Millimeter/submillimeter Array. To search for a faint spectral line feature of HC 5 N, we used a spectral stacking method that integrates different transition data. Integrating six independent observation data sets with a total integration time of ∼15,000 s, we found no statistically significant detection of an HC 5 N spectral feature. Using the Planetary Spectrum Generator for radiative transfer modeling and employing the previously predicted vertical distribution proposed by V. Vuitton et al., E. Lellouch et al., J. C. Loison et al., and V. A. Krasnopolsky, we derived the upper limit column density of HC 5 N, corresponding to 3 σ noise level, to be 0.3–10 × 10 13 molecules cm –2 above ∼60 km altitude (depending on the assumed vertical profile). The corresponding upper limit of [HC 5 N]/[HC 3 N] ratio value derived from three of four models seem to be lower than that measured in high-mass protostellar objects, possibly due to the lower abundance of C 4 H 2 .