Detection of CH3C3N in Titan's Atmosphere


Disk-averaged spectra of Titan from SG2 and SG3 spectral windows 31 (top) and 37 (bottom), respectively. Strong spectral lines of various molecular species are marked with black or gray lines; spectral line parameters are detailed in Table 1. Additional unlabeled transitions of C2H5CN and C2H3CN are present. Insets in purple show the detections of the CH3C3N K = 0–3 lines in the J = 64 → 63 and J = 62 → 61 bands. Both detected transitions (purple) and undetected or blended transitions (red) are marked, with marker heights proportional to the line intensities (calculated at 160 K). An additional, blended feature (B) is shown in the inset of SPW 37 at ∼ 256.024 GHz, most likely a combination of C3H8, C2H5CN, and CH3C3N.

Titan harbors a dense, organic-rich atmosphere primarily composed of N2 and CH4 , with lesser amounts of hydrocarbons and nitrogen-bearing species.

As a result of high sensitivity observations by the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 6 (∼230-272 GHz), we obtained the first spectroscopic detection of CH3C3N (methylcyanoacetylene or cyanopropyne) in Titan's atmosphere through the observation of seven transitions in the J=64→63 and J=62→61 rotational bands.

The presence of CH3C3N on Titan was suggested by the Cassini Ion and Neutral Mass Spectrometer detection of its protonated form: C4H3NH+, but the atmospheric abundance of the associated (deprotonated) neutral product is not well constrained due to the lack of appropriate laboratory reaction data. Here, we derive the column density of CH3C3N to be (3.8-5.7)×1012 cm−2 based on radiative transfer models sensitive to altitudes above 400 km Titan's middle atmosphere.

When compared with laboratory and photochemical model results, the detection of methylcyanoacetylene provides important constraints for the determination of the associated production pathways (such as those involving CN, CCN, and hydrocarbons), and reaction rate coefficients. These results also further demonstrate the importance of ALMA and (sub)millimeter spectroscopy for future investigations of Titan's organic inventory and atmospheric chemistry, as CH3C3N marks the heaviest polar molecule detected spectroscopically in Titan's atmosphere to date.

A. E. Thelen, M. A. Cordiner, C. A. Nixon, V. Vuitton, Z. Kisiel, S. B. Charnley, M. Y. Palmer, N. A. Teanby, P. G. J. Irwin

Comments: 17 pages, 6 figures, 2 tables. Accepted in ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2010.08654 [astro-ph.EP] (or arXiv:2010.08654v1 [astro-ph.EP] for this version)
Submission history
From: Alexander Thelen
[v1] Fri, 16 Oct 2020 22:27:14 UTC (412 KB)
https://arxiv.org/abs/2010.08654
Astrobiology, Astrochemistry,

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