Detection of Cyclopropenylidene on Titan
Authors: Nixon Conor , Thelen Alexander , Cordiner Martin , Kisiel Zbigniew , Charnley Steven , Molter Edward , Serigano Joseph , Irwin Patrick , Teanby Nicholas , Kuan Yi-Jehng 
Published: October 2020

Titan, Saturn&#8217;s largest moon, has long been known to harbor a thick atmosphere [1] that evolves a complex array of organic molecules through atmospheric photochemistry [2, 3]. Especially from the 1970s onwards, successive waves of investigation with ground-based telescopes, spacecraft including Voyager 1 and Cassini-Huygens, and space telescopes have revealed the molecular inventory of its atmosphere through remote sensing at UV to radio wavelengths, and in situ mass spectroscopy [4, 5]. Since coming online in 2012, the ALMA (Atacama Large Millimeter/submillimeter Array) telescope has added importantly to our knowledge of Titan&#8217;s atmospheric composition, especially through first detections of propionitrile (ethyl cyanide, C2H5CN) and acrylonitrile (vinyl cyanide, C2H3CN) in the neutral atmosphere [6, 7]. Such new measurements are of vital importance for constraining photochemical models [8-10] and helping us unravel the steps to building even larger molecules and haze particles [11], with important repercussions for astrobiology [12].In recent years we have continued the search for new molecules in Titan&#8217;s atmosphere, acquiring high-sensitivity observations with ALMA to search for larger hydrocarbons, nitriles and other species. In 2016 we acquired 129 mins of integration on Titan in ALMA Band 6 that exhibited many lines of C2H5CN (ethyl cyanide) and other known species. In addition we found several weak lines that we identified as c-C3H2 (cyclopropenylidene), a small cyclic molecule frequently seen in the interstellar medium [13, 14], but not previously seen in a planetary atmosphere. The spectrum was modeled using the NEMESIS radiative transfer and inversion computer model [15] yielding a best-fit mixing column abundance of 5.29x1012 molecule cm-2, somewhat greater than predicted by recent photochemical models (1.41x1012 [8]; 7.71x1011 [16]).Cyclopropenylidene is now only the second cyclic molecule to be detected in a planetary atmosphere after benzene. Its measurement will provide vital constraints on the chemistry of important intermediate-size radicals such as C3H3 (propargyl and its isomers) whose chemistry may lead to either c-C3H2 (by hydrogen loss) or to benzene (e.g. by self-reaction). Ultimately, a better understanding of cyclic molecule chemistry will lead to a better understanding of haze formation, and Titan&#8217;s potential for astrobiology.&#160;References[1]&#160;&#160;&#160;&#160;&#160;&#160; G. P. Kuiper, "Titan: A satellite with an atmosphere," Astrophysical Journal, vol. 100, no. 3, pp. 378-383, Nov 1944, doi: 10.1086/144679.[2]&#160;&#160;&#160;&#160;&#160;&#160; Y. L. Yung, M. Allen, and J. P. Pinto, "Photochemistry of the Atmosphere of Titan - Comparison Between Model and Observations," Astrophysical Journal Supplement Series, vol. 55, no. 3, pp. 465-506, 1984, doi: 10.1086/190963.[3]&#160;&#160;&#160;&#160;&#160;&#160; Y. L. Yung, "An Update of Nitrile Photochemistry on Titan," Icarus, vol. 72, no. 2, pp. 468-472, Nov 1987, doi: 10.1016/0019-1035(87)90186-2.[4]&#160;&#160;&#160;&#160;&#160;&#160; B. Bezard, R. V. Yelle, and C. A. Nixon, "The composition of Titan's atmosphere," (in English), Titan: Interior, Surface, Atmosphere, and Space Environment, Article; Book Chapter no. 14, pp. 158-189, 2014.[5]&#160;&#160;&#160;&#160;&#160;&#160; S. M. Horst, "Titan's atmosphere and climate," Journal of Geophysical Research-Planets, vol. 122, no. 3, pp. 432-482, Mar 2017, doi: 10.1002/2016je005240.[6]&#160;&#160;&#160;&#160;&#160;&#160; M. A. Cordiner et al., "ETHYL CYANIDE ON TITAN: SPECTROSCOPIC DETECTION AND MAPPING USING ALMA," Astrophysical Journal Letters, vol. 800, no. 1, Feb 10 2015, Art no. L14, doi: 10.1088/2041-8205/800/1/l14.[7]&#160;&#160;&#160;&#160;&#160;&#160; M. Y. Palmer et al., "ALMA detection and astrobiological potential of vinyl cyanide on Titan," Science Advances, vol. 3, no. 7, Jul 2017, Art no. e1700022, doi: 10.1126/sciadv.1700022.[8]&#160;&#160;&#160;&#160;&#160;&#160; V. Vuitton, R. V. Yelle, S. J. Klippenstein, S. M. Horst, and P. Lavvas, "Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model," Icarus, vol. 324, pp. 120-197, May 2019, doi: 10.1016/j.icarus.2018.06.013.[9]&#160;&#160;&#160;&#160;&#160;&#160; K. Willacy, M. Allen, and Y. Yung, "A NEW ASTROBIOLOGICAL MODEL OF THE ATMOSPHERE OF TITAN," Astrophysical Journal, vol. 829, no. 2, Oct 2016, Art no. 79, doi: 10.3847/0004-637x/829/2/79.[10]&#160;&#160;&#160;&#160; J. C. Loison et al., "The neutral photochemistry of nitriles, amines and imines in the atmosphere of Titan," Icarus, vol. 247, pp. 218-247, Feb 2015, doi: 10.1016/j.icarus.2014.09.039.[11]&#160;&#160;&#160;&#160; J. C. Loison, M. Dobrijevic, and K. M. Hickson, "The photochemical production of aromatics in the atmosphere of Titan," Icarus, vol. 329, pp. 55-71, Sep 2019, doi: 10.1016/j.icarus.2019.03.024.[12]&#160;&#160;&#160;&#160; S. M. Horst et al., "Formation of Amino Acids and Nucleotide Bases in a Titan Atmosphere Simulation Experiment," Astrobiology, vol. 12, no. 9, pp. 809-817, Sep 2012, doi: 10.1089/ast.2011.0623.[13]&#160;&#160;&#160;&#160; P. Thaddeus, J. M. Vrtilek, and C. A. Gottlieb, "LABORATORY AND ASTRONOMICAL IDENTIFICATION OF CYCLOPROPENYLIDENE, C3H2," Astrophysical Journal, vol. 299, no. 1, pp. L63-L66, Dec 1985, doi: 10.1086/184581.[14]&#160;&#160;&#160;&#160; D. Fosse, J. Cernicharo, M. Gerin, and P. Cox, "Molecular carbon chains and rings in TMC-1," Astrophysical Journal, vol. 552, no. 1, pp. 168-174, May 2001, doi: 10.1086/320471.[15]&#160;&#160;&#160;&#160; P. G. J. Irwin et al., "The NEMESIS planetary atmosphere radiative transfer and retrieval tool," Journal of Quantitative Spectroscopy &amp; Radiative Transfer, vol. 109, no. 6, pp. 1136-1150, Apr 2008, doi: 10.1016/j.jqsrt.2007.11.006.[16]&#160;&#160;&#160;&#160; E. Hebrard, M. Dobrijevic, J. C. Loison, A. Bergeat, K. M. Hickson, and F. Caralp, "Photochemistry of C3Hp hydrocarbons in Titan's stratosphere revisited," Astronomy &amp; Astrophysics, vol. 552, Apr 2013, Art no. A132, doi: 10.1051/0004-6361/201220686.