Spectroscopic studies of non-volatile residue formed by photochemistry of solid C 4 N 2 : A model of condensed aerosol formation on Titan Isabelle Couturier-Tamburelli a,⇑ , Murthy S. Gudipati b,⇑ , Antti Lignell b,c , Ronen Jacovi b,d , Nathalie Piétri a a Aix-Marseille Université, CNRS, PIIM, UMR 7345, 13013 Marseille, France b Ice Spectroscopy Lab, Science Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA c Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, MC 127-72, 1200 East California Boulevard, Pasadena, CA 91125, USA 1 d Flight Control Group, Urban Aeronautics Ltd., Nahal-Snir 10, Yavne 81224, Israel 2 article info Article history: Received 15 May 2013 Revised 10 February 2014 Accepted 17 February 2014 Available online 28 February 2014 We dedicate this article to the memory of Bishun Khare, the co-discoverer of ‘‘Tholins’’ Keywords: Titan Astrobiology Ices, IR Spectroscopy Photochemistry Prebiotic environments abstract Following our recent communication (Gudipati, M.S. et al. [2013]. Nat. Commun. 4, 1648. http:// dx.doi.org/10.1038/ncomms2649) on the discovery of condensed-phase non-volatile polymeric material with similar spectral features as tholins, we present here a comprehensive spectroscopic study of photo- chemical formation of polymeric material from condensed dicyanoacetylene (C 4 N 2 ) ice films. C 4 N 2 is chosen as starting material for the laboratory simulations because of the detection of this and similar molecules (nitriles and cyanoacetylenes) in Titan’s atmosphere. UV–Vis and infrared spectra obtained during long-wavelength (>300 nm) photon irradiation and subsequent warming of the ice films are used to analyze changes in C 4 N 2 ice, evolution of tholins, and derive photopolymerization mechanisms. Our data analysis revealed that many processes occur during the photolysis of condensed Titan’s aerosol analogs, including isomerization and polymerization leading to the formation of long-chain as well as aromatic cyclic polymer molecules. In the light of tremendous new data from the Cassini mission on the seasonal variations in Titan’s atmosphere, our laboratory study and its results provide fresh insight into the formation and evolution of aerosols and haze in Titan’s atmosphere. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Titan, the largest saturnian satellite, has a dense atmosphere essentially composed of N 2 (97%) and CH 4 (2%) (Cui et al., 2009). The solar UV radiation and the energetic particles on this satellite induce a rich chemistry by dissociation of nitrogen and methane. Among the molecules formed, HCN and C 2 H 2 play a key role under UV irradiation because they yield complex molecules in the lower part of the atmosphere as illustrated in Fig. 1. When HCN and C 2 H 2 are formed in the upper part of the atmosphere and transported to a lower altitude where shorter wavelength UV photons no longer penetrate but longer wavelength UV photons are available (Raulin et al., 2012), the CH bond continues to be broken and extremely reactive radicals C 2 H and CN are formed. These intermediates can react with neutral molecules, forming new compounds. Numerous molecules have been detected in the atmosphere of Titan, including members of the cyanopolyy- nes family (HC n N or NC m N with n P 3 or m P 4). These molecules are incorporated into the atmosphere, haze, and aerosols, and eventually rain down to the surface, contributing to the formation of the Titan’s solid surface layer. Four gas-phase nitriles, members of the cyanopolyyne family, have already been identified in Titan’s atmosphere, namely HCN, CH 3 CN, HC 3 N, C 2 N 2 (Hanel et al., 1981; Kunde et al., 1981; Marten et al., 2002). Two more members of this family, dicyanoacetylene (C 4 N 2 ) and propionitrile (C 3 H 5 N), have been detected only as a solid (Coustenis et al., 1999), although C 3 H 5 N, which has the same molecular formula of propionitrile, has been detected from the Cassini-INMS measurements in the upper atmosphere (Vuitton et al., 2007). The same class of cyanop- olyynes, which are easily converted to amino acids, the building blocks of life (Oro, 1960; Oro and Kamat, 1961), are also detected in the interstellar medium (Fosse et al., 2001; Cleaves and Dworkin, 2010). Understanding the composition and formation processes of aerosols developed within the planetary atmosphere of Titan still represents an enigma. Many experimental laboratory simulations of Titan’s atmosphere have been carried out and reviewed recently (Coll et al., 2013; Cable et al., 2012). These experiments typically lead to the production of orange-brownish http://dx.doi.org/10.1016/j.icarus.2014.02.016 0019-1035/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding authors. E-mail addresses: isabelle.couturier@univ-amu.fr (I. Couturier-Tamburelli), gudipati@jpl.nasa.gov (M.S. Gudipati). 1 Present address. 2 Present address. Icarus 234 (2014) 81–90 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus