Chemical Characterization of Titan’s Tholins: Solubility, Morphology and Molecular Structure Revisited † N. Carrasco,* ,‡ I. Schmitz-Afonso, § J-Y. Bonnet, | E. Quirico, | R. Thissen, | Odile Dutuit, | A. Bagag, § O. Lapre ´vote, § A. Buch, ⊥ A. Giulani, #,∇ Gilles Adande ´, | F. Ouni, ‡ E. Hadamcik, ‡ C. Szopa, ‡ and G. Cernogora ‡ UniVersite ´ de Versailles St-Quentin, UPMC UniV. Paris 06 CNRS/INSU, LATMOS-IPSL, Route des Ga ˆtines, 91371 Verrie `res le Buisson Cedex, France, Institut de Chimie des Substances Naturelles, CNRS, Ba ˆt. 27, AVenue de la Terrasse 91198 Gif-sur-YVette Cedex, France, Laboratoire de Plane ´tologie de Grenoble, CNRS, UJF, UMR 5109, BP 53, 38041 Grenoble Cedex 9, France, Laboratoire de Ge ´nie des Proce ´de ´s et Mate ´riaux, Ecole Centrale Paris, Grande Voie des Vignes - 92295 Chatenay-Malabry Cedex, France, SOLEIL, L’Orme des merisiers, Saint Aubin BP48, 91192 Gif sur YVette Cedex, France, and Cepia, Institut National de la Recherche Agronomique (INRA), BP 71627, F-44316 Nantes Cedex 3 France ReceiVed: May 20, 2009; ReVised Manuscript ReceiVed: August 25, 2009 In this work Titan’s atmospheric chemistry is simulated using a capacitively coupled plasma radio frequency discharge in a N 2 -CH 4 stationnary flux. Samples of Titan’s tholins are produced in gaseous mixtures containing either 2 or 10% methane before the plasma discharge, covering the methane concentration range measured in Titan’s atmosphere. We study their solubility and associated morphology, their infrared spectroscopy signature and the mass distribution of the soluble fraction by mass spectrometry. An important result is to highlight that the previous Titan’s tholin solubility studies are inappropriate to fully characterize such a heterogeneous organic matter and we develop a new protocol to evaluate quantitatively tholins solubility. We find that tholins contain up to 35% in mass of molecules soluble in methanol, attached to a hardly insoluble fraction. Methanol is then chosen as a discriminating solvent to characterize the differences between soluble and insoluble species constituting the bulk tholins. No significant morphological change of shape or surface feature is derived from scanning electron microscopy after the extraction of the soluble fraction. This observation suggests a solid structure despite an important porosity of the grains. Infrared spectroscopy is recorded for both fractions. The IR spectra of the bulk, soluble, and insoluble tholins fractions are found to be very similar and reveal identical chemical signatures of nitrogen bearing functions and aliphatic groups. This result confirms that the chemical information collected when analyzing only the soluble fraction provides a valuable insight representative of the bulk material. The soluble fraction is ionized with an atmospheric pressure photoionization source and analyzed by a hybrid mass spectrometer. The congested mass spectra with one peak at every mass unit between 50 and 800 u confirm that the soluble fraction contains a complex mixture of organic molecules. The broad distribution, however, exhibits a regular pattern of mass clusters. Tandem collision induced dissociation analysis is performed in the negative ion mode to retrieve structural information. It reveals that (i) the molecules are ended by methyl, amine and cyanide groups, (ii) a 27 u neutral moiety (most probably HCN) is often released in the fragmentation of tholin anions, and (iii) an ubiquitous ionic fragment at m/z 66 is found in all tandem spectra. A tentative structure is proposed for this negative ion. 1. Introduction Titan, the largest satellite of Saturn, has a dense atmosphere composed of nitrogen (95 to 98%), methane, molecular hydro- gen, and traces of hydrocarbons. Several nitrogen-bearing organic compounds such as hydrogen cyanide (HCN), cy- anoacetylene (HC 3 N), and cyanogen (C 2 N 2 ) have been detected in its atmosphere. 1,2 A further chemical growth, initiated by the activation of N 2 and CH 4 is at work, leading to strong interest and relevance for astrobiological questions. The atmospheric chemistry is so active that it eventually produces macroscopic particles that sediment slowly in the atmosphere, leading to the brownish haze permanently surrounding the satellite. These organic aerosols play an important role in the properties and evolution of Titan’s atmosphere. 3 Indeed, aerosols absorb a significant fraction of the incoming sunlight, generating an antigreenhouse effect that cools down Titan’s surface. 4 However, information on these aerosols composition, their formation, and their growth is still very limited due to the difficulty to perform observations of Titan’s upper atmosphere from the Earth or from space probes. 5 A complementary strategy to remote observations was developed. The study of Titan’s aerosols was refined by producing analogous materials in laboratory, the so-called Titan tholins. The most efficient production methods are plasma discharges in gaseous N 2 -CH 4 mixtures. 6-13 In the present work, the discharge is based on a radio frequency capacitively coupled plasma. 14 † Part of the special section “Chemistry: Titan Atmosphere”. * Corresponding author. E-mail: nathalie.carrasco@latmos.ipsl.fr. ‡ Universite ´ de Versailles St-Quentin. § Institut de Chimie des Substances Naturelles CNRS. | Laboratoire de Plane ´tologie de Grenoble, CNRS. ⊥ Ecole Centrale Paris. # SOLEIL. ∇ Institut National de la Recherche Agronomique. J. Phys. Chem. A 2009, 113, 11195–11203 11195 10.1021/jp904735q CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009 Downloaded by BIUS JUSSIEU on October 15, 2009 | http://pubs.acs.org Publication Date (Web): September 23, 2009 | doi: 10.1021/jp904735q