The Titan Haze Simulation experiment: a laboratory study to explore the role of PAHs and PANHs in Titan's atmospheric chemistry

A complex organic chemistry between Titan's two main constituents, N2 and CH4, leads to the production of more complex molecules and subsequently to solid organic aerosols. These aerosol particles form the haze layers giving Titan its characteristic orange color. In situ measurements by the Ion Neutral Mass Spectrometer (INMS) and Cassini Plasma Spectrometer (CAPS) instruments onboard Cassini have revealed the presence of large amounts of neutral, positively and negatively charged heavy molecules in the ionosphere of Titan[1,2]. In particular, benzene (C6H6) and toluene (C6H5CH3)[3], which are critical precursors of polycyclic aromatic hydrocarbon (PAH) compounds, have been detected, suggesting that PAHs might play a role in the production of Titan's aerosols. Moreover, results from numerical models[4,5] as well as laboratory simulations[6,7] of Titan's atmospheric chemistry are also suggesting chemical pathways that link the simple precursor molecules resulting from the first steps of the N2-CH4 chemistry (C2H2, C2H4, HCN...) to benzene, and to PAHs and nitrogen-containing PAHs (or PANHs) as precursors to the production of solid aerosols. The aim of the Titan Haze Simulation (THS) experiment is to study the chemical formation pathways, going from simple gas phase molecules to the more complex gas phase precursors of aerosols; and more specifically, to investigate the role of PAHs and PANHs in the nitrogen-methane enriched Titan atmosphere. In the THS experiment, Titan's atmospheric chemistry is simulated by a plasma jet expansion at pressure and temperature conditions close to those of Titan's atmosphere in the regions where aerosols are formed. The products of the chemistry are detected and studied using two complementary techniques: Cavity Ring Down Spectroscopy[8] and Time-Of-Flight Mass Spectrometry[9]. We will present the results of ongoing studies on different aspects of Titan's chemistry using various gas mixtures: from N2-CH4 gas mixtures at different CH4 concentrations to more targeted gas mixtures including trace elements present in Titan's atmosphere such as, for example, N2-C2H2, N2-C6H6 and N2-C2H2-C6H6 to study specific pathways to the production of PAHs and PANHs and large organic aerosols.