TACTIC · Titan Atmospheric Composition: Tholins and Ionospheric Chemistry

Titan Atmospheric Composition: Tholins and Ionospheric Chemistry

Titan, the largest moon of Saturn, is a fascinating object, with its dense and cold atmosphere, mainly made of nitrogen and methane. Solar photons and Saturn's magnetospheric electrons induce a complex atmospheric chemistry leading to the formation of neutral hydrocarbons and nitrogen-bearing species that eventually end up as organic aerosols. The Cassini-Huygens space mission recently revealed the presence in the upper atmosphere of positively and negatively charged ions with a mass-to-charge ratio (m/z) reaching up to a few hundred atomic mass units for cations and up to a few thousand for anions. Cassini also observed that very energetic OHx+ ions penetrate in the upper atmosphere and are at the origin of a few oxygenated compounds. Then, Titan appears as a natural laboratory to study the formation and the composition of complex organic matter, from the first steps in the gas phase to solid macromolecules. The objective of the TACTIC project was to study the photochemical evolution of the organic matter present in the atmosphere of Titan through a combination of models in silico and laboratory experiments. This research activity has an acedemic interest and has an impact in the field of space exploration of the solar system, and more widely in Astrophysics. Modeling of Titan's atmosphere was performed in the framework of an international consortium involving researchers from the University of Arisona, Imperial College London and CNRS (French Center for Scientific Research). In order to model properly the atmospheric chemistry, it is important to characterise the energy being deposited in the atmosphere. We demonstrated that on the dayside, the solar ultraviolet flux is the main ionisation source in the upper atmosphere. We then developed the first photochemical model of Titan's atmosphere including neutral and both positive and negative ion chemistry. This model first lead to the identification of the low mass anions observed in the atmosphere. We also showed that the production of ammonia is the result of a complex chemical network involving both radicals and positive ions. Finally, we included for the first time in the reaction list reactions of radiative association of neutral species and found that they are fast enough to have an impact on the concentration of hydrocarbons in the upper atmosphere. The processes leading to the formation of the atmospheric aerosols are very poorly characterised and the latter are still not taken into account in photochemical models. We showed that the negatively charged aerosols observed in the upper atmosphere are formed by electronic attachment on sub-nanometer particles and that molecular growth occurs through collision with positive ions. Moreover, these particles induce an electric field that decreases their sedimentation velocity, increasing their coagulation rate even more. A second aspect of the TACTIC project is the characterisation by very-high resolution mass spectrometry of laboratory analogs of the Titan aerosols, called "tholins". We first confirmed the molecular complexity of these samples and thoroughly characterised them with custom-made computer tools. For example, we demonstrated that they are partially constituted of HCN polymers. We then studied how oxygen can be incorporated into tholins and detected the presence of nuclear bases and simple amino acids, thus showing for the first time that prebiotic molecules can be synthesized in the absence of liquid water. We also focused on structural analysis by tandem mass spectrometry (HRMS/MS) of the species contained in tholins. We found that C2N3-is a molecular building block of tholins. We built a database of the fragmentation spectra of 25 standard CxHyNz compounds allowing the interpretation of the fragmentation patterns observed in the tholins. This work is also supported by ab initio calculations to rationalise on theoretical grounds the fragmentation observed. Photochemical modeling implies a continuous need for validation of chemical networks and determination of new kinetic rate constants to improve them. We validated the capability of models to reproduce the ion chemistry occurring in the upper atmosphere of Titan by constraining the model against laboratory experiments simulating the first steps of molecular growth in a high-resolution mass spectrometer coupled to synchrotron radiation. We performed kinetic experiments in order to better characterise some reactions of importance for our understanding of Titan's atmosphere. This aspect of the project required some external expertise and we developed new collaborations with the Laboratoire de Chimie Physique, Universit? Paris-Sud and the Heyrovsk? Institute in Prague. First, we studied the reactions of O+ with CH4 and found that the distribution of products strongly depends on the electronic state of O+ considered. Second, we investigated charge reversal processes upon the impact of keV OHx+ (x = 0-3) with N2 and CH4. We also measured cross sections of photodetachment of an electron upon interaction of a photon with several anions CxHyNz-. Finally, we extrapolated down to low temperature rate constants of interest by using a theoretical approach based on semi-empirical capture. These new data will be included in the next generation of Titan's atmospheric models. contact: V?ronique Vuitton Institut de Plan?tologie et d'Astrophysique de Grenoble Institut de Plantologie et d'Astrophysique de Grenoble (UMR 5274) BP 53 F-38041 GRENOBLE C?dex 9 (France) Phone: + 33 (0) 476635278 veronique. vuitton@obs. ujf-grenoble. fr

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