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Simulation expérimentale de la chimie atmosphérique de Titan : Suivi des espèces produites et comparaison à un modèle cinétique

Abstract : Since several years, GPCOS team (Groupe de Physico-Chimie Organique Spatiale) of LISA has developed an experimental program, of which the goal is to simulate Titan's atmospheric chemistry. Analytical techniques has been developed in order to detect and to quantify compounds produced during these experimental simulations : gaseous compounds by IR spectrometry and GC-MS, solid compounds (deposited on the walls of the reactor) by elemental analysis and pyrolysis coupled with GC-MS. However, due to the complexity of the studied chemistry, production mechanisms (especially of Titan's aerosols analogues) were not correctly described. In addition the representativity of the energy in this kind of glow discharge (electrons in place of UV photons), in order to simulate the Titan's stratospheric conditions was seriously contested. Using a kinetic model of the glow discharge for analysing experimental results, we managed to explain mechanisms taking place inside the reactor. We have determined the energy deposited in the discharge by the electrons; detected in situ short time species (radicals, ions, excited species) by UV-Vis emission spectrometry; and compared the evolution of relative abundances of species with the modeling results Even if we detected CH radical, it is not predicted by photochemical models of Titan's atmosphere. Thus, if it is present in Titan's atmosphere, it may participate to the C2H2 formation, which present an underestimated abundance with about 30% by the photochemical models. We also have detected ammonia (NH3) in the major products. Its possible presence in condensed phase on Titan could explain Titan's albedo at about 5 µm. Effectively NH3 ices have a strong absorption feature around 5.25 µm. If its presence is confirmed by CIRS, the IR spectrometer onboard Cassini-Huygens mission, this exo/astrobiological compound could participate to the grow of Titan's aerosols. Notably, it could react with hydrogen cyanide (HCN) in order to form NH4CN which can product, in presence of water, purine bases like adenine and diaminopurine. Coming from cometary and meteoritic impacts, oxygenated compounds (CO, CO2 and H2O) are present in Titan's atmosphere. Thus, we have performed the first experimental simulation with an initial mixture made of N2/CH4/CO (98/1,99/0,01) in order to verify the impact of carbon monoxide (CO), major O-containing compound in Titan's atmosphere, on the produced gaseous phase. We have identified by two analysis techniques (IR spectrometry and CG-MS) oxirane (or ethylene oxide, an O-cyclic molecule), the major O-containing organic product. This compound has been detected in the Interstellar Medium and its possible presence on Titan will be confirmed by CIRS (with a strong feature at 11.4 µm). The evolution as a function of the experimental parameters of : 1. species abundances in the discharge 2. atomic composition of tholins has allowed to propose a mechanism concerning the gaseous organic compounds : an exchange of an hydrogen fixed on a carbon by a C≠N radical. This hypothesis is in agreement with reactions proposed by the atmospheric chemical models : HCN + CN  C2N2 + H k = 6,31.10-17 Tg1,57 exp (-50/Tg) cm3.s-1 C2H2 + CN  HC3N + H k = 5,67.10-9 Tg-0,55 exp(-4/Tg) cm3 s-1 C2H4 + CN  CH2CHCN + H k = 1,25.10-10 (Tg/300)0,7 exp(-30/Tg) cm3 s-1 This kind of reactions could be possible with polyynes in order to product cyanopolyynes : C4H2 + CN  HC5N + H k = 2.10-10 cm3 s-1 C6H2 + CN  HC7N + H k = 2.10-10 cm3 s-1 C8H2 + CN  HC9N + H k = 2.10-10 cm3 s-1 Concerning tholins, the same kind of mechanism could take place on a chemical structure made of conjugated systems. For the first time, the comparison between the energy deposited in the reactor and the one arriving in Titan's atmosphere has been realised, allowing to discuss the energetic representativity of the glow discharge. The power provided by the electrons in the plasma is 108 times bigger than the one brought by photons responsible of CH4 and N2 dissociations (> 10eV ; < 150 nm). A comparison with the production rates of solid compounds shows that the experimental simulation has a production rate about 104 times smaller than the one of Titan (rate relative to the deposited power in the two cases).
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Jean-Michel Bernard. Simulation expérimentale de la chimie atmosphérique de Titan : Suivi des espèces produites et comparaison à un modèle cinétique. Astrophysique [astro-ph]. Université Paris-Diderot - Paris VII, 2004. Français. ⟨tel-00008510⟩

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