An Adequate Kinetic Model of the photochemical formation of hydrocarbon aerosols in Titan's atmosphere

Titan, Saturn's outermost moon, is unique in the solar system in having a massive atmosphere, consisting of 15 bar of N-2 and about 35% of CH4. Methane photolysis by the feeble solar irradiation at Saturn (similar to 100 times smaller than the irradiation at the Earth) converts the methane into a plethora of hydrocarbons. Some unsaturated hydrocarbons polymerize, to form long chains, which condense into aerosols. Indeed, three aerosol layers shroud Titan, as observed by the Voyager spacecraft. In the present paper, we try to construct an adequate kinetic model, to describe the experimentally observed methane photolysis products and high hydrocarbon polymer formation, as well as their chemical structures. The Adequate Kinetic Model (AKM) of chemical processes in Titan's atmosphere possesses three main properties: it is valid for the adequate description of any phenomena of the process investigated and describe these phenomena with a preassigned and known accuracy as well as having a prognostic capability. An adequate model is designed by a special procedure which consists of the determination of all possible (non-prohibited) elementary reactions - the so-called maximal mechanism. This mechanism is further successively decreased on the basis of stoichiometric, thermodynamic and kinetic restrictions. As a result, the adequate model does not contain any inner hidden contradictions. The most comprehensive AKM of chemical activity in Titan's atmosphere is found to consist of 732 elementary reactions and involves 147 chemical species. These species are saturated and unsaturated low hydrocarbons (such as C2H2, C2H4, C2H6 etc.), active intermediate-radical (H, CH2, CH3., etc.) and high molecular weight hydrocarbons, belonging to the Polyacetylene family (PA), Vinylacetylene family (VA) Polyvinyl family (PV) and Allenes (Polyenes). A row of low-dimensional partial AKM (Like AKM-98, AKM-145, etc.) are subtracted from the complete AKM-732. The validity of these models was verified by various testing procedures including direct comparison of the theoretical data with the available experimental data. Some general results of this AKM are: the overall time of establishment of chemical equilibrium (i.e. complete termination of chemical activity in Titan's atmosphere) is not less than 1.7 x 10(7) years, the current net-rate of production of the stable components of chemical evolution is 4.3 x 10(18) molec/m(3) sec; the ''coefficient of efficiency of the chemical machine'' converting the initial composition into the final stable products is similar to 0.82. The final products consist mainly of PA similar to(30-35)%, VA similar to(20-25)%, PV similar to(35-40)%, equilibrium abundance of low hydrocarbons (C2H6, C3H8, etc.) <10% and a small amount of Allenes <5%. These results will be used, further, to determine the composition and structure of Titan's three aerosol layers.