AN ADEQUATE KINETIC-MODEL OF THE PHOTOCHEMICAL FORMATION OF HYDROCARBON AEROSOLS IN TITANS ATMOSPHERE

Titan, Saturn's outermost moon, is unique in the solar system in havin g a massive atmosphere, consisting of 15 bar of N-2 and about 35% of C H4. Methane photolysis by the feeble solar irradiation at Saturn (simi lar to 100 times smaller than the irradiation at the Earth) converts t he methane into a plethora of hydrocarbons. Some unsaturated hydrocarb ons polymerize, to form long chains, which condense into aerosols. Ind eed, three aerosol layers shroud Titan, as observed by the Voyager spa cecraft. In the present paper, we try to construct an adequate kinetic model, to describe the experimentally observed methane photolysis pro ducts and high hydrocarbon polymer formation, as well as their chemica l structures. The Adequate Kinetic Model (AKM) of chemical processes i n 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 poss ible (non-prohibited) elementary reactions - the so-called maximal mec hanism. 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 chem ical species. These species are saturated and unsaturated low hydrocar bons (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 fa mily (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 procedu res including direct comparison of the theoretical data with the avail able experimental data. Some general results of this AKM are: the over all time of establishment of chemical equilibrium (i.e. complete termi nation 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 comp onents of chemical evolution is 4.3 x 10(18) molec/m(3) sec; the ''coe fficient of efficiency of the chemical machine'' converting the initia l composition into the final stable products is similar to 0.82. The f inal products consist mainly of PA similar to(30-35)%, VA similar to(2 0-25)%, PV similar to(35-40)%, equilibrium abundance of low hydrocarbo ns (C2H6, C3H8, etc.) <10% and a small amount of Allenes <5%. These re sults will be used, further, to determine the composition and structur e of Titan's three aerosol layers.