Kinetic Pathways in the Atmospheric Chemistry of Titan – a Generalized Analysis

Abstract

Titan, the sixth Saturnine moon, is a unique celestial body in many respects, including the existence of a high-density atmosphere over a relatively small astro-physical object, chemical activity in the low-potential reducing medium, the presence of an extensive aerosol domain, etc. Despite many observations, simulation experiments and theoretical models, the general picture of Titan's atmospheric photochemistry is still imprecise. This study of the most general features of chemical activity in Titan's atmosphere by means of Generalized Kinetic Analysis (GKA) is based on the point that both the probability and efficiency of kinetic trends are estimated solely on the basis of energy/material restrictions and general kinetic laws. Only the quantity (intensity) and quality (spectrum) of the external driving force are considered closely, while both the particular kinetic demands and low internal energy resources of Titan's background are discounted. What this means is that the main inferences of GKA should be valid for any given kinetic model. Only a small part Lch of the total external energy flux Labs∼12·6 W m−2 is photochemically active Lch = (L1ion + L2ion + L1dis) + L2ch = (1·5 X 10−3 + 0·22 X 10−3 + 10·6 X 10−3) + 0·69 W m−2. The secondary energy L2ch (1440<λ<3500Å) meets the common energy requirements, while the primary energies L1ion, L2ion and L1dis define kinetic pathways of the chemical process, i.e. L1ion (790<λ<980Å) and L2ion (λ<790Å) initiate ionic photochemistry via ionization of CH4 and N2, respectively, while L1dis (980<λ<1440Å) provides photodissociation of CH4 to neutral species. Because of severe energy/material restrictions, the general chemical process proceeds in the form of a self-sustaining Diels-Alder diene low-temperature synthesis to give telomerization and polymerization. GKA proves that the main kinetic pathways (photodissociation to neutrals and charged photoionization) play different roles with respect to the quantitative and qualitative formation of the final stable products of Titan's atmospheric photochemistry. The neutral pathway governs the bulk (overall yield) of the final products while ionic chemistry is responsible for its wide chemical composition (variety of chemical species). Species identification in terms of hydrocarbon type content results in the following weight ratio composition of the final products: dienes (0·60–0·65) + saturated/unsaturated acyclic pure hydrocarbons (0·16–0·19) + tholins (0·07–0·08) + isocyclics (0·03–0·05) + miscellaneous (0·05). The elemental composition of this bulk material is (C/H/N)∼1·00/ 1·12/ 0·08.