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 L_ch of the total external energy flux L^abs∼12·6 W m⁻² is photochemically active L_ch = (L₁^ion + L₂^ion + L₁^dis) + L₂^ch = (1·5 X 10⁻³ + 0·22 X 10⁻³ + 10·6 X 10⁻³) + 0·69 W m⁻². The secondary energy L₂^ch (1440<λ<3500Å) meets the common energy requirements, while the primary energies L₁^ion, L₂^ion and L₁^dis define kinetic pathways of the chemical process, i.e. L₁^ion (790<λ<980Å) and L₂^ion (λ<790Å) initiate ionic photochemistry via ionization of CH₄ and N₂, respectively, while L₁^dis (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.