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We present a one-dimensional coupled ion-neutral photochemical kinetics and diffusion model to study the atmospheric composition of Titan in light of new theoretical kinetics calculations and scientific findings from the Cassini-Huygens mission. The model extends from the surface to the exobase. The atmospheric background, boundary conditions, vertical transport and aerosol opacity are all constrained by the Cassini-Huygens observations. The chemical network includes reactions between hydrocarbons, nitrogen and oxygen bearing species. It takes into account neutrals and both positive and negative ions with masses extending up to about 100 u. We incorporate high-resolution isotopic photoabsorption and photodissociation cross sections for N 2 as well as new photodissociation branching ratios for CH 4 and C 2 H 2 . Ab initio transition state theory calculations are performed in order to estimate the rate coefficients and products for critical reactions.Main reactions of production and loss for neutrals and ions are quantitatively assessed and thoroughly discussed. The vertical distributions of neutrals and ions predicted by the model generally reproduce observational data, suggesting that for the small species most chemical processes in Titan's atmosphere and ionosphere are adequately described and understood; some differences are highlighted. Notable remaining issues include (i) the total positive ion density (essentially HCNH + ) in the upper ionosphere, (ii) the low mass negative ion densities (CN -, C 3 N -/C 4 H -) in the upper atmosphere, and (iii) the minor oxygen-bearing species (CO 2 , H 2 O) density in the stratosphere. Pathways towards complex molecules and the impact of aerosols (UV shielding, atomic and molecular hydrogen budget, nitriles heterogeneous chemistry and condensation) are evaluated in the model, along with lifetimes and solar cycle variations.

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[1] We present a study of the formation and distribution of benzene (C 6 H 6 ) on Titan. Analysis of the Cassini Mass Spectrometer (INMS) measurements of benzene densities on 12 Titan passes shows that the benzene signal exhibits an unusual time dependence, peaking $20 s after closest approach, rather than at closest approach. We show that this behavior can be explained by recombination of phenyl radicals (C 6 H 5 ) with H atoms on the walls of the instrument and that the measured signal is a combination of (1) C 6 H 6 from the atmosphere and (2) C 6 H 6 formed within the instrument. In parallel, we investigate Titan benzene chemistry with a set of photochemical models. A model for the ionosphere predicts that the globally averaged production rate of benzene by ion-molecule reactions is $10 7 cm À2 s À1 , of the same order of magnitude as the production rate by neutral reactions of $4 Â 10 6 cm À2 s À1 . We show that benzene is quickly photolyzed in the thermosphere, and that C 6 H 5 radicals, the main photodissociation products, are $3 times as abundant as benzene. This result is consistent with the phenyl/benzene ratio required to match the INMS observations. Loss of benzene occurs primarily through reaction of phenyl with other radicals, leading to the formation of complex aromatic species. These species, along with benzene, diffuse downward, eventually condensing near the tropopause. We find a total production rate of solid aromatics of $10 À15 g cm À2 s À1 , corresponding to an accumulated surface layer of $3 m.

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[1] We present Cassini Ion and Neutral Mass Spectrometer (INMS) measurements of ion densities on the nightside of Titan from April 16, 2005, and show that a substantial ionosphere exists on the nightside and that complex ion chemistry is operating there. The total ionospheric densities measured both by the INMS and the Cassini Radio and Plasma Wave (RPWS) experiments on Cassini suggest that precipitation from the magnetosphere into the atmosphere of electrons with energies ranging from 25 eV up to about 2 keV is taking place. The absence of ionospheric composition measurements has been a major obstacle to understanding the ionosphere. Seven ''families'' of ion species, separated in mass-to-charge ratio by 12 Daltons (i.e., the mass of carbon), were observed and establish the importance of hydrocarbon and nitrile chains in the upper atmosphere. Several of the ion species measured by the INMS were predicted by models (e.g., HCNH + and C 2 H 5 + ). But the INMS also saw high densities at mass numbers not predicted by models, including mass 18, which we suggest will be ammonium ions (NH 4 + ) produced by reaction of other ion species with neutral ammonia.

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