Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusm.p31a..07k&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #P31A-07
Physics
5405 Atmospheres (0343, 1060), 5410 Composition (1060, 3672), 5435 Ionospheres (2459), 5455 Origin And Evolution, 6281 Titan
Scientific paper
A global-mean model of coupled neutral and ion chemistry on Titan has been developed. Unlike the previous models, the model involves ambipolar diffusion and escape of ions, hydrodynamic escape of light species with molecular mass less than 20, and calculates the H2 and CO densities near the surface that were assigned previously. We tried to reduce the numbers of species and reactions in the model and remove all species and reactions that weakly affect balances of the observed atmospheric components. However, all new species observed or derived from the Cassini observations and related reactions are included in the model. Hydrocarbon chemistry is extended to bicyclic aromatic hydrocarbons (up to C12H10 for neutrals and C10H11+ for ions) but does not include PAHs. The model involves 375 reactions of 81 neutrals and 33 ions. Chemistry is driven by the solar UV and EUV photons, magnetospheric electrons, and cosmic rays. Absorption of the solar UV radiation by Titan's haze was calculated using the data on the haze particles from the optical observations at the Huygens probe and a code for the aggregate particles. Hydrocarbon, nitrile, and ion chemistries are strongly coupled on Titan. Therefore the approach in some previous models when at first hydrocarbons, then nitriles, and finally ions were calculated may result in significant error. Similarly, models of ionospheric composition may be in error because they neglect effects of ion reactions on the neutral atmosphere. The model densities of various species are in reasonable agreement with the observations. However, the calculated vertical profiles in the stratosphere are steeper than those retrieved from the CIRS limb observations. The ionosphere includes an E-layer at 700-900 km and F1-layer above 900 km with a peak electron density of 3700 cm-3 at 1120 km (SZA = 60°). A narrow peak at 80 km is due to the cosmic ray ionization. The calculated densities of major ions in the nighttime ionosphere at 1100 km are in good agreement with the observed INMS mass spectrum. Ion chemistry dominates in the production of bicyclic aromatic hydrocarbons (indene and naphthalene) above 750 km. This production peaks at 820 km where [C9H11+] = 450 cm-3. However, the major production of the polymer blocks is in the reaction C6H + C4H2 → C10H3 which peaks at 440 km. Polymerizations of HC3N and HCN peak at 320 and 220 km, respectively, and the bulk condensation of hydrocarbons occurs below 100 km. Overall, precipitation rate of the photochemical products is equal to 7.5 kg cm-2 Byr-1. Escape rates of methane and hydrogen are 2.2 and 1.5 kg cm-2 Byr-1, respectively. The ion escape is small, and we do not consider nonthermal escape processes in our model. The escape of CH4 and H2 for the age of Titan corresponds to a loss of a methane ocean 0.5 km deep and may be compared to the global-mean depth of the hydrocarbon lakes and seas of ~1 m on Titan. The model does not support the low C/N ratio observed by the Huygens ACP in Titan's haze.
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