Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.3119k&link_type=abstract
American Astronomical Society, DPS meeting #40, #31.19; Bulletin of the American Astronomical Society, Vol. 40, p.450
Mathematics
Logic
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, and calculates the H2 density near the surface that was assigned previously. We tried to reduce the numbers of species and reactions in the model and remove all species and reactions that weakly affect the observed species. Hydrocarbon chemistry is extended to C12H10 for neutrals and C10H11+ for ions but does not include PAHs. The model involves 386 reactions of 83 neutrals and 33 ions, effects of magnetospheric electrons and cosmic rays. UV absorption by Titan's haze was calculated using the Huygens observations and a code for aggregate particles. Hydrocarbon, nitrile, and ion chemistries are strongly coupled on Titan, and attempt to calculate them separately (e.g. in models of ionospheric composition) may result in significant error. The model densities of various species are typically in good agreement with the observations except vertical profiles in the stratosphere that are steeper than the CIRS limb data. Influx of O+ 106 cm-2 s-1 from Saturn's magnetosphere is sufficient to support CO at the observed level of 50 ppm without a surface source. The ionosphere includes a peak at 80 km formed by the cosmic rays, a steplike layer at 700-900 km and a peak at 1120 km (SZA = 60º). Nighttime densities of major ions agree with the INMS data. Ion chemistry dominates in the production of bicyclic aromatic hydrocarbons (indene and naphthalene) above 750 km. Precipitation rate of the photochemical products by polymerization and condensation is capable to fill the observed lakes and seas for a geologically short period of 10-100 Myr. The model does not support the low C/N ratio observed by the Huygens ACP in Titan's haze.
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