Photochemistry of Saturn's atmosphere - I. Hydrocarbon chemistry and comparisons with ISO observations

To investigate the details of hydrocarbon photochemistry on Saturn, we have developed a one-dimensional diurnally averaged model that couples hydrocar bon and oxygen photochemistry, molecular and eddy diffusion, radiative tran sfer, and condensation. The model results are compared with observations fr om the Infrared Space Observatory (ISO) to place tighter constraints on mol ecular abundances, to better define Saturn's eddy diffusion coefficient pro file, and to identify important chemical schemes that control the abundance s of the observable hydrocarbons in Saturn's upper atmosphere. From the ISO observations, we determine that the column densities of CH3, CH3C2H, and C 4H2 above 10 mbar are 4(-1.5)(+2) x 10(13) cm(-2), (1.1 +/- 0.3) x 10(15) c m(-2), and (1.2 +/- 0.3) x 10(14) cm(-2), respectively. The observed ISO em ission features also indicate C2H2 mixing ratios of 1.2(-0.6)(+0.9) x 10(-6 ) at 0.3 mbar and (2.7 +/- 0.8) x 10(-7) at 1.4 mbar, and a C2H6 mixing rat io of (9 +/- 2.5) x 10(-6) at 0.5 mbar. Upper limits are provided for C2H4, CH2CCH2, C3H8, and C6H2. The sensitivity of the model results to variation s in the eddy diffusion coefficient profile, the solar flux, the CH4 photol ysis branching ratios, the atomic hydrogen influx, and key reaction rates a re discussed in detail. We find that C4H2 and CH3C2H are particularly good tracers of important chemical processes and physical conditions in Saturn's upper atmosphere, and C2H6 is a good tracer of the eddy diffusion coeffici ent in Saturn's lower stratosphere. The eddy diffusion coefficient must be smaller than similar to 3 x 10(4) cm(2) s(-1) at pressures greater than 1 m bar in order to reproduce the C2H6 abundance inferred from ISO observations . The eddy diffusion coefficients in the upper stratosphere could be constr ained by observations of CH3 radicals if the low-temperature chemistry of C H3 were better understood. We also discuss the implications of our modeling for aerosol formation in Saturn's lower stratosphere-diacetylene, butane, and water condense between similar to 1 and 300 mbar in our model and will dominate stratospheric haze formation at nonauroral latitudes. Our photoche mical models will be useful for planning observational sequences and for an alyzing data from the upcoming Cassini mission. (C) 2000 Academic Press.