Photochemistry of Saturn's Atmosphere. I. Hydrocarbon Chemistry and Comparisons with ISO Observations

Abstract

To investigate the details of hydrocarbon photochemistry on Saturn, we have developed a one-dimensional diurnally averaged model that couples hydrocarbon and oxygen photochemistry, molecular and eddy diffusion, radiative transfer, and condensation. The model results are compared with observations from the Infrared Space Observatory (ISO) to place tighter constraints on molecular abundances, to better define Saturn's eddy diffusion coefficient profile, and to identify important chemical schemes that control the abundances of the observable hydrocarbons in Saturn's upper atmosphere. From the ISO observations, we determine that the column densities of CH ₃, CH ₃C ₂H, and C ₄H ₂ above 10 mbar are 4 ⁺²_-1.5×10 ¹³ cm ^-2, (1.1±0.3)×10 ¹⁵ cm ^-2, and (1.2±0.3)×10 ¹⁴ cm ^-2, respectively. The observed ISO emission features also indicate C ₂H ₂ mixing ratios of 1.2 ^+0.9_-0.6×10 ^-6 at 0.3 mbar and (2.7±0.8)×10 ^-7 at 1.4 mbar, and a C ₂H ₆ mixing ratio of (9±2.5)×10 ^-6 at 0.5 mbar. Upper limits are provided for C ₂H ₄, CH ₂CCH ₂, C ₃H ₈, and C ₆H ₂. The sensitivity of the model results to variations in the eddy diffusion coefficient profile, the solar flux, the CH ₄ photolysis branching ratios, the atomic hydrogen influx, and key reaction rates are discussed in detail. We find that C ₄H ₂ and CH ₃C ₂H are particularly good tracers of important chemical processes and physical conditions in Saturn's upper atmosphere, and C ₂H ₆ is a good tracer of the eddy diffusion coefficient in Saturn's lower stratosphere. The eddy diffusion coefficient must be smaller than ∼3×10 ⁴ cm ² s ^-1 at pressures greater than 1 mbar in order to reproduce the C ₂H ₆ abundance inferred from ISO observations. The eddy diffusion coefficients in the upper stratosphere could be constrained by observations of CH ₃ radicals if the low-temperature chemistry of CH ₃ 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 ∼1 and 300 mbar in our model and will dominate stratospheric haze formation at nonauroral latitudes. Our photochemical models will be useful for planning observational sequences and for analyzing data from the upcoming Cassini mission.