New photochemical model of Saturn's atmosphere

Abstract

A new photochemical model of Saturn's atmosphere, which includes hydrocarbon (up to C ₄ compounds) and oxygen compounds, is presented. This model derives from Neptune's model established by Dobrijévic et al. (2000. Planet. Space Sci., submitted). In this one-dimensional model, we consider vertical transport driven by molecular and eddy diffusion. Downward flux of atomic hydrogen from the upper atmosphere and the ionosphere, and external fluxes of oxygenated material are included. Concerning hydrocarbons, calculated abundances agree with observations for methane and acetylene, whereas the ethane and methyl radical disagree. Methyl (CH ₃) abundance is widely overestimated, suggesting an underevaluation of methyl recombination rates. Moreover, further investigations of the chemical scheme and dynamics are needed. Water and carbon dioxide, whose column abundances in Saturn's stratosphere have been inferred by ISO observations ( Feuchtgruber et al., 1997. Nature 389, 159-162), originate in an external flux of oxygenated material. We find an agreement with ISO values ( η_{H 2O =6.8×10 ¹⁴ molecules cm ^-2 and ηCO 2=7.8×10 ¹⁴ molecules cm ^-2) for an external flux of material with a cometary-like composition (Φ H 2O =10 ⁶ molecules cm ^-2 s ^-1, Φ CO=2×10 ⁵ molecules cm ^-2 s ^-1 and Φ CO 2=6×10 ⁴ molecules cm ^-2 s ^-1). These external fluxes cannot reproduce the abundance of CO inferred by Noll and Larson (1991. Icarus 89, 168-189), suggesting that CO has an internal origin. More generally, we find that a total influx of oxygen atoms between 10 ⁶ and 10 ⁷ atoms cm ^-2 s ^-1 is necessary to reproduce oxygen compound abundances. The precise determination of this flux depends on the nature of the entering materials.}