Photochemical Modeling of the Venus Middle Atmosphere

F.P. Mills, M.A. Allen, Y.L. Yung, A. Lin (Caltech)

The primary photochemical cycle of the Venus middle atmosphere is the photolysis of CO tex2html_wrap_inline23 to form CO and oxygen atoms on the dayside, and the re-formation of CO tex2html_wrap_inline23 from CO and oxygen via catalytic cycles. Previous modeling used ClO tex2html_wrap_inline27 [Krasnopolsky & Parshev 1983, Yung & DeMore 1982], SO tex2html_wrap_inline27 [Winick & Stewart 1980], and HO tex2html_wrap_inline27 [Sze & McElroy 1975] radicals to catalyze the re-formation of CO tex2html_wrap_inline23 . These models qualitatively explained the stability of Venus' CO tex2html_wrap_inline23 atmosphere, but, despite the powerful catalytic cycles introduced, none could quantitatively explain either the low column abundance of molecular oxygen (two-sigma upper limit of tex2html_wrap_inline37 molecules cm tex2html_wrap_inline39 depending on the assumed altitude for optical depth unity, Trauger & Lunine 1983) or the intense nightside airglow in the O tex2html_wrap_inline23 ( tex2html_wrap_inline43 ) band.

We have developed a revised one-dimensional, steady-state model based on the latest kinetic and photoabsorption data and observations of the abundances of HCl, SO tex2html_wrap_inline23 , SO, and H tex2html_wrap_inline23 O. The vertical eddy diffusion profile and the abundance of SO tex2html_wrap_inline23 at the lower boundary (58-km altitude) were adjusted to simultaneously reproduce (within the stated error bars and temporal/spatial variability) the retrieved SO profile [Na et al. 1994], the retrieved SO tex2html_wrap_inline23 abundance and scale height at the cloud tops [Na et al. 1994], and the retrieved CO profile [Clancy & Muhleman 1991]. Using only gas-phase chemistry, the predicted column abundance of molecular oxygen (above 58-km altitude) has been reduced to tex2html_wrap_inline53 molecules cm tex2html_wrap_inline39 by adjusting the rates for selected reactions within their one-sigma uncertainties. Although still larger than the observed upper limit, this column abundance is a factor of 10 smaller than would have been predicted using the Yung & DeMore 1982 model with the currently accepted abundance for HCl (0.4 ppm, Connes et al. 1967, Pollack et al. 1993).

Our preliminary results affirm the importance of the ClO tex2html_wrap_inline27 catalytic cycles in the chemistry of the Venus middle atmosphere but imply some additional catalytic process, such as heterogeneous chemistry or BrO tex2html_wrap_inline27 chemistry, may also be important.