Photochemistry of the Stratosphere of Venus: Implications for Atmospheric Evolution

Abstract

The photochemistry of the stratosphere of Venus was modeled using an updated and expanded chemical scheme, combined with the results of recent observations and laboratory studies. We examined three models, with H2 mixing ratio equal to 2 × 10^(−5), 5 × 10^(−7), and 1 × 10^(−13), respectively. All models satisfactorily account for the observations of CO, O_2, O_2(^1Δ), and SO_2 in the stratosphere, but only the last one may be able to account for the diurnal behavior of mesospheric CO and the uv albedo. Oxygen, derived from CO_2 photolysis, is primarily consumed by CO_2 recombination and oxidation of SO_2 to H_2SO_4. Photolysis of HCl in the upper stratosphere provides a major source of odd hydrogen and free chlorine radicals, essential for the catalytic oxidation of CO. Oxidation of SO_2 by O occurs in the lower stratosphere. In the high-H_2 model (model A) the O-O bond is broken mainly by S + O_2 and SO + HO_2. In the low-H_2 models additional reactions for breaking the O-O bond must be invoked: NO + HO_2 in model B and ClCO + O_2 in model C. It is shown that lightning in the lower atmosphere could provide as much as 30 ppb of NO_x in the stratosphere. Our modeling reveals a number of intriguing similarities, previously unsuspected, between the chemistry of the stratosphere of Venus and that of the Earth. Photochemistry may have played a major role in the evolution of the atmosphere. The current atmosphere, as described by our preferred model, is characterized by an extreme deficiency of hydrogen species, having probably lost the equivalent of 10^2–10^3 times the present hydrogen content.