Photochemistry of HCl and Other Minor Constituents in the Atmosphere of Venus

Abstract

The photodissociation of HCl in the atmosphere of Venus and its various implications are discussed in some detail. From a photochemical steady-state study a new and appealing viewpoint to answer problems associated with Venusian photochemistry emerges. First, the principal source for the hydrogen observed in the Venus thermosphere may be H2 produced from HCl photochemistry low in the atmosphere. Second, the reason Venus has lost most of its water (assuming that Earth and Venus had similar accretion histories), may be because water is converted to the more photo-labile HCl by reactions between the atmosphere and NACl and other minerals at the very hot Venusian surface. This HCl even today is producing H2 above the visible clouds at rates approaching those necessary to deplete H2O over geologic time. Third, the remarkable stability of the predominantly CO2 atmosphere of Venus may be due to the chlorine-atom catalysed-combination of O2, and CO by the cycle of reactions.These Cl atoms are an important steady-state product of HCl photochemistry. This catalytic cycle, which occurs above the visible clouds, will reform CO2 at rates equal to the total CO2 photodissociation rate even if it is only ?1% effective. These Cl atoms may also catalyse the known very slow oxidation of CO to CO2 by HO2 if the reactionis even moderately fast. The effect of a particulate haze extending to the 50-mb level on these various conclusions is discussed, and it is shown that such a low density haze will increase the total gaseous HCl absorption possible above the assumed cloud deck at 200 mb. If these haze particles are composed of HCl-H2O solution droplets, then UV irradiation and dissolved products of HCl photochemistry will produce the pale yellow OCl? and Cl3? ions in these droplets. Finally, we discuss the effect of including gases such as COS above the visible clouds. This gas has been proposed to be present in the lower atmosphere. Since COS would be irreversibly converted to SO2, which has a stringent spectroscopic upper limit in the visible atmosphere, we must conclude that COS is an extremely rare species in the upper atmosphere.