Measurements of the Venus lower atmosphere composition: A comparison of results

From the Pioneer Venus, as well as the Venera 11 and 12 spacecrafts, data on the chemical composition of the Venus atmosphere from about 700 km to the surface were obtained. In general, reasonable consistency can be found among the results from the seven gas analyzers that encountered Venus in December 1978. One of the more significant findings is the large excess abundance of primordial gases on Venus. The O-2 abundances below 30 km must be very small, since none of the instruments detected O-2 there. In and near the clouds the 02 mixing ratio may be as much as a few tens of parts per million. The CO mixing ratio is 20-30 ppm. The existence of COS, originally reported to be a major sulfur compound in the lowest part of the atmosphere, is doubtful there, and its mixing ratio at higher altitudes does not exceed a few parts per million. On the other hand, SO2 may exist below the clouds at a mixing ratio of the order of 100 ppm. In the clouds it is less than 10 ppm. A serious discrepancy in the data still exists for the case of the water vapor abundance. As a result of the cross-checks provided by the number and types of instruments flown to Venus, confidence in the validity of these results on its atmospheric composition has progressed to a reasonable degree of certitude.