Composition of the Venus lower atmosphere from the Pioneer Venus Mass Spectrometer

Data from the Pioneer Venus sounder probe neutral mass spectrometer confirm that the major constituents of the lower atmosphere of Venus are CO2 and N-2, with the latter having a mass abundance of about 3%. The data allow the order of up to 1000 ppm of water vapor and 500 ppm of SO2 and COS below the clouds, but lower bounds for the abundances of these compounds cannot be established because they appeared in large amounts as lingering products of the reaction of cloud droplets deposited on the gas inlet leak to the mass spectrometer. The presence of several minor constituents on Venus is more certain. Of particular significance is the discovery that there is a hundred-fold excess of nonradiogenic argon and neon on Venus over terrestrial abundances in addition to a small deficit of radiogenic '' Ar. The isotopic ratios of carbon, oxygen, and nonradiogenic argon are similar to those on earth. Other minor constituents detected on Venus include helium and ethane, and there is strong, but presently inconclusive, evidence for H2S. Through laboratory simulation it has been shown that the measured pattern of volatile release during the evaporation of cloud materials blocking the inlet leak correspond to the volatiles produced when a leak is coated with an 85% concentration solution of H2SO4 and immersed in a Venus atmosphere simulator.