The Structure of the Clouds of Venus: Results of the Pioneer Venus Nephelometer Experiment

The results of the nephelometer experiments conducted aboard the four probes of the Pioneer Venus mission are presented. The vertical structure of the clouds exhibits an upper haze region leading to three more or less clearly differentiated layers within the main cloud bank. Below the main cloud banks are variable series of strata followed by a lower atmosphere haze region with occasional larger concentrations of particulates at lower altitudes. The general structures, especially the middle cloud layers, are remarkably constant planetary features but do exhibit some local variation from one site to another. Concentrated sulfuric acid appears to be the principal constituent of most of the particulate matter in all of the cloud structure with the possible exceptions of the largest particles in the lower cloud layers, whose composition is still uncertain, and the hazes and particulate matter in the lower atmosphere. Near -UV radiation is absorbed throughout the cloud structure, primarily in the highest layers and, perhaps, below it. Absorbing species are not yet identified, but a combination of SO2 and an unspecified absorber look promising. Particles ejected from the surface of the planet on probe impact appear to be in the 10- to 100 mu m size range. The source of a faint glow detected by the night and north probes in the lower atmosphere is as yet uncertain.