A reexamination of the evidence for large, solid particles in the clouds of Venus

Abstract

The Pioneer Venus particle size spectrometer (LCPS) data revealed a large size (15–35 μm) mode of particles resident within the nominal H 2SO 4 cloud forming the third mode of what appears to be a trimodal size distribution. The composition of these mode 3 particles has previously been suggested as solid since an asymmetric particle was desired to interpret the LCPS particle imaging data and reduce the scattering and extinction cross-sections to more reasonable values. Recently this interpretation has been challenged, favoring instead an error or shift in the calibration of the second size range of the LCPS which removes this third size distribution mode and obviates the need for a crystalline particle species. In this paper the evidence for the existence of these mode 3 particles and their crystalline composition interpretation has been reexamined. A thorough examination of the calibration data and instrumental behavior is presented. This study suggests the following: (1) the LCPS was operating under nearly optimal “instrument health” conditions; (2) the magnitude of the required error, or shift in calibration proposed by others, is beyond what this author considers as acceptable limits; (3) calibration data with snow crystals produce distribution artifacts similar to those in the Venus data while water droplet populations do not; (4) the scattering and extinction cross-sections dominated by mode 3 particles can only be reduced by undersizing and not oversizing, a reduction of over a factor of 2 is admissable assuming mode 3 are H 2SO 4 droplets; (5) the mode 3 size distribution features persist unless unreasonable sizing errors are permitted; (6) the evidence for a third size mode is much stronger than for a crystalline species.