A Search for the Near-Surface Particulate Layer Using Venera 13 In Situ Spectroscopic Observations

Whether or not there is a particulate layer in the lowest 10 km of the Venusian atmosphere is still an open question. Some of the past in situ experiments showed the presence of a detached particulate layer, and a few suggested the existence of finely dispersed aerosols, while other instruments supported the idea of no particulate matter in the deep atmosphere. In this work, we investigate the presence of a near-surface particulate layer (NSPL) using in situ data from the Venera 13 mission. While the original spectrophotometric data from Venera 13 were lost, we have reconstructed a part of this data by digitizing the old graphic material and selected the eight most reliable Venera 13 downward radiance profiles from 0.48 to 0.8 $\mu$m for our retrievals. The retrievals suggest the existence of the particulate layer with a peak in the altitude range of 3.5–5 km. They further indicate a log-normal particle size distribution with a mean radius between 0.6 and 0.85 $\mu$m. The retrievals constrain the real refractive index of the particles to lie around the range of 1.4–1.6, with the imaginary refractive index of a magnitude of ${10}^{-3}$. Based on refractive index retrievals, uplifted basalt particles or volcanic ash could be responsible for near-surface particulates. In comparison, volatile condensates appear less likely to be behind the formation of NSPL.