The VMC/VEx photometry at small phase angles: Glory and the physical properties of particles in the upper cloud layer of Venus

With the Venus Monitoring Camera onboard the Venus Express orbiter, for the first time full glory on the upper cloud layer of Venus has been imaged. Images were acquired in three wavelengths of 0.365, 0.513, and 0.965 μm. Glory is an optical phenomenon that poses stringent constraints on the cloud properties. The fact of the observed glory itself requires that the scattering medium at a level where the radiance comes from is rather homogeneous, its particles are spherical and their size distribution is narrow. From the angular position of the glory features in specified wavelengths, one may infer the size of scattering particles, which yields the estimate of their effective radius Reff from 1.0 to 1.4 μm for different regions of the cloud deck. This corresponds to the so-called 1-μm mode of the cloud particles of Venus, which, as is known from the previous investigations, are droplets of concentrated sulfuric acid. However, in some cases, the details of the glory feature cannot be explained by scattering at purely sulfuric acid droplets and require presence of an additional component with a higher value of the real part of the refractive index. We suppose that this material can be ferric chloride or sulfur; both are also candidates for the so-called unknown ultraviolet absorber in the upper cloud layer of Venus. We suggest that this component is in the submicron particles that, under proper conditions, participate in the condensation of sulfuric acid droplets in the clouds and form the complex UV absorbing particles with an increased real refractive index. For a number of UV dark and bright regions observed at small phase angles, it is shown that UV contrasts are caused by variations in the portion of absorbing particles of the submicron mode in the clouds. From changing of the angular position of the glory maximum in the UV phase profiles, we infer a decrease of the sizes of sulfuric acid droplets (Reff changes from 1.05 to 0.8-0.9 μm) with increasing latitude (from 40 °S to 60 °S) before the local noon. The increase in the amount of 0.9-μm particles may also cause the UV-bright feature often observed at about 50 °S.