A Model of UV-Blue Absorbance in Bulk Liquid of Venusian Cloud Aerosols Is Consistent with Efficient Organic Absorbers at High Concentrations

Abstract

At visible wavelengths, Venus appears serene and pale-yellow, but since the 1920s, observers have noted high-contrast features in the ultraviolet. These features track the about 4-day superrotation of the upper cloud deck and vary widely over time and space. The identity of the UV absorber(s), active between at least 280 and 500 nm, remains unknown, as no proposed candidate fully matches all observational data. From remote observations of Venus, and accounting for light scattering by sub-micrometer droplets, we modeled the 365-455 nm absorbance per cm of the bulk liquids forming Venus's clouds. Assuming a uniform distribution in mode 1 and 2 particles across a 6 km layer below the cloud top at 65 km, we constrain the we constrained the decadic absorption coefficient within the modeled range to a peak at 375 nm to be 1278 per cm. This extremely high absorption coefficient implies the presence of a highly efficient absorber, for example, conjugated organics, at relatively high concentration, e.g., approx. 12 g/L for porphyrin-type pigments with molar absorption coefficient of approx. 100,000 per mole per cm. Inorganic absorbers, typically below 10,000 would either need to constitute a large portion of the aerosols or still not be sufficiently light-absorbing, even in pure form. We emphasize that all candidate absorbers must be evaluated against Venus's reflectance curve using (i) known molar absorption coefficients, (ii) realistic atmospheric distributions, and (iii) appropriate particle size distributions. The planned Rocket Lab mission will test the hypothesis of organics in Venus's clouds.