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

TL;DR

This study models the UV-blue absorbance of Venusian cloud aerosols, suggesting that highly efficient organic compounds at high concentrations could explain observed spectral features, guiding future missions to test this hypothesis.

Contribution

It introduces a model linking UV-blue absorbance in Venus's clouds to high-concentration organic absorbers, providing constraints and guiding future observational tests.

Findings

High bulk absorbance at 375 nm (A375 nm = 2942 per cm) implies presence of efficient organic absorbers.

Inorganic absorbers are less likely due to insufficient absorption coefficients at relevant concentrations.

Upcoming missions will test the organic absorber hypothesis in Venus's clouds.

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 bulk absorbance with a peak at A375 nm is 2942 per cm. This extremely high absorbance implies the presence of a highly efficient absorber, for example, conjugated organics, at relatively high concentration, e.g., approx. 25 g/L for porphyrin-type pigments with molar absorption coefficient of approx. 100,000 per mole per cm. Inorganic absorbers, typically below 10,000 per mole per cm, 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 upcoming Rocket Lab mission will test the hypothesis of organics in Venus's clouds.