Altered Cosmic Organics as Venus' Ultraviolet Absorbers

Abstract

Venus' ultraviolet (UV) absorbers significantly contribute to the atmosphere's energy budget. However, the composition of these absorbers remains a mystery. Here, we show that mixtures of polycyclic aromatic hydrocarbons (PAHs) and iron-bearing compounds, analogs of altered cometary dust, excellently match Venus' spectra from the cloud tops to the sub-cloud atmosphere across the UV and visible wavelengths. The molecular compositions for the cloud tops (5-10 ring PAHs and ferric chloride), decomposed cloud aerosols (3-4 ring PAHs and acid ferric sulfate), and sub-cloud atmosphere (5-10 PAHs and ferric hydroxy sulfates) are consistent with the stepwise alteration of cosmic dust. These steps include sourcing of the PAHs and iron from thermally unablated and ablated dust particles, respectively, reactions with sulfuric acid in the clouds, and thermal decomposition below the clouds. Geologically short timelines of cosmic influx (minimum of 600 and 3 kyr) would respectively yield the cloud top carbon and iron concentrations. Hence, we propose a unified origin for Venus' absorbers, which may arise from cometary dust via altitude-dependent alteration pathways from the mesosphere to the surface, with PAHs serving as the primary UV absorbers and iron compounds as the dominant absorbers by mass.