The ultraviolet absorber on Venus: Amorphous sulfur

The large backscattering cross section of the particles composing the upper clouds on Venus suggests that a small quantity of high refractive index material is present in the clouds. We propose that this material is elemental sulfur and that sulfur also accounts for the absorption of uv-visible radiation at wavelengths outside of the SO₂ absorption bands. A physical-chemical model of the clouds shows that sulfur, with a mass comparable to that of the observed Mode 1 particles, can be produced in oxygen-poor regions of the upper clouds and in rising air columns. Sulfur production from SO₂ can be rapid, which explains the observed correlation between SO₂ and the uv absorber. The sulfur is properly located to be the uv absorber uv absorber since its calculated concentration rapidly increases with depth in the upper clouds, but it is largely absent in the middle and lower clouds. Sulfur nucleation provides a means of generating the observed bimodal particle size distribution in the upper clouds. Chemical modeling shows that the sulfur vapor is rich in short-chain allotropes such as S₃ and S₄. These allotropes have absorption bands centered near 4000 and 5300 Å, respectively. We suggest that the sulfur particles on Venus are largely composed of S₈, but also contain a few percent of S₃ and S₄. Such particles could account for the wavelength dependence of the albedo of Venus and for the solar energy deposition profile in the clouds. These allotropes are metastable and relax to S₈ over periods of hours to days, providing a simple explanation for the relatively short lifetime of the uv absorber.