The decrease in the reflectivity of Venus in the near UV can be explained if the clouds contain particles of elemental sulfur in addition to sulfuric acid. The low-resolution McDonald-Pittsburgh spectrum can be fitted by two sulfur-containing, multiple-scattering cloud models: 1) a mixed cloud consisting of one particle of elemental sulfur of radius 10 µm for every 670 particles of sulfuric acid of radius 1 µm, and 2) a layered cloud of optical thickness τ = 1.0 consisting of 1 µm particles of sulfuric acid overlying a thick cloud of elemental sulfur particles of radius 3.6 µm: Some of the sulfur is incompletely polymerized. The source of the sulfur is photo-dissociation of COS, although some may also be recycled from the lower atmosphere. The sulfur plays a crucial role in the planetary meteorology of Venus since it is responsible for the bulk of the absorption of solar energy.

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