Are the clouds of venus sulfuric acid?

Abstract

Water solutions of sulfuric acid, containing about 75% H 2SO 4 by weight, have a refractive index within 0.01 of the values deduced from polarimetric observations of the Venus clouds. These solutions remain liquid at the cloud temperature, thus explaining the spherical shape of the cloud particles (droplets). The equilibrium vapor pressure of water above such solutions is 0.01 that of liquid water or ice, which accounts for the observed dryness of the cloud region. Furthermore, H 2SO 4 solutions of such concentration have spectra very similar to Venus in the 8–13 μm region; in particular, they explain the 11.2 μm band. Cold sulfuric acid solutions also seem consistent with Venus spectra in the 3–4 μm region. The amount of acid required to make the visible clouds is quite small, and is consistent with both the cosmic abundance of sulfur and the degree of out-gassing of the planet indicated by known atmospheric constituents. Sulfuric acid occurs naturally in volcanic gases, along with known constituents of the Venus atmosphere such as CO 2, HCl, and HF ; it is produced at high temperature by reactions between these gases and common sulfate rocks. The great stability and low vapor pressure of H 2SO 4 and its water solutions explain the lack of other sulfur compounds in the atmosphere of Venus—a lack that is otherwise puzzling. Sulfuric acid precipitation may explain some peculiarities in Venera and Mariner data. Because sulfuric acid solutions are in good agreement with the Venus data, and because no other material that has been proposed is even consistent with the polarimetric and spectroscopic data, H 2SO 4 must be considered the most probable constituent of the Venus clouds.