The case for ice clouds on Venus

A paper by Rea and O'Leary, claiming that ice clouds are inconsistent with the near-infrared spectra of Venus, is examined. Ice crystals a few microns in diameter are found to be consistent with polarimetric observations and with all ground-based and high-altitude observations of Venus near 1.5-, 2-, and 3-μ wavelengths. However, reliable extraction of cloud parameters from the spectral details seems impossible because the uncertainties of absolute reflectivity in existing spectra are 20% or more. To explain the low 3-μ reflectivity for particles of dimensions compatible with polarization data requires a 3-μ absorption coefficient ≥ several hundred cm−1. Ice crystals satisfy this requirement without contradicting data for other wavelengths; various common carbonates, silicates, and oxides, as well as dry ice, do not. Adsorbed water also seems unlikely. The Venera 4 water vapor observations, if reliable, independently demonstrate the existence of ice clouds on Venus, and with optical depths consistent with those obtained from analysis of the near-infrared spectra. Some dust may nevertheless be mixed with the ice.