A Determination of the Composition of the Venus Clouds from Aircraft Observations in the Near Infrared

We summarize the evidence showing that the first optical depth of the Venus cloud layer is composed of a water solution of sulfuric acid, including our earlier aircraft observations of Venus’ reflectivity in the 1–4 μm region obtained at a phase angle of 120° (Pollack et al.). Analyses of these aircraft results indicated that of all the proposed cloud candidates only a sulfuric acid solution with a concentration of 75% or more H2SO4, by weight was consistent with the observed 3 µm cloud feature. We present new aircraft observations of Venus obtained in the 1–4 µm region at a phase angle of 40° and in the 3–6 µm region at a phase angle of 136°. Comparing the two sets of observations in the 1–4 µm region, we find a striking phase effect: the reflectivity is much lower in the 3 µm region and there is a much more marked decline between 1.3 and 2.5 µm for the data obtained at the smaller phase angle. The observations made at the 40° phase angle are consistent with the theoretical behavior of a sulfuric acid cloud and imply that the sulfuric acid is present to at least many tens of optical depth below the cloud tops. Arguments concerning the concentration of the solution are reviewed and we conclude that the best current estimate is about 85% H2SO4 by weight.