Cloud Structure and Heating Rates in the Atmosphere of Venus

Abstract

Ground-based observations and Venera 8 entry probe measurements are used to infer the vertical distribution of cloud particles in the atmosphere of Venus. In the cloud-top region, from a few millibars to a few hundred millibars pressure, the mixing ratio of cloud particles to gas increases with depth. The visible clouds are diffuse with a scale height about one-half of the gaseous atmosphere. Although the presence of significant vertical structure could escape detection by available observations, the diffuse haze appears to extend over at least 20 km in altitude. The Venera 8 measurements suggest considerable vertical structure in the deep atmosphere. A unique solution for the cloud structure is not possible, but if it is assumed that the cloud optical properties are independent of height then some characteristics of the relative cloud structure can be deduced. Under this assumption the results show a maximum cloud density near 40 km, a nearly homogeneous particle mixing in the region from ?40 to 50 km, and a fairly sharp cloud bottom near 30 km. Relative maxima in the cloud density are also implied near ?55 and 10 km, but with much greater uncertainty. From ground-based observations we find that Venus absorbs ?22.5% of the incident solar flux, ?4% in the UV (? < 0.4 µm), ?5% in the visible (0.4 < ? < 0.7 µm), and ?13.5% in the IR (? > 0.7 µm). Only ?1% of the incident flux (?5% of the absorbed flux) is associated with the UV contrast differences. Most of the solar energy is absorbed above 55 km, with the maximum heating probably near the τ = 1 level. The heating rate has a strong dependence on the cloud particle distribution, and can exhibit considerable vertical structure. The solar heating at the ground is in the range ?0.1 to 1% of the incident solar flux, unless the ground albedo is near unity.