Models of the Venus Clouds

Abstract

This paper describes an attempt to find a self-consistent radiative-convective model of the Venus clouds, with the sun as the only external source of heat. The transfer of heat and cloud particles by thermal convection are treated by means of Priestley's theory of free convection and Kraichnan's theory of a convective boundary layer. Approximate equations are used to describe solar and thermal radiative transfer. We constrain the model to give the correct planetary albedo, and suitable line-formation properties, and the cloud particles conform to Mie's theory. We find that a transition region, from fully developed turbulence to radiative control, exists at the cloud tops. There is only negligible diurnal variation of temperature inside the cloud. The constraints imposed are so restrictive that no type of cloud is fully satisfactory. Dust clouds do not convect at depth. High vapor pressure condensates, such as water, cannot satisfy optical constraints. Low vapor pressure condensates do not convect at all levels in the cloud, which could not therefore exist in a steady state. We conclude that the Venus clouds are not in a local radiative-convective steady state. The possibility of an internal planetary heat source is found unattractive as an alternative. It is, however, plausible that the interior of the cloud is dominated by planetary-scale dynamical heat transfer while the cloud top conforms to the model described in this paper. Some properties of the cloud top are described.