A Discussion of the Deep Circulation of the Atmosphere of Venus

If high temperatures ( 600 K) exist at the base of the Venus cloud layer, the question arises as to whether deep penetration of solar radiation is the only possible mechanism for their maintenance. We have therefore examined a model in which the atmosphere is completely opaque to both solar and planetary radiation but which is in motion on account of the differential heating between subsolar and antisolar points. The nature of the solution depends upon the magnitudes of the turbulent eddy coefficients of viscosity and thermal conductivity. It is argued that the geometry of the system favors moderate vertical mixing coefficients ( 104 cm2s- ) and that the inflow of solar energy is large enough to maintain an essentially non-linear boundary layer near to the cloud tops. This flow has to be supported by an upwelling motion over most of the planet, whose velocity is of such a magnitude that the deep interior currents are adiabatic. This conclusion is also valid for turbulent mixing coefficients smaller than those postulated, and for radiative transfer, provided that the radiation mean free path is less than 2 km. The less probable case of very large turbulent coefficients is also considered, and it is shown that the interior will remain adiabatic, although the nature of the flow will differ. Thus a temperature increase with depth of about 9 K km-1 is characteristic of a model in which no penetration of solar radiation takes place, and the high temperatures reported could be a simple consequence of the planetary dynamics. The model has other features closely related to the observations: (i) The predicted temperature change from subsolar to antisolar point is small relative to the absolute temperature. (ii) Coriolis forces are strong enough to exert a secondary influence on the flow despite the slow rotation rate, and a zonal thermal structure of the observed magnitude is expected. (iii) The almost complete cover of what appears to be ice clouds is comprehensible in terms of the model discussed, although it is at variance with terrestrial experience for condensation clouds. In the Appendix it is shown that these conclusions are not greatly modified if the atmosphere is predominantly water at low levels.