Structure of the lower atmosphere of Venus

Abstract

If the centimeter microwave emission from Venus arises from its surface, the radar reflectivities and microwave brightness temperatures give mean darkside surface temperatures of about 640° K. Extrapolations of the phase data to small phase angles indicate mean brightside surface temperatures of about 750° K. If the cloudtop pressures and temperatures are known in both hemispheres, the surface pressures and darkside subadiabatic indices can be derived. A reanalysis of the CO 2 absorption bands near 0.8 and 1.6 μ and of the Regulus occultation data indicates: (1) that the same cloud level, at T c ∽234° K, is responsible for the reflection and emission throughout the visible and infrared, and (2) that the brightside cloudtop pressure is at least as great as the darkside cloudtop pressure, the most probable values being 0.6 atm and 90 mb, respectively. Even with a small phase effect these cloudtop pressures give surface pressures ⋍50 atm. The darkside lapse rates are substantially subadiabatic, in contradiction to the Aeolosphere Model. Failure of the Urey equilibrium on Venus results in surface pressures of this order or greater; and similar values are obtained from the atmospheric structure deduced from Spinrad's measurements of the near infrared CO 2 band at 7820 Å. The altitude of the cloudtops on the dark side is then ⋍80 km, and is possibly even higher in the bright hemisphere. The surface pressures and phase effect lead to a sidereal period of rotation which exceeds 170 days, and is quiet possibly equal to the period of revolution. For nonsynchronous rotation, the specific heat capacity of the atmosphere controls the nocturnal cooling. There is a smaller contribution from subsurface conduction. For synchronous rotation, the atmospheric circulation must supply the radiation emitted to space from the dark hemisphere. The effect of Rayleigh scattering on a cloudless day on Venus is to yellow the sky and redden the sun. The radiation scattered back to space will also have a yellow cast, and may explain the apparent color of Venus. The color index should therefore be a function of phase. In short visual wavelengths, the surface of Venus cannot be seen from space, even on a cloudness day. The observations of permanent dark markings at these wavelenghts possibly represent clouds connected with surface features far below; they cannot be the surface features themselves. But near infrared photography has the promise of detecting surface markings on Venus. The high surface temperatures and pressures lead to melting and vaporization of surface material, and to greatly enhanced infrared opacities, facilitating the operation of the Greenhouse Effect on Venus. Direct exploration of the surface of Venus would seen to be a very difficult engineering problem.