On the structure of the Venus atmosphere

Abstract

We compare the results of the Mariner 5 and Venera 4 spacecraft measurements with one another and with ground-based observations to learn about the structure and composition of the Venus atmosphere. Several independent arguments imply that the carbon dioxide mixing ratio lies between 50 and 85%. Thus some other gas, perhaps nitrogen, must be present in significant amounts. The atmosphere exhibits an adiabatic temperature gradient at temperatures in excess of 400°K and has a slightly subadiabatic value between the 400°K level and a level 15°K warmer than stratospheric temperatures. The final measurements of Venera 4 appear to have been made not close to the surface, but rather some 25 km above. According to this view, the altitude measurement by the Venera 4 radar altimeter was made when the spacecraft was almost exactly twice the reported 26 km above the surface, and so raises questions as to possible ambiguities in the radar system. A simple reflecting layer model of line formation yields pressures consistent with polarimetric and space vehicle results; the dependence of the equivalent width on phase angle is not necessarily incompatible with a modified version of this model. Estimates of pressures near the cloudtops from polarization data indicate that there is an appreciable quantity of aerosols in the stratosphere. Finally, the weak water vapor lines observed in the near-infrared are not incompatible with the presence of ice clouds.