Venus: A perspective at the beginning of planetary exploration

Abstract

In situ measurements of the Venus atmosphere, made by the entry probes Venera 4, 5, 6, and 7, and data from the Mariner 5 flyby, have provided essentially new and reliable information and have powerfully contributed to our understanding of the nearest planet. The abundances of the principal atmospheric constituents and the temperature and pressure profiles down to the Venus surface were obtained for the first time. It was shown that the atmosphere is composed primarily of CO 2 and that N 2 (if any) and H 2O are relatively minor admixtures. In the region of the Venera 7 landing, the temperature and pressure at the Venus surface were established as equal to 747 ± 20°K and 90 ± 15 kgcm −2. Space vehicles have also provided limited but quite important information on the physical properties of the Venus upper atmosphere and ionosphere, and on the interaction of the planet with the interplanetary environment. The main characteristics of the Venus atmosphere are discussed here with emphasis on the Venera results, including instrumentation, data processing, and altitude profiles. As a result of both direct measurements and analysis of refractivity and microwave data, a thermodynamic gas state in the Venus troposphere was evaluated that corresponds as a whole to adiabatic equilibrium. Nevertheless, analysis of the Venus troposphere reveals some peculiarities in the height profiles that apparently are not explained in terms of an adiabatic lapse rate in pure CO 2 for all levels where measurements were performed. Minor ingredients evaporated into the atmosphere from the hot surface could influence the thermodynamic gas state. While the data on the nature of the clouds are rather scarce, some problems on their structure and composition seem worthwhile to discuss, and in so doing we emphasize the possibilities of ice crystals and/or H 2O solutions and mixtures. A multilayered structure for the Venus clouds, composed of different volatiles at several levels, is considered to be plausible as well. The mechanism of radiative and convective heat transfer in the Venus lower atmosphere and the greenhouse and deep circulation models are discussed. A model of the Venus atmosphere up to several hundred kilometers above the surface, based on results of Venera and Mariner 5 measurements and data from ground-based optical and radio observations, is developed. Progress is radar astronomy allows a more precise understanding of the relief of the Venus surface, and Venera 7 uncovered the very first information on the strength of the surface material. Some evidence on the origin and evolution of Venus as a planet, as well as some prospective problems for further study, are also briefly discussed.