Venera 9 and 10: Thermal radiometry

Abstract

New data about the top clouds of Venus were obtained during the radiometric experiment on-board the Venera 9 and Venera 10 orbiters. A diurnal component of the ir thermal radiation was determined for the latitude range −40, +50°. The brightness temperature of radiation referred to the normal was measured; it was 244°K at night and 239°K at the subsolar point for the 7- to 13-, 17- to 30-μm bands. Minimum temperatures correspond to the meridian of local time 16.00 h and are 232°K. There is also a zone of lower temperatures in the region of local time 7.5 h. Absolute temperatures were measured with an accuracy of −1.9° +1.2°. Thermal radiation has no distinct latitudinal dependence but has a day-night asymmetry, with the night radiation flux exceeding that on the day side by 17%. The limb-darkening law for thermal radiation is rather complicated, depending on the time of day. There are at least two states of the radiating cloud cover: day and night. The extinction coefficient is close to 0.24 km −1. The analysis shows that the source function of the medium is close to Planck's function. During the day the flux of thermal radiation is assumed to be weakened by an aerosol medium forming by photochemical processes. Comparison of experimental and calculated data yields a particle concentration in the radiating cloud cover of about 95 cm −3. Experimental data and the results of ground-based measurements were used to determine the radiometric albedo of Venus, 0.79 −0.01 +0.02.