Venus spacecraft infrared radiance spectra and some aspects of their interpretation

Fourier spectrometers aboard the Venera 15 and 16 orbiters measured the outgoing spectral radiance of Venus in the 250–1600-cm−1 (Venera 15) and 400–1600-cm−1 (Venera 16) regions with a spectral resolution of 5–7 cm−1 depending on the data handling procedure. This paper gives a short description of the experimental design and discusses those aspects that can be suitably tackled by wide range spectral measurements. Different Venusian regions can be characterized by typical and significantly different radiance and brightness temperature spectra based on the magnitude of brightness temperatures and also on secondary spectral signatures caused by H2SO4 bands as well as isotopic and hot CO2 bands. At least five groups can be identified, one typical for equatorial regions, one for mid-latitudes, and three for the subpolar and polar regions. Estimates of H2O and SO2 abundances were obtained (∼30 ppm at 58 km and ∼2 ppm at 65 km, respectively). SO2 is first identified in the infrared by our measurements. A detailed comparison between computed and measured spectra for the transition zone between mid-latitudes and the cold subpolar region gives arguments that the cloud top is situated within the temperature inversion and not above or below it.