Measurements of the flux of sunlight in the atmosphere of Venus

Abstract

The solar flux radiometer (LSFR) aboard the Pioneer Venus (PV) sounder probe measured the intensity of sunlight in five directions to the vertical using narrow angular fields of view. The measurements in a narrow spectral channel (0.59–0.67 µm) and two broad channels (0.4–1.0 and 0.4–1.8 µm) were reduced to yield the profiles of upward and downward solar flux with a vertical resolution of 100–500 m. All the flux profiles show three distinct cloud layers with bottoms at altitudes of 57.5, 49.7, and 47.9 km. The azimuthal structure of the intensity samples at the highest altitudes implies a cloud optical depth between 3.5 and 4.0 above 64.3 km. The narrow‐band data were interpreted to give the optical depths and single scattering albedos of the clouds at a wavelength of 0.63 µm. The results for the upper cloud are generally consistent with those reported by the cloud particle size spectrometer (LCPS) experiment on PV. In the middle and lower cloud the LSFR data imply that the largest particle size mode has about one third the optical depth deduced from the LCPS data under the assumption that these particles are spherical. The total optical depth of the clouds was found to be about 25. Models of the broad‐band fluxes confirm these conclusions and indicate that about half the sunlight absorbed by Venus is absorbed above our first measurements. In addition, absorption of solar radiation occurs in the upper cloud and below 35 km; the middle and lower cloud layers absorb remarkably little sunlight. Models consistent with our measured flux profiles and the spherical albedo of Venus were used to scale our measured net fluxes to a bolometric globally averaged solar energy deposition profile, assuming that the sounder site is typical of planetwide conditions. Averaged over the planet, about 17 W/m² are absorbed at the ground (some 2.5% of the total solar energy incident on the planet). The solar net flux profile in the lower atmosphere is not very sensitive to changes in the thickness of the nearly conservative lower cloud.