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.

Atmospheric composition, chemistry, and clouds

“…In addition, these other absorbers must explain the phase angle dependence of the uV dark markings as well as their short lifetime above the clouds (from hours to days, see Esposito et al [1983]). They must also be consistent with the visible-wavelength solar flux observations of Tomasko et al [1980] which show absorption at 58-62 km, and little absorption below. near uV solar flux absorption results from VEnErA 14 [Ekonomov et al, 1983;1984] provide an additional constraint.…”

Venus Atmospheric Thermal Structure and Radiative Balance

“…Several comprehensive radiative transfer models consistent with the data were developed at that time (Tomasko et al 1980a. The authors calculated the global balance of radiative energy and solar heating rates in the atmosphere.…”

“…The measurements of the vertical profiles of downward solar fluxes by the Venera and Pioneer-Venus descent probes showed that UV and blue radiation are strongly absorbed in the upper cloud layer above 57 km. To find the global mean net solar flux, numerical models must be used to extend the spectral and spatial coverage provided by the experimental results (Tomasko et al 1980a;. The measurements and subsequent radiative transfer modeling revealed two regions where deposition of the solar energy is significant.…”

“…The solar flux radiometer (LSFR) aboard the Pioneer Venus Large Probe measured solar fluxes during descent in a series of channels covering a broad range from 0.4 through 1.8 μm (Tomasko et al 1980a). The Venera-13 and 14 descent probes also carried a photometric experiment to measure the distribution of UV radiation (Ekonomov et al 1984).…”

Multiple Equilibrium States Appearing in a Venus-Like Atmospheric General Circulation Model

“…The heating rate is almost equal to that used in YT in the cloud layer, but the value is small about onesixth below the cloud layer, because YT's heating rate is too large for the real Venus near the ground. The real heating rate is estimated as about 3.0 × 10 −3 K day −1 at 0−10 km altitude, and the heating rate provided in this study is, nonetheless, too high for the real Venus near the ground (Tomasko et al 1980). Figure 2 shows the reference temperature T 0 (z), which is based on an observed value (Seiff et al 1980).…”