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D. Crisp (Jet Propulsion Laboratory, California Institute of Technology)
Even though the H2SO4 clouds that shroud Venus reflect ~80% of the incident sunlight and absorb more than half of the rest, entry probes show that the atmosphere is well illuminated between the cloud base (~47.5 km) and the surface. For example, the Pioneer Venus Large Probe entered near the dawn terminator (7:38 AM) and measured downward solar fluxes decreasing from ~200 W/m2 to ~20 W/m2 between the cloud base and surface (Tomasko et al., JGR, 85, 1980). The Venera spectrophotometers showed that the sunlight is most intense at 0.5 <\lambda< 1.0 \mum (Ekonomov et al., VENUS, 632, 1983). At shorter wavelengths, it is attenuated by Rayleigh scattering, SO2 absorption, and the unknown, cloud-top UV absorber. Strong CO2 and H2O bands absorb most of the sunlight at \lambda >1 \mum, except in the near-IR spectral windows at 1.0, 1.1, 1.18, 1.27, 1.31, 1.74, and 2.3 \mum. The sub-cloud region is also illuminated by thermal emission from hot surface and lower atmosphere. This emission is most intense in these spectral windows, where it contributes 0.1 to 1 W/m2/sr/ \mum.
A spectrum resolving multiple scattering model was used to study the spectral and angular distribution of this radiation to assess the feasibility of descent imaging from entry probes. The radiation field is azimuthally uniform everywhere below the clouds, even when the sun is on the horizon. At \lambda < 0.6 \mum, Rayleigh scattering optical depths between the cloud base and the surface are comparable to the optical depth of the main cloud deck (\tauR~25 at 0.5 \mum). This scattering will dramatically reduce the contrast in high-altitude (>10 km) images of the surface at visible wavelengths. Fortunately, Rayleigh scattering decreases as 1/\lambda4, such that \tauR<1 at 1.0\mum. This spectral region is also relatively free of absorption by gases. It therefore may provide the best opportunity to image the surface from a falling probe.
This work is supported by the NASA Planetary Atmospheres Program.