Scattering of visible radiation around the spherical atmosphere of Venus

Abstract

Horak and Harris found that the observed visual brightness of Venus at phase angles α > 120° exceeds the predicted theoretical brightness for isotropic and Rayleigh phase functions. Following a suggestion by Harris that the excess brightness is caused by the diffusion of radiation around the edge of the spherical atmosphere of Venus, we have made approximate computations of the flux of this diffused radiation at various phase angles by assuming a Rayleigh phase function. By an appropriate geometrical consideration we have attempted to resolve this spherical problem into a series of separate problems which can be treated by the ordinary plane-parallel technique. This procedure allows us to utilize the available tables of scattering functions for plane Rayleigh atmospheres of different optical thicknesses. The standard model of Kaplan and a similar new model based on the recent Venus 4 and Mariner V data were used for obtaining the required optical depths. It is found that the computed fluxes account for only a small part of the excess observed brightness for α > 120°. It is concluded that the main contribution to the observed brightness of Venus at inferior conjunction comes from particulate matter, which scatters one order of magnitude more efficiently in the forward direction than a Rayleigh scatterer. From the variation of the efficiently factor with color, which is partly caused by the variation of phase function with wavelength and partly by the variation of extinction coefficient with wavelength, some conclusions about the possible nature of the scattering particles are derived. The effective geometrical thickness of the scattering layer is found to be 15 to 30 km, in good agreement with the minimum thickness of 15 km derived by Schilling and Moore from the observed cusp extensions of Venus.