On the Origin of Ultraviolet Contrasts on Venus

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Abstract
Models for the origin of the contrasts in the ultraviolet images of Venus are examined in an attempt to determine the physical differences between light and dark regions fundamental to a clear understanding of the apparent cloud motions. To evaluate the meaning of the wavelength dependence of the contrasts, an improved determination of the spherical albedo curve for Venus in the 0.225 ≤ λ ≤ 1.06 μm range is made by fitting appropriate theoretical models to the observations of monochromatic magnitude as a function of phase angle. It is shown that, because of differences between the spectral dependences of spherical albedo and contrasts, at least one major absorber other than the one causing the contrasts is almost certainly required. A popular model employing differential Rayleigh scattering due to variations in cloud height can be ruled out, but several classes of models are compatible with present observational evidence. The contrasts and the absorption associated with them may in fact be occurring below, within or above the main visible cloud layer, and thus an unambiguous interpretation of the apparent cloud motions is not possible. Ground-based observations of the polarization for the regions of contrast may permit the field of acceptable models to be narrowed. Observations planned for the Pioneer Venus orbiter and entry probes should provide the information on local cloud properties and vertical structure necessary to reveal the physical nature of the UV markings.

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