Venus phase function and forward scattering from [H.sub.2]S[O.sub.4]

Ground-based and spacecraft photometry covering phase angles from 2[degrees] to 179[degrees] has been acquired in wavelength bands from blue to near infrared. An unexpected brightness surge is seen in the B and V bands when the disk of Venus is less than 2% illuminated. This excess luminosity appears to be the result of forward scattering from droplets of [H.sub.2]S[O.sub.4] (sulfuric acid) in the high atmosphere of Venus. The fully sunlit brightness of Venus, adjusted to a distance of one AU from the Sun and observer, was found to be V = -4.38, and the corresponding geometric albedo is 67%. The phase integral is 1.35 and the resulting spherical albedo is 90%. Comparison between our data and photometry obtained over the past 50 years indicates a bias in the older photoelectric results, however atmospheric abundance variations suggest that brightness changes may have occurred too. Keywords: Venus; Atmospheres; Photometry