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Bruce Hapke

Abstract

The elimination of the residual image problem in the Mariner 10 vidicons allowed photometry of moderately high accuracy to be carried out during the 5 February 1974 flyby of Venus. Photometry of celestial objects allowed in-flight verification of camera linearity, shading and absolute photometric calibration. Due to the rapid rotation of the upper atmosphere the planet exhibits temporal brightness variations in the UV greater than 10% over a few hours. The observed terminator is 4° past the geometric terminator because of the detached haze layers at altitudes around 85 km. No indications of local cloud-top elevation variations greater than a few hundred meters were found in the Mariner 10 data. The cloud tops are probably diffuse. The distribution of brightness across the planet at 23° phase angle is described better by a cloud of isotropic scatters than by Mie spheres. In the UV the bright and dark regions both have low albedos in all scales, showing that the UV absorber is not just confined to the dark markings. Correlated contrasts can be seen on frames taken through the ultraviolet (UV), blue and orange filters. The outlines of the UV markings are diffuse even at the highest resolution; the contrast gradients are characterized by lengths of the order of 10–15 km. Regions which are brighter in the UV also have higher polarizations than darker areas. A cloud model which is consistent with the Mariner 10 observations has a third type of particle present in the clouds in addition to atmospheric gas molecules and sulfuric acid droplets. This particle is UV-absorbing, scatters nearly isotropically and is weakly polarizing; elemental sulfur has these properties. The UV absorbers are well-mixed vertically but incompletely mixed horizontally, thus causing the UV markings. The UV markings apparently represent stratospheric rather than tropospheric processes. There is little in the Mariner 10 pictures to suggest evaporation or condensation processes or strong horizontal wind shears.

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Bruce Hapke and Robert Nelson

Abstract

The decrease in the reflectivity of Venus in the near UV can be explained if the clouds contain particles of elemental sulfur in addition to sulfuric acid. The low-resolution McDonald-Pittsburgh spectrum can be fitted by two sulfur-containing, multiple-scattering cloud models: 1) a mixed cloud consisting of one particle of elemental sulfur of radius 10 µm for every 670 particles of sulfuric acid of radius 1 µm, and 2) a layered cloud of optical thickness τ = 1.0 consisting of 1 µm particles of sulfuric acid overlying a thick cloud of elemental sulfur particles of radius 3.6 µm: Some of the sulfur is incompletely polymerized. The source of the sulfur is photo-dissociation of COS, although some may also be recycled from the lower atmosphere. The sulfur plays a crucial role in the planetary meteorology of Venus since it is responsible for the bulk of the absorption of solar energy.

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Edwin S. Barker, Jerry H. Woodman, Michael A. Perry, Bruce A. Hapke, and Robert Nelson

Abstract

The relative spectral reflectivity from 3067 Å to 5960 Å for the integrated disk of Venus is presented. The reflectivity is essentially flat from 5960 to about 5200 Å then decreases smoothly to a flat region between 3950 and 3400 Å at 55% of the value at 5960 Å. Below 3300 Å the reflectivity appears to drop again to possibly another flat region between 3200 and 3100 Å.

Temporal changes in the reflectivity curve are of the same magnitude as the changes over a range of phase angle from 40° to 76°. These changes appear to be only in the amount of UV absorption and not in the shape of the reflectivity curve. The narrow-band data of Irvine et al. are compared to the average reflectivity curve.

The relative reflectivity curve of a dark UV feature compared to a bright UV feature has the same shape as the curve for the integrated disk of Venus. The comparison of these two curves leads to the conclusion that at least a significant amount of the UV absorption must occur above even the bright UV features.

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