Near-infrared images of Venus, obtained from a global network of ground-based observatories during January and February 1990, document the morphology and motions of the night-side near-infrared markings before, during, and after the Galileo Venus encounter. A dark cloud extended halfway around the planet at low latitudes (>+/-40 degrees ) and persisted throughout the observing program. It had a rotation period of 5.5 +/- 0.15 days. The remainder of this latitude band was characterized by small-scale (400 to 1000 kilometers) dark and bright markings with rotation periods of 7.4 +/- 1 days. The different rotation periods for the large dark cloud and the smaller markings suggests that they are produced at different altitudes. Mid-latitudes (+/-40 degrees to 60 degrees ) were usually occupied by bright east-west bands. The highest observable latitudes (+/-60 degrees to 70 degrees ) were always dark and featureless, indicating greater cloud opacity. Maps of the water vapor distribution show no evidence for large horizontal gradients in the lower atmosphere of Venus.

Information about the nature of the UV absorbing agents in Venus' atmosphere has been obtained from a comparison of model predictions with a variety of photometric data obtained from the Pioneer Venus probes and orbiter as well as from the earth. The theoretical simulations were performed with numerically accurate radiative transfer codes that incorporated spacecraft constraints on the properties of aerosols and gases in Venus' atmosphere. We find that at least two UV absorbers are needed: gaseous SO2, which is a major source of opacity at wavelengths shortward of 0.32 µm, and a second absorber, which dominates above 0.32 µm. The concentration of the second absorber increases sharply with increasing optical depth within the first few optical depths and then has a relatively flat profile throughout most of the remainder of the upper main cloud layer; but it is largely absent from the middle and lower cloud regions. Several mechanisms appear to be responsible for producing UV contrast features. These include changes in the optical depth of the upper haze layer, which leads to brightness differences between equatorial and polar regions, and changes in the depth over which the second UV absorber is depleted in the uppermost part of the main clouds, which creates UV features at equatorial latitudes. Atmospheric dynamics acts as a driving force for the above variations in the radiative properties of the clouds through dynamically induced temperature changes and changes in vertical mixing. Many UV‐absorbing materials, including sulfur, various iron compounds, and nitrogen dioxide gas, have spectral absorption properties that are in serious disagreement with those deduced for the second UV absorber. One possibly tenable candidate for the second absorber is gaseous Cl2, whose spectral absorption characteristics closely match the empirically derived ones and whose expected altitude profile is crudely consistent with the sharp decreases in the concentration of the second absorber near the cloud tops and bottom of the upper clouds. Approximately 1 ppm of Cl2 is required. Whether this amount of Cl2 can be generated from the photodissociation of HCl is not clear, with the peak Cl2 mixing ratio being strongly dependent on the exact amount of HCl and other gases that are present in the upper clouds.

Abstract. The results of the nephelometer experiment conducted aboard the probe of the Galileo mission to Jupiter are presented. The tenuous clouds and sparse particulate matter in the relatively particle-free 5-/xm "hot spot" region of the probe's descent were documented from about 0.46 bar to about 12 bars. Three regions of apparent coherent structure were noted, in addition to many indications of extremely small particle concentrations along the descent path. From the first valid measurement at about 0.46 bar down to about 0.55 bar, a feeble decaying lower portion of a cloud, corresponding with the predicted ammonia particle cloud, was encountered. A denser, but still very modest, particle structure was present in the pressure regime extending from about 0.76 bar to a distinctive base at 1.34 bars and is compatible with the expected ammonium hydrosulfide cloud. No massive water cloud was encountered, although below the second structure, a small, vertically thin layer at about 1.65 bars may be detached from the cloud above, but may also be water condensation, compatible with reported measurements of water abundance from other Galileo Mission experiments. A third small signal region, extending from about 1.9 to 4.5 bars, exhibited quite weak but still distinctive structure and, although the identification of the light scatterers in this region is uncertain, may also be a water cloud, perhaps associated with lateral atmospheric motion and/or reduced to a small mass density by atmospheric subsidence or other causes. Rough descriptions of the particle size distributions and cloud properties in these regions have been derived, although they may be imprecise because of the small signals and experimental difficulties. These descriptions document the small number densities of particles, the moderate particle sizes, generally in the slightly submicron to few micron range, and the resulting small optical depths, mass densities due to particles, column particle number loading, and column mass loading in the atmosphere encountered by the Galileo probe during its descent. IntroductionOne of the principal objectives of the Galileo Mission to Jupiter was to determine the locations, horizontal and vertical extent, microphysical properties, and composition of the clouds of Jupiter. A nephelometer (NEP) instrument measuring light scattering from the ambient atmosphere at five scattering angles was included as part of the experiments package on the Galileo Mission probe that entered the Jovian atmosphere on December 7, 1995. The purpose of this experiment was to establish the vertical location and to attempt to document the microphysical properties of the clouds along the probe descent trajectory. It was hoped that the data would yield values of parameters required for radiation modeling studies, such as cloud particle size distributions, opacities, scattering and extinction cross sections, and local total mass den-