Ground-based near-infrared imaging observations of venus during the galileo encounter.

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.

The nature of the near-infrared features on the venus night side.

Near-infrared images of the Venus night side show bright contrast features that move from east to west, in the direction of the cloud-top atmospheric superrotation. Recently acquired images of the Venus night side along with earlier spectroscopic observations allow identification of the mechanisms that produce these features, their level of formation, and the wind velocities at those levels. The features are detectable only at wavelengths near 1.74 and 2.3 micrometers, in narrow atmospheric windows between the CO(2) and H(2)O bands. The brightest features have brightness temperatures near 480 Kelvin, whereas the darkest features are more than 50 Kelvin cooler. Several factors suggest that this radiation is emitted by hot gases at altitudes below 35 kilometers in the Venus atmosphere. The feature contrasts are produced as this thermal radiation passes through a higher, cooler, atmospheric layer that has horizontal variations in transparency. The 6.5-day east-west rotation period of the features indicates that equatorial wind speeds are near 70 meters per second in this upper layer. Similar wind speeds have been measured by entry probes and balloons at altitudes between 50 and 55 kilometers in the middle cloud layer. The bright features indicate that there are partial clearings in this cloud deck. The presence of these clearings could decrease the efficiency of the atmospheric greenhouse that maintains the high surface temperatures on Venus.