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 and persisted throughout the observing program. It had a rotation period of 5.5 + or - 0.15 days. The remainder of this latitude band was characterized by small-scale (400 to 1000 km) dark and bright markings with rotation periods of 7.4 + or - 1 days. The different rotation periods for the large dark cloud and the smaller markings suggest that they are produced at different altitudes. Midlatitudes (+ or - 40 to 60 deg) were usually occupied by bright east-west bands. The highest observable latitudes (+ or - 60 deg to 70 deg) 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.