Polar Cloud Structure as Derived From the Pioneer Venus Orbiter

Pioneer Venus orbiter radio occultation data taken on orbits 9, 18, and 19 at S band (13.06 cm) are used to obtain vertical absorption coefficient profiles of the Venus clouds in the north polar regions. Resulting profiles show dense cloud decks at the 1.5- to 4.7-bar level in the Venus atmosphere. The cloud decks are lower in altitude and more extensive vertically than those found in the equatorial region by Mariner 10 and the Pioneer Venus probes, which have peaks at about the 1-bar level. These lower clouds appear to be fairly uniform in absorption properties throughout the polar regions. Pioneer Venus orbiter infrared radiometer data are combined with the occultation data in interpreting the north polar cloud structure. The three occultation profiles represent three different regions of the north pole in terms of vertical temperature structure and proximity to the polar hot spots. Regions near the hot spots appear to have depressed upper cloud heights as well as increased optical density. These areas lack the temperature inversions characteristic of the north polar regions far from the hot spots. In addition, the cloud tops at the pole are about 100 mbar deeper in the atmosphere than at the equator.