Cloud morphology and motions from Pioneer Venus images

Abstract

The orbiter cloud photopolarimeter on the Pioneer Venus spacecraft obtained over 300 images of Venus in the ultraviolet during a 3‐month period of extensive observations from January through March 1979. The planetary scale and small‐scale markings in the images provide information on the horizontal and vertical cloud structure, atmospheric waves, and wind speeds at the cloud top level. The images show evidence of long‐term evolution of cloud properties and the atmospheric dynamics in addition to rapid small‐scale changes in the cloud morphology. The polar regions are substantially brighter than at the time of Mariner 10 imaging. The planetary scale dark Y feature, previously identified in earth‐based and Mariner 10 images, rotates about the planet with a period of 4–5 days but often changes dramatically from one appearance to the next and sometimes disappears entirely. Bow‐shaped features and cellular features occur at all longitudes but are observed more easily near and downwind from the subsolar point. All of the cloud features exhibit a globally coordinated oscillation of their orientation relative to latitude circles. Retrograde zonal winds of ∼100 m s−1 near the equator, similar to wind speeds observed from Mariner 10, are inferred from tracking of small‐scale cloud features, but the two hemispheres exhibit significant differences. In the southern hemisphere the zonal wind speed decreases toward the poles at a rate similar to that for solid body rotation; the mid‐latitude jet stream observed by Mariner 10 is not present. In the northern hemisphere, two jet streams are present with the mid‐latitude jet stream weaker than that of the single northern hemisphere jet stream observed by Mariner 10. Meridional wind velocities are an order of magnitude weaker than the zonal velocities and are poleward in both hemispheres. The meridional wind speeds increase with increasing latitude up to mid‐latitudes, with those in the northern hemisphere attaining higher values.