Images of the Venus cloud deck from Galileo.

Kurzfassung

Images of Venus taken in spectral bands centered at 418 (violet) and 986 (NIR) nanometers show that the morphology and motions of large-scale features change with depth into the cloud deck. Equatorial zonal velocities of 101+-1 m.sec -1 are seen in the violet and 78+-2m.sec -1 in the NIR. Poleward meridional velocities are seen in both spectral regions but are much reduced in the NIR. Inthe south polar region the dominant markings in the two wavelength bands are strongly anticorrelated, while in the equatorial region the motion of a large-scale meridional NIR feature appears to be associated with the equatorial wave (Venus "horizontal" Y) feature seen in the violet. The images follow the changing state of the upper cloud layer downwind of the subsolar point, and several equatorial strucutres are seen to evolve rapidly. The zonal flow field shows a longitudinal periodicity that may be coupled to the initiation of limited regions of small scale markings near the subsolar region. In midlatitudes the shapes of small features are seen to evolve as they move along the region that forms the arms of the "Y", indicating that advection, and notsuperimposed wave motion, is the probable cause of the striated pattern that is seen there. Limb hazes between 83 and 96 km altitude show similar behavior at both wavelengths indicating that te particulates above the main cloud deck are at least a few tenth of a micron in size. The vertical structure is similar to that previously observed by Mariner 10 but displaced to higher altitudes with less prominent layering. A search was made for optical lightning but no events were detected. The limiting optical energy/flash for lightning to show in the SSI frames is estimated at 4.10^9 J.