TEMPORAL BEHAVIOR OF CLOUD MORPHOLOGIES AND MOTIONS IN SATURNS ATMOSPHERE

Saturn's atmosphere displays a variety of temporal changes in its clou d morphology at synoptic and planetary scales. Hemispheric long-term a lbedo variations, which are more pronounced at ultraviolet and blue wa velengths, have been well established and are apparently linked to the seasonal insolation cycle. Short-term changes, of the order of months , have been observed in the spectral reflectivity of the belt/zone pat tern mainly at mid-latitudes. Some appear to be related to discrete sp ot activity, but for the majority of the changes, no such signs of spo t activity were detected up to a resolution approximately 2000 km. In particular, the ''ribbon'' structure at 45-degrees-N could have been a ffected by this activity and probably is a transient phenomenon. The p olar belts at approximately 65-degrees north and south and those at 75 -degrees north (hexagon) and south show a long-term stability in their location, perhaps for more than a century. The most prominent belts l ocated in the Equator and the Equatorial Zone are persistent features of Saturn's globe; their main changes took place during the developmen t of a rare, planetary-scale disturbance known as Great White Spots. O nly five such events have been observed at repetitive regular interval s of about one Saturn year since the first report in 1876, and at diff erent latitudes from the equator to 60-degrees-N. They are probably co nvective in origin, with the seasonal heating of the upper atmosphere acting as a trigger mechanism. During its life cycle of 2-3 years, the disturbance shows turbulent planetary-scale patterns of clouds that m ight be related to a wave dynamical phenomenon. Other distinct nonaxis ymmetric cloud systems at synoptic scale are very scarce in Saturn's a tmosphere. We observed new isolated features at several latitudes duri ng 1990 and 1991. These spots, together with those available from hist orical records, are used as tracers for determining the latitude depen dence and time variability of Saturn's zonal winds. The winds tend to exhibit a long-term stability in their latitudinal positions and avera ge wind speed, although some significant departures from the zonal mea n Voyager profile have been detected during the development of the equ atorial disturbance in 1990 and 1991.