Mankindís Explanation: Jupiter's Red Spot


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   ďTHE CLOUDS AND THE GREAT RED SPOT. Even a modest telescope can show much detail on Jupiter. The region of the planetís atmosphere seen from the Earth contains several different types of clouds that are separated both vertically and horizontally. Changes in these cloud systems can occur in a few hours, but an underlying pattern of latitudinal currents has maintained its stability for decades. It has become traditional to describe the appearance of the planet in terms of a standard nomenclature for alternating dark belts and bright zones. The currents, however, seen to have a greater persistence than this pattern. 

   The close-up views of Jupiter from the Voyager spacecraft revealed a variety of cloud forms, with a predominance of elliptical features reminiscent of cyclonic and anticyclonic storm systems on the Earth.  All these systems are in motion, appearing and disappearing on time scales dependent on their sizes and locations. Also observed to vary are the pastel shades of various colours present in the cloud layers--from the tawny yellow that seems to characterize the main layer, through browns and blue-grays to the well-known salmon-coloured Great Red Spot, Jupiterís largest, most prominent, and longest-lived feature (see Figure 44). This vertical and horizontal segregation of the cloud systems is evidently accompanied by chemical differences as well.

   Jovian meteorology can be compared with the global circulation of the Earthís atmosphere. On the Earth huge spiral cloud systems often stretch over many degrees of latitude and are associated with motion around high-and low-pressure regions. These cloud systems are much less zonally confined than the cloud systems on Jupiter and move in latitude as well as longitude.  Local weather on the Earth is often closely tied to the local environment, which in turn is determined by the varied nature of the planetís surface. 

   Jupiter has no solid surface, hence no topographic features, and the planetís large-scale circulation is dominated by latitudinal currents. The lack of a solid surface with physical boundaries and regions with different heat capacities makes the persistence of these currents and their associated cloud patterns all the more remarkable. The Great Red Spot, for example, moves in longitude with respect to all three of the rotation systems, yet it does not move in latitude.  The three white ovals found at a latitude just south of the Great Red Spot exhibit similar behaviour; white ovals of this size are found nowhere else on the planet. The dark brown clouds, evidently holes in the tawny cloud layer, are found almost exclusively at latitudes near +18 degrees.  The blue-gray or purple areas, from which the strongest thermal emission is detected, only occur in the equatorial region of the planet. 

   THE NATURE OF THE GREAT RED SPOT. The true nature of Jupiterís unique Great Red Spot was still unknown by the early 1990ís, despite extensive observations from the Voyager spacecraft. On a planet whose cloud patterns have lifetimes often counted in days, the Great Red Spot has survived as long as detailed observations of Jupiter have been made--at least 100, and perhaps 300, years. There is some evidence that the spot may be slowly shrinking, but a longer series of observations is needed to confirm this suggestion. Its present dimensions are about 26,000 by 14,000 kilometers, making it large enough to accommodate, side by side, two planets the size of the Earth. These huge dimensions are probably responsible for the featureís longevity and possibly for its distinct colour. 

   The rotation period of the Great Red Spot with respect to the rotation of Jupiter itself shows a variability that has not been successfully correlated with other Jovian phenomena. Earth-based observations in 1966 and 1967 revealed the counterclockwise circulation of the material within the spot itself to have a period of 12 days. This period was confirmed by the Voyager observations, which recorded a large number of interactions between the Great Red Spot and much smaller disturbances moving in the current at the same latitude (Figure 43). The Voyager pictures showed the interior of the spot to be remarkably tranquil, with no clear evidence for the expected upwelling (or divergence) of material from lower depths. 

   The Great Red Spot, therefore, appears to be a huge anticyclone, a vortex, or eddy, whose lateral dimensions are greater that the Earthís diameter. This lateral size is presumably accompanied by a huge vertical extent that allows the feature to reach well below and well above the main cloud layers. The extension above the main clouds can be observed directly, and it also is manifested by lower temperatures and by less gas absorption above the Great Red Spot than at neighboring regions on the planet.Ē 

Information acquired within the quotes is from: (Encyclopedia Britannica copyright 1997 15th edition macropaedia volume 27 pages 502-503) 


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