Nature of the Great Red Spot

The true nature of Jupiter's unique Great Red Spot was still unknown at the start of the 21st century, despite extensive observations from the Voyager and Galileo 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 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 20,000 by 12,000 km (12,400 by 7,500 miles), making it large enough to accommodate both Earth and Mars. These huge dimensions are probably responsible for the feature's longevity and possibly for its distinct colour.

The rotation period of the Great Red Spot around the planet does not match any of Jupiter's three rotation periods. It shows a variability that has not been successfully correlated with other Jovian phenomena. Voyager observations revealed that the material within the spot circulates in a counterclockwise direction once every seven days, corresponding to superhurricane-force winds of 400 km (250 miles) per hour at the periphery. The Voyager images also recorded a large number of interactions between the Great Red Spot and much smaller disturbances moving in the current at the same latitude. The interior of the spot is 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 diameter is presumably accompanied by a great depth that allows the feature to reach well below and well above the main cloud layers. Its extension above the main clouds is manifested by lower temperatures and by less gas absorption above the Great Red Spot than at neighbouring regions on the planet. Its lower extension remains to be observed.

Jupiter's Great Red Spot (top right) and the surrounding region, as seen from Voyager 1 on March 1,1979. Below the spot is one of the large white ovals associated with the feature. NASA/JPL

These cloud systems are much less zon-ally confined than the cloud systems on Jupiter and move in latitude as well as longitude. Local weather on Earth is often closely tied to the local environment, which in turn is determined by the varied nature of the planet's surface.

Jupiter has no solid surface—hence, no topographic features—and the planet's 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 planet's rotation systems, yet it does not move in latitude. The 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 near 18° N latitude. The blue-gray or purple areas, from which the strongest thermal emission is detected, occur only in the equatorial region of the planet.

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