GREAT RED SPOT

False-colour infrared image of the Great Red Spot. A gigantic storm system larger than the Earth, 1/4 atmospheric feature on the planet Jupiter that moves in longitude but remains centred at about latitude 23 S. It is oval-shaped, with a variable east-west dimension that is about twice the north-south one. Its north-south extent is approximately 14,000 km (8,700 miles), or about one-fifth of Jupiter's equatorial radius, and thus is about equal to the diameter of the Earth. The Great Red Spot was discovered in 1665 by the French astronomer Gian Domenico Cassini with the aid of one of the earliest telescopes, and it has existed at least since that time. Detailed observations and measurements have been made by the Voyager and Galileo spacecraft. Its colour varies from brick red to brownish and may blend indistinguishably into the colouring of the other cloud belts; the colour also tends to change over a period of years. Dynamically, the Great Red Spot is an anticyclonic circulation system-i.e., a high-pressure centre. Scientific instruments carried by the U.S. Voyager 1 and 2 spacecraft revealed in 1979 that individual patches of cloud rotate around the spot. It takes about six Earth days to complete one circulation, which corresponds to a wind speed of about 80 metres per second (180 miles per hour). Many smaller spots on Jupiter appear to be dynamically similar to the Great Red Spot, though their circulation is weaker and their colour is usually white instead of red. The cause of the red coloration is unknown. It seems probable that the strong circulation draws certain trace-chemical constituents from deep within the planet. These could contaminate the cloud particles, which consist mainly of ammonia crystals. Sulfur is relatively abundant and could possibly contribute to the red coloration, as it does on the surface of Jupiter's satellite Io. The Great Red Spot is not anchored to any solid surface feature, because Jupiter is fluid throughout except for a small core representing about 20 percent of the planet's radius. In effect, the spot and its smaller counterparts are free atmospheric eddies that sustain themselves for long periods of time in the face of dissipative processes. In this respect, they resemble certain special fluid-dynamic configurations (e.g., vortices or solitons) that exhibit long-term stability. The details, however, are not understood for the Great Red Spot.