Great Red Spot

Great Red Spot

(GRS) An immense oval feature centered 22° south of Jupiter's equator. It is variable in size and color. At its largest it can be 40 000 by 14 000 km, but at the time of the Voyager flybys, it was only slightly larger than the Earth. Its color – possibly associated with the conversion of phosphene into red phosphorus – has varied from pale pink to bright red. Often visible from Earth through small telescopes, it has been persisted as a feature of Jupiter ever since astronomers made the first telescopic observations in the 17th century. Early explanations involved a solid island adrift in Jupiter's atmosphere or an atmospheric disturbance above a Jovian mountain or basin; the latter was disputed by evidence that it drifts in longitude. Infrared data from Pioneer, Voyager, and Galileo spacecraft confirm that the spot is an anticyclonic high-pressure region that is much colder than its surroundings and at a higher elevation. In effect it is a vast high-pressure storm. Examination of cloud motions in and around the GRS reveal that the spot rotates counterclockwise with a period of about six days. The winds to the north of the spot are blowing to the west, the winds to the south move toward the east.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006