A large oval feature in Jupiter’s clouds at about 22° south latitude. Its dimensions are currently about 18 000 km east–west and 14 000 km north–south; it has been shrinking in length fairly steadily for the past century. It was first recorded by S. H. Schwabe in 1831 but attracted little attention until 1878–82, when it became a striking dark red. Since then it has varied greatly in size, colour, and intensity, sometimes being so faint that it is detectable only by the hollow it makes in the South Temperate Belt (the Red Spot Hollow). A prominent, slightly smaller dark spot at a similar latitude to the Great Red Spot was seen in 1664 by the English scientist Robert Hooke (1635–1703), and lasted until 1713. This was probably an earlier manifestation of a similar phenomenon. Space probes have shown the Great Red Spot to be a vast vortex rotating anticlockwise (anticyclonically), equivalent to a storm or hurricane. At the time of the Voyager encounters in 1979 its rotation period was about 7 days, but 30 years later had speeded up to about 4.5 days. Its top lies a few kilometres above the surrounding cloud deck, and its red colour may be due either to compounds such as phosphine (PH₃) rising from lower layers or to ammonia and acetylene being broken down at altitude by solar ultraviolet light. A smaller version of the Great Red Spot arose at 34° south following the merger of three white oval storms in 1998 and 2000. Initially it remained white, like the storms from which it formed, but took on a ruddy colour in late 2005, presumably as complex chemicals were brought up from deeper within Jupiter’s clouds.

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