The most prominent and permanent feature of the visible surface of Jupiter is the GREAT BED SPOT . It is an elongated area of variable size; at its largest,-in the 1880s, it was about 38500km long by 13800km wide and had a surface area about equal to that of the Earth. The spot lies in the south tropical zone but protrudes somewhat into the latitudes occupied by the south equatorial belt, to form what is called the BED SPOT HOLLOW in this belt. In 1878 it first became really prominent when it developed from a pink oval object into a brick red area. This startling red color only lasted until 1882 when the spot began to fade. Since then the spot itself has from time to time been invisible but has always been recognizable by its characteristic hollow, which has persisted at all times. This hollow has been found on a drawing by Schwabe made in 1831. It is probably on another drawing made by Robert Hooke in 1664 and was recorded intermittently between then and 1713. The spot revives from time to time, as, for example, it did in 1939. Typical of the changes in the spot are those observed during the years 1962-9. It was very prominent in 1962 and 1963 and remained prominent until 1965. In 1966 it started to fade and by February 1968 it was very weak. It then suddenly began to strengthen and quickly resumed its prominence of four years earlier.

The most remarkable feature of the spot is the fact that it does not appear to be fixed to a solid surface. Measurements of its longitude reveal that its rotation period is not constant so that relative to its moan position it has wandered over a wide range of longitude, amounting to about. 1200″ (more than three revolutions) during the last century. The latitude of the spot has always remained nearly fixed at about 22° south; it deviates by up to l° in latitude. Various associated features have been seen in addition to the hollow in the south equatorial belt. For example, in January 1966 a small dark spot was soon moving along the edge of the south temperate belt. It approached the red spot along its southern side and than circulated round it, with a period of about nine days, about one and a half times before disappearing. During 1967 four similar spots appeared although at least two of these came from the south equatorial belt. These spots had periods of about 12 days. They were only soon to move part way round the red spot.

The spot has a lower temperature than its surroundings and there is less gas above; it. These differences show that it extends about H km above the surrounding cloud tops. Material is observed to rise in the centre of the spot, move outwards and then descend at the edge. During its outward motion the material also moves round the spot somewhat in an anti-clockwise direction.

The nature of the spot is not known with certainty. At one time, many astronomers thought that it was some kind of solid island floating in a dense atmosphere. It is now thought that no solid substance can have a sufficiently low density to float in the upper atmosphere of .Jupiter. If an island were to have a larger vertical extent so that it reached down to a region of much higher density it would be disrupted and would also move towards the equator. As a result the theory of a floating island is no longer accepted.

A more recent suggestion is that the spot is the top of a TAYLOR COLUMN. This is a stagnant column of fluid that can result when an atmospheric flow is unable to surmount some topographic feature. Because of the spot’s motion in longitude this feature cannot be attached to a rigid solid surface. In any case it is believed that Jupiter has at most only a small solid core. A further suggestion is that the cause of the column is some local feature of the magnetic field or of the internal convective motions. There are many objections to this theory and it is not even certain that such a column would form in Jupiter’s atmosphere, even if there were a suitable topographic feature to disturb the atmospheric flow.

The most widely accepted explanation of the spot is that it is the vortex of a long-lived cyclonic storm – a suggestion first made ‘by Gerard Kuiper. For this theory to be acceptable it is necessary that the lifetime of such a cyclone or eddy be longer than the 300 or so years for which the spot is known to have existed. Recent calculations suggest that major changes in the circulation of Jupiter’s atmosphere take at least 300 000 years. If the great red spot is a long-lived eddy as suggested, then it would last about this long, which is indeed longer than the observed minimum age of 300 years. This theory also explains other spots in the same way. These smaller spots would have shorter lifetimes in accordance with observations.

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