The Great Red Spot is a persistent anticyclonic storm on the planet Jupiter, 22° south of the equator, which has lasted at least 340 years. The storm is large enough to be visible through Earth-based telescopes. It was probably first observed by Cassini, who described it around 1665.
This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on February 25, 1979, when the spacecraft was 9.2 million km (5.7 million miles) from Jupiter. Cloud details as small as 160 km (100 miles) across can be seen here. The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex and variable wave motion. To give a sense of Jupiter's scale, the white oval storm directly below the Great Red Spot is approximately the same diameter as Earth.
The oval object rotates counterclockwise, with a period of about 6 days. The Great Red Spot's dimensions are 24–40,000 km × 12–14,000 km. It is large enough to contain two or three planets of Earth size. The cloudtops of this storm are about 8 km above the surrounding cloudtops.
Storms such as this are not uncommon within the turbulent atmospheres of gas giants. Jupiter also has white ovals and brown ovals, which are lesser unnamed storms. White ovals tend to consist of relatively cool clouds within the upper atmosphere. Brown ovals are warmer and located within the "normal cloud layer". Such storms can last hours or centuries.
Before the Voyager missions, astronomers were highly uncertain of its nature. Many believed it to be a solid or liquid feature on Jupiter's surface. It is thought it comes from the collision between Jupiter and comet Shoemaker-Levy 9.
Color and visibility
It is not known exactly what causes the Great Red Spot's reddish color. Theories supported by laboratory experiments suppose that the color may be caused by any of "complex organic molecules, red phosphorus, or yet another sulfur compound" , but a consensus has yet to be reached.
The Great Red Spot varies greatly in prominence, from almost brick-red to pale salmon, or even white. In fact, the Spot occasionally "disappears", becoming evident only through the Red Spot Hollow, which is its niche in the South Equatorial Belt (SEB). Interestingly, its visibility is apparently coupled to the SEB; when the Belt is bright white, the Spot tends to be dark, and when it is dark the Spot is usually light. These periods when the Spot is dark or light occur at irregular intervals; in the last 50 years the Spot was darkest from 1961-66, 1968-75, 1989-90, and 1992-93.(Beebe 38-41)
A smaller spot, designated Oval BA, formed recently from the merger of three white ovals, has turned reddish in color.
The Great Red Spot appears at first to be remarkably stable, and most sources concur that it has been continuously observed for 300 years. However, the situation is more complex than that; the present Spot was first seen only in 1830, and well studied only after a prominent apparition in 1879. A long gap separates its period of current study after 1830 from its 17th-century discovery; whether the original Spot dissipated and reformed, or whether it faded, or even if the observational record was simply poor are all unknown.(Beebe 38-41)
Several factors may be responsible for its longevity, such as the fact that it never encounters solid surfaces over which to dissipate its energy and that its motion is driven by Jupiter's internal heat. Simulations suggest that the Spot tends to absorb smaller atmospheric disturbances.
At the start of 2004, the Great Red Spot was approximately half as large as it was 100 years ago. It is not known how long the Great Red Spot will last, or whether this is a result of normal fluctuations.
The Great Red Spot should not be confused with the Great Dark Spot, famously seen in the atmosphere of Neptune by Voyager 2 in 1989. The Great Dark Spot may have been an atmospheric hole rather than a storm, and it was no longer present as of 1994 (although a similar spot had appeared farther to the north).
As the hot gasses that comprise Jupiter's atmosphere rise from lower levels to higher levels, eddies form and converge together. A Coriolis force forms and forces cooler air to fall back into a swirling motion that may be many kilometers in diameter. These eddies can last for a long time, because there is no solid surface to provide friction and colder cloud tops above the eddy allow little energy to escape by radiation. Once formed, such eddies are free to move, merging with or affecting the behaviour of other storm systems in the atmosphere. It is theorized that this mechanism formed the great red spot. According to this theory, many adjacent eddies are engulfed and merge with the spot, adding to the energy of the storm and contributing to its longevity.
As of June 5 2006, the NASA Science website reported that the Great Red Spot and Oval BA might converge. The storms pass each other about every two years, but the passings of 2002 and 2004 did not produce anything exciting. But Dr. Amy Simon-Miller, of the Goddard Space Flight Center, predicted the storms would have their closest passing on July 4. Simon-Miller had been working with Dr. Imke de Pater and Dr. Phil Marcus of UC Berkeley, and a team of professional astronomers since April, studying the storms using the Hubble Space Telescope. On July 20, the two storms were photographed  passing each other by the Gemini Observatory. No convergence occurred.
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