The Great Red Spot is a persistent high-pressure region in the atmosphere of Jupiter, producing an anticyclonic storm that is one of the largest in the Solar System. Located 22 degrees south of Jupiter’s equator, it produces wind-speeds up to 432 km/h (268 mph). Observations from 1665 to 1713 are believed to be of the same storm; if this is correct, it has existed for at least 357 years. It was next observed in September 1831, with 60 recorded observations between then and 1878, when continuous observations began.
The Great Red Spot may have existed since before 1665, but it could also be the case that the present spot was first seen only in 1830, and well-studied only after a prominent apparition in 1879. The storm that was seen in the 17th century may have been different than the storm that exists today. A long gap separates its period of current study after 1830 from its 17th century discovery. Whether the original spot dissipated and reformed, whether it faded, or if the observational record was simply poor is unknown.
For example, the first sighting of the Great Red Spot is often credited to Robert Hooke, who described a spot on the planet in May 1664. However, it is likely that Hooke’s spot was not only in another belt altogether (the North Equatorial Belt, as opposed to the current Great Red Spot’s location in the South Equatorial Belt), but also that it was the shadow of a transiting moon, most likely that of Callisto. Far more convincing is Giovanni Cassini’s description of a “permanent spot” the following year. With fluctuations in visibility, Cassini’s spot was observed from 1665 to 1713, but the 118-year observational gap makes the identity of the two spots inconclusive. The older spot’s shorter observational history and slower motion than the modern spot makes it difficult to conclude that they are the same.
A minor mystery concerns a Jovian spot depicted in a 1711 canvas by Donato Creti, which is exhibited in the Vatican.
Part of a series of panels in which different (magnified) heavenly bodies serve as backdrops for various Italian scenes, and all overseen by the astronomer Eustachio Manfredi for accuracy, Creti’s painting is the first known to depict the Great Red Spot as red (albeit raised to the Jovian Northern hemisphere due to an optical inversion inherent to the era’s telescopes). No Jovian feature was explicitly described in writing as red before the late 19th century.
The Great Red Spot has been observed since 5 September 1831. By 1879 over 60 observations were recorded. After it came into prominence in 1879, it has been under continuous observation.
In the 21st century, the Great Red Spot was seen to be shrinking in size. At the start of 2004, it had approximately half the longitudinal extent it had had a century ago, when it reached a size of 40,000 km (25,000 mi), about three times the diameter of Earth. At the present rate of reduction, it would become circular by 2040. It is not known how long the spot will last, or whether the change is a result of normal fluctuations.
Modern Era and Future
In 2019, the Great Red Spot began “flaking” at its edge, with fragments of the storm breaking off and dissipating. The shrinking and “flaking” fueled concern from some astronomers that the Great Red Spot could dissipate within 20 years. However, other astronomers believe that the apparent size of the Great Red Spot reflects its cloud coverage and not the size of the actual, underlying vortex, and they also believe that the flaking events can be explained by interactions with other cyclones or anticyclones, including incomplete absorptions of smaller systems; if this is the case, this would mean that the Great Red Spot is not in danger of dissipating.
A smaller spot, designated Oval BA, formed in March 2000 from the merging of three white ovals, has turned reddish in color. Astronomers have named it the Little Red Spot or Red, Jr. As of 5 June 2006, the Great Red Spot and Oval BA appeared to be approaching convergence. The storms pass each other about every two years but the passings of 2002 and 2004 were of little significance. Amy Simon-Miller, of the Goddard Space Flight Center, predicted the storms would have their closest passing on 4 July 2006. She worked with Imke de Pater and Phil Marcus of UC Berkeley and a team of professional astronomers since April 2006 to study the storms using the Hubble Space Telescope; on 20 July 2006, the two storms were photographed passing each other by the Gemini Observatory without converging.
In May 2008, a third storm turned red.
The Great Red Spot should not be confused with the Great Dark Spot, a feature observed near the northern pole of Jupiter in 2000 with the Cassini–Huygens spacecraft. There is also a feature in the atmosphere of Neptune also called the Great Dark Spot. The latter feature was imaged by Voyager 2 in 1989 and may have been an atmospheric hole rather than a storm. It was no longer present as of 1994, although a similar spot had appeared farther to the north.