Jupiter’s Great Red Spot is probably the best known atmospheric feature and a popular icon among solar system objects. Its large oval shape, contrasted red color and longevity, have made it an easily visible target for small telescopes. From historical measurements of size and motions, new research led by University of the Basque Country scientists shows that most likely the current Great Red Spot was first reported in 1831 and is not the Permanent Spot observed by Giovanni Domenico Cassini and others between 1665 and 1713.
The Permanent Spot (PS) and the early Great Red Spot (GRS): (a) drawing of the PS by G.D. Cassini, January 19, 1672; (b) drawing by S. Swabe on May 10, 1851, showing the GRS area as a clear oval with limits marked by its Hollow (draw by a red dashed line); (c) photograph by A.A. Common obtained in Ealing (London) on September 3, 1879 using a 91-cm reflector; the GRS shows prominently as a ‘dark’ oval due to its red color and photographic plate sensibility to violet-blue wavelengths; (d) photograph from Observatory Lick with a yellow filter on October 14, 1890. All figures show the astronomical view of Jupiter (south up, east left) to preserve notes on the drawings. Image credit: Sánchez-Lavega et al., doi: 10.1029/2024GL108993.
Jupiter’s Great Red Spot is the largest and longest-lived known vortex of all solar system planets.
The formation mechanism that gave rise to this feature is unknown, and its longevity is a matter of debate.
It was also not clear if the Great Red Spot was the dark oval, nicknamed Permanent Spot, reported by the astronomer Giovanni Domenico Cassini and others from 1665 to 1713.
“Speculation about the origin of the Great Red Spot dates back to the first telescopic observations made by Giovanni Domenico Cassini, who in 1665 discovered a dark oval at the same latitude as the Great Red Spot and named it the Permanent Spot, since it was observed by him and other astronomers until 1713,” said University of the Basque Country’s Professor Agustín Sánchez-Lavega.
“Track of it was subsequently lost for 118 years and it was not until 1831 and later years that S. Schwabe again observed a clear structure, roughly oval in shape and at the same latitude as the Great Red Spot; that can be regarded as the first observation of the current Great Red Spot, perhaps of a nascent Great Red Spot.”
“Since then, the Great Red Spot has been observed regularly by means of telescopes and by the various space missions that have visited the planet right up to the present day.”
In the study, the authors analyzed the evolution of Great Red Spot’s size over time, its structure and the movements of both meteorological formations, the former Permanent Spot and the Great Red Spot.
To do so, they used historical sources dating back to the mid-17th century, shortly after the invention of the telescope.
“From the measurements of sizes and movements we deduced that it is highly unlikely that the current Great Red Spot was the Permanent Spot observed by Cassini,” Professor Sánchez-Lavega said.
“The Permanent Spot probably disappeared sometime between the mid-18th and 19th centuries, in which case we can say that the longevity of the Red Spot now exceeds 190 years at least.”
“The Red Spot, which in 1879 was 39,000 km in size at its longest axis, has been shrinking to about the current 14,000 km and simultaneously becoming more rounded.”
“What is more, since the 1970s several space missions have studied this meteorological phenomenon closely.”
“Recently, various instruments on board the Juno mission in orbit around Jupiter have shown that the Great Red Spot is shallow and thin when compared to its horizontal dimension, as vertically it is about 500 km long.”
To find out how this immense vortex could have formed, the astronomers carried out numerical simulations using two types of complementary models of the behavior of thin vortices in Jupiter’s atmosphere.
Predominating on the giant planet are intense wind currents that flow along the parallels alternating in their direction with the latitude.
To the north of the Great Red Spot, winds blow in a westerly direction at speeds of 180 km per hour while to the south, they blow in the opposite direction, in an easterly direction, at speeds of 150 km per hour.
This generates a huge north-south shear in wind speed, which is a basic ingredient enabling the vortex to grow inside it.
In the research a range of mechanisms were explored to explain the genesis of the Great Red Spot, including the eruption of a gigantic superstorm, similar to those rarely observed on the twin planet Saturn, or the merging of multiple smaller vortices produced by wind shear.
The results indicate that, although an anticyclone forms in both cases, it differs in terms of shape and dynamic properties from those of the present Great Red Spot.
“We also think that if one of these unusual phenomena had occurred, it or its consequences in the atmosphere must have been observed and reported by astronomers at the time,” Professor Sánchez-Lavega said.
In a third set of numerical experiments, the researchers explored the generation of the Great Red Spot from a known instability in the winds that is thought to be capable of producing an elongated cell that encloses and traps them.
Such a cell would be a proto-Great Red Spot, a nascent Red Spot, whose subsequent shrinkage would give rise to the compact and rapidly rotating Great Red Spot observed in the late 19th century.
The formation of large elongated cells has already been observed in the genesis of other major vortices on Jupiter.
“In our simulations, supercomputers enabled us to discover that the elongated cells are stable when they rotate around the periphery of the Great Red Spot at the speed of Jupiter’s winds, as would be expected when they form because of this instability,” said Dr. Enrique García-Melendo, an astronomer at the Universitat Politècnica de Catalunya.
Using two different types of numerical models, the scientists concluded that if the rotational speed of the proto-Great Red Spot is lower than that of the surrounding winds, the proto-Great Red Spot will break up, making the formation of a stable vortex impossible.
And, if it is very high, the properties of the proto-Great Red Spot differ from those of the current Great Red Spot.
“Future research will aim to try and reproduce the shrinkage of the Great Red Spot over time in order to find out, in greater detail, the physical mechanisms underlying its sustainability over time,” the authors said.
“At the same time it will try to predict whether the Great Red Spot will disintegrate and disappear when it reaches a size limit, as might have occurred to Cassini’s Permanent Spot, or whether it will stabilise at a size limit at which it may last for many more years.”
The results appear in the journal Geophysical Research Letters.
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Agustín Sánchez-Lavega et al. 2024. The Origin of Jupiter’s Great Red Spot. Geophysical Research Letters 51 (12): e2024GL108993; doi: 10.1029/2024GL108993
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