Jupiter’s Great Red Spot is not the permanent spot reported by Cassini, a new study suggests

Jupiter’s Great Red Spot is probably the best-known atmospheric feature and a popular symbol of solar system objects. Its large oval shape, iridescent red color, and long life make it an easy-to-see target for small telescopes. From historical measurements of size and movements, new research led by scientists at the University of the Basque Country shows that the current Great Red Spot may have been first reported in 1831 and not the permanent spot observed by Giovanni Domenico Cassini and others between 1665 and 1713.

Permanent Space (PS) and Early Great Red Spot (GRS): (a) PS drawing by GD Cassini, January 19, 1672; (b) May 10, 1851 s. Schwabe’s drawing shows the GRS region as a clear oval shape with its halo-marked boundaries (draw red line); (c) photograph taken by AA Common on September 3, 1879 at Ealing (London) using a 91 cm reflector; GRS shows prominently as a 'dark’ oval due to its red color and photographic plate sensitivity to violet-blue wavelengths; (d) Photo from Observatory Lick with yellow filter on October 14, 1890. All figures show Jupiter’s astronomical view (south top, east left) to preserve references to maps. Image credit: Sánchez-Lavega and many others., doi: 10.1029/2024GL108993.

Jupiter’s Great Red Spot is the largest and longest known spiral of all the Solar System planets.

The mechanism by which this feature is produced is unknown, and its longevity is debatable.

It is not clear whether the Great Red Spot is the dark oval nicknamed the Permanent Point reported by astronomer Giovanni Domenico Cassini and others from 1665 to 1713.

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„Speculations about the origin of the Great Red Spot date back to the first telescope observations 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 as it was observed. By him and other astronomers until 1713, University of the Basque Country professor Augustine said. Sanchez-LaVega said.

„Its track was then lost for 118 years, and in 1831 and later S. Schwabe again saw a clear structure, roughly oval in shape and at the same latitude as the Great Red Spot; it may be considered the first observation of the present Great Red Spot, and perhaps a new Great Red Spot. „

„Since then, the Great Red Spot has been continuously monitored by telescopes and by various spacecraft that have visited the planet to date.”

In the study, the authors examined the evolution of the Great Red Spot over time, its structure, and the movements of two weather systems, 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 measurements of magnitudes and motions, we found that the current Great Red Spot is unlikely to be the permanent location observed by Cassini,” Professor Sánchez-LaVega said.

„Permanent spotting probably disappeared in the 18th and mid-19th centuries, at which point the Red Spot’s longevity now exceeds at least 190 years.”

„The Red Spot, which was 39,000 km along its long axis in 1879, has shrunk to its current 14,000 km and simultaneously become rounder.”

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„What’s more, several space missions since the 1970s have closely studied this weather phenomenon.”

„Recently, various instruments aboard the Juno mission in orbit around Jupiter have shown that the Great Red Spot is shallow and thin compared to its horizontal dimension, because vertically it is about 500 km long.”

To figure out how this massive eddy might have formed, the astronomers ran numerical simulations using two complementary models of the behavior of thin eddies in Jupiter’s atmosphere.

Dominant on the giant planet are intense wind currents that alternate in their direction with latitude and flow along the parallels.

North of the Great Red Spot, winds blow in a westerly direction at a speed of 180 km/h, and in the south, they blow in the opposite direction, in an easterly direction, at a speed of 150 km/h.

This creates a large north-south shear in wind speed, which is a basic ingredient that helps the vortex grow inside it.

In the research, several mechanisms were explored to explain the origin of the Great Red Spot, including the explosion of a giant superstorm, rarely seen on the binary planet Saturn, or the merger of many small eddies produced by wind shear.

The results indicate that although an anticyclone forms in both cases, it differs from the current Great Red Spot in terms of its shape and dynamical characteristics.

„If one of these unusual events had occurred, it or its effects on the atmosphere should have been observed and reported by astronomers at the time,” Professor Sánchez-LaVega said.

In a third set of numerical experiments, the researchers investigated the generation of the Great Red Spot from a known instability in the wind, which is thought to create an elongated cell and trap them.

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Such a cell would be a proto-Great Red Spot, a new red spot whose subsequent contraction would produce the compact and rapidly rotating Great Red Spot seen in the late 19th century.

The formation of large elongated cells has already been observed at the origin of other large gyres on Jupiter.

„In our simulations, the supercomputers were able to find that the elongated cells are stable as they rotate around the Great Red Spot at the speed of Jupiter’s wind, as would be expected when they form because of this instability,” said Dr. Enrique García-Melendo is an astronomer at the Universitat Politecnica de Catalunya.

Using two different types of numerical models, the scientists concluded that if the rotation speed of the proto-great red spot was less than that of the surrounding air, it would be impossible for the proto-great red spot to break up and form a stable vortex. .

And, if it is too high, the properties of the proto-Great Red Spot differ from those of the present Great Red Spot.

„Future research will attempt to reproduce the shrinking of the Great Red Spot over time to uncover in more detail the physical mechanisms underlying its persistence 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 certain size limit, as happened with Cassini’s permanent location, or whether it will remain stable at a size limit where it will last. Many more years.”

The Results appear in the magazine Geophysical Research Letters.

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Agustín Sanchez-Laveca and many others. 2024. The appearance of Jupiter’s Great Red Spot. Geophysical Research Letters 51 (12): e2024GL108993; doi: 10.1029/2024GL108993

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