New study suggests Jupiter’s Great Red Spot may not be the permanent feature reported by Cassini

Jupiter’s Great Red Spot is perhaps the best-known atmospheric feature and a popular icon among the solar system’s objects. Its large oval shape, contrasting red color, and long lifespan make it easily visible with a small telescope. A new study led by scientists from the University of the Basque Country, based on historical measurements of its size and motion, shows that the present-day Great Red Spot was probably first reported in 1831 and is not a permanent spot observed by Giovanni Domenico Cassini and others between 1665 and 1713.

Jupiter’s Great Red Spot is the largest and longest-lasting known vortex of any planet in the solar system.

The formation mechanism that produced this feature is unknown, and its longevity is controversial.

It was also unclear whether the Great Red Spot was the dark oval nicknamed the “Eternal Spot” that astronomer Giovanni Domenico Cassini and others reported between 1665 and 1713.

“Speculation about the origin of the Great Red Spot dates back to the first telescopic observations by Giovanni Domenico Cassini, who in 1665 discovered a dark oval at the same latitude as the Great Red Spot, which he named a permanent spot, because it was observed by Cassini and other astronomers until 1713,” said Professor Agustin Sánchez Lavega from the University of the Basque Country.

“For the next 118 years, traces of it were lost, and it was only after 1831 that S. Schwabe again observed a clear, almost elliptical structure at the same latitude as the GRS. This can be considered the first observation of the present-day GRS, possibly of the infant GRS.”

“Since then, the Great Red Spot has been regularly observed by telescopes and by various space probes that have visited the planet up to the present day.”

In their study, the authors analysed the change in the size of the Great Red Spot over time, its structure, and the behaviour of two meteorological structures, 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 telescope was invented.

“Based on our measurements of its size and motion, we infer that it is highly unlikely that the current Great Red Spot is the permanent spot observed by Cassini,” Professor Sanchez LaVega said.

“The permanent spot probably disappeared sometime between the mid-18th and 19th centuries, which would put the lifespan of the red spot at least 190 years.”

“The Red Spot, which in 1879 measured 39,000 kilometres along its longest axis, has now shrunk to about 14,000 kilometres and is becoming rounder at the same time.”

“Furthermore, since the 1970s, several space missions have studied this weather phenomenon in detail.”

“Recently, various instruments on the Juno spacecraft in orbit around Jupiter have shown that the Great Red Spot is shallow and thin compared to its horizontal length. Its vertical length is about 500 km.”

To understand how this giant whirlpool formed, the astronomers ran numerical simulations using two complementary models of the behavior of thin vortices in Jupiter’s atmosphere.

Powerful winds prevail on this giant planet, flowing along parallels that alternate in direction and latitude.

To the north of the Great Red Spot, winds blow westward at 180 km/h, while to the south, winds blow in the opposite direction, eastward at 150 km/h.

This creates huge north-south shear in the wind speed, which is the fundamental element that allows vortices to grow internally.

The study explored a variety of mechanisms to explain the formation of the Great Red Spot, including the eruption of a giant superstorm like those rarely observed around its twin planet Saturn, or the merging of several smaller vortices caused by sheared winds.

The results show that although anticyclones form in both cases, their shapes and dynamic characteristics are different from those of the present-day Great Red Spot.

“We believe that if one of these anomalies had occurred, it, or its effects in the atmosphere, would have been observed and reported by astronomers at the time,” Prof Sanchez Lavega said.

In a third set of numerical experiments, the researchers investigated how the GRS may arise from known instabilities in the winds that they believe could produce elongated cells that surround and trap the GRS.

Such cells were early red spots, the proto-Great Red Spot, whose subsequent shrinkage would give rise to the compact, rapidly rotating Great Red Spot observed in the late 19th century.

The formation of large elongated cells has already been observed during the emergence of other major vortices on Jupiter.

“In our simulations, thanks to supercomputers, we were able to find that elongated cells are stable when they rotate around the Great Red Spot at the speed of Jupiter’s winds, which is what you would expect to form due to this instability,” said Dr Enrique García Melendo, an astronomer at the Polytechnic University of Catalonia.

Using two different numerical models, the scientists concluded that if the GRS rotated slower than the surrounding winds, it would break up and the formation of a stable vortex would be impossible.

And if it were very high, the properties of the primordial Great Red Spot would be different from those of the current Great Red Spot.

“Future studies will aim to reconstruct the Great Red Spot’s shrinkage over time and elucidate in more detail the physical mechanisms underlying its persistence,” the authors wrote.

“At the same time, we try to predict whether the Great Red Spot will collapse and disappear when it reaches its size limit, as happened with Cassini’s permanent spot, or whether it will remain stable at its size limit and persist for many years.”

of result Published in a journal Geophysical Research Letters.

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Agustin Sanchez Lavega others2024. Origin of Jupiter’s Great Red Spot. Geophysical Research Letters 51(12):e2024GL108993; doi:10.1029/2024GL108993