Jupiter’s Great Red Spot (GRS) is one of the defining features of the Solar System. It is a massive storm that astronomers have observed since the 1600s. However, its date of origin and longevity will be debated. Have we been seeing the same phenomenon all along?
A GRS is a gigantic anticyclonic (counter-clockwise rotating) storm that is larger than Earth. Its wind speed exceeds 400 km/h (250 mp/h). It’s an icon that people have been observing since at least the 1800s, maybe even earlier. Its history, along with how it came to be, is a mystery.
Its earliest observations may have been in 1632, when the German Abbott used his telescope to look at Jupiter. 32 years later, another observer reported seeing the GRS move from east to west. Then, in 1665, Giovanni Cassini examined Jupiter with a telescope and noted the presence of a storm at the same latitude as the GRS. Cassini and other astronomers observed it continuously until 1713 and called it the Permanent Spot.
Unfortunately, the astronomers lost the place. No one had seen the GRS for 118 years until the astronomer S. Schwabe observed a clear structure, roughly oval and at the same latitude as the GRS. This sighting is thought by some to be the first sighting of the current GRS and that the storm has re-formed at the same latitude. But the details disappear the farther back in time we look. There are also questions about the earlier storm and its relationship to the current GRS.
New research in Geophysical Research Letters combined historical records with GRS computer simulations to try to understand this chimerical meteorological phenomenon. Its title is “The Origin of Jupiter’s Great Red Spot” and the lead author is AgustÃn Sánchez-Lavega. Sánchez-Lavega is a professor of physics at the University of the Basque Country in Bilbao, Spain. He is also head of the planetary science group and the department of applied physics at the university.
“Jupiter’s Great Red Spot (GRS) is the largest and longest-lived known vortex of any planet in the Solar System, but its lifetime is debated and the mechanism of its formation remains obscure,” the authors write in their paper.
The researchers started with historical sources dating back to the mid-17th century, just after the telescope was invented. They analyzed the size, structure and movement of both PS and GRS. But this is not an easy task. “The appearance of the GRS and its depression throughout Jupiter’s observational history has been highly variable due to changes in size, albedo, and contrast with surrounding clouds,” they write.
“From measurements of magnitudes and motions, we conclude that it is highly unlikely that the present GRS is the PS observed by GD Cassini. The PS probably disappeared sometime between the mid-18th and mid-19th centuries, in which case we can say that the lifetime of the Red Spot now exceeds at least 190 years,” said lead author Sánchez-Lavega. The GRS was 39,000 km long in 1879 and has since shrunk to 14,000 km. She also became more rounded.
The historical record is valuable, but we now have other tools at our disposal. Space telescopes and spacecraft have studied the GRS in ways that would have been unimaginable to Cassini and others. NASA’s Voyager 1 took our first close-up image of the GRS in 1979, when it was just over 9,000,000 km from Jupiter.
Since the Voyager image, both Galileo and Juno have imaged the GRS. In particular, Juno provided us with more detailed images and data about Jupiter and the GRS. He took pictures of the planet from a height of only 8,000 km above the surface. Juno takes raw images of the planet with its Junocam, and NASA invites anyone to process the images, resulting in refined GRS images like the one below.
Juno also measured the depth of the GRS, which previous efforts could not achieve. Recently, “various instruments on board the Juno mission in orbit around Jupiter showed that the GRS is shallow and thin compared to its horizontal dimension, as it is about 500 km long vertically,” explained Sánchez-Lavega.
Jupiter’s atmosphere contains winds running in opposite directions at different latitudes. North of the GRS, winds blow from the west and reach a speed of 180 km/h. South of the GRS, the wind is blowing in the opposite direction at a speed of 150 km/h. These winds create strong wind shear that promotes the vortex.
In their supercomputer simulations, the researchers explored the different forces that could produce GRS under these circumstances. They considered the eruption of a gigantic superstorm similar to the one that occurs, albeit rarely, on Saturn. They also investigated the phenomenon of smaller eddies created by wind shear that coalesce to form GRS. Both produced anticyclonic storms, but their shapes and other characteristics did not match the current GRS.
“From these simulations, we conclude that superstorms and merging mechanisms, although generating a single anticyclone, likely did not form the GRS,” the researchers write in their paper.
The authors also point out that if any of this happened, we should have seen it. “We also think that if any of these unusual phenomena had occurred, it or its consequences in the atmosphere must have been observed and reported by astronomers at the time,” Sánchez-Lavega said.
However, other simulations have been shown to be more accurate in reproducing the GRS. Jupiter’s winds are known to have instabilities called the South Tropical Disturbance (STrD). When the researchers ran supercomputer simulations of the STRD, they created an anticyclonic storm very similar to the GRS. The STRD captured the various winds in the region and trapped them in an elongated envelope like the GRS. “We therefore propose that the GRS was generated from a long cell arising from the STRD, which gained cohesion and compactness as it shrank,” the authors write.
The simulations show that over time the GRS would rotate faster as it contracted and became more coherent and compact until the elongated cell more closely resembled the present GRS. Since this is what the GRS looks like now, scientists agree on this explanation.
This process probably started in the mid-1800s, when the GRS was much larger than it is now. This leads to the conclusion that the GRS is only about 150 years old.