Jupiter’s Great Red Spot (GRS) is one of the defining features of the solar system. It is a huge storm that astronomers have observed since the 17th century. However, the date of founding and lifespan are subject to debate. Have we been seeing the same phenomenon all this time?
The GRS is a giant anticyclonic (counterclockwise) storm that is larger than Earth. Wind speeds exceed 400 km/h (250 mph). It is an icon that people have observed since the 19th century, possibly even earlier. Its history, and how it came to be, is a mystery.
The first observations were possible in 1632, when a German Abbott used his telescope to look at Jupiter. 32 years later, another observer reported seeing the GRS moving 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 he named it the Permanent Spot.
Unfortunately, astronomers have lost sight of the spot. No one saw the GRS for 118 years until astronomer S. Schwabe observed a distinct structure, roughly oval and at the same latitude as the GRS. Some consider that observation to be the first observation of the current GRS and that the storm re-formed at the same latitude. But the details fade as we look further back in time. There are also questions about the earlier storm and its relationship to the current GRS.
New research in Geophysical Research Letters combined historical data with computer simulations from the GRS to try to understand this chimeric meteorological phenomenon. The title is ‘The Origin of Jupiter’s Great Red Spot’ and the lead author is Agustín 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 Sciences Group and the Applied Physics department of 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 its formation mechanism remains hidden,” 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 the PS and GRS. But that is no easy task. “The appearance of the GRS and its cavity throughout the history of Jupiter observations has been highly variable due to changes in size, albedo, and contrast with surrounding clouds,” they write.
“From the size and motion measurements, we concluded that it is highly unlikely that the current GRS was the PS observed by GD Cassini. The PS probably disappeared sometime between the mid-18th and 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. It has also become rounder.
The historical record is valuable, but we now have several tools at our disposal. Space telescopes and spacecraft have studied the GRS in ways that would have been unthinkable to Cassini and others. NASA’s Voyager 1 took our first detailed image of the GRS in 1979, when it was just over 9,000,000 km from Jupiter.
Since Voyager was captured, the Galileo and Juno spacecraft have both imaged the GRS. Juno in particular has given us more detailed images and data about Jupiter and the GRS. It captured images of the planet from just 8,000 km above the surface. Juno takes raw images of the planet with its Junocam, and NASA invites everyone to process the images, leading to artistic images of the GRS like the one below.
Juno also measured the depth of the GRS, something previous efforts could not achieve. Recently, “several instruments on board the Juno mission in orbit around Jupiter showed that the GRS is shallow and thin compared to its horizontal size, as it is about 500 km vertically long,” Sánchez-Lavega explains.
Jupiter’s atmosphere contains winds that flow in opposite directions at different latitudes. North of the GRS the wind blows in a westerly direction and reaches speeds of 180 km/h. South of the GRS the wind is blowing in the opposite direction at speeds of 150 km/h. These winds generate powerful wind shear that promotes the vortex.
In their supercomputer simulations, the researchers investigated various forces that could cause the GRS under these conditions. They were thinking of the eruption of a gigantic superstorm, like the kind that occurs, albeit rarely, on Saturn. They also investigated the phenomenon of smaller eddies created by the wind shear that merged to form the GRS. Both produced anticyclonic storms, but their shapes and other properties did not correspond to the current GRS.
“From these simulations, we conclude that the superstorm and the merger mechanisms are unlikely to have formed the GRS, even if they generate a single anticyclone,” the researchers write in their paper.
The authors also point out that if any of these events had happened, we should have seen them. “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 the astronomers at the time,” says Sánchez-Lavega.
However, other simulations proved more accurate in reproducing the GRS. Jupiter’s winds are known to have instabilities called South Tropical Disturbance (STrD). When the researchers ran supercomputer simulations of the STrD, they created an anticyclonic storm that was very similar to the GRS. The STrD captured the different winds in the region and confined them in an elongated shell like the GRS. “We therefore propose that the GRS was generated from a long cell that emerged from the STrD, which acquired coherence and compactness as it shrank,” the authors write.
The simulations show that the GRS would rotate faster over time as it became smaller and more coherent and compact until the elongated cell became more similar to the current GRS. Because that is what the GRS looks like now, the researchers came to this explanation.
That process probably started in the mid-19th century, when the GRS was much larger than it is today. This leads to the conclusion that the GRS is only about 150 years old.