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 formation date and longevity are open to debate. Have we been witnessing the same phenomenon all this time?
The GRS is a gigantic anticyclonic (counterclockwise rotating) storm larger than Earth. Its wind speeds exceed 400 km/h (250 mph). It’s an icon that humans have observed since at least the 1800s, perhaps earlier. Its history, as well as how it was formed, remains a mystery.
Its first observations may date back to 1632, when a German Abbott used his telescope to observe 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 named it the Permanent Spot.
Unfortunately, astronomers have lost track of the location. No one saw the GRS for 118 years until astronomer S. Schwabe observed a clear, roughly oval structure at the same latitude as the GRS. Some believe this observation is the first sighting of the current GRS and that the storm formed again at the same latitude. But the details fade as we go back in time. Questions also arise about the previous storm and its relationship to the current GRS.
New research in Geophysical Research Letters combined historical records with computer simulations of the GRS to try to understand this quixotic weather phenomenon. Its title is “The Origin of Jupiter’s Great Red Spot” and the main author is Agustín Sánchez-Lavega. Sánchez-Lavega is a professor of physics at the University of the Basque Country in Bilbao, Spain. He also heads the university’s planetary sciences group and applied physics department.
“Jupiter’s Great Red Spot (GRS) is the largest and oldest 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 article.
The researchers started with historical sources dating back to the mid-1600s, just after the invention of the telescope. They analyzed the size, structure and movements of the PS and GRS. But it is not a simple task. “The appearance of the GRS and its Hollow throughout the history of Jupiter observations has been highly variable due to changes in size, albedo, and contrast with surrounding clouds,” they write.
“From measurements of sizes and movements, we deduced that it is very unlikely that the current GRS is the PS observed by GD Cassini. The PS probably disappeared between the mid-18th and 19th centuries, in which case we can say that the longevity of the Red Spot today exceeds at least 190 years,” said lead author Sánchez-Lavega. The GRS measured 39,000 km in 1879 and has since been reduced to 14,000 km. It has also become more rounded.
Historical records are valuable, but we now have different tools. Space telescopes and spacecraft have studied the GRS in ways that would have been unimaginable to Cassini and others. NASA’s Voyager 1 captured our first detailed image of the GRS in 1979, when it was just over 9,000,000 km from Jupiter.
Since the Voyager image, the Galileo and Juno spacecraft have both photographed 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 anyone to process the images, resulting in nifty GRS images like the one below.
Juno also measured the depth of the GRS, something previous efforts could not achieve. Recently, “various instruments on board the Juno mission orbiting Jupiter showed that the GRS is shallow and thin compared to its horizontal dimension, because vertically it is about 500 km long,” Sánchez-Lavega explained.
Jupiter’s atmosphere contains winds blowing in opposite directions at different latitudes. North of the GRS, winds blow in a westerly direction and reach speeds of 180 km/h. South of the GRS, winds blow in the opposite direction at speeds of 150 km/h. These winds generate powerful wind shear which promotes vortexing.
In their computer simulations, the researchers examined different forces that could produce GRS under these circumstances. They considered the eruption of a gigantic superstorm like the one that occurs, although rarely, on Saturn. They also looked at the phenomenon of smaller vortices created by wind shear that coalesced to form the GRS. Both produced anticyclonic storms, but their shapes and other properties did not match the current GRS.
“From these simulations, we conclude that the superstorm and merger mechanisms, although generating a single anticyclone, are unlikely to have formed the GRS,” the researchers write in their paper.
The authors also point out that if either of these events had happened, we should have seen them. “We also believe that if any of these unusual phenomena had occurred, its consequences in the atmosphere should have been observed and reported by astronomers of the time,” Sánchez-Lavega said.
However, other simulations were found to be more accurate in reproducing the GRS. Jupiter’s winds are known to exhibit instabilities called southern tropical disturbances (STrD). When researchers ran computer simulations of the STrD, they created an anticyclonic storm very similar to the GRS. The STrD captured the region’s different winds and trapped them in an elongated hull like the GRS. “We therefore propose that the GRS is generated from a long cell resulting from the STrD, which acquired coherence and compactness by shrinking,” the authors write.
The simulations show that over time, the GRS would rotate faster as it shrinks and becomes more cohesive and compact until the elongated cell more closely resembles the current GRS. Since this is what GRS looks like today, the researchers opted for this explanation.
This process probably began in the mid-1800s, when the GRS was much larger than it is today. This leads us to conclude that the GRS is only about 150 years old.