Jupiter's Great Red Spot (GRS) is one of the most distinctive features of our solar system. It is a powerful storm that astronomers have been observing since the 17th century, but its date of origin and duration are disputed. Have we been observing the same phenomenon all this time?
The GRS is a gigantic anticyclonic (anti-clockwise rotating) storm that is larger than the Earth. Its wind speeds exceed 400 km/h. It is a symbol that humanity has observed since at least the 19th century, possibly earlier. Its history and formation are a mystery.
The first observations are believed to have taken place in 1632, when a German Abbott observed Jupiter with his telescope. Thirty-two years later, another observer reported seeing the GRS moving from east to west. Then, in 1665, Giovanni Cassini examined Jupiter with a telescope and noticed 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 lost track of the location. For 118 years, no one saw the GRS until astronomer S. Schwabe spotted a clear structure that was roughly oval and located at the same latitude as the GRS. Some believe that this observation was the first observation of the current GRS and that the storm formed again at the same latitude. But the details fade the further back we look 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 records with computer simulations from the GRS to understand this chimeric meteorological phenomenon. The title of the study 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 at the university.
“Jupiter’s Great Red Spot (GRS) is the largest and longest-lived vortex known of any planet in the Solar System, but its lifetime is controversial and its formation mechanism remains hidden,” the authors write in their article.
The researchers started with historical sources from the mid-17th century, shortly after the invention of the telescope. They analyzed the size, structure and motion of both the PS and the GRS. But this is not an easy task. “The appearance of the GRS and its hollow has been highly variable throughout the history of Jupiter observations due to changes in size, albedo and contrast with the surrounding clouds,” they write.
This research image compares the Permanent Spot (PS) and the current GRS. a, b, and c are drawings by Cassini from 1677, 1690, and 1691, respectively. d is a current image of the GRS from 2023. Image credit: Sánchez-Lavega et al. 2024.
“From the measurements of size and motion, we concluded that it is highly unlikely that the current GRS is the PS observed by GD Cassini. The PS probably disappeared sometime between the mid-18th and 19th centuries. In this 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 more rounded.
Four views of Jupiter and its GRS. a is a drawing of the Permanent Spot by G. D. Cassini dated 19 January 1672. b is a drawing by S. Swabe dated 10 May 1851. It shows the GRS region as a clear oval with boundaries marked by its trough (drawn by a red dashed line). c is a photograph by AA Common dated 1879. d is a photograph taken by Observatory Lick with a yellow filter dated 14 October 1890. Each image is an astronomical picture of Jupiter with south above and east below. Image credit: Sánchez-Lavega et al. 2024.
The historical record is valuable, but today we have other tools at our disposal. Space telescopes and space probes have studied the GRS in ways that would have been unimaginable for Cassini and others. NASA's Voyager 1 captured our first detailed image of the GRS in 1979, when it was just 9 million kilometers from Jupiter.
Jupiter's Great Red Spot, taken by Voyager 1 in 1979. The complex wave patterns had never been seen before this image. Image credit: From NASA – http://photojournal.jpl.nasa.gov/catalog/PIA00014, Public Domain, https://commons.wikimedia.org/w/index.php?curid=86812
Since Voyager's image, both the Galileo and Juno spacecraft have taken images of the GRS. Juno in particular has given us more detailed images and data of Jupiter and the GRS. It has taken pictures 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 edit the images, resulting in artistic images of the GRS like the one below.
A different view of Jupiter and its GRS. Image credit: NASA / SwRI / MSSS / Navaneeth Krishnan S © CC BY
Juno also measured the depth of the GRS, something that was not possible in previous attempts. Recently, “various instruments on board the Juno mission in orbit around Jupiter showed that the GRS is flat and thin compared to its horizontal extent, as it is about 500 km long vertically,” explained Sánchez-Lavega.
In Jupiter's atmosphere, winds blow in opposite directions at different latitudes. North of the GRS, the winds blow in a westerly direction and reach speeds of 180 km/h. South of the GRS, the winds blow in the opposite direction at speeds of 150 km/h. These winds create a strong wind shear that promotes the vortex.
In their supercomputer simulations, the researchers examined various forces that could produce the GRS under these circumstances. They considered the outbreak of a gigantic superstorm, such as those that occur on Saturn, albeit rarely. They also examined the phenomenon of smaller vortices generated by the 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 fusion mechanisms, although they produce a single anticyclone, probably did not form the GRS,” the researchers write in their article.
The authors also point out that if any of these phenomena had happened, we should have seen them. “We also believe that if any of these unusual phenomena occurred, it or its consequences in the atmosphere must have been observed and reported by the astronomers of the time,” said Sánchez-Lavega.
However, other simulations proved more accurate at reproducing the GRS. Jupiter's winds are known for their instabilities, called the South Tropical Disturbance (STrD). When researchers ran supercomputer simulations of the STrD, they produced 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 arose from a long cell that resulted from the STrD and acquired coherence and compactness as it shrank,” the authors write.
These research images show how the GRS was formed. a is a drawing of the STrD by TER Phillips in 1931-1932. The red arrows indicate the flow direction with longitude marked. b and c are maps created from images from the New Horizons spacecraft. The yellow arrows mark position-velocity changes in the STrD. The STrD caught winds and created a long cell that produced the Great Red Spot. Image credit: Sánchez-Lavega et al. 2024.
The simulations show that the GRS spins faster over time as it shrinks and becomes more coherent and compact, until the elongated cell becomes more similar to the current GRS. Since this is what the GRS looks like now, the researchers settled on this explanation.
This process probably began in the mid-19th century, when the GRS was much larger than it is today. This suggests that the GRS is only about 150 years old.
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