Astronomers believe they have discovered how and when Jupiter’s Red Spot was formed

The planet Jupiter is particularly known for its so-called Great Red Spot, a swirling vortex in the gas giant’s atmosphere that has existed since at least 1831. But how it formed and how old it is remains a matter of debate. Astronomers in the 17th century, including Giovanni Cassini, also reported a similar spot, which they called the “Permanent Spot,” during their observations of Jupiter. This led scientists to question whether the spot observed by Cassini is the same one we see today. Now we have an answer to that question: According to a new paper published in the journal Geophysical Research Letters, the spots are not the same.

“From the measurements of size and motion, we concluded that it is highly unlikely that the current Great Red Spot was the ‘Permanent Spot’ observed by Cassini,” said co-author Agustín Sánchez-Lavega of the University of the Basque Country in Bilbao, Spain. “The ‘Permanent Spot’ probably disappeared sometime between the mid-18th and 19th centuries. In this case, we can now say that the lifetime of the Red Spot exceeds 190 years.”

The planet Jupiter was known to Babylonian astronomers in the 7th and 8th centuries BC, as well as to ancient Chinese astronomers. The latter’s observations eventually led to the Chinese zodiac, with its 12-year cycle based on the gas giant’s orbit around the Sun, in the 4th century BC. In 1610, Galileo Galilei, aided by the invention of telescopes, observed Jupiter’s four largest moons, thus reinforcing the Copernican heliocentric model of the solar system.

Robert Hooke may have observed the Permanent Spot as early as 1664, Cassini followed him a year later and observed it several more times until 1708. Then it disappeared from the astronomical record. A pharmacist named Heinrich Schwabe made the first known drawing of the Red Spot in 1831, and in 1878 it was again quite conspicuous in observations of Jupiter, but disappeared again in 1883 and at the beginning of the 20th century.

Maybe the place is not the same…

But was this the same Permanent Spot that Cassini had observed? Sánchez-Lavega and his co-authors set out to answer that question. They combed through historical sources – including Cassini’s 17th-century notes and drawings – as well as more recent astronomical observations, and quantified the results. They made a year-by-year measurement of the size, ellipticity, area and motion of both the Permanent Spot and the Great Red Spot, from the earliest recorded observations to the 21st century.

The team also ran several numerical computer simulations to test different models of the vortex behavior in Jupiter’s atmosphere, the likely cause of the Great Red Spot. Essentially, it is a massive, persistent anticyclonic storm. In one of the models the authors tested, the spot forms in the wake of a massive superstorm. Alternatively, several smaller vortices created by wind shear could have merged together, or there could have been an instability in the planet’s wind currents that led to an elongated atmospheric cell in the shape of the spot.

Sánchez-Lavega et al. concluded that the current Red Spot is probably not the same one observed by Cassini and others in the 17th century. They argue that the Permanent Spot had faded by the beginning of the 18th century and a new spot formed in the 19th century – the one we observe today, making it over 190 years old.

But maybe that’s it?

Others, such as astronomer Scott Bolton of the Southwest Research Institute in Texas, are not convinced by this conclusion. “I think what we’re seeing is not so much that the storm disappeared and then a new one appeared in almost the same place,” he told New Scientist. “It would be a huge coincidence if it appeared at exactly the same or even a similar latitude. It could be that we’re actually watching the storm evolve.”

The numerical simulations ruled out the merging vortex model as the cause of the spot’s formation; it is much more likely that it is due to wind currents that create an elongated atmospheric envelope. In addition, the Red Spot was about 39,000 kilometers long along its longest axis in 1879 and is about 14,000 kilometers long today. The spot has thus shrunk and become more rounded in the decades that followed. The recent observations from the Juno mission also showed that the spot is thin and flat.

The question of why the Great Red Spot is shrinking remains controversial. The team plans to conduct further simulations to reproduce the shrinkage dynamics and predict whether the spot will stabilize at a certain size and stay there, or eventually disappear, as Cassini’s Permanent Spot is thought to have done.

Geophysical Research Letters, 2024. DOI: 10.1029/2024GL108993 (About DOIs).