Astronomers think they’ve figured out how and when Jupiter’s red spot formed

The planet Jupiter is best 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 1600s, including Giovanni Cassini, also reported a similar spot when observing Jupiter, which they called the “Permanent Spot”. This has led scientists to question whether the spot observed by Cassini is the same as the one we see today. Now we have an answer to that question: the spots are not all the same, according to a new paper published in the journal Geophysical Research Letters.

“From measurements of magnitudes and motions, we conclude that it is highly unlikely that the current Great Red Spot is 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 middle of the 18th and 19th centuries, in which 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 were ancient Chinese astronomers; observations of the latter would eventually give rise to the Chinese zodiac in the 4th century BC, with its 12-year cycle based on the gas giant’s orbit around the Sun. In 1610, aided by the advent of telescopes, Galileo Galilei famously observed the four largest moons of Jupiter, strengthening the Copernican heliocentric model of the solar system.

It is possible that Robert Hooke may have observed the “Permanent Spot” as early as 1664, followed by Cassini a year later and several more observations until 1708. It then 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 prominent in observations of Jupiter, fading again in 1883 and in the early 20th century.

Maybe the place is not the same…

But was it the same Permanent Spot observed by Cassini? Sánchez-Lavega and his co-authors set out to answer this question, combing through historical sources—including Cassini’s 17th-century notes and drawings—and more recent astronomical observations, and quantifying the results. They made annual measurements of the sizes, ellipticities, area and motions of both the Permanent Spot and the Great Red Spot from the earliest recorded observations into the 21st century.

The team also performed several numerical computer simulations testing different models of the vortex behavior in Jupiter’s atmosphere that is the likely cause of the Great Red Spot. It is essentially a massive, persistent anticyclonic storm. In one of the models the authors tested, the spot forms as a result of a massive superstorm. Alternatively, several smaller eddies created by wind shear may have merged, or there may have been an instability in the planet’s wind currents that resulted in an elongated atmospheric cell shaped like this spot.

Sánchez-Lavega et al. concluded that the current red spot is probably not the same as the one observed by Cassini and others in the 17th century. They claim that the Permanent Spot faded in the early 18th century and a new spot formed in the 19th century – the one we see today, making it more than 190 years old.

But maybe it is?

Others, such as astronomer Scott Bolton of the Southwest Research Institute in Texas, are not convinced by this conclusion. “I think what we can see is not so much that the storm went away and then a new one came in almost exactly the same place,” he told New Scientist. “It would be a very big coincidence if it happened at the same time. exact latitude or even a similar latitude. It’s possible that what we’re really watching is the evolution of the storm.”

Numerical simulations ruled out a vortex-merging model for the formation of the spot; it is much more likely to be caused by wind currents creating an elongated atmospheric envelope. Additionally, in 1879 the Red Spot measured about 24,200 miles (39,000 kilometers) on its longest axis, and is now about 8,700 miles (14,000 kilometers). Thus, the site shrank and rounded over the following decades. The most recent observations by the Juno mission also revealed that the spot is thin and shallow.

The question of why the Great Red Spot is shrinking remains a matter of debate. The team plans more simulations to reproduce the shrinking dynamics and predict whether the spot will settle to a certain size and remain stable, or eventually disappear, as Cassini’s Permanent Spot likely did.

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