Astronomers think they have 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 1831. But how it originated and how old it is remains a matter of debate. 17th century astronomers, including Giovanni Cassini, also reported a similar spot in their observations of Jupiter, which they called the ‘Permanent Spot’. This led scientists to wonder whether the spot Cassini observed is the same one we see today. We now have an answer to that question: the spots are not the same, according to a new paper published in the journal Geophysical Research Letters.

“From the size and motion measurements, we concluded that it is highly unlikely that today’s 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 which case we can now say that the lifespan of the Red Spot is more than 190 years.”

The planet Jupiter was known to Babylonian astronomers in the 7th and 8th centuries BCE, as well as to ancient Chinese astronomers; His observations would eventually give rise to the Chinese zodiac in the 4th century BC, with its twelve-year cycle based on the orbit of the gas giant around the sun. In 1610, Galileo Galilei, aided by the advent of telescopes, famously observed Jupiter’s four largest moons, reinforcing the Copernican heliocentric model of the solar system.

It is possible that Robert Hooke observed the ‘Permanent Spot’ as early as 1664, while Cassini followed suit a year later and made several more observations through 1708. He then disappeared from the astronomical record. A pharmacist named Heinrich Schwabe made the earliest known drawing of the Red Spot in 1831, and by 1878 it was again quite prominent in observations of Jupiter, before fading again in 1883 and into the early 20th century.

Maybe the place isn’t the same…

But was this the same permanent spot that Cassini had observed? Sánchez-Lavega and his co-authors attempted to answer this question by combining historical sources (including Cassini’s notes and drawings from the 17th century) and more recent astronomical observations and quantifying the results. They took measurements of the sizes, ellipticity, area and motions of both the Permanent Spot and the Great Red Spot year after year from the earliest observations into the 21st century.

The team also ran several numerical computer simulations, testing different models for vortex behavior in Jupiter’s atmosphere, which are likely the 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 in the aftermath of a massive superstorm. Alternatively, several smaller vortices caused 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 in the shape of the spot.

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

But maybe that is true?

Others remain unconvinced of that conclusion, like astronomer Scott Bolton of the Southwest Research Institute in Texas. “What I think we might be seeing is not so much that the storm died down and then a new one came in almost the same place,” he told New Scientist. exact latitude, or even a similar latitude. It may be that we are really looking at the evolution of the storm.”

The numerical simulations excluded the merging eddy model for the formation of the spot; it is much more likely that this is due to wind currents that produce an elongated atmospheric shell. Furthermore, the Red Spot was about 39,000 kilometers long on its longest axis in 1879 and is now about 14,000 kilometers long. So the place has become smaller and rounder in recent decades. The most recent observations from the Juno mission also showed the spot to be thin and shallow.

The question of why the Great Red Spot is shrinking remains a matter of debate. The team plans further simulations to reproduce the shrinking dynamics and predict whether the spot will settle at 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).