planet Jupiter The Sun is particularly known for the so-called Great Red Spot, a swirling vortex in the gas giant’s atmosphere that has been around since at least 1831. But how it formed and how old it is remains a matter of debate. Astronomers in the 1600s, Giovanni Cassinireported a similar speck in its observations of Jupiter, which it named the “permanent speck.” This led scientists to wonder whether the speck Cassini observed was the same one we see today. That question now has an answer: the speck is not the same, the researchers say. New paper The results were published in the journal Geophysical Research Letters.
“Based on our measurements of its size and motion, we infer that it is highly unlikely that the current Great Red Spot is the ‘permanent spot’ observed by Cassini.” Co-author Agustin Sanchez Lavega said: “The ‘permanent spot’ probably disappeared between the mid-18th and 19th centuries, which would mean the lifespan of the red spot would exceed 190 years,” said researchers from the University of the Basque Country in Bilbao, Spain.
Jupiter was also known to Babylonian astronomers in the 7th and 8th centuries BC, and to ancient Chinese astronomers. The latter’s observations led to the Chinese Zodiac, a 12-year cycle based on the gas giant’s orbit around the Sun, in the 4th century BC. In 1610, aided by the advent of the telescope, Galileo Galilei famously observed Jupiter’s four largest moons, thereby strengthening the Copernican heliocentric model of the solar system.
Robert Hooke may have observed the “permanent spot” as early as 1664, followed a year later by Cassini, and it was observed many times until 1708. After that, the Red Spot disappeared from the astronomical record. A pharmacist named Heinrich Schwabe made the earliest known drawing of the Red Spot in 1831, and it became prominent again in observations of Jupiter by 1878, fading again by 1883 and the early 20th century.
Maybe it’s not the same place…
But was this the same permanent spot observed by Cassini? To answer this question, Sanchez-Lavega and her co-authors combed through historical sources, such as Cassini’s notes and drawings from the 17th century, as well as more recent astronomical observations, and quantified their results. They measured the size, ellipticity, area, and motion of both the permanent spot and the Great Red Spot by year, from the earliest recorded observations through the 21st century.
The team also ran several numerical computer simulations testing different models for the behavior of vortices in Jupiter’s atmosphere that could be the cause of the Great Red Spot. The Great Red Spot is essentially a large, persistent anticyclonic storm. In one model the authors tested, the Great Red Spot formed after a massive superstorm. Alternatively, several smaller vortices generated by wind shear could have merged, or the planet’s wind currents could have become unstable, forming elongated atmospheric cells shaped like the Great Red Spot.
Sánchez-Lavega and his colleagues conclude that the current Red Spot is probably not the same as the one observed by Cassini and his crew in the 17th century: they argue that the permanent spot had faded by the early 18th century, and that a new spot (the one seen today) formed in the 19th century, making it more than 190 years old.
But maybe that’s true?
Some, like astronomer Scott Bolton of the Southwest Research Institute in Texas, aren’t convinced by that conclusion: “I suspect what we’re seeing is not a storm passing and then a new storm coming in roughly the same place.” he told New Scientist.“That it occurs at exactly the same latitude, or a similar latitude, is a tremendous coincidence. What we’re actually observing may be the evolution of the storm.”
Numerical simulations show that the vortex-merging model of sunspot formation does not apply; it is much more likely that wind currents have created an elongated shell of atmosphere. Furthermore, in 1879, the Red Spot was about 24,200 miles (39,000 km) across on its longest axis; today, it is about 8,700 miles (14,000 km), meaning that it has shrunk and become rounder over the decades since. Recent observations from the Juno spacecraft also reveal that the Red Spot is thinner and shallower.
The question of why the Great Red Spot is shrinking remains controversial, and the team plans further simulations aiming to reproduce the dynamics of the shrinkage and predict whether the spot will remain stable at a constant size or whether it will eventually disappear, as Cassini’s permanent spot likely did.
Geophysical Research Letters, 2024. DOI: 10.1029/2024GL108993 (About DOIs)