A team of researchers has examined observations of Jupiter's Great Red Spot and concluded that the famous anticyclone is not actually as old as previously thought. The research group is led by Agustin Sánchez-Lavega (Universidad del País Vasco, Spain) Geophysical Research Letters The results of the research are mentioned in its columns.
The research was based on observations of the Great Red Spot recorded in recent years, according to which the spot has shrunk in size to a fraction of its former size. In 50 years, the anticyclone has changed from a huge, elongated oval shape to a faint, round spot. It seems to be getting smaller and smaller. Could the Great Red Spot finally disappear? Researchers have been searching for an answer to this.
For those familiar with the history of astronomy, Jupiter's Great Red Spot is a perennial phenomenon, much like the planet's four moons. It is widely accepted that the Great Red Spot has existed for nearly 400 years. Giovanni Cassini charted the spot in 1665, which he described as a “permanent spot,” and another astronomer, Leander Bandtius, reported a large oval spot as early as 1632.
There is no evidence of the Great Red Spot between 1713 and 1831, although it was observed by two famous astronomers, Messier and Herschel, and many of their contemporaries. Could the patch captured by Cassini have disappeared by 1713 and the anticyclone we know today formed more than a hundred years later?
It is not easy to prove that the spot we see today only formed in 1831 and does not match what Cassini saw. In such situations, it makes sense to resort to Ockham's razor and rule out the overly complicated options. But on what basis should we choose? Is the simplest assumption that there was only one spot on Jupiter all along (and observers have not seen it once for over a hundred years), or perhaps there were several spots (which disappeared, but at the same latitude and evolved in exactly the same size as before)?
Sanchez-Lavega and his colleagues used two methods to find an answer. First, they projected past observations onto a sphere and measured the widths and proportions of past spots. They also took into account the filters used to image them, and found that the spot appeared differently depending on whether it was viewed through a blue or red filter. With a blue filter, the red areas remained dark, but when viewed through a red filter, the red parts were fainter and blended in with the white cloud bands that surrounded them.
Observations during Cassini's time suggest a smaller, round spot similar to the one we see today. In contrast, 19th-century observations are of a relatively large phenomenon. Could the spot shrink and then grow again? Or did it disappear completely and a new, larger spot form in its place?
These questions led to the next direction of research: How could a large spot form, and does what forms fit the observed pattern? The researchers created simulations based on some storms on Jupiter and Saturn.
The most closely studied new storm is the Great White Spot, which appeared on Saturn in 2010 while being observed by the Cassini spacecraft. However, simulations of a similar event on Jupiter have not produced a storm as large as the one observed on the planet in the early 19th century. The one that did form had very high wind speeds and never appeared at the latitude of the Great Red Spot.
Three white ovals on Jupiter were recently observed to merge to form an atmospheric feature known as the “BA oval.” This process occurred over a period of about 60 years. If a similar merger created the Great Red Spot in the 19th century, the merging storms would surely have been as large as the spot we see now. Could Messier or Herschel have seen more Great Spots before their merger?
To figure out how such a large bubble could form out of thin air, the researchers ran simulations based on another phenomenon observed on Jupiter, the Southern Equatorial Disturbance. In this case, a bulge forms in a dark belt, blocking the flow of the lighter-colored southern equatorial region. If two such disturbances were to occur, they could erode a region of material in the southern equatorial region about a third of the planet’s size. The large, rotating cell, isolated from outside influences, would become stronger, narrower, and more compact, and its rotation would accelerate over time. They have also been observed in the Great Red Spot, visible today.
Simulations don’t prove anything by themselves, but they can provide plausible answers to more difficult questions, and with their help we can make predictions that we can later verify with observations. According to the researchers, the Great Red Spot currently visible is only the second permanent Great Spot observed by humanity. According to predictions, it will disappear, and a third, giant spot will form in its place at some point.
source: sky and telescope
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