Jupiter’s Great Red Spot is one of the most famous and spectacular sights in the Solar Systems. Wider than the diameter of the Earth, the spot is a giant vortex of winds up to 400 kilometers per hour. Its reddish color probably comes from complex organic molecules that form in its upper atmosphere, although nobody is quite sure.

The Spot may have first been seen by the Italian astronomer Giovanni Cassini in 1665 and then observed throughout his life until his death in 1712. But after that something strange happened: the spot disappeared and was not seen again for over a hundred years, despite observations by some of the leading astronomers of the time, such as Charles Messier and William Herschel. Then in 1831 it re-appeared and has since been studied in detail from Earth and from various spacecraft.

This hundred-year gap raises an interesting question. Some astronomers have suggested that Cassini didn’t see the Great Red Spot at all but some other atmospheric storm that dispersed soon after his death. The Spot we see today, they say, formed later. However, working out the truth of this conjecture has been hard.

Spot the Difference

Until now. Agustín Sánchez-Lavega at the University of the Basque Country in Bilbao, Spain, and colleagues say they have finally solved the mystery by simulating how the Great Red Spot evolved. This also revealed how the Spot probably formed in the first place.

The team begin by re-examining the drawings that Cassini made of his spot and the drawings that others made later. The most obvious evidence that Cassini’s spot is not the Great Red Spot is that the drawings of Herschel, Messier, Schroeder and others record nothing like them at the same latitudes in their drawings. “It would be surprising if, had it existed, none of the astronomers of the time had reported [Cassini’s Spot],” say Sánchez-Lavega and co.

Next, they look at the characteristics of Cassini’s spot and say it was 2-3 times smaller than the Great Red Spot in 1873. If these were the same objects, Cassini’s spot must have grown in diameter at the rate of 160 kilometers per year for 166 years.

“This is highly unlikely,” say Sánchez-Lavega and colleagues, pointing out that this kind of growth has never been observed in Jupiter’s vortices.

What’s more, rather than growing, the Great Red Spot has been consistently shrinking, particularly since 2010. Today, it is about the same size as the spot Cassini observed.

All that makes it highly unlikely that Cassini’s Spot is the Great Red Spot. Instead, say the team, Cassini’s Spot must have dispersed, and the Great Red Spot must have formed during the hundred years after Cassini died. But how did it form?

Astronomers have identified three formation mechanisms. The first is that the Spot formed as a “super storm” in the same way as storms form on Earth — as convective cells of energetic, moist air that rise from within the atmosphere forming a vortex of rotating air.

Raging Vortex

Sánchez-Lavega and co say there are various historical examples of this kind of mechanism, such as the origin of the Great White Spot, which still rages today.

But when the team simulated the formation of the Great Red Spot in this way, it was much smaller than observed in 1831. The simulated spot also formed into unrealistic oval shapes and rotated faster.

The second formation mechanism is that two or more anticyclones could have merged to form a bigger one. Again, astronomers have observed several examples of this on Jupiter.

However, Sánchez-Lavega and co say the simulations suggest this would have required anticyclones that were at least as large as the Great Red Spot today and there is no evidence that these existed on Jupiter before 1831.

That leaves the third and last formation mechanism, which is that the Spot formed from the interaction between adjacent bands of winds travelling across Jupiter at different speeds. It would have begun as an elongated rotating cell and then evolved.

In this case, the simulations more or less exactly match the historical observations of the Great Red Spot. Back in 1831, the spot was considerably elongated and his since become more compact and oval. “We therefore propose that the GRS generated from a long cell that acquired coherence and compactness as it shrank,” say Sánchez-Lavega and co. That makes the Spot about 200 years old.

That’s fascinating work that finally lays to rest at least some of the mysteries of the Great Red Spot and how it formed. Cassini would surely approve!

Ref: The Origin of Jupiter’s Great Red Spot : arxiv.org/abs/2406.13222