The Red Spot on Jupiter is a massive, persistent high-pressure storm system, located in the planet’s southern hemisphere. It measures over 16,000 kilometers (10,000 miles) in width, which makes it larger than Earth. The storm has been raging for over 400 years — or so we thought.

According to new research, the spot reported in the 1670s is “highly unlikely” to be the same one we’re observing today.

That’s not the spot

The discovery of the Great Red Spot is often credited to British polymath Robert Hooke, although Italian mathematician Giovanni Cassini made a much more convincing description. Whichever of the two it was, they made the Red Spot known to the scientific community in the 17th century.

Since 1831, astronomers have constantly observed and monitored the Red Spot. They’ve measured and calculated it size, but its age and formation have been more difficult to determine.

Researchers in Spain have now modeled the physical characteristics and movement of the spot. According to their analysis, what we’re seeing now is “very likely” not the same spot that was observed by Cassini.

“From the measurements of sizes and movements, we deduced that it is highly unlikely that the current Great Red Spot was the ‘Permanent Spot’ observed by Cassini,” said Agustín Sánchez-Lavega, a planetary scientist at the University of the Basque Country in Bilbao, Spain, who led this research. “The ‘Permanent Spot’ probably disappeared sometime between the mid-18th and 19th centuries, in which case we can now say that the longevity of the Red Spot exceeds 190 years.”

A surprising vertex

The Great Red Spot is an anticyclonic storm. It rotates counterclockwise in Jupiter’s southern hemisphere. This storm system is incredibly large, but also remarkably stable. It’s so stable because it lies between two jet streams flowing in opposite directions. This helps maintain its position and structure.

The storm’s wind speeds can reach up to 432 kilometers per hour (268 miles per hour), significantly stronger than terrestrial hurricanes. Despite its immense power and size, the Great Red Spot has been gradually shrinking over the past few decades. Its shape has also changed from an oval to a more circular form.

Researchers now believe that the spot will disappear soon, and Jupiter might have had several such spots in its history.

In the new study, researchers looked at historical sources to analyze the evolution of the spot’s size, structure, and location. In 1879, the vertex was 39,000 km (24,200 miles) at its longest axis, over two times bigger than it is today. But we know less about how big it was one or even two centuries earlier.

New Perspectives

“It has been very motivating and inspiring to turn to the notes and drawings of Jupiter and its Permanent Spot made by the great astronomer Jean Dominique Cassini, and to his articles of the second half of the 17th century describing the phenomenon,” Sánchez-Lavega said. “Others before us had explored these observations, and now we have quantified the results.”

In addition to analyzing historical sources, the researchers conducted numerical simulations on supercomputers using two models. The models examined the behavior of thin vortices within Jupiter’s atmosphere. The researchers theorized that the spot might have formed from a massive superstorm, akin to those seen on Jupiter’s sister planet, Saturn.

Alternatively, it could have emerged from the merger of numerous smaller vortices. These would have formed through wind shear from intense, alternating wind currents flowing parallel to each other at different latitudes. Another possibility is that an instability in the winds led to the formation of an elongated atmospheric cell resembling the Great Red Spot. Whatever the case was, however, it’s unlikely for this spot to have been around in Cassini’s time.

The simulation results showed that although an anticyclone develops in the first two scenarios, it does not match the current Great Red Spot in terms of shape and dynamic properties. However, the instability-induced wind cell could have generated a “proto-Great Red Spot”. This then gradually shrank, eventually evolving into the compact, rapidly rotating Great Red Spot observed in the late 19th century.

It’s not impossible for the same Red Spot to be over four centuries old, but it’s unlikely.

The team is now calculating the predicted evolution of the Red Spot and the likelihood of a new vertex appearing after this one disappears. The current Red Spot may disappear within decades, according to some models.

This study was published in Geophysical Research Letters, an open-access AGU journal. View and download a pdf of the study here.