Jupiter’s Great Red Spot is different from what an Italian astronomer observed in 1665

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Jupiter’s iconic Great Red Spot is a massive storm that has swirled within the atmosphere of the largest planet in the solar system for years.

But astronomers have debated just how old the vortex really is, as well as when and how it formed. Some experts believed it was centuries old and first observed by Italian astronomer Giovanni Domenico Cassini in the 17th century, while others thought the storm was more recent.

Now, new research suggests that the Great Red Spot formed about 190 years ago, which means that Cassini observed something else on Jupiter in 1665. And despite being younger than previously believed, the storm remains both the largest and longest-lived vortex known across our solar system, according to researchers.

A study detailing the findings appeared June 16 in the journal Geophysical Research Letters.

An eye on the storm

Jupiter’s striking appearance features stripes and spots composed of planet-encircling cloud bands and cyclonic storms. Its colors come from the composition of different atmospheric layers, which are individually made of ammonia, water ice, sulfur and phosphorus gases, according to NASA. Swift jet streams sculpt and stretch the clouds into long bands.

Cyclonic storms on Jupiter can last for years because the gas planet doesn’t have a solid surface, which can slow storms down.

The Great Red Spot is a massive vortex within Jupiter’s atmosphere that is about 10,159 miles (16,350 kilometers) wide, which is similar to Earth’s diameter, according to NASA. The storm towers at a height more than 200 miles (322 kilometers) tall.

Screaming winds zip by at 280 miles per hour (450 kilometers per hour) along the storm’s boundaries. And its signature red hue comes from atmospheric chemical reactions.

The iconic feature is visible, even through small telescopes.

And it sounded similar to a dark oval at the same latitude that Cassini first spotted as he looked through his telescope in the mid-1600s. He referred to the feature he had spied as the “Permanent Spot,” and Cassini and other astronomers observed it until 1713, when they lost sight of the storm.

Then, in 1831, astronomers spotted a large, oval-shaped storm at the same latitude on Jupiter, which has persisted and is still observed today. But astronomers have long questioned whether it was possible that the storms were the same phenomenon, or two different vortices that manifested in the same place more than a century apart.

A team of researchers aiming to solve the enigma gathered a wealth of data, analyzing historical drawings and images that depict the spot’s structure, location and size over time. The data was used to create numerical models recreating the storm’s potential longevity.

“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 lead study author Agustín Sánchez-Lavega, a professor of applied physics at the University of the Basque Country in Bilbao, Spain, in a statement. “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.”

The Permanent Spot persisted for about 81 years, and none of the drawings the team analyzed mentioned any specific color for the storm, according to the study authors.

“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 (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.”

Shrinking in size

While going through the historical data, the researchers also explored how the storm originated by carrying out simulations on supercomputers using models of how vortices behave in Jupiter’s atmosphere.

The team ran simulations to see whether the Great Red Spot formed from a gigantic superstorm, the merging of smaller vortices produced by Jupiter’s intense and alternating winds, or from an instability in the winds that could produce an atmospheric storm cell. A storm cell is an air mass sculpted by updrafts and downdrafts of wind that moves as one entity.

While the first two scenarios resulted in cyclones, they differed in shape and other characteristics witnessed in the Great Red Spot.

“We also think that if one of these unusual phenomena had occurred, it or its consequences in the atmosphere must have been observed and reported by the astronomers at the time,” Sánchez-Lavega said.

But the researchers believe that the persistent atmospheric storm cell, which resulted from intense wind instability, produced the Great Red Spot.

The storm measured about 24,200 miles (about 39,000 kilometers) at its longest point, according to data from 1879, but it has been shrinking and becoming more rounded over time, and is now about 8,700 miles (14,000 kilometers).

Previous research, published in March 2018, has shown that the Great Red Spot is growing taller as it shrinks in size overall. The 2018 study also used archival data to study how the storm has changed over time.

Data from modern space missions, such as NASA’s Juno spacecraft, has given astronomers unprecedented looks at the storm’s shape.

“Various instruments on board the Juno mission in orbit around Jupiter have shown that the (Great Red Spot) is shallow and thin when compared to its horizontal dimension, as vertically it is about 500 km (310.7 miles) long,” Sánchez-Lavega said.

Going forward, the researchers will try to recreate the storm’s shrink rate over time to understand the processes that keep the storm stable, as well as determine whether it will persist for years to come or disappear when reaching a certain size — which might have been the fate of Cassini’s Permanent Spot.

“I love articles like this that delve into pre-photographic observations,” said Michael Wong, research scientist at the University of California, Berkeley and coauthor of the 2018 paper, after reading Sánchez-Lavega’s research. “(Our) paper used tracking data back to 1880, but the new Sánchez-Lavega paper went back further and used data from hand drawings. The supplemental materials for this article are great, too.”

Wong was not involved in the new study.

“We have a lot to learn about these planets by making continuous long-term observations of their weather and climate.”

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