Scientists have discovered how old Jupiter’s Great Red Spot really is

Centuries ago, a huge red spot disappeared on Jupiter. But years later, a new one was born.

Today we know this striking feature as the “Great Red Spot”, a swirling storm wider than Earth. Curiously, earlier astronomers, such as Giovanni Domenico Cassini in 1665, also observed a colossal red storm at the same latitude on Jupiter – raising the possibility that it is in fact the same storm.

But in newly published research, astronomers examined historical drawings and early telescope observations of Jupiter to conclude that today’s spot is indeed a separate storm from its predecessor, inappropriately known as the “Permanent Spot.” It probably disappeared between the mid-18th and mid-19th centuries.

“What is certain is that no astronomer of the time reported any spot at this latitude for 118 years,” Agustín Sánchez-Lavega, a planetary scientist at the University of the Basque Country in Spain, told Mashable.

Then, in 1831, astronomers began seeing the conspicuous red spot again. New research, published in Geophysical Research Lettersconcludes that this latest spot is at least 190 years old.

That’s an impressive storm. Not only has it been spinning counterclockwise for nearly two centuries, but it also has winds reaching speeds of 400 miles per hour. Planetary scientists at NASA and elsewhere are working to understand what gives the space storm its brilliant red hue.

Centuries-old documentation of the Persistent Spot also shows that it was much smaller than the Great Red Spot in the 19th century (and later), meaning that this earlier storm would have to triple in size. But that’s not something astronomers have ever witnessed in a storm on Jupiter, Sánchez-Lavega explained.

a: A 1711 painting of Jupiter by Donato Creti depicting the Permanent Spot. b: Drawing by French artist EL Trouvelot in November 1880, depicting the Great Red Spot. c: November 1881 drawing by TG Elger showing the Great Red Spot.
Credit: Donato Creti / EL Trouvelot / TG ​​Elger

You may be wondering how the Great Red Spot, so unique in its color and size, came to be. You are not alone. To find out, the research team also ran computer simulations based on the behavior of vortices (or storms) in the Jovian atmosphere. The most compelling result—which created a larger “proto-Great Red Spot” that would collapse into a more compact storm—was unstable winds and atmospheric disturbances in this region of Jupiter’s atmosphere. Another leading candidate was the possibility of several storms merging, but this did not produce anything resembling the Great Red Spot.

For more than 150 years, the Great Red Spot has been shrinking. In 1879, when it looked more like a sausage, it was about 24,200 miles (39,000 kilometers) across. It is now 8,700 miles (14,000 kilometers) wide, about the size of its predecessor. The next stages of the spot are uncertain.

The Great Red Spot as seen in April 2017, with Earth overlaid.
Credit: NASA/JPL-Caltech/SwRI/MSSS/Christopher Go

“We don’t know what the future will be like [Great Red Spot] is,” Sánchez-Lavega said. If it continues to shrink, it can split. Or, he added, “It can reach a stable size and last for a long time.”

One thing’s for sure: We’ll be watching from our perch hundreds of millions of miles away.