Jupiter’s Great Red Spot isn’t as permanent as we thought

Research has revealed that the crimson-hued spot visible today is, on average, larger than the one Italian astronomer Giovanni Cassini espied for the first time in 1665 and called the “Permanent Spot."

Both Cassini’s spot and the current one are anticyclones, or vortexes of winds in high pressure areas. Today’s spot swirls in the gas giant’s Southern Hemisphere, and its winds can reach speeds of nearly 300 miles an hour.

But the 17th century tempest viewed by Cassini likely dissipated and was replaced, according to researchers from Spain, who found that observations of the storm’s size and motions from the mid-1600s and those of today’s storm don’t match. Indeed, astronomers lost track of Cassini’s spot after 1713, and it wasn’t until 1831 that observations of a storm on Jupiter resurfaced.

According to the new research, led by Agustín Sánchez‐Lavega, the storm observed in 1831 is the Great Red Spot we see today. The findings were published earlier this year in the journal Geophysical Research Letters.

Sánchez‐Lavega, a physicist and planetary scientist at the University of the Basque Country in Bilbao, Spain, said it is reasonable to think that although such spots are long-lived, they can form, disappear and reform cyclically in the same area.

Today’s spot is trapped at its current mid-latitude locale between two windy jet streams to the north and south, which flow parallel to Jupiter’s equator in opposite directions. Previously, researchers hypothesized that smaller storms merged to form the Great Red Spot, but, based on simulations of disturbances in the wind currents and behavior, Sánchez‐Lavega’s group concluded that the spot formed because of disturbances between these two jet streams.

The source of the storm’s red color remains a mystery, but National Aeronautics and Space Administration scientists posit the color is formed by chemicals being broken up by sunlight in Jupiter’s upper atmosphere.

Though the Great Red Spot appears stationary and unchanging through a telescope, an October study, published in the Planetary Science Journal, indicated that it is anything but stable and jiggles as it gets squeezed between the two jet streams.

Observations of the spot by the Earth-orbiting Hubble Space Telescope between December 2023 and March 2024 found that, over a 90-day timeline, the storm can look like a skinnier or fatter oval, according to Amy Simon, a senior planetary scientist at NASA’s Goddard Space Flight Center and lead author of the study.

Simon said she agrees that the current spot isn’t the same as the smaller one discovered by Cassini: “Given that storms on Jupiter can’t move in latitude, and generally can’t grow much larger, it doesn’t seem likely to be the same storm."