Jupiter’s Great Red Spot jiggles like JELL-O

If the moon is said to be made of cheese (it’s not), then Jupiter’s famous Great Red Spot (GRS) is more like a bowl of JELL-O. Recent observations of this massive anticyclone on our solar system’s largest planet show it jiggling. These observations were made using NASA’s Hubble Space Telescope over a period of 90 days between December 2023 and March 2024, allowing astronomers to create a time-lapse video of GRS’ wobbly movements. The findings from these observations are detailed in a study published on October 9 in The Planetary Science Journal.

“While we knew that its motion varied slightly in longitude, we did not expect to see changes in its size. To our knowledge, this phenomenon has not been observed before,” said study co-author and astronomer Amy Simon from NASA’s Goddard Space Flight Center. “This is the first time we have had such detailed imaging of the GRS. With Hubble’s high resolution, we can confirm that the GRS is indeed expanding and contracting while also moving at varying speeds. This was a surprising discovery, and currently, there are no hydrodynamic explanations for it.”

The GRS is an anticyclone—a large wind system that rotates counterclockwise around a center of high pressure in a planet’s southern hemisphere and clockwise in its northern hemisphere. Jupiter’s GRS is large enough to engulf the Earth and subject it to stormy conditions for over 150 years. Hubble continuously monitors Jupiter and the other outer planets of the solar system through the Outer Planet Atmospheres Legacy program (OPAL). However, these recent observations were part of a program dedicated solely to studying the GRS and its intriguing behavior. Understanding the mechanisms behind the solar system’s largest storms helps place the theory of hurricanes on Earth in a broader cosmic perspective and can provide insights into meteorology on planets around other stars.

In their study, Simon’s team used Hubble to zoom in on the GRS and examine its size, shape, and any subtle changes in color. Numerous aspects of the GRS change on a daily basis, including observations in ultraviolet light that reveal the distinct core of the storm becoming brightest when the GRS reaches its maximum size in its oscillation cycle. According to the team, this indicates less absorption of haze in the upper atmosphere.

“As the GRS speeds up and slows down, it interacts with the powerful jet streams to its north and south,” explained study co-author and planetary scientist Mike Wong from the University of California at Berkeley. “It’s like a sandwich where the slices of bread bulge out when there’s too much filling in the middle.”

In contrast, the planet Neptune features dark spots that can move extensively in latitude without being held in place by strong jet streams. Jupiter’s GRS has remained at a southern latitude, trapped between these jet streams, for as long as Earth-based telescopes have been observing it.

[Related: Jupiter’s Great Red Spot keeps shrinking.]

The team has been monitoring the gradual shrinking of the GRS since the inception of the OPAL program a decade ago. They anticipate that it will continue to decrease in size until it assumes a stable and less elongated shape.

“Currently, the GRS is expanding beyond its latitude band relative to the wind field. Once it contracts within that band, the winds will effectively hold it in place,” Simon explained. The team predicts that the GRS will eventually stabilize in size, but as of now, Hubble has only observed it for one oscillation cycle.