The venerable Hubble Space Telescope has witnessed a remarkable phenomenon occurring within Jupiter’s iconic Great Red Spot (GRS). This colossal anticyclone, a storm that has been shrinking over the decades and is currently about 9,165 miles (14,750 kilometers) across, appears to be pulsating, oscillating in and out as if squeezed by an invisible hand. This perplexing behavior is occurring roughly every 90 days, leaving scientists puzzled about the driving forces behind it.
“With Hubble’s high resolution, we can say that the GRS is definitely squeezing in and out at the same time as it moves faster and slower,” said Amy Simon, of NASA’s Goddard Space Flight Center in Maryland, in a statement. “This was very unexpected, and at present there are no hydrodynamic explanations.”
Astronomers led by Simon observed the Great Red Spot with Hubble for 88.5 days between December 2023 and March 2024. A time-lapse of images taken during this period reveals a fascinating pattern: the GRS periodically expands and contracts along its semi-major axis (the widest part of an ellipse). “While we know its motion varies slightly in longitude, we didn’t expect to see the size oscillate as well,” Simon added.
This giant vortex, located 22 degrees south of Jupiter’s equator on the edge of the South Equatorial Belt, is constantly buffeted from above and below by powerful jet streams that race around the planet at speeds of 266 mph (428 kph). These jet streams prevent the GRS from wandering into other latitudes, although it does drift westward with respect to the rest of the Jovian atmosphere. This drift is not steady but exhibits a cyclical acceleration and deceleration over an approximate 90-day oscillation.
This 90-day oscillation in westward drift seems to be directly linked to the squeezing of the GRS’s shape observed by Hubble. “It’s similar to a sandwich where the slices of bread are forced to bulge out when there’s too much filling in the middle,” explained Mike Wong of the University of California, Berkeley.
The degree of squeezing appears to be inversely related to the rate of the GRS’s westward drift. During periods when the drift decelerates, the width of the vortex and the size of its core reach their maximum. The core also shines brighter in ultraviolet light during these periods, suggesting less haze absorption in the atmosphere above it. Conversely, when the drift accelerates, the width of the GRS and the size of its core shrink. This may be due to the GRS interacting with the surrounding atmosphere as its drift rate increases.
Hubble has only observed one full oscillatory period of the GRS so far. Simon leads the Outer Planet Atmospheres Legacy (OPAL) program, which utilizes Hubble to image the four giant outer solar system planets – Jupiter, Saturn, Uranus, and Neptune – at least once annually. However, the Hubble observations of the GRS were a separate project.
Interestingly, top amateur astronomers, such as Damian Peach, regularly capture high-resolution images of Jupiter, and their data is so precise that Simon and the OPAL team often collaborate with them. It’s possible that the squeezing of the Great Red Spot is evident in amateur images, although Simon believes it might be too subtle for amateur data to capture reliably. Nevertheless, now that we know this squeezing is occurring, amateur astronomers might be able to refine their image capture techniques to identify it.
Simon is also planning to revisit the GRS with the James Webb Space Telescope, which previously imaged the Jovian storm in near-infrared light earlier this year and detected atmospheric waves above the GRS. By probing deeper into the GRS at longer mid-infrared wavelengths, Simon hopes to determine if wind velocities within the storm are also changing in sync with the oscillations.
The combination of the GRS’s overall shrinking and its newfound oscillatory squeezing reveals that this iconic storm is undergoing intriguing changes. What will its fate be? “Right now, it’s over-filling its latitude band relative to the wind field,” Simon said. “Once it shrinks inside that band, the winds will really be holding it in place.” When this happens, the GRS may stabilize in size, but for now, this remains speculation until more data can be gathered. The discovery was reported on Oct. 9 in a paper published in The Planetary Science Journal.