Hubble discovers decagonal wave at Saturn's south pole

New Hubble images reveal a ten-sided jet pattern around Saturn's south pole, a feature never seen before in the planet's southern hemisphere.

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Astronomers using the Hubble Space Telescope have spotted a ten-sided atmospheric wave, or decagon, encircling Saturn's south pole. The feature, reported in Science Advances on Wednesday, is the first large, regular-sided jet pattern observed in the planet's southern hemisphere. It resembles the long-known northern hexagon but is distinctly different, researchers say.

The discovery was made as part of Hubble's Outer Planet Atmospheres Legacy (OPAL) program, which has been imaging the outer planets every year for over a decade. Hubble data show the structure has been present since at least 2023, with earlier observations hinting at its emergence.

"We've never seen anything quite like this in Saturn's southern hemisphere," said Amy Simon, an OPAL principal investigator at NASA's Goddard Space Flight Center. She added, "The northern hexagon has been there every time we've looked for more than 40 years. This feature is different — it appears to be strengthening, giving us the rare opportunity to watch a giant atmospheric pattern develop."

The initial clues came from ground-based images taken in 2024, where a subtle undulating band was noticed along the southern pole by lead author Agustín Sánchez-Lavega, a researcher at the University of the Basque Country in Spain, and amateur astronomers Trevor Barry and Jean-Paul Oger. Additional ground imagery from 2025 pointed more strongly toward the decagon structure. Hubble's space-based view provides unmatched sharpness and spatial resolution over full rotations of Saturn, without the blurring caused by Earth's atmosphere.

Sánchez-Lavega noted that the search for such a feature has been ongoing for decades. "Given Saturn's symmetry in its north-south jet stream system, we have been searching for a counterpart to Saturn's northern hexagon on the south pole in Hubble images since 1990," he said.

The Cassini spacecraft, which orbited Saturn from 2004 to 2017, showed no sign of a long-lived formation at the south pole. The wave sits within one of Saturn's powerful jet streams and extends through multiple atmospheric layers. Its apparent position shifts because Hubble captures images at different wavelengths that probe different altitudes.

"The most intriguing part to me is that this seems to have just formed recently," Simon said. "The question is, why did it suddenly form now when we haven't seen one before?"

Further study is planned using Hubble and the James Webb Space Telescope, along with computer models, to understand the phenomenon. Mike Wong, a co-author at the University of California, Berkeley, reflected on the program's broader mission: "When we started the OPAL program, we expected compelling surprises, but we didn't know what to expect specifically."