Astronomers have discovered an enormous 10-sided atmospheric wave encircling Saturn's south pole, the first large regular polygonal jet pattern identified in the planet's southern hemisphere. The Spanish-led team reported the finding on September 2 in Science Advances after combining NASA Hubble Space Telescope observations with images from ground-based professionals and amateur astronomers. Researchers say the decagon spans several atmospheric layers and may still be developing.
Hubble data trace faint signs of the structure to 2023, when Saturn's seasonal tilt gradually brought its southern polar region back into view from Earth. Ground observers noticed a subtle wavy band in 2024, while observations during 2025 made its 10 vertices increasingly clear. Earlier Hubble images and measurements by NASA's Cassini spacecraft, which orbited Saturn from 2004 to 2017, showed no comparable long-lived southern feature, suggesting that it formed between 2017 and 2023.
The decagon lies within a powerful jet stream whose peak winds reach about 400 kilometers per hour, according to reporting based on the study. Yet the whole shape drifts eastward relative to Saturn's rotation at only about 10 kilometers per hour. Its vertices also oscillate on a cycle of roughly 32 days. A single side extends more than 16,700 kilometers, and unevenly defined corners indicate that the structure may be less stable than Saturn's famous northern hexagon.
Voyager observations revealed the northern hexagon in the early 1980s, and spacecraft have continued to see it for more than 40 years, longer than one Saturn year of about 30 Earth years. The northern formation is nearly stationary, unlike the moving southern decagon. Hubble observations at different wavelengths and infrared measurements from the European Southern Observatory's Very Large Telescope in Chile indicate vertical cloud layering and wind speeds that change with altitude.
Scientists do not yet know whether the two polygons are rooted deep below Saturn's visible clouds or arise mainly in upper-level weather. Preliminary numerical simulations show that a 10-sided wave can form in several ways and persist on its jet stream, but no single model yet explains the observation. The team plans continued Hubble and James Webb Space Telescope monitoring as southern visibility improves, seeking to learn whether the decagon stabilizes, changes shape or fades and what Saturn's geometric storms reveal about giant-planet atmospheres.
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