Astronomers using the Hubble Space Telescope have found a growing 10-sided storm pattern around Saturn's south pole, a younger counterpart to the famous six-sided hexagon that has spun at the planet's north pole for more than four decades. The discovery, published this week in the journal Science Advances, is the first such polygonal wave found in the planet's southern hemisphere.
A shape that almost never repeats
Saturn's northern hexagon was first spotted in images from NASA's Voyager flybys in the early 1980s. The storm is roughly 30,000 kilometers across, wide enough to swallow Earth twice over, and its jet streams rise about 300 kilometers above the cloud tops. Scientists long expected to find a similar wave in the south, but decades of searching turned up nothing.
A restless new feature
Saturn's tilted axis hid the south pole from Earth after 2012. When the region came back into view in 2023, Hubble images from October of that year showed a faint 10-sided polygon. The shape grew clearer by August 2024, and by 2025 the decagon's ten corners were fully developed, though uneven: three sides stood out sharply, four were weaker and three were barely defined.
Unlike the rigid-looking northern hexagon, the decagon's corners shift back and forth on a cycle of about 32 days. That movement suggests the shape is not a solid structure but a meandering atmospheric wave, guided by the curved path of a powerful jet stream. Computer simulations indicate the wave may have formed from a repeating disturbance along the jet, possibly driven by a large counterclockwise-rotating vortex to its north.
An evolving mystery
The researchers, led by a team from the University of the Basque Country, say the decagon shows "complex dynamical behavior, never observed in the hexagon." If the wave is tied to a passing storm, it could be a short-lived feature, unlike the hexagon, which has held its shape for over 40 years. Either way, the discovery gives scientists a rare live example of how giant storms take shape on other worlds.