Saturn's north pole hosts a hexagonal jet stream pattern that has persisted for more than forty years since Voyager first observed it. For decades, it was assumed to be a quirk unique to the northern hemisphere—Cassini orbited Saturn for thirteen years without finding any polygonal trace in the south. On September 2, a paper in *Science Advances* announced that at 63° south latitude, a ten-vertex standing wave is now forming around the south pole.
How It Was Seen
Saturn's axial tilt means its southern hemisphere faced away from Earth for over a decade. The turning point came in October 2023, when Hubble captured a faint ten-vertex pattern in a jet stream at 63°S. By August 2024, the outline had sharpened. On August 29, 2025, Hubble took definitive images in the 763-nanometer band, with ground-based telescopes confirming all ten vertices fully formed, with brightness varying unevenly around the circle. NASA's OPAL program, which takes annual portraits of the outer planets, provided more than a decade of archival data that proved essential. The feature is new.
A Newcomer with a Different Temperament
Corresponding author Agustín Sánchez-Lavega of the University of the Basque Country has led a team searching archival imagery for a southern counterpart since 1990, reasoning that Saturn's north and south jet streams look symmetric. Cassini's images revealed nothing. What makes the decagon remarkable is its age—Hubble's data trace it back at most to 2023. It wasn't there before; it was born recently, less than three years old.
Amy Simon of NASA's Goddard Space Flight Center, OPAL's lead, expressed near-astonishment in the press release: nothing like this had ever been seen in the southern hemisphere, while the northern hexagon is there every time we look. What fascinates her most is why it formed now.
Why a Gas Planet Draws Polygons
Saturn has no coastlines, mountains, or obstacles to block wind. In such a world, stable geometry can only emerge from the jet stream itself: when the velocity shear across a polar jet exceeds a threshold, the flow destabilizes and waves arrange themselves along latitude circles—six peaks make a hexagon, ten peaks make a decagon. This explanatory framework was refined over decades on the northern hexagon, with many competing models. But this time researchers aren't rushing to apply them, because the newcomer is vertical: the wave structure penetrates multiple atmospheric layers, not just a skin at the cloud tops.
How it penetrates, why it formed now, and how long it will last remain open questions for Hubble and Webb to pursue.
The comparison is striking: the northern hexagon has 46 years of observations; the southern feature is at most three years old—a gap of more than a decade in age, four more corners. A new experiment in planetary atmospheres has just begun, and we have seats for opening night. The next time someone says gas giant atmospheres are boring, show them this decagon—a planet learning a new shape with a mouth 100,000 kilometers wide.
Source: Paper published in *Science Advances*, September 2.