Saturn hid a 10-sided south pole storm for years — now it’s getting bigger

Saturn’s south pole is doing something nobody has seen before on the planet: forming a geometric storm. It has ten sides. It sits inside one of Saturn’s powerful atmospheric jet streams, locked into shape just as a ruler-drawn polygon would be, and it has been steadily growing stronger. Scientists have named it a decagon, they have tracked it across multiple years of Hubble observations, and they cannot yet explain why it exists.

The discovery is surprising partly because of what it echoes. Saturn’s northern pole is already famous for its hexagon — a six-sided storm that has persisted, unchanged, for at least 40 years. Voyager photographed it in the 1980s. Cassini spent 13 years studying it. It is one of the solar system’s most striking geometric structures, and the working assumption has always been that something very specific about Saturn’s northern jet stream made it exceptional. The south pole decagon has now quietly retired that assumption.

Two geometric storms on the same planet, at opposite poles, each locked to a different number of sides. Whatever is happening, it is not a fluke.

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How Hubble caught the decagon forming

The Hubble Space Telescope has been monitoring the outer solar system annually for more than a decade through its Outer Planet Atmospheres Legacy program. In 2023, the program’s images of Saturn’s south pole showed faint traces of a ten-vertex shape — subtle enough to read as noise, interesting enough to keep watching. Ground-based observers independently noted an undulating band in southern Saturn’s atmosphere in 2024. By August and September 2025, the pattern had resolved into an unmistakable, fully formed decagon: ten straight sides, ten corners, spinning steadily at Saturn’s south pole.

The research was led by Agustín Sánchez-Lavega of the University of the Basque Country in Spain, working with Amy Simon of NASA’s Goddard Space Flight Center and a team from the Planetary Virtual Observatory Laboratory. Their analysis, published in Science Advances, confirmed that the decagon is not a surface-level cloud pattern but extends through multiple layers of Saturn’s atmosphere — a deep, vertically structured feature anchored inside the jet stream at that latitude.

“We’ve never seen anything quite like this in Saturn’s southern hemisphere,” Simon said. “This feature is different — it appears to be strengthening.”

The hexagon and the decagon: two poles, two numbers

Saturn’s northern hexagon is six-sided and slightly smaller than the southern decagon appears to be. It has remained essentially stable across four decades of observation — same shape, same location, same rotation. No planetary scientist who has studied it considers that stability unremarkable. When a feature endures through seasons that each last seven Earth years, through the death of the spacecraft that studied it most closely, it has earned its reputation as one of the solar system’s most durable structures.

The south pole decagon is the opposite: freshly born, actively strengthening, and ten-sided rather than six. Both structures form within jet streams — high-speed rivers of wind that circle Saturn at fixed latitudes. The jet stream’s interplay with atmospheric waves can trap them into repeating geometric patterns, which is broadly how both polygons are thought to form. The number of sides reflects the wave frequency. Six at the north pole; ten at the south pole. Why different numbers at different poles of the same planet is the question neither team can yet answer.

Possible explanations under consideration include differences in latitude between the two poles, differences in Saturn’s background wind structure at southern versus northern latitudes, and the presence of a high-pressure vortex near the south pole that might be driving the wave into a ten-pointed resonance. All three remain speculative. The paper does not endorse one over the others.

Why it formed — and why no one knows

There is an observational gap at the heart of this story. Saturn’s south pole spent most of the 2010s tilted away from Earth, leaving it essentially invisible from 2012 until 2023. The last detailed look at the southern hemisphere came from Cassini, which was intentionally steered into Saturn’s atmosphere and destroyed in September 2017. Since then, no spacecraft has observed the planet from up close.

This means the decagon almost certainly formed between 2017 and 2023, in a window where Earth-based telescopes could not see it and no orbiting instrument existed to record it. Scientists cannot track its birth backward. They know what it looks like now; they do not know what triggered it or how quickly it appeared.

“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?”

The limitation is real. Hubble can image the outer atmosphere with extraordinary clarity, but it cannot measure wind temperatures and speeds at depth — the quantities that would directly test the proposed mechanisms. Proposals for a Saturn orbiter successor to Cassini exist within both NASA and ESA planning documents, but none has been approved or funded. For now, the annual Hubble surveys provide the only systematic data on what the decagon is doing.

What the team does conclude is that polygonal jet stream waves on Saturn may not be exceptional at all. The paper notes that the northern hexagon is “not as extraordinary as we thought.” The decagon suggests Saturn might generate such structures more readily than assumed — and raises the question of whether gas giants elsewhere do the same.

Common questions about Saturn’s south pole decagon

What exactly is the Saturn south pole decagon?

A ten-sided atmospheric wave sitting inside Saturn’s southern polar jet stream. Both the shape and its location inside a fast-moving wind channel are what make it a geometric storm rather than a random cloud formation. The same type of structure (but six-sided) has existed at Saturn’s north pole for at least 40 years.

Why does the jet stream produce a polygon instead of a circle?

Atmospheric waves trapped inside a jet stream can lock into repeating geometric patterns. The number of sides reflects the wave’s spatial frequency — how many peaks and troughs fit around the planet at that latitude. Why Saturn’s northern jet stream locks into six and the southern into ten is the central open question the paper raises but does not answer.

How does the decagon compare to the hexagon?

The northern hexagon is six-sided, has been stable for over 40 years, and is slightly smaller. The southern decagon is ten-sided, has existed for no more than a decade, and is still growing stronger. They form the same type of structure through similar mechanisms, but at different latitudes and with different geometries.

Could the decagon disappear?

It could. The hexagon has persisted for at least four decades and may continue indefinitely; the decagon is too young to have shown whether it will stabilize or change shape. Continued Hubble monitoring will track whether it behaves like the hexagon — holding steady — or evolves into something different.

The Hubble program that captured the decagon’s emergence will continue its annual surveys of the outer solar system. Researchers expect to observe whether the storm stabilizes or keeps changing. Should a future Saturn mission receive funding, it would provide the temperature and wind-speed data at depth that Hubble cannot — and likely settle, at last, why two poles on the same planet chose different numbers.

Reference: Sánchez-Lavega et al., “A ten-sided polygonal wave at Saturn’s south pole,” Science Advances, 2026.

Tags: astronomy, agustín sánchez-lavega, amy simon, atmospheric wave, hubble space telescope, saturn

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