Saturn's Hidden Decagon—And Why It Changes Everything We Knew About Polar Storms

A second polar mystery, now in color

For over forty years, the hexagon at Saturn’s north pole has been one of the solar system’s most striking weather anomalies—a rigid, six-sided storm pattern wrapping around the pole that has refused to dissipate since Voyager first spotted it in 1981. Models have chased explanations: barotropic instability, Rossby waves, deep convective coupling. None has settled.

Now there is a companion. A research team led by Agustín Sánchez-Lavega of the University of the Basque Country has identified a ten-sided polygonal wave encircling Saturn’s south pole, reported this week in Science Advances. It is the first time such a regular polygonal structure has been observed on Saturn’s southern hemisphere at all.

The timing is the part that matters most.

How it was found—and why that’s unusual

The pattern came to light through a combination of amateur and professional observation, which is rare for a finding of this scale. In 2024, Sánchez-Lavega, along with amateur astronomers Trevor Barry and Jean-Paul Oger, scanned images uploaded to PVOL (the Planetary Virtual Observatory and Laboratory), a citizen-science platform run by the university. They spotted a faint banded structure near the south pole in ground-based data.

Hubble follow-up confirmed it. Images from August 2025 show the decagon—the team’s working name, from the Greek for “ten angles”—clearly resolved. Retrospective analysis revealed it was already present in Hubble data from 2023.

This matters for a simple reason: the south pole had been largely invisible to Earth-based observers for decades. Saturn’s axial tilt means the southern hemisphere spends long stretches pointed away from us. Only as Saturn’s seasons progressed did the south pole rise into view, turning a blind spot into a discovery window. The delay between the pattern’s emergence and its detection was not a limitation of instruments—it was a limitation of geometry.

The numbers

The decagon sits at roughly 63°S latitude, tracing the axis of a jet stream there. Its vertices average about 16,800 kilometers apart—large enough to encompass a region larger than Earth itself. The structure extends across a broad atmospheric depth, from about 10 millibars up to 2 bars of pressure.

But the kinematics are where the pattern gets strange.

The jet stream at that latitude blows eastward at roughly 116 meters per second. The decagon wave itself drifts eastward at only about 2.5 m/s. Relative to the wind, the pattern lags westward by some 113 m/s—effectively fighting against the current that surrounds it. Meanwhile, each of the ten vertices oscillates in longitude with a period of about 32.5 days, wobble that the team is still working to characterize.

A structure that resists being swept along by the wind it rides is a structure that is being held in place by something deeper—either a Rossby-wave mechanism anchored at depth, a resonance with a subsurface feature, or a forcing mechanism not yet identified.

The anticyclone connection

The researchers are focusing on a high-pressure anticyclone located near 55°S, roughly 8° equatorward of the decagon. Using a shallow-water model—a simplified fluid-dynamics framework that treats the atmosphere as a thin layer—they found that the anticyclone cannot be ruled out as a catalyst for the wave pattern.

This is preliminary but significant. If a pre-existing anticyclone seeded or stabilized the decagon, it would represent a different formation pathway from the north polar hexagon, which lacks a comparable neighboring vortex. Two polar storms, two different birth stories.

That possibility alone reframes the debate. For years, the prevailing hypothesis has been that the hexagon and any southern counterpart share a common origin—a symmetry waiting to be confirmed. The decagon suggests the symmetry may be more apparent than real. Saturn’s poles do not have to tell the same story.

Why the south pole showed up now

Amy Simon at NASA’s Goddard Space Flight Center, a co-author on the paper, noted that no one can yet say why the decagon became visible only in recent years. Several scenarios are on the table.

It could be seasonal. Saturn’s year is nearly 30 Earth years; its southern hemisphere winter ended only recently, and the planet’s axial tilt has been gradually exposing the south pole to both Earth-based and orbital observers. Heating changes in the upper atmosphere could alter jet-stream structure enough to make a previously invisible pattern emerge.

It could be transient. Unlike the north polar hexagon—which has persisted for over four decades—the decagon might be a short-lived instability. The shallow-water simulations do not rule out a jet-stream instability origin, and such patterns can collapse without warning. The team explicitly flags this as an open question.

Or it could simply be that the decagon has always been there, hidden beneath cloud layers or below the resolution threshold of earlier instruments, and only now has the geometry of observation aligned to reveal it.

Each of those possibilities carries different implications for how we understand Saturn’s interior.

What this means for the hexagon

The north polar hexagon remains the outlier in every sense. It is stable. It is persistent. It is approximately regular despite the chaotic environment that should tear it apart. Whatever maintains it must draw energy from deep within Saturn’s atmosphere—likely from the angular momentum of the planet’s differential rotation, transferred upward through the column.

If the decagon is ephemeral, that makes the hexagon’s longevity all the more remarkable, not less. It suggests the north polar configuration sits in a dynamical sweet spot—a resonance that is harder to achieve than the south pole’s momentary coincidence.

If the decagon turns out to be stable as well, then we are looking at a more general class of polar wave structure than anyone had modeled, one that can manifest with different vertex counts depending on local conditions. That would mean the hexagon was never special—it was just the first one we could see.

The reading between the data

The paper does not settle any of these questions. It documents a discovery, characterizes its basic properties, and offers a plausible formation scenario that still needs testing. The real work—the years-long monitoring required to determine whether the decagon persists—has only just begun.

What the discovery does do is widen the aperture. For decades, the hexagon was treated as a singular curiosity. The decagon tells us it is not singular. Saturn’s poles produce polygonal atmospheric patterns. The question now is not whether the south pole will join the north—but how closely their behaviors will mirror, and how far they will diverge.

Every extra year of observation will narrow that gap. Or open a deeper one.