Saturn May Be Growing a Decagon at Its South Pole

At Saturn's south pole, the wind and the shape are moving at radically different speeds.

The eastward jet near 60.5°S moves at about 116 meters per second. The ten-sided wave embedded in it drifts east at only about 2.5 meters per second. The air is moving more than 46 times faster than the pattern.

That distinction is useful because the new feature is easy to mistake for a giant object drawn on Saturn's clouds. It is not. Agustín Sánchez-Lavega and colleagues report in Science Advances that the decagon is a large atmospheric wave associated with one of Saturn's jet streams. Hubble images from 2023 and 2024 show weaker traces of its vertices; by 2025 the ten-sided structure had become much clearer.

Cassini, which orbited Saturn until 2017, never saw a comparable long-lived southern polygon. The observational gap means nobody can say exactly when the decagon formed. But the evidence is consistent with something planetary scientists rarely get to watch: a giant atmospheric pattern still developing.

The sides are longer than Earth's diameter

The decagon sits roughly between 58°S and 63°S. Its mean zonal wavelength is about 16,800 kilometers, so the spacing from one vertex to the next is larger than Earth's roughly 12,700-kilometer diameter.

The geometry is less rigid than the word decagon suggests. The ten vertices oscillate in longitude with a period of about 32 days and amplitudes of roughly 4.6° to 8.4°. The whole wave also migrates eastward. Saturn is not maintaining ten fixed corners while gas simply rushes around them.

That behavior immediately separates the new feature from the famous northern hexagon. Sánchez-Lavega and colleagues showed in 2014 that the northern hexagon had an extraordinarily steady rotation period of about 10 hours 39 minutes 23 seconds and that its associated jet remained essentially unchanged through large seasonal changes. The southern decagon is younger in the data, slower-moving as a pattern, and visibly evolving.

The discovery depended on having old pictures

Saturn takes about 29.5 Earth years to orbit the Sun. Its tilt periodically makes one pole much easier to observe from Earth than the other. The south polar region had been poorly placed for years, which is why “newly observed” cannot automatically mean “newly formed.”

Ground-based observers using the University of the Basque Country's Planetary Virtual Observatory and Laboratory noticed an undulating southern band in 2024. Stronger evidence appeared in 2025. Hubble's sharper images then confirmed the structure and allowed the team to trace weaker signatures back into 2023.

This is one of those cases where the archive is part of the instrument. Hubble's Outer Planet Atmospheres Legacy program has been taking comparable images of the giant planets for more than a decade. Once the decagon became obvious, researchers could ask whether it had been hiding in earlier observations.

Different Hubble filters also probe different atmospheric altitudes. The apparent position of the wave shifts slightly between wavelengths, which indicates that the structure extends vertically through several atmospheric layers rather than being only a superficial cloud marking.

Why does a jet turn into a polygon?

The new paper does not settle that question.

The authors' shallow-water simulations show that a large meandering wave can be trapped by the curvature of the jet and confined in latitude. They found two plausible ways to excite something like the observed decagon: a periodic disturbance near the jet maximum, or forcing from a dark anticyclonic vortex immediately to the north.

The northern hexagon already has competing physical explanations. A 2020 Proceedings of the National Academy of Sciences study showed that deep rotating convection can spontaneously generate strong zonal jets and polygonal structures in a gas-giant model. Other work treats the northern hexagon as a vertically trapped wave associated with its jet.

The southern decagon gives those ideas a second object to explain. A theory that can reproduce one polygon on one pole is interesting. A theory that can explain why the same planet supports a persistent six-sided wave in the north and an evolving ten-sided wave in the south would be more convincing.

The next few years should make the comparison better. NASA says the southern feature appears to be strengthening, and Saturn's south pole is becoming easier to observe. If the vertices keep oscillating, the pattern changes speed, or the vertical structure changes as the season advances, those measurements will constrain which models remain plausible.

Sources

• Agustín Sánchez-Lavega et al., “A decagon wave around Saturn's south pole,” Science Advances 12 (36), eaee4251 (September 2, 2026)
• NASA Hubble Mission Team, “NASA's Hubble Tracks New Decagon Encircling Saturn's South Pole”
• A. Sánchez-Lavega et al., “The long-term steady motion of Saturn's hexagon and the stability of its enclosed jet stream under seasonal changes,” Geophysical Research Letters 41 (2014)
• Rakesh K. Yadav and Jeremy Bloxham, “Deep rotating convection generates the polar hexagon on Saturn,” Proceedings of the National Academy of Sciences 117 (2020)