A Mysterious Decagon Unexpectedly Appeared Around Saturn's South Pole. The Ten-Sided Shape Seems to Be a Wave in the Atmosphere

A newly identified ten-sided pattern is circling Saturn’s south polar region, giving planetary scientists an unexpected second example of striking geometry in the giant planet’s atmosphere. The feature, described in a new Science Advances study, has the form of a decagon rather than a smooth ring. Researchers interpret it as an immense atmospheric wave associated with a jet stream, not as a solid structure on or above the planet.

The observation is notable because Saturn was already known for another unusual polar outline: a six-sided formation at its north pole, first recognized decades ago. The southern discovery therefore adds a contrasting shape at the opposite end of the planet. While planets are commonly pictured as round worlds moving along oval paths, the new finding is a reminder that their moving atmospheres can organize themselves into far less familiar forms.

The south-polar decagon came into view through a combination of recent Hubble Space Telescope images and observations made by amateur astronomers. That mix of professional space-based imaging and work from observers on Earth helped reveal a feature that had not previously been apparent. The imagery credits include NASA, the European Space Agency, the Space Telescope Science Institute, Agustín Sánchez-Lavega of the University of the Basque Country, Amy Simon of NASA’s Goddard Space Flight Center, Michael Wong of the University of California, Berkeley, and image processor A. Pagan of STScI.

Its timing is central to the mystery. Earlier Hubble views did not show the ten-sided arrangement, according to the researchers. Nor was it visible in pictures acquired by NASA’s Cassini mission while the spacecraft orbited Saturn from 2004 through 2017. The absence from those long-running records suggests the pattern emerged comparatively recently, rather than being a permanent southern-polar landmark that had simply escaped notice.

Sánchez-Lavega, the paper’s lead author and a planetary scientist in Spain, said the wave was an unexpected find. That assessment reflects the unusually extensive visual record available for Saturn: astronomers can compare newer observations with both Hubble’s earlier images and Cassini’s close-up coverage. Even so, the available comparison establishes what was not seen before more clearly than it explains why the decagon has appeared now.

The study’s interpretation places the shape within one of Saturn’s jet streams. In that view, the decagon is a changing atmospheric wave traced by the circulation around the south pole. This distinguishes it from a fixed geometric object and also means its visible boundaries may depend on how the wave evolves. The researchers have not identified the trigger that produced the pattern, leaving the origin of the apparent transformation unresolved.

There is also no answer yet to a second practical question: whether the feature will persist. A wave that is evolving could endure, alter its outline or disappear, but the study does not determine its future lifespan. Continued observations will be needed to establish whether the decagon settles into a recurring part of Saturn’s weather or proves to be a temporary episode in the planet’s atmosphere. Such follow-up could separate a durable configuration from a short-lived feature visible only during a limited period.

The northern hexagon provides an important point of context, but it does not supply a ready explanation for the southern result. The two phenomena share the unusual quality of turning atmospheric circulation into polygon-like outlines, yet the newly reported southern figure has a different number of sides and a different observational history. Treating them as identical would go beyond the evidence currently available.

For now, the decagon offers scientists a fresh target for tracking how large-scale winds behave at Saturn’s poles. Future images can test whether its ten-sided form remains stable and may narrow the period in which it developed. Each additional observation may clarify whether changes occur in the wave itself or in how it can be detected. Until then, the discovery is best understood as a newly visible, planet-sized atmospheric pattern whose cause and durability remain open questions.

Source: Smithsonian Magazine