Simulation Helps Explain Saturn’s Mysterious Hexagon

A new study of the mysterious hexagon-shaped storm at Saturn’s north pole suggests this phenomenon is actually the result of activity occurring across the entire planet.

In a new paper published in the Proceedings of the National Academy of Sciences, scientists say that the unnatural-looking hurricane occurs when atmospheric flows across Saturn create large and small cyclone-like vortices. These storms very likely swirl deep within planet and the storms’ convection funnels and “pinches” the atmospheric flows, confining it to the top of Saturn into a hexagonal shape.

“The hexagonal flow pattern on Saturn is a striking example of turbulent self-organization,” the researchers wrote in their paper. “Our model simultaneously and self-consistently produces alternating zonal jets, the polar cyclone, and hexagon-like polygonal structures similar to those observed on Saturn.”

The bizarre six-sided cloud structure circling Saturn’s entire north pole was hinted at when the Voyager spacecraft flew by the planet 40 years ago in November of 1980. The Cassini spacecraft arrived in Saturn’s orbit in 2004, but wasn’t able to take images of that region until 2007. Stunning images showed the hexagonal structure in detail.

“We see storms on Earth regularly and they are always spiraling, sometimes circular, but never something with hexagon segments or polygons with edges,” said Rakesh K. Yadav, a research associate at Harvard University’s Department of Earth and Planetary Sciences, a co-author on the paper. “That is really striking and completely unexpected. The question on Saturn is, how did such a large system form and how can such a large system stay unchanged on this large planet?”

The six-sided storm is only at the north pole. While the south pole of Saturn also has a large storm, it looks more like a hurricane with a giant eye. Cassini data showed the hexagon extended much deeper than scientists previously expected, with initial estimates reaching 100 km (60 miles) below the cloud tops.

But the new model created by Yadav and geophysics professor Jeremy Bloxham suggests the storm could be thousands of kilometers deep.

The scientists ran a computer simulation of Saturn’s atmospheric flows for a month. It showed that a phenomenon called deep thermal convection — which happens when heat is transferred from one place to another by the movement of fluids or gases — can unexpectedly give rise to atmospheric flows that create large polar cyclones and a high-latitude eastward jet pattern. When these mix at the top of the planet, it forms the unexpected shape. And because the storms form deep within the planet, the scientists said it makes the hexagon furious and persistent.

The simulation, based on actual data from the Cassini mission, revealed that smaller storms in Saturn’s atmosphere surround a larger horizontal jet stream blowing east near the planet’s north pole.  The smaller storms interact with the larger jet stream system and as a result effectively pinch the eastern jet and confine it to the top of the planet. The pinching process warps the stream into a hexagon.

“This jet is going around and around the planet, and it has to coexist with these localized [smaller] storms,” said Yadav. “Think of it like this: Imagine we have a rubber band and we place a bunch of smaller rubber bands around it and then we just squeeze the entire thing from the outside. That central ring is going to be compressed by some inches and form some weird shape with a certain number of edges. That’s basically the physics of what’s happening. We have these smaller storms and they’re basically pinching the larger storms at the polar region and since they have to coexist, they have to somehow find a space to basically house each system. By doing that, they end up making this polygonal shape.”

The researchers said their computer model didn’t produce a perfect hexagon. Instead, the shape they saw was a nine-side polygon that moved faster than Saturn’s storm. Still, they said, the shape serves as proof of concept for the overall thesis on how the gigantic storm formed and why it has been relatively unchanged for almost 40 years.

Yadev and Bloxham said they need more atmospheric data from Saturn to further refine their model to create a more accurate picture of what’s happening long-term with the storm.

“The scientific motivation is basically understanding how Saturn came to be and how it evolves over time,” Yadav said.

Further reading:

Harvard University
PNAS