Subsurface vortexes could be behind Saturn’s mysterious hexagon

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Saturn’s bold rings are not its only signature feature.

Saturn also boasts a six-sided feature at its north pole, which scientists first noticed when NASA’s Voyager 2 spacecraft visited Saturn in 1981. Saturn’s hexagon is mesmerizing but also odd: the strange phenomenon only appears at the planet’s north pole. In a new study, a researching duo investigated the dynamics of Saturn’s churning gas to figure out what sorts of planetary fluid dynamics are creating this mystifying structure. 

Last year, Rakesh Yadav and Jeremy Bloxham from the Department of Earth and Planetary Sciences at Harvard University in Cambridge, Massachusetts, performed a monthlong computer simulation to understand the physics behind Saturn’s hexagon. 

According to their paper, atmospheric flows within Saturn create large and small vortexes near the ringed planet’s north pole. A strong horizontal jet that runs about 60 degrees latitude above the equator gets pinched and confined by these vortexes, which defines the rim of the hexagon. 

Video: How does Saturn’s hexagon-shaped jet stream form?
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One of the main points of the paper, according to Yadav, is that the vortexes do exist, but churn deep beneath the cloud tops and are therefore mostly invisible to spacecraft that have flown over Saturn like Voyager 2 and, more recently, NASA’s Cassini mission. After the Voyager 2 data came back to Earth, scientists hypothesized that the hexagon could have been caused by subsurface cyclones like this new study suggests. 

Although Cassini’s observations of Saturn from 2004 to 2017 helped scientists learn more about the planet’s fluid dynamics, the theory about subsurface vortexes didn’t hold much footing when the spacecraft didn’t spot prominent signs of their existence, Yadav said. 

Yadav performed model simulations of deep turbulent compressible convection, a phenomenon that happens when material at the bottom of a thick, multilayered substance of liquid or gas is hot at the bottom and cool at the top. That difference in temperature causes material to move in a circular up-down motion. Convection is pervasive throughout nature and is what creates hurricanes and tornadoes on Earth.

Yadav said the phenomenon is similar to what happens when someone puts cold water into a pot, ignites their stovetop and begins to heat up this water. The heat from the bottom of the pan stirs up the water as the water transfers heat upwards to the colder surface. 

Yadav said their simulation captured the physics behind the one-pole hexagon and offered up the possibility that in the past, Saturn may have had a hexagon in the south pole and not in the north pole, or maybe a hexagon at each pole. 

The simulation didn’t achieve a hexagonal shape; it produced an angled object with nine sides instead of six. But the mechanics behind their simulation suggest that subsurface vortexes are behind the geometrical feature, Yadav said. “It’s very possible that with different conditions we can easily get six edges instead of nine.”

The computer simulations were challenging to run and researchers can only do small studies within existing parameters. This paper reported one case and Yadav hopes that researchers can run more cases in the future to better understand Saturn’s beauty mark.

The paper detailing this work was published June 8 in the journal Proceedings on the National Academy of the Sciences. 

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