Saturn boasts one of the solar system's most geometrical features: a giant hexagon encircling its north pole. Though not as famous as Jupiter’s Great Red Spot, Saturn's Hexagon is equally mysterious. Now researchers have recreated this formation in the lab using little more than water and a spinning table—an important first step, experts say, in finally deciphering this cosmic mystery.
Saturn’s striped appearance comes from jet streams that fly east to west through its atmosphere at different latitudes. Most jets form circular bands, but the Voyager spacecraft snapped pictures of an enormous hexagonally shaped one (each side rivals Earth's diameter) when it passed over the planet's north pole in 1988. Stumped scientists first attributed the shape to a huge, stormlike vortex along one of the hexagon’s sides, which Voyager also spotted during its journey. Astronomers believed this gyre was altering the jet stream’s course, much in the same way a large rock would change a nearby river’s path. But when the Cassini mission returned to Saturn and photographed Saturn's north pole in 2006, the vortex was gone, yet the hexagon was still there.
Physicists Ana Claudia Barbosa Aguiar and Peter Read of the University of Oxford in the United Kingdom wanted to see if they could recreate the hexagon in the lab. They placed a 30-liter cylinder of water on a slowly spinning table; the water represented Saturn’s atmosphere spinning with the planet’s rotation. Inside this tank, they placed a small ring that whirled more rapidly than the cylinder. This created a miniature artificial "jet stream" that the researchers tracked with a green dye.
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The faster the ring rotated, the less circular the green jet stream became. Small eddies formed along its edges, which slowly became larger and stronger and forced the fluid within the ring into the shape of a polygon. By altering the rate at which the ring spun, the scientists could generate various shapes. “We could create ovals, triangles, squares, almost anything you like,” says Read. The bigger the difference in the rotation between the planet and the jet steam—that is the cylinder and the ring—the fewer sides the polygon had, the team reports in this month's issue of Icarus. Barbosa Aguiar and Read suggest that Saturn’s north polar jet stream spins at a rate relative to the rest of the atmosphere that favors a six-sided figure, hence the hexagon.
Such polygonal formations have been observed in the center of major hurricanes on Earth, says Barbosa Aguiar, though they quickly dissipate. “Most planetary scientists are not aware of how ubiquitous these sorts of patterns are in fluid dynamics.”
Planetary scientist Kevin Baines of NASA’s Jet Propulsion Laboratory in Pasadena, California, is impressed. “These results are very intriguing,” he says. “The team has formed what I think is a landmark paper that could stand the test of time.” Although the lab experiment does not explain what force is driving this particular jet stream, he says that the results can give real insight into what might be going on in Saturn’s atmosphere.
*This article has been corrected. The original version stated that a greater difference in speed between the laboratory cylinder and ring produced a polygon with more sides. It actually produced a fewer-sided polygon.