In the 1980s, the Voyager spacecraft flew past the planet Saturn, sending back images that showed something scientists never expected: a giant hexagon, centered around Saturn’s north pole. Later, more recent images taken by the Cassini spacecraft confirmed that this was not just a fluke, but a persistent phenomena- a six-sided polygon that appears in the currents around the north pole of this gas giant, four times as large as the planet Earth.
It was a surprise that the weather patterns of Saturn could produce such a standard shape, especially one that persists over time. Similar patterns can sometime be seen on earth, such as in the eye of hurricanes, but usually dissipate in a matter of weeks or less, losing energy to friction when they run over land masses. Fluids on the surface of Saturn have no underlying solid masses to run into, allowing for the persistence of long-lasting storms and formations, such at Jupiter’s giant red spot. Still, the consistency of this giant shape perplexed scientists, who went looking for an explanation for why a polygon could appear on Saturn’s round surface.
Solving the Mystery
A theory developed proposing that the hexagon was a standing wave in Saturn’s atmosphere. Standing waves are a type of waves that vibrate in the same position, like a guitar string or jump rope. Specifically, Saturn’s hexagon was thought to be an example of Rossby waves, which are the product of two large masses of air travelling against each other at different speeds. Another example of Rossby waves are currents around the Earth’s north pole, which contributed to the especially cold winter of 2014-2015 that blanketed the American Northeast in snow. Initially, it was thought that that Saturn’s standing wave was propagated by the driving force of a big dark spot, believed to be a storm, seen near the Hexagon in the 1980s Voyager images. However, the images sent back by Cassini showed that this dark spot has since disappeared, requiring a better explanation of this phenomena.
One set of researchers have attempted to find an alternate explanation, and provide evidence for it in the laboratory. They think that the appearance of this hexagon has to do with the gradient in wind speeds. By analyzing wind speeds at different latitudes on Saturn, they found that the hexagon appears right around the latitude where there is the sharpest change in wind speed. These sorts of extreme changes in wind speed can cause turbulence and instabilities to form in the air currents, which could give rise to standing waves and persistent shapes like Saturn’s hexagon.
To test this theory, researchers built a 60-centimeter wide tank of water filled with different-colored liquids that they could rotate at different speeds. At low speeds, nothing interesting happened, but at higher speed differences, instabilities formed that sometimes stabilized into distinct shapes. At certain speeds, they were able to replicate a hexagonal shape like the one seen on Saturn. Based on their experiments, it’s entirely possible that we could one day discover a planet sporting an enormous fluid triangle around one of its poles.
A Closer Look
To further investigate the hexagon, the Cassini spacecraft took false-color photographs, which allowed it to get a good idea of the sort of particles floating about in Saturn’s wind. The images indicated that inside the hexagon, smaller particles were more prevalent, while outside the hexagon larger particles were more prevalent. This difference is enforced by the hexagonal jet stream, in the same way that the Earth’s jet stream around the south pole allows a hole in the ozone layer to persist- when conditions are right for ozone-destroying chemicals to form, the fast-flowing jet stream above Antarctica prevents ozone in the rest of the atmosphere from filling in the hole.
Saturn’s hexagon is an interesting example of the sort of phenomena that can occur in planetary atmospheres. This extraterrestial polygon provides a good test of theories of fluid dynamics and weather patterns, and also goes to show that we can find all sorts of peculiarities hiding in our solar system.