Saturn's hexagon

• 1. Introduction

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  Saturn's hexagon is a cloud formation sitting above the planet's north pole, and it is unlike anything else in the known solar system. Its six sides each stretch roughly 14,500 kilometres, making every single side about 2,000 kilometres longer than Earth's full diameter. It sits at about 78 degrees north latitude, rising some 300 kilometres high. What force of nature could carve a nearly perfect six-sided shape out of swirling atmospheric gas, and hold that shape steady for decades? Those are the questions that have occupied planetary scientists since the pattern was first glimpsed by a spacecraft passing through the outer solar system in 1981.

• 2. Discovery and Return

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  David Godfrey identified the hexagon in 1987 by piecing together flyby images collected during the 1981 Voyager mission. The shape had been sitting there in the raw data, waiting to be assembled into a coherent picture. Nearly a quarter of a century passed before another spacecraft arrived to take a closer look. When the Cassini-Huygens mission reached Saturn in 2006, the hexagon was still there, unchanged in position. Cassini could only capture thermal infrared images of the structure at first, because the north pole was still cloaked in the long polar night of Saturn's winter. That changed in January 2009, when sunlight finally reached the hexagon. Once illuminated, Cassini recorded video of the pattern while matching Saturn's rotation speed, which meant the camera captured only the hexagon's own motion rather than the planet spinning beneath it. Saturn's south pole, examined through Hubble observations, has no matching hexagon; it has a vortex, but no polygon.

• 3. Motion and Timing

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  One of the hexagon's most striking properties is its stubbornness. Other cloud systems in Saturn's visible atmosphere drift in longitude over time, but the hexagon stays put. It rotates with a period of 10 hours, 39 minutes, and 24 seconds. That figure is notable because it matches the period of Saturn's radio emissions from its interior. The link between an atmospheric cloud pattern and the deep interior of the planet is not fully explained, and it stands as one of the formation's enduring puzzles. The pattern may itself be a jet stream, with atmospheric gases moving at 320 kilometres per hour along those six sides.

• 4. A Shifting Palette

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  Between 2012 and 2016, the hexagon's color shifted. What had been a mostly blue hue gradually became more golden. Cassini's instruments tracked the change across those years. One working hypothesis ties the color shift to the arrival of sunlight at the pole as Saturn moved through its seasons. Researchers suggest that the new solar exposure may be generating a photochemical haze, altering the mix of particles scattering light above the pole. The color change does not appear to have altered the hexagon's shape or its rotational timing.

• 5. Why Six Sides

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  Oxford University researchers proposed that the hexagon forms where the atmospheric winds in Saturn's atmosphere show a steep change in speed across latitude. They tested this idea in a laboratory by spinning a circular tank of liquid at different rates near its center and its outer edge. Regular polygons appeared in the transition zone between the fast and slow regions. Six sides was the most common result, though shapes with anywhere from three to eight sides also formed. A ring of similarly sized vortices assembles along the slower, southern edge of the fluid boundary. These vortices push each other apart until they are evenly spaced around the perimeter, and where each vortex sits, it pulls the boundary northward, bending it into an angular shape. Polygons only form when the difference in wind speed and the fluid's viscosity fall within specific limits, which is why Saturn's south pole and the poles of Jupiter do not show the same pattern. Some other researchers contest the Oxford interpretation, arguing that laboratory versions produce vortex streets, which are spiraling chains of vortices, rather than the clean jetstream behavior seen in Saturn's actual hexagon.

• 6. The Role of the Polar Vortex

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  Research into barotropic instability suggests that the north polar vortex sitting inside the hexagon plays a decisive part in keeping it stable. Studies found that a jet stream on its own, without that central vortex, does not produce a long-lived hexagonal structure. The interaction between the circumpolar jet and the north polar vortex together creates the conditions for a durable polygon. Separately, a 2020 mathematical study from the California Institute of Technology found that a stable geometric arrangement of storms can emerge on any planet when a central storm is encircled by winds rotating in the opposite direction, a configuration called an anticyclonic ring or anticyclonic shielding. That shielding creates a vorticity gradient that pushes neighboring cyclones apart. On Saturn, however, the polar cyclone is large and its winds are slow, which means it cannot sustain a ring of circumpolar cyclones the way Jupiter does. The Cassini mission's own wind speed measurements rule out certain classes of instability, at least as they stood during the time Cassini was observing, leaving barotropic or possibly baroclinic instabilities as the strongest remaining candidates for what sustains the hexagon across decades.