Saturn's hexagon is a persisting hexagonal cloud pattern around the north pole of Saturn, located at about 78°N. The sides of the hexagon are about 13,800 km (8,600 mi) long, which is more than the diameter of Earth (about 12,700 km (7,900 mi)). It rotates with a period of 10h 39m 24s, the same period as Saturn's radio emissions from its interior. The hexagon does not shift in longitude like other clouds in the visible atmosphere.
Saturn's polar hexagon discovery was made by the Voyager mission in 1981–82, and it was revisited since 2006 by the Cassini mission. Cassini was only able to take thermal infrared images of the hexagon, until it came in the sunlight in January 2009. Cassini was also able to take a video of the hexagonal weather pattern while traveling at the same speed as the planet, therefore recording only the movement of the hexagon. After its discovery, and after it came back into the sunlight, amateur astronomers managed to get images showing the hexagon from Earth.
One hypothesis, developed at Oxford University, is that the hexagon forms where there is a steep latitudinal gradient in the speed of the atmospheric winds in Saturn's atmosphere. Similar regular shapes were created in the laboratory when a circular tank of liquid was rotated at different speeds at its centre and periphery. The most common shape was six sided, but shapes from three to eight sided were also produced. The shapes form in an area of turbulent flow between the two different rotating fluid bodies with dissimilar speeds. A number of stable vortices of similar size form on the slower (south) side of the fluid boundary and these interact with each other to space themselves out evenly around the perimeter. The presence of the vortices influences the boundary to move northward where each is present and this gives rise to the polygon effect. Polygons do not form at wind boundaries unless the speed differential and viscosity parameters are within certain margins and so are not present at other likely places, such as Saturn's south pole or the poles of Jupiter.
Saturn's north pole , true color
View from about 630,000 kilometres (390,000 mi) away at 938 nm (near-IR)
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|Wikimedia Commons has media related to Poles of Saturn.|
- Saturn Revolution 175, Cassini images, November 27, 2012
- Saturn’s Strange Hexagon – In Living Color! - Universe Today
- Edge of the hexagon from Planetary Photojournal
- Saturn's Hexagon Comes to Light, APOD January 22, 2012
- In the Center of Saturn's North Polar Vortex, Astronomy Picture of the Day - December 4, 2012
- Video of hexagon's rotation from NASA
- NASA's Cassini Spacecraft Obtains Best Views of Saturn Hexagon (December 4, 2013)
- Animated vortex view (TPS)
- Saturn's Hexagon Replicated In Laboratory, video