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 14,500 km (9,000 mi) long, which is more than the diameter of Earth (about 12,700 km (7,900 mi)). The hexagon may be a bit greater than 29,000 km (18,000 mi) wide, may be 300 km (190 mi) high, and may be a jet stream made of atmospheric gases moving at 320 km/h (200 mph). 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 hexagon was discovered during the Voyager mission in 1981 and was later revisited by Cassini-Huygens in 2006. During the Cassini mission, the hexagon changed from a mostly blue color to more of a golden color. Saturn's south pole does not have a hexagon, according to Hubble observations; however, it does have a vortex, and there is also a vortex inside the northern hexagon. Multiple hypotheses for the hexagonal cloud pattern have been developed.
Cassini was only able to take thermal infrared images of the hexagon until it passed into 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.
Between 2012 and 2016, the hexagon changed from a mostly blue color to more of a golden color. One theory for this is that sunlight is creating haze as the pole is exposed to sunlight due to the change in season. These changes were observed by the Cassini spacecraft.
Explanations for hexagon shape
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 with three to eight sides 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.
Other researchers claim that lab studies exhibit vortex streets, a series of spiraling vortices not observed in Saturn's hexagon. Simulations show that a shallow, slow, localized meandering jetstream in the same direction as Saturn's prevailing clouds is able to match the observed behaviors of Saturn's Hexagon with the same boundary stability.
Developing barotropic instability of Saturn's North Polar hexagonal circumpolar jet (Jet) plus North Polar vortex (NPV) system produces a long-living structure akin to the observed hexagon, which is not the case of the Jet-only system, which was studied in this context in a number of papers in literature. The north polar vortex (NPV), thus, plays a decisive dynamical role to stabilize hexagon jets. The influence of moist convection, which was recently suggested to be at the origin of Saturn's north polar vortex system in the literature, is investigated in the framework of the barotropic rotating shallow water model and does not alter the conclusions.
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|Wikimedia Commons has media related to Poles of Saturn.|
- on YouTube
- 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)
- Hexagon image
- Saturn's Hexagon Replicated In Laboratory, video
- Hexagon Changes Color (October 21, 2016)