Saturn - North polar hexagon and vortex as well as rings (April 2, 2014).
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 longer than the Earth's diameter. It rotates with a period of 10h 39m 24s, the same period as Saturn's radio emissions from its interior. However, the hexagon does not shift in longitude like other clouds in the visible atmosphere.
Saturn's south pole does not have a hexagon, according to Hubble observations. But it does have a vortex, and there is also a vortex inside the northern hexagon.
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 started to become visible by light in January 2009. Cassini also recently was 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 sunlight, amateur astronomers managed to get a blurry view of the hexagon from Earth.
One hypothesis, developed at Oxford University, is that the hexagon forms where there is a steep latitudinalgradient 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 two 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.