Atmosphere
The outer atmosphere of Saturn consists of 96.3% molecular hydrogen and 3.25% helium. Trace amounts of ammonia, acetylene, ethane, phosphine, and methane have also been detected. The upper clouds on Saturn are composed of ammonia crystals, while the lower level clouds appear to be composed of either ammonium hydrosulfide (NH4SH) or water. The atmosphere of Saturn is significantly deficient in helium relative to the abundance of the elements in the Sun.
The quantity of elements heavier than helium are not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System. The total mass of these elements is estimated to be 19–31 times the mass of the Earth, with a significant fraction located in Saturn's core region.
Cloud layers
Saturn's atmosphere exhibits a banded pattern similar to Jupiter's (the nomenclature is the same), but Saturn's bands are much fainter and are also much wider near the equator. At depth, extending for 10 km and with a temperature of −23 °C, is a layer made up of water ice. Above this layer is likely a layer of ammonium hydrosulfide ice, which extends for another 50 km and is approximately −93 °C. Eighty kilometers above that layer are ammonia ice clouds, where the temperatures are roughly −153 °C. Near the top of the atmosphere, extending for some 200 km to 270 km above the visible ammonia clouds, are gaseous hydrogen and helium. Saturn's winds are among the Solar System's fastest. Voyager data indicate peak easterly winds of 500 m/s (1800 km/h). Saturn's finer cloud patterns were not observed until the Voyager flybys. Since then, however, Earth-based telescopy has improved to the point where regular observations can be made.
Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990, the Hubble Space Telescope observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters, and, in 1994, another smaller storm was observed. The 1990 storm was an example of a Great White Spot, a unique but short-lived phenomenon which occurs once every Saturnian year, or roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice. Previous Great White Spots were observed in 1876, 1903, 1933, and 1960, with the 1933 storm being the most famous. If the periodicity is maintained, another storm will occur in about 2020.
In recent images from the Cassini spacecraft, Saturn's northern hemisphere appears a bright blue, similar to Uranus, as can be seen in the image below. This blue color cannot currently be observed from Earth, because Saturn's rings are currently blocking its northern hemisphere. The color is most likely caused by Rayleigh scattering
Infrared imaging has shown that Saturn has a warm polar vortex, the only example of such a phenomenon known to date in the Solar System. Whereas temperatures on Saturn are normally −185 °C, temperatures on the vortex often reach as high as −122 °C, believed to be the warmest spot on Saturn.
North pole hexagon cloud pattern
A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images. Unlike the north pole, HST imaging of the south polar region indicates the presence of a jet stream, but no strong polar vortex nor any hexagonal standing wave. However, NASA reported in November 2006 that the Cassini spacecraft observed a 'hurricane-like' storm locked to the south pole that had a clearly defined eyewall. This observation is particularly notable because eyewall clouds had not previously been seen on any planet other than Earth (including a failure to observe an eyewall in the Great Red Spot of Jupiter by the Galileo spacecraft).
The straight sides of the northern polar hexagon are each about 13 800 km long. The entire structure rotates with a period of 10h 39 m 24s, the same period as that of the planet's radio emissions, which is assumed to be equal to the period of rotation of Saturn's interior. The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere.
The pattern's origin is a matter of much speculation. Most astronomers seem to think it was caused by some standing-wave pattern in the atmosphere; but the hexagon might be a novel aurora. Polygonal shapes have been replicated in spinning buckets of fluid in a laboratory.