The outer atmosphere of Saturn consists of about 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.
Saturn's celestial body 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 the bottom, extending for 10 km and with a temperature of -23 °C, is a layer made up of water ice. After that comes a layer of ammonium hydrosulfide ice, which extends for another 50 km and is approximately at -93 °C. Eighty kilometers above that are ammonia ice clouds, where the temperatures are about -153 °C. Near the top, extending for some 200 km to 270 km above the clouds, come layers of visible cloud tops and a hydrogen and helium atmosphere. 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.
Astronomers using infrared imaging have shown that Saturn has a warm polar vortex and that it is the only such feature known in the solar system. This, they say, is the warmest spot on Saturn. Whereas temperatures on Saturn are normally -185 °C, temperatures on the vortex often reach as high as -122 °C.
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 some sort of standing-wave pattern in the atmosphere; but the hexagon might be a novel sort of aurora. Polygon shapes have been replicated in spinning buckets of fluid in a laboratory.
- Main article: Magnetosphere of Saturn
Saturn has an intrinsic magnetic field that has a simple, symmetric shape—a magnetic dipole. Its strength at the equator—0.2 gauss (20 µT)—is approximately one twentieth than that of the field around Jupiter and slightly weaker than Earth's magnetic field. As a result the cronian magnetosphere is much smaller than the jovian and extends slightly beyond the orbit of Titan. Most probably, the magnetic field is generated similarly to that of Jupiter—by currents in the metallic-hydrogen layer, which is called a metallic-hydrogen dynamo. Similarly to those of other planets, this magnetosphere is efficient at deflecting the solar wind particles from the Sun. The moon Titan orbits within the outer part of Saturn's magnetosphere and contributes plasma from the ionized particles in Titan's outer atmosphere.
- ↑ Saturn. Universe Guide. Accessed 29 March 2009.
- ↑ Courtin, R.; Gautier, D.; Marten, A.; Bezard, B. (1967). "The Composition of Saturn's Atmosphere at Temperate Northern Latitudes from Voyager IRIS spectra". Bulletin of the American Astronomical Society 15: 831, http://adsabs.harvard.edu/abs/1983BAAS...15..831C. Retrieved on 4 February 2007.
- ↑ Martinez, Carolina (September 5, 2005). "Cassini Discovers Saturn's Dynamic Clouds Run Deep". NASA. Retrieved on 2007-04-29.
- ↑ Guillot, Tristan (1999). "Interiors of Giant Planets Inside and Outside the Solar System". Science 286 (5437): 72–77. doi:10.1126/science.286.5437.72. PMID 10506563, http://www.sciencemag.org/cgi/content/full/286/5437/72. Retrieved on 27 April 2007.
- ↑ "Saturn". MIRA. Retrieved on 2007-07-27.
- ↑ Hamilton, Calvin (1997). "Voyager Saturn Science Summary". Solarviews. Retrieved on 2007-07-05.
- ↑ S. Pérez-Hoyos, A. Sánchez-Lavega, R.G. Frenchb, J.F. Rojas (2005). "Saturn’s cloud structure and temporal evolution from ten years of Hubble Space Telescope images (1994–2003)" (PDF). Retrieved on 2007-07-24.
- ↑ Patrick Moore, ed., 1993 Yearbook of Astronomy, (London: W.W. Norton & Company, 1992), Mark Kidger, "The 1990 Great White Spot of Saturn", pp. 176-215.
- ↑ Watanabe, Susan (March 27, 2007). "Saturn's Strange Hexagon". NASA. Retrieved on 2007-07-06.
- ↑ "Warm Polar Vortex on Saturn". Merrillville Community Planetarium (2007). Retrieved on 2007-07-25.
- ↑ Godfrey. "A hexagonal feature around Saturn's North Pole". Icarus. Retrieved on 2007-07-09.
- ↑ Sanchez-Lavega, A.. "Ground-based observations of Saturn's north polar SPOT and hexagon". Bulletin of the American Astronomical Society. Retrieved on 2007-07-30.
- ↑ "Hubble Space Telescope Observations of the Atmospheric Dynamics in Saturn's South Pole from 1997 to 2002". The American Astronomical Society (October 8, 2002). Retrieved on 2007-07-06.
- ↑ "NASA catalog page for image PIA09187". NASA Planetary Photojournal. Retrieved on 2007-05-23.
- ↑ "NASA Sees into the Eye of a Monster Storm on Saturn". NASA (November 9, 2006). Retrieved on 2006-11-20.
- ↑ "Geometric whirlpools revealed". Nature (May 19, 2006). Retrieved on April 27 2007. Bizarre geometric shapes that appear at the centre of swirling vortices in planetary atmospheres might be explained by a simple experiment with a bucket of water but correlating this to Saturn's pattern is by no means certain.
- ↑ 17.0 17.1 Russell, C. T.; Luhmann, J. G. (1997). "Saturn: Magnetic Field and Magnetosphere". UCLA - IGPP Space Physics Center. Retrieved on 2007-04-29.
- ↑ 18.0 18.1 McDermott, Matthew (2000). "Saturn: Atmosphere and Magnetosphere". Thinkquest Internet Challenge. Retrieved on 2007-07-15.