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Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant. Together, these four planets are sometimes referred to as the Jovian planets, where Jovian is the adjectival form of Jupiter.
Saturn is named after the Roman god Saturnus (that became the namesake of Saturday), equated to the Greek Kronos (the Titan father of Zeus) the Babylonian Ninurta and to the Hindu Shani. Saturn's symbol represents the god's sickle (Unicode: ♄).
The planet Saturn is composed of hydrogen, with small proportions of helium and trace elements.The interior consists of a small core of rock and ice, surrounded by a thick layer of metallic hydrogen and a gaseous outer layer. The outer atmosphere is generally bland in appearance, although long-lived features can appear. Wind speeds on Saturn can reach 1800 km/h, significantly faster than those on Jupiter. Saturn has a planetary magnetic field intermediate in strength between that of Earth and the more powerful field around Jupiter.
Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. Sixty known moons orbit the planet. Titan, Saturn's largest and the Solar System's second largest moon (after Jupiter's Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere.
Due to a combination of its lower density, rapid rotation, and fluid state, Saturn is an oblate spheroid; that is, it is flattened at the poles and bulges at the equator. Its equatorial and polar radii differ by almost 10%—60 268 km vs. 54 364 km. The other gas planets are also oblate, but to a lesser extent. Saturn is the only planet of the Solar System that is less dense than water. Although Saturn's core is considerably denser than water, the average specific density of the planet is 0.69 g/cm³ due to the gaseous atmosphere. Saturn is only 95 Earth masses, compared to Jupiter, which is 318 times the mass of the Earth but only about 20% larger than Saturn.
Internal structure
Though there is little direct information about Saturn's internal structure, it is thought that its interior is similar to that of Jupiter, having a small rocky core surrounded mostly by hydrogen and helium. The rocky core is similar in composition to the Earth, but denser. Above this, there is a thicker liquid metallic hydrogen layer, followed by a layer of liquid hydrogen and helium, and in the outermost 1000 km a gaseous atmosphere. Traces of various ices are also present. The core region is estimated to be about 9–22 times the mass of the Earth. Saturn has a very hot interior, reaching 11 700 °C at the core, and it radiates 2.5 times more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen.
The outer atmosphere of Saturn consists of about 93.2% molecular hydrogen and 6.7% 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 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.
