Saturn is the sixth
planet from the
Sun and the second-largest in the
Solar System, after
Jupiter. It is a
gas giant with an average radius of about nine and a half times that of
Earth.
It has only one-eighth the average density of Earth; however, with its larger volume, Saturn is over 95 times more massive.
Saturn's interior is most likely composed of a core of
iron–nickel and rock (
silicon and
oxygen compounds). Its core is surrounded by a deep layer of
metallic hydrogen, an intermediate layer of
liquid hydrogen and
liquid helium, and finally, a gaseous outer layer. Saturn has a pale yellow hue due to
ammonia crystals in its upper atmosphere. An
electrical current
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
within the metallic hydrogen layer is thought to give rise to Saturn's planetary
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
, which is weaker than Earth's, but which has a
magnetic moment 580 times that of Earth due to Saturn's larger size. Saturn's magnetic field strength is around one-twentieth of Jupiter's.
The outer
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
is generally bland and lacking in contrast, although long-lived features can appear.
Wind speeds on Saturn can reach , higher than on Jupiter but not as high as on
Neptune
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
.
The planet's most notable feature is its prominent
ring system, which is composed mainly of ice particles, with a smaller amount of rocky debris and
dust. At least 83
moons are known to orbit Saturn, of which 53 are officially named; this does not include the hundreds of
moonlets in its rings.
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
, Saturn's largest moon and the second largest in the Solar System, is larger than the planet
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
, although less massive, and is the only moon in the Solar System to have a substantial atmosphere.
Name and symbol
Saturn is named after the Roman
god of wealth and agriculture and father of Jupiter. Its
astronomical symbol (
16px, ♄) has been traced back to the Greek
Oxyrhynchus Papyri, where it can be seen to be a Greek
kappa
Kappa (uppercase Κ, lowercase κ or cursive ; el, κάππα, ''káppa'') is the 10th letter of the Greek alphabet, representing the voiceless velar plosive sound in Ancient and Modern Greek. In the system of Greek numerals, has a value o ...
-
rho
Rho (uppercase Ρ, lowercase ρ or ; el, ρο or el, ρω, label=none) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician letter res . Its uppercase form uses the sa ...
with a
horizontal stroke, as an abbreviation for ''Κρονος'' (
Cronus
In Ancient Greek religion and mythology, Cronus, Cronos, or Kronos ( or , from el, Κρόνος, ''Krónos'') was the leader and youngest of the first generation of Titans, the divine descendants of the primordial Gaia (Mother Earth) and ...
), the Greek name for the planet (
).
It later came to look like a lower-case Greek
eta
Eta (uppercase , lowercase ; grc, ἦτα ''ē̂ta'' or ell, ήτα ''ita'' ) is the seventh letter of the Greek alphabet, representing the close front unrounded vowel . Originally denoting the voiceless glottal fricative in most dialects, ...
, with the cross added at the top in the 16th century to Christianize this pagan symbol.
The Romans named the seventh day of the week
Saturday, ''Sāturni diēs'' ("Saturn's Day"), for the planet Saturn.
Physical characteristics
Saturn is a
gas giant composed predominantly of hydrogen and helium. It lacks a definite surface, though it is likely to have a solid core.
[ Saturn's rotation causes it to have the shape of an ]oblate spheroid
A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has circ ...
; that is, it is flattened at the poles
Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a West Slavic nation and ethnic group, who share a common history, culture, the Polish language and are identified with the country of Poland in Ce ...
and bulges at its equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
. Its equatorial and polar radii differ by almost 10%: 60,268 km versus 54,364 km. Jupiter, Uranus, and Neptune, the other giant planets in the Solar System, are also oblate but to a lesser extent. The combination of the bulge and rotation rate means that the effective surface gravity along the equator, , is 74% of what it is at the poles and is lower than the surface gravity of Earth. However, the equatorial escape velocity of nearly is much higher than that of Earth.
Saturn is the only planet of the Solar System that is less dense than water—about 30% less.[ Although Saturn's core is considerably denser than water, the average specific density of the planet is due to the atmosphere. Jupiter has 318 times ]Earth's mass
An Earth mass (denoted as M_\mathrm or M_\oplus, where ⊕ is the standard astronomical symbol for Earth), is a unit of mass equal to the mass of the planet Earth. The current best estimate for the mass of Earth is , with a relative uncertainty ...
, and Saturn is 95 times Earth's mass. Together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System.[
]
Internal structure
Despite consisting mostly of hydrogen and helium, most of Saturn's mass is not in the gas phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
, because hydrogen becomes a non-ideal liquid when the density is above , which is reached at a radius containing 99.9% of Saturn's mass. The temperature, pressure, and density inside Saturn all rise steadily toward the core, which causes hydrogen to be a metal in the deeper layers.[
Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium, with trace amounts of various ]volatiles
Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances.
On planet Earth, the term ' ...
.[ Analysis of the distortion shows that Saturn is substantially more centrally condensed than Jupiter and therefore contains a significantly larger amount of material denser than hydrogen near its centre. Saturn’s central regions contain about 50% hydrogen by mass, while Jupiter’s contain approximately 67% hydrogen.
This core is similar in composition to Earth, but is more dense. The examination of Saturn's ]gravitational moment
In classical mechanics, the gravitational potential at a point in space is equal to the work (energy transferred) per unit mass that would be needed to move an object to that point from a fixed reference point. It is analogous to the electric po ...
, in combination with physical models of the interior, has allowed constraints to be placed on the mass of Saturn's core. In 2004, scientists estimated that the core must be 9–22 times the mass of Earth, which corresponds to a diameter of about 25,000 km. However, measurements of Saturn's rings suggest a much more diffuse core with a mass equal to about 17 Earths and a radius equal to around 60% of Saturn's entire radius. This is surrounded by a thicker liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude. The outermost layer spans 1,000 km and consists of gas.
Saturn has a hot interior, reaching 11,700 °C at its core, and radiates 2.5 times more energy into space than it receives from the Sun. Jupiter's thermal energy is generated by the Kelvin–Helmholtz mechanism
The Kelvin–Helmholtz mechanism is an astronomical process that occurs when the surface of a star or a planet cools. The cooling causes the internal pressure to drop, and the star or planet shrinks as a result. This compression, in turn, heats t ...
of slow gravitational compression, but such a process alone may not be sufficient to explain heat production for Saturn, because it is less massive. An alternative or additional mechanism may be generation of heat through the "raining out" of droplets of helium deep in Saturn's interior. As the droplets descend through the lower-density hydrogen, the process releases heat by friction and leaves Saturn's outer layers depleted of helium.[ These descending droplets may have accumulated into a helium shell surrounding the core.][ Rainfalls of diamonds have been suggested to occur within Saturn, as well as in Jupiter] and ice giants Uranus and Neptune.
Atmosphere
The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. The proportion of helium is significantly deficient compared to the abundance of this element in the Sun.[ The quantity of elements heavier than helium (]metallicity
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...
) is not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System
The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a ...
. The total mass of these heavier elements is estimated to be 19–31 times the mass of the Earth, with a significant fraction located in Saturn's core region.
Trace amounts of ammonia, acetylene
Acetylene (systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure ...
, ethane, propane
Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used a ...
, phosphine, and methane have been detected in Saturn's atmosphere. The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide () or water. Ultraviolet radiation from the Sun causes methane photolysis
Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies
In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid beh ...
and diffusion. This photochemical cycle
Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible light (400–7 ...
is modulated by Saturn's annual seasonal cycle.[ ''Cassini'' observed a series of cloud features found in northern latitudes, nicknamed the "String of Pearls". These features are cloud clearings that reside in deeper cloud layers.
]
Cloud layers
Saturn's atmosphere exhibits a banded pattern similar to Jupiter's, but Saturn's bands are much fainter and are much wider near the equator. The nomenclature used to describe these bands is the same as on Jupiter. Saturn's finer cloud patterns were not observed until the flybys of the ''Voyager
Voyager may refer to:
Computing and communications
* LG Voyager, a mobile phone model manufactured by LG Electronics
* NCR Voyager, a computer platform produced by NCR Corporation
* Voyager (computer worm), a computer worm affecting Oracle ...
'' spacecraft during the 1980s. Since then, Earth-based telescopy
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
has improved to the point where regular observations can be made.[
The composition of the clouds varies with depth and increasing pressure. In the upper cloud layers, with the temperature in the range 100–160 K and pressures extending between 0.5–2 ]bar
Bar or BAR may refer to:
Food and drink
* Bar (establishment), selling alcoholic beverages
* Candy bar
* Chocolate bar
Science and technology
* Bar (river morphology), a deposit of sediment
* Bar (tropical cyclone), a layer of cloud
* Bar (u ...
, the clouds consist of ammonia ice. Water ice clouds begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185 to 270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 190–235 K. Finally, the lower layers, where pressures are between 10 and 20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in aqueous solution.[
Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990, the Hubble Space Telescope imaged an enormous white cloud near Saturn's equator that 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 that occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere's ]summer solstice
The summer solstice, also called the estival solstice or midsummer, occurs when one of Earth's poles has its maximum tilt toward the Sun. It happens twice yearly, once in each hemisphere ( Northern and Southern). For that hemisphere, the summer ...
.[ 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.
The winds on Saturn are the second fastest among the Solar System's planets, after Neptune's. ''Voyager'' data indicate peak easterly winds of .] In images from the '' Cassini'' spacecraft during 2007, Saturn's northern hemisphere displayed a bright blue hue, similar to Uranus. The color was most likely caused by Rayleigh scattering. Thermography has shown that Saturn's south pole has a warm polar vortex, the only known example of such a phenomenon in the Solar System. Whereas temperatures on Saturn are normally −185 °C, temperatures on the vortex often reach as high as −122 °C, suspected to be the warmest spot on Saturn.[
]
Hexagonal cloud patterns
A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the ''Voyager'' images. The sides of the hexagon are each about long, which is longer than the diameter of the Earth. The entire structure rotates with a period of (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 scientists think it is a ]standing wave
In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect ...
pattern in the atmosphere. Polygonal shapes have been replicated in the laboratory through differential rotation of fluids.[ Laboratory experiment of spinning disks in a liquid solution forms vortices around a stable hexagonal pattern similar to that of Saturn's.]
HST imaging of the south polar region indicates the presence of a jet stream
Jet streams are fast flowing, narrow, meandering thermal wind, air currents in the Atmosphere of Earth, atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are west ...
, but no strong polar vortex nor any hexagonal standing wave. NASA reported in November 2006 that ''Cassini'' had observed a " hurricane-like" storm locked to the south pole that had a clearly defined eyewall. Eyewall clouds had not previously been seen on any planet other than Earth. For example, images from the ''Galileo
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
'' spacecraft did not show an eyewall in the Great Red Spot
The Great Red Spot is a persistent high-pressure region in the atmosphere of Jupiter, producing an anticyclonic storm that is the largest in the Solar System. Located 22 degrees south of Jupiter's equator, it produces wind-speeds up to 432 ...
of Jupiter.
The south pole storm may have been present for billions of years. This vortex is comparable to the size of Earth, and it has winds of 550 km/h.
Magnetosphere
Saturn has an intrinsic magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
that has a simple, symmetric shape—a magnetic dipole. Its strength at the equator—0.2 gauss ( µT)—is approximately one twentieth of that of the field around Jupiter and slightly weaker than Earth's magnetic field. As a result, Saturn's magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
is much smaller than Jupiter's. When '' Voyager 2'' entered the magnetosphere, the solar wind pressure was high and the magnetosphere extended only 19 Saturn radii, or 1.1 million km (712,000 mi), although it enlarged within several hours, and remained so for about three days. Most probably, the magnetic field is generated similarly to that of Jupiter—by currents in the liquid metallic-hydrogen layer called a metallic-hydrogen dynamo. 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's magnetosphere, like Earth's, produces aurorae.
Orbit and rotation
The average distance between Saturn and the Sun is over 1.4 billion kilometers (9 AU). With an average orbital speed of 9.68 km/s, it takes Saturn 10,759 Earth days (or about years) to finish one revolution around the Sun. As a consequence, it forms a near 5:2 mean-motion resonance with Jupiter. The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth. The perihelion and aphelion distances are, respectively, 9.195 and 9.957 AU, on average. The visible features on Saturn rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions (as in Jupiter's case).
Astronomers use three different systems for specifying the rotation rate of Saturn. ''System I'' has a period of (844.3°/d) and encompasses the Equatorial Zone, the South Equatorial Belt, and the North Equatorial Belt. The polar regions are considered to have rotation rates similar to ''System I''. All other Saturnian latitudes, excluding the north and south polar regions, are indicated as ''System II'' and have been assigned a rotation period of (810.76°/d). ''System III'' refers to Saturn's internal rotation rate. Based on radio emissions from the planet detected by '' Voyager 1'' and ''Voyager 2'', System III has a rotation period of (810.8°/d). System III has largely superseded System II.[
A precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004, ''Cassini'' found that the radio rotation period of Saturn had increased appreciably, to approximately .][ An estimate of Saturn's rotation (as an indicated rotation rate for Saturn as a whole) based on a compilation of various measurements from the ''Cassini'', ''Voyager'' and ''Pioneer'' probes is .][ Studies of the planet's C Ring yield a rotation period of .]
In March 2007, it was found that the variation of radio emissions from the planet did not match Saturn's rotation rate. This variance may be caused by geyser activity on Saturn's moon Enceladus. The water vapor emitted into Saturn's orbit by this activity becomes charged and creates a drag upon Saturn's magnetic field, slowing its rotation slightly relative to the rotation of the planet.[
An apparent oddity for Saturn is that it does not have any known ]trojan asteroid
In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger body, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of its Lagrangian points and . Trojans can sha ...
s. These are minor planets that orbit the Sun at the stable Lagrangian point
In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of th ...
s, designated L4 and L5, located at 60° angles to the planet along its orbit. Trojan asteroids have been discovered for Mars, Jupiter, Uranus, and Neptune. Orbital resonance mechanisms, including secular resonance A secular resonance is a type of orbital resonance between two bodies with synchronized precessional frequencies. In celestial mechanics, secular refers to the long-term motion of a system, and resonance is periods or frequencies being a simple nu ...
, are believed to be the cause of the missing Saturnian trojans.
Natural satellites
Saturn has 83 known moons, 53 of which have formal names. In addition, there is evidence of dozens to hundreds of moonlets with diameters of 40–500 meters in Saturn's rings, which are not considered to be true moons. Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
, the largest moon, comprises more than 90% of the mass in orbit around Saturn, including the rings.[ Saturn's second-largest moon, Rhea, may have a tenuous ring system of its own,] along with a tenuous atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
.[
Many of the other moons are small: 34 are less than 10 km in diameter and another 14 between 10 and 50 km in diameter.] Traditionally, most of Saturn's moons have been named after Titans of Greek mythology. Titan is the only satellite in the Solar System with a major atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
, in which a complex organic chemistry occurs. It is the only satellite with hydrocarbon lakes.
On 6 June 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan, a possible precursor for life. On 23 June 2014, NASA claimed to have strong evidence that nitrogen in the atmosphere of Titan came from materials in the Oort cloud, associated with comets, and not from the materials that formed Saturn in earlier times.
Saturn's moon Enceladus, which seems similar in chemical makeup to comets, has often been regarded as a potential habitat for microbial life.[ Evidence of this possibility includes the satellite's salt-rich particles having an "ocean-like" composition that indicates most of Enceladus's expelled ice comes from the evaporation of liquid salt water. A 2015 flyby by ''Cassini'' through a plume on Enceladus found most of the ingredients to sustain life forms that live by ]methanogenesis
Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group ...
.
In April 2014, NASA scientists reported the possible beginning of a new moon within the A Ring, which was imaged by ''Cassini'' on 15 April 2013.
Planetary rings
Saturn is probably best known for the system of planetary rings that makes it visually unique. The rings extend from outward from Saturn's equator and average approximately in thickness. They are composed predominantly of water ice, with trace amounts of tholin
Tholins (after the Greek (') "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon ...
impurities and a peppered coating of approximately 7% amorphous carbon. The particles that make up the rings range in size from specks of dust up to 10 m. While the other gas giants also have ring systems, Saturn's is the largest and most visible.
There are two main hypotheses regarding the origin of the rings. One hypothesis is that the rings are remnants of a destroyed moon of Saturn, for which a research team at MIT has proposed the name "Chrysalis
A pupa ( la, pupa, "doll"; plural: ''pupae'') is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their ...
". The second hypothesis is that the rings are left over from the original nebula
A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...
r material from which Saturn was formed. Some ice in the E ring comes from the moon Enceladus's geysers.[ The water abundance of the rings varies radially, with the outermost ring A being the most pure in ice water. This abundance variance may be explained by meteor bombardment.
Beyond the main rings, at a distance of 12 million km from the planet is the sparse Phoebe ring. It is tilted at an angle of 27° to the other rings and, like ]Phoebe Phoebe or Phœbe may refer to:
__NOTOC__ People and characters
* Phoebe (given name), a list of people, mythological, biblical and fictional characters
* Phoebe (Greek myth), several characters
* Phoebe, an epithet of Artemis/ Diana and Selene/ L ...
, orbits in retrograde fashion.
Some of the moons of Saturn, including Pandora
In Greek mythology, Pandora (Greek: , derived from , ''pān'', i.e. "all" and , ''dōron'', i.e. "gift", thus "the all-endowed", "all-gifted" or "all-giving") was the first human woman created by Hephaestus on the instructions of Zeus. As Hes ...
and Prometheus, act as shepherd moon
A shepherd moon (also herder moon or watcher moon) is a small natural satellite that clears a gap in planetary-ring material or keeps particles within a ring contained. The name is a result of the fact they limit the "herd" of the ring particle ...
s to confine the rings and prevent them from spreading out.[ Pan and Atlas cause weak, linear density waves in Saturn's rings that have yielded more reliable calculations of their masses.
]
History of observation and exploration
The observation and exploration of Saturn can be divided into three phases. The first phase is ancient observations (such as with the naked eye), before the invention of modern telescopes. The second phase began in the 17th century, with telescopic observations from Earth, which improved over time. The third phase is visitation by space probes, in orbit or on flyby. In the 21st century, telescopic observations continue from Earth (including Earth-orbiting observatories
An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been constructed. His ...
like the Hubble Space Telescope) and, until its 2017 retirement, from the '' Cassini'' orbiter around Saturn.
Ancient observations
Saturn has been known since prehistoric times, and in early recorded history it was a major character in various mythologies. Babylonian astronomers
Babylonian astronomy was the study or recording of celestial objects during the early history of Mesopotamia.
Babylonian astronomy seemed to have focused on a select group of stars and constellations known as Ziqpu stars. These constellations m ...
systematically observed and recorded the movements of Saturn.[ In ancient Greek, the planet was known as ''Phainon'', and in Roman times it was known as the "star of ]Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
". In ancient Roman mythology, the planet Phainon was sacred to this agricultural god, from which the planet takes its modern name. The Romans considered the god Saturnus the equivalent of the Greek god Cronus
In Ancient Greek religion and mythology, Cronus, Cronos, or Kronos ( or , from el, Κρόνος, ''Krónos'') was the leader and youngest of the first generation of Titans, the divine descendants of the primordial Gaia (Mother Earth) and ...
; in modern Greek, the planet retains the name ''Cronus''—: ''Kronos''.
The Greek scientist Ptolemy based his calculations of Saturn's orbit on observations he made while it was in opposition.[ In Hindu astrology, there are nine astrological objects, known as Navagrahas. Saturn is known as " Shani" and judges everyone based on the good and bad deeds performed in life.][ Ancient ]Chinese
Chinese can refer to:
* Something related to China
* Chinese people, people of Chinese nationality, citizenship, and/or ethnicity
**''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation
** List of ethnic groups in China, people of va ...
and Japanese culture designated the planet Saturn as the "earth star" (). This was based on Five Elements which were traditionally used to classify natural elements.
In ancient Hebrew, Saturn is called ''Shabbathai''. Its angel is Cassiel. Its intelligence or beneficial spirit is 'Agȋȇl ( he, אגיאל, ʿAgyal), and its darker spirit (demon
A demon is a malevolent supernatural entity. Historically, belief in demons, or stories about demons, occurs in religion, occultism, literature, fiction, mythology, and folklore; as well as in media such as comics, video games, movies, ani ...
) is Zȃzȇl ( he, זאזל, Zazl). Zazel has been described as a great angel, invoked in Solomonic magic, who is "effective in love conjurations". In Ottoman Turkish
Ottoman Turkish ( ota, لِسانِ عُثمانى, Lisân-ı Osmânî, ; tr, Osmanlı Türkçesi) was the standardized register of the Turkish language used by the citizens of the Ottoman Empire (14th to 20th centuries CE). It borrowed extens ...
, Urdu, and Malay
Malay may refer to:
Languages
* Malay language or Bahasa Melayu, a major Austronesian language spoken in Indonesia, Malaysia, Brunei and Singapore
** History of the Malay language, the Malay language from the 4th to the 14th century
** Indonesi ...
, the name of Zazel is 'Zuhal', derived from the Arabic language ( ar, زحل, Zuhal).
European observations (17th–19th centuries)
Saturn's rings require at least a 15-mm-diameter telescope[ to resolve and thus were not known to exist until ]Christiaan Huygens
Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists of ...
saw them in 1655 and published about this in 1659. Galileo
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
, with his primitive telescope in 1610,[ incorrectly thought of Saturn's appearing not quite round as two moons on Saturn's sides.][ It was not until Huygens used greater telescopic magnification that this notion was refuted, and the rings were truly seen for the first time. Huygens also discovered Saturn's moon Titan; Giovanni Domenico Cassini later discovered four other moons: ]Iapetus
In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other ...
, Rhea, Tethys and Dione Dione may refer to:
Astronomy
*106 Dione, a large main belt asteroid
*Dione (moon), a moon of Saturn
*Helene (moon), a moon of Saturn sometimes referred to as "Dione B"
Mythology
*Dione (Titaness), a Titaness in Greek mythology
*Dione (mythology) ...
. In 1675, Cassini discovered the gap now known as the Cassini Division
The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirel ...
.[
No further discoveries of significance were made until 1789 when William Herschel discovered two further moons, ]Mimas
Mimas may refer to:
*Mimas (Giant), son of Gaia in Greek mythology, one of the Gigantes
* Mimas (''Aeneid''), a son of Amycus and Theono, born the same night as Paris, who escorted Aeneas to Italy
*Karaburun, a town and district in Turkey, formerl ...
and Enceladus. The irregularly shaped satellite Hyperion
Hyperion may refer to:
Greek mythology
* Hyperion (Titan), one of the twelve Titans
* ''Hyperion'', a byname of the Sun, Helios
* Hyperion of Troy or Yperion, son of King Priam
Science
* Hyperion (moon), a moon of the planet Saturn
* ''Hyp ...
, which has a resonance with Titan, was discovered in 1848 by a British team.[
In 1899 William Henry Pickering discovered Phoebe, a highly irregular satellite that does not rotate synchronously with Saturn as the larger moons do.][ Phoebe was the first such satellite found and it takes more than a year to orbit Saturn in a retrograde orbit. During the early 20th century, research on Titan led to the confirmation in 1944 that it had a thick atmosphere – a feature unique among the Solar System's moons.][
]
Modern NASA and ESA probes
''Pioneer 11'' flyby
''Pioneer 11
''Pioneer 11'' (also known as ''Pioneer G'') is a robotic space probe launched by NASA on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, solar winds, and cosmic rays. It was the first probe to encounter ...
'' made the first flyby of Saturn in September 1979, when it passed within 20,000 km of the planet's cloud tops. Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. The spacecraft also studied Saturn's rings, revealing the thin F-ring and the fact that dark gaps in the rings are bright when viewed at high phase angle (towards the Sun), meaning that they contain fine light-scattering material. In addition, ''Pioneer 11'' measured the temperature of Titan.
Voyager flybys
In November 1980, the '' Voyager 1'' probe visited the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites. Surface features of various moons were seen for the first time. ''Voyager 1'' performed a close flyby of Titan, increasing knowledge of the atmosphere of the moon. It proved that Titan's atmosphere is impenetrable in visible wavelength
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye will respond to wave ...
s; therefore no surface details were seen. The flyby changed the spacecraft's trajectory out from the plane of the Solar System.
Almost a year later, in August 1981, '' Voyager 2'' continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's turnable camera platform stuck for a couple of days and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus.
The probes discovered and confirmed several new satellites orbiting near or within the planet's rings, as well as the small Maxwell Gap
The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entire ...
(a gap within the C Ring) and Keeler gap (a 42 km-wide gap in the A Ring).
''Cassini–Huygens'' spacecraft
The ''Cassini–Huygens
''Cassini–Huygens'' ( ), commonly called ''Cassini'', was a space research, space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, i ...
'' space probe entered orbit around Saturn on 1 July 2004. In June 2004, it conducted a close flyby of Phoebe Phoebe or Phœbe may refer to:
__NOTOC__ People and characters
* Phoebe (given name), a list of people, mythological, biblical and fictional characters
* Phoebe (Greek myth), several characters
* Phoebe, an epithet of Artemis/ Diana and Selene/ L ...
, sending back high-resolution images and data. ''Cassini'' flyby of Saturn's largest moon, Titan, captured radar images of large lakes and their coastlines with numerous islands and mountains. The orbiter completed two Titan flybys before releasing the ''Huygens'' probe on 25 December 2004. ''Huygens'' descended onto the surface of Titan on 14 January 2005.
Starting in early 2005, scientists used ''Cassini'' to track lightning on Saturn. The power of the lightning is approximately 1,000 times that of lightning on Earth.
In 2006, NASA reported that ''Cassini'' had found evidence of liquid water reservoirs no more than tens of meters below the surface that erupt in geyser
A geyser (, ) is a spring characterized by an intermittent discharge of water ejected turbulently and accompanied by steam. As a fairly rare phenomenon, the formation of geysers is due to particular hydrogeological conditions that exist only in ...
s on Saturn's moon Enceladus. These jets of icy particles are emitted into orbit around Saturn from vents in the moon's south polar region. Over 100 geysers have been identified on Enceladus. In May 2011, NASA scientists reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".
''Cassini'' photographs have revealed a previously undiscovered planetary ring, outside the brighter main rings of Saturn and inside the G and E rings. The source of this ring is hypothesized to be the crashing of a meteoroid off Janus
In ancient Roman religion and myth, Janus ( ; la, Ianvs ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janu ...
and Epimetheus. In July 2006, images were returned of hydrocarbon lakes near Titan's north pole, the presence of which were confirmed in January 2007. In March 2007, hydrocarbon seas were found near the North pole, the largest of which is almost the size of the Caspian Sea. In October 2006, the probe detected an 8,000 km diameter cyclone-like storm with an eyewall at Saturn's south pole.
From 2004 to 2 November 2009, the probe discovered and confirmed eight new satellites. In April 2013 ''Cassini'' sent back images of a hurricane at the planet's north pole 20 times larger than those found on Earth, with winds faster than . On 15 September 2017, the ''Cassini-Huygens'' spacecraft performed the "Grand Finale" of its mission: a number of passes through gaps between Saturn and Saturn's inner rings. The atmospheric entry of ''Cassini'' ended the mission.
Possible future missions
The continued exploration of Saturn is still considered to be a viable option for NASA as part of their ongoing New Frontiers program of missions. NASA previously requested for plans to be put forward for a mission to Saturn that included the Saturn Atmospheric Entry Probe, and possible investigations into the habitability and possible discovery of life on Saturn's moons Titan and Enceladus by ''Dragonfly
A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of true dragonfly are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threate ...
''.
Observation
Saturn is the most distant of the five planets easily visible to the naked eye from Earth, the other four being Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
, Venus, Mars and Jupiter. (Uranus, and occasionally 4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
, are visible to the naked eye in dark skies.) Saturn appears to the naked eye in the night sky as a bright, yellowish point of light. The mean apparent magnitude of Saturn is 0.46 with a standard deviation of 0.34. Most of the magnitude variation is due to the inclination of the ring system relative to the Sun and Earth. The brightest magnitude, −0.55, occurs near in time to when the plane of the rings is inclined most highly, and the faintest magnitude, 1.17, occurs around the time when they are least inclined. It takes approximately 29.5 years for the planet to complete an entire circuit of the ecliptic against the background constellations of the zodiac. Most people will require an optical aid (very large binoculars or a small telescope) that magnifies at least 30 times to achieve an image of Saturn's rings in which clear resolution is present. When Earth passes through the ring plane, which occurs twice every Saturnian year (roughly every 15 Earth years), the rings briefly disappear from view because they are so thin. Such a "disappearance" will next occur in 2025, but Saturn will be too close to the Sun for observations.
Saturn and its rings are best seen when the planet is at, or near, opposition, the configuration of a planet when it is at an elongation of 180°, and thus appears opposite the Sun in the sky. A Saturnian opposition occurs every year—approximately every 378 days—and results in the planet appearing at its brightest. Both the Earth and Saturn orbit the Sun on eccentric orbits, which means their distances from the Sun vary over time, and therefore so do their distances from each other, hence varying the brightness of Saturn from one opposition to the next. Saturn also appears brighter when the rings are angled such that they are more visible. For example, during the opposition of 17 December 2002, Saturn appeared at its brightest due to a favorable orientation of its rings relative to the Earth, even though Saturn was closer to the Earth and Sun in late 2003.
From time to time, Saturn is occulted
An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
by the Moon (that is, the Moon covers up Saturn in the sky). As with all the planets in the Solar System, occultations of Saturn occur in "seasons". Saturnian occultations will take place monthly for about a 12-month period, followed by about a five-year period in which no such activity is registered. The Moon's orbit is inclined by several degrees relative to Saturn's, so occultations will only occur when Saturn is near one of the points in the sky where the two planes intersect (both the length of Saturn's year and the 18.6-Earth year nodal precession Nodal precession is the precession of the orbital plane of a satellite around the rotational axis of an astronomical body such as Earth. This precession is due to the non-spherical nature of a rotating body, which creates a non-uniform gravitational ...
period of the Moon's orbit influence the periodicity).
See also
* Gas giant
* Stats of planets in the Solar System
Notes
References
Further reading
*
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External links
Saturn overview
by NASA's Science Mission Directorate
The Science Mission Directorate (SMD) of the National Aeronautics and Space Administration (NASA) engages the United States’ science community, sponsors scientific research, and develops and deploys satellites and probes in collaboration with NAS ...
Saturn fact sheet
at the NASA Space Science Data Coordinated Archive
Saturnian System terminology
by the IAU Gazetteer of Planetary Nomenclature
''Cassini-Huygens'' legacy website
by the Jet Propulsion Laboratory
Saturn
at SolarViews.com
Interactive 3D gravity simulation of the Cronian system
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Astronomical objects known since antiquity
Gas giants
Outer planets