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This article is about the planet. For the deity, seeSaturn (mythology).For other uses, seeSaturn (disambiguation).
"Sixth planet" redirects here. For other systems of numbering planets, seePlanet § History and etymology.

Saturn
Saturn and its prominentrings,as captured by theCassiniorbiter[a]
Designations
Pronunciation/ˈsætərn/ⓘ[1]
Named after
Saturn
AdjectivesSaturnian/səˈtɜːrniən/,[2]Cronian[3]/ Kronian[4]/ˈkroʊniən/[5]
Symbol♄
Orbital characteristics[6]
EpochJ2000.0
Aphelion1,514.50 million km (10.1238 AU)
Perihelion1,352.55 million km (9.0412 AU)
1,433.53 million km (9.5826 AU)
Eccentricity0.0565
378.09 days
9.68 km/s (6.01 mi/s)
317.020°[8]
Inclination
113.665°
2032-Nov-29[10]
339.392°[8]
Knownsatellites146with formal designations; innumerable additionalmoonlets.[11][12]
Physical characteristics[6]
58,232 km (36,184 mi)[b]
9.1402 Earths
  • 60,268 km (37,449 mi)[b]
  • 9.449Earths
Polarradius
  • 54,364 km (33,780 mi)[b]
  • 8.552 Earths
Flattening0.09796
Circumference365882.4km (227348.8mi) (equatorial)[13]
  • 4.27×1010km2(1.65×1010sq mi)[14][b]
  • 83.703 Earths
Volume
  • 8.2713×1014km3(1.9844×1014cu mi)[b]
  • 763.59 Earths
Mass
  • 5.6834×1026kg
  • 95.159 Earths
0.687g/cm3(0.0248lb/cu in)[c]
0.1246Earths
0.22[15]
35.5 km/s (22.1 mi/s)[b]
10 h 32 m 36 s;
10.5433 hours,[16]10 h 39 m;
10.7 hours[7]
10h33m38s+1m52s
−1m19s [17][18]
Equatorial rotation velocity
9.87 km/s (6.13 mi/s; 35,500 km/h)[b]
26.73°(to orbit)
North poleright ascension
40.589°;2h42m21s
North poledeclination
83.537°
Albedo
Surfacetemp. min mean max
1bar 134 K
0.1bar 88 K[21] 97 K[22] 151 K[21]
−0.55[23]to +1.17[23]
−9.7[24]
14.5″ to 20.1″ (excludes rings)
Atmosphere[6]
Surfacepressure
140 kPa[25]
59.5 km (37.0 mi)
Composition by volume
  • 96.3%±2.4%hydrogen
  • 3.25%±2.4%helium
  • 0.45%±0.2%methane
  • 0.0125%±0.0075%ammonia
  • 0.0110%±0.0058%hydrogen deuteride
  • 0.0007%±0.00015%ethane
  • Icyvolatiles:

Saturnis the sixthplanetfrom theSunand the second-largest in theSolar System,afterJupiter.It is agas giantwith an average radius of about nine-and-a-half times that ofEarth.[26][27]It has only one-eighth the average density of Earth, but is over 95 times more massive.[28][29][30]Even though Saturn is nearly the size of Jupiter, Saturn has less than one-third of Jupiter's mass. Saturn orbits the Sun at a distance of 9.59AU(1,434 millionkm) with anorbital periodof 29.45 years.

Saturn's interior is thought to be composed of a rocky core, surrounded by a deep layer ofmetallic hydrogen,an intermediate layer ofliquid hydrogenandliquid helium,and finally, a gaseous outer layer. Saturn has a pale yellow hue due toammoniacrystals in its upper atmosphere. Anelectrical currentwithin the metallic hydrogen layer is thought to give rise to Saturn's planetarymagnetic field,which is weaker than Earth's, but which has amagnetic moment580 times that of Earth due to Saturn's larger size. Saturn's magnetic field strength is around one-twentieth of Jupiter's.[31]The outeratmosphereis generally bland and lacking in contrast, although long-lived features can appear.Wind speedson Saturn can reach 1,800 kilometres per hour (1,100 miles per hour).

The planet has a bright and extensivering systemcomposed mainly of ice particles, with a smaller amount of rocky debris anddust.At least146 moons[32]are known to orbit the planet, of which 63 are officially named; this does not include the hundreds ofmoonletsin its rings.Titan,Saturn's largest moon and the second largest in the Solar System, is larger (while less massive) than the planetMercuryand is the only moon in the Solar System to have a substantial atmosphere.[33]

Name and symbol

Saturn is named after the Romangod of wealth and agricultureand father of Jupiter. Itsastronomical symbol(♄)has been traced back to the GreekOxyrhynchus Papyri,where it can be seen to be a Greekkappa-rholigature with ahorizontal stroke,as an abbreviation forΚρονος(Cronus), the Greek name for the planet ().[34]It later came to look like a lower-case Greeketa,with the cross added at the top in the 16th century to Christianize this pagan symbol.

The Romans named the seventh day of the weekSaturday,Sāturni diēs( "Saturn's Day" ), for the planet Saturn.[35]

Physical characteristics

Saturn is agas giantcomposed predominantly of hydrogen and helium. It lacks a definite surface, though it is likely to have a solid core.[36]Saturn's rotation causes it to have the shape of anoblate spheroid;that is, it isflattenedat thepolesandbulgesat itsequator.Its equatorial radius is more than 10% larger than its polar radius: 60,268 km versus 54,364 km (37,449 mi versus 33,780 mi).[6]Jupiter,Uranus,andNeptune,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,8.96 m/s2,is 74% of what it is at the poles and is lower than the surface gravity of Earth. However, the equatorialescape velocityof nearly36 km/sis much higher than that of Earth.[37]

Saturn is the only planet of the Solar System that is less dense than water—about 30% less.[38]Although Saturn'scoreis considerably denser than water, the averagespecific densityof the planet is0.69 g/cm3due to the atmosphere. Jupiter has 318 timesEarth's mass,[39]and Saturn is 95 times Earth's mass.[6]Together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System.[40]

Internal structure

Diagram of Saturn, to scale

Despite consisting mostly of hydrogen and helium, most of Saturn's mass is not in thegasphase,because hydrogen becomes anon-ideal liquidwhen the density is above0.01 g/cm3,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.[40]

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 variousvolatiles.[41]Analysis of the distortion shows that Saturn is substantially more centrally condensed thanJupiterand therefore contains a significantly larger amount of material denser thanhydrogennear its centre. Saturn's central regions contain about 50% hydrogen by mass, while Jupiter's contain approximately 67% hydrogen.[42]

This core is similar in composition to Earth, but is more dense. The examination of Saturn'sgravitational moment,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,[43][44]which corresponds to a diameter of about 25,000 km (16,000 mi).[45]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.[46]This is surrounded by a thicker liquidmetallic hydrogenlayer, followed by a liquid layer of helium-saturatedmolecular hydrogenthat gradually transitions to a gas with increasing altitude. The outermost layer spans about 1,000 km (620 mi) and consists of gas.[47][48][49]

Saturn has a hot interior, reaching 11,700 °C (21,100 °F) at its core, and radiates 2.5 times more energy into space than it receives from the Sun. Jupiter'sthermal energyis generated by theKelvin–Helmholtz mechanismof slowgravitational 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 the 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 byfrictionand leaves Saturn's outer layers depleted of helium.[50][51]These descending droplets may have accumulated into a helium shell surrounding the core.[41]Rainfalls ofdiamondshave been suggested to occur within Saturn, as well as in Jupiter[52]andice giantsUranus and Neptune.[53]

Atmosphere

The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume.[54]The proportion of helium is significantly deficient compared to the abundance of this element in the Sun.[41]The quantity of elements heavier than helium (metallicity) is not known precisely, but the proportions are assumed to match the primordial abundances from theformation of the Solar System.The total mass of these heavier elements is estimated to be 19–31 times the mass of Earth, with a significant fraction located in Saturn's core region.[55]

Trace amounts of ammonia,acetylene,ethane,propane,phosphine,andmethanehave been detected in Saturn's atmosphere.[56][57][58]The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of eitherammonium hydrosulfide(NH4SH) or water.[59]Ultraviolet radiationfrom the Sun causes methanephotolysisin the upper atmosphere, leading to a series ofhydrocarbonchemical reactions with the resulting products being carried downward byeddiesanddiffusion.Thisphotochemical cycleis modulated by Saturn's annual seasonal cycle.[58]Cassiniobserved 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.[60]

Cloud layers

A global storm girdles the planet in 2011. The storm passes around the planet, such that the storm's head (bright area) passes its tail.

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 theVoyagerspacecraft during the 1980s. Since then, Earth-basedtelescopyhas improved to the point where regular observations can be made.[61]

The composition of the clouds varies with depth and increasing pressure. In the upper cloud layers, with temperatures in the range of 100–160 K and pressures extending between 0.5–2bar,the clouds consist of ammonia ice. Waterice cloudsbegin 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.[62]

Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990, theHubble Space Telescopeimaged an enormous white cloud near Saturn's equator that was not present during theVoyagerencounters, and in 1994 another smaller storm was observed. The 1990 storm was an example of aGreat White Spot,a short-lived phenomenon that occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere'ssummer solstice.[63]Previous Great White Spots were observed in 1876, 1903, 1933, and 1960, with the 1933 storm being the best observed.[64]The latest giant storm was observed in 2010. In 2015, researchers usedVery Large Arraytelescope to study Saturnian atmosphere, and reported that they found "long-lasting signatures of all mid-latitude giant storms, a mixture of equatorial storms up to hundreds of years old, and potentially an unreported older storm at 70°N".[65]

The winds on Saturn are the second fastest among the Solar System's planets, after Neptune's.Voyagerdata indicate peak easterly winds of 500 m/s (1,800 km/h).[66]In images from theCassinispacecraft during 2007, Saturn's northern hemisphere displayed a bright blue hue, similar to Uranus. The color was most likely caused byRayleigh scattering.[67]Thermographyhas shown that Saturn's south pole has a warmpolar vortex,the only known example of such a phenomenon in the Solar System.[68]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.[68]

Hexagonal cloud patterns

Main article:Saturn's hexagon
Saturn's north and south pole ininfrared

A persistinghexagonalwave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in theVoyagerimages.[69][70][71]The sides of the hexagon are each about 14,500 km (9,000 mi) long, which is longer than the diameter of the Earth.[72]The entire structure rotates with a period of10h39m24s(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.[73]The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere.[74]The pattern's origin is a matter of much speculation. Most scientists think it is astanding wavepattern in the atmosphere. Polygonal shapes have been replicated in the laboratory through differential rotation of fluids.[75][76]

HSTimaging of the south polar region indicates the presence of ajet stream,but no strong polar vortex nor any hexagonal standing wave.[77]NASAreported in November 2006 thatCassinihad observed a "hurricane-like "storm locked to the south pole that had a clearly definedeyewall.[78][79]Eyewall clouds had not previously been seen on any planet other than Earth. For example, images from theGalileospacecraft did not show an eyewall in theGreat Red Spotof Jupiter.[80]

The south pole storm may have been present for billions of years.[81]This vortex is comparable to the size of Earth, and it has winds of 550 km/h.[81]

Magnetosphere

Main article:Magnetosphere of Saturn
Auroral lights at Saturn's north pole

Saturn has an intrinsicmagnetic fieldthat has a simple, symmetric shape—a magneticdipole.Its strength at the equator—0.2gauss(20μT)—is approximately one twentieth of that of the field around Jupiter and slightly weaker than Earth's magnetic field.[31]As a result, Saturn'smagnetosphereis much smaller than Jupiter's.[82]

WhenVoyager 2entered the magnetosphere, thesolar windpressure was high and the magnetosphere extended only 19 Saturn radii, or 1.1 million km (684,000 mi),[83]although it enlarged within several hours, and remained so for about three days.[84]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.[82]This magnetosphere is efficient at deflecting thesolar windparticles from the Sun. The moon Titan orbits within the outer part of Saturn's magnetosphere and contributes plasma from theionizedparticles in Titan's outer atmosphere.[31]Saturn's magnetosphere, likeEarth's,producesaurorae.[85]

Orbit and rotation

Animation of Saturn and the Solar System'souter planetsorbiting around the Sun
Simulated appearance of Saturn as seen from Earth (atopposition) during an orbit of Saturn, 2001–2029

The average distance between Saturn and the Sun is over 1.4 billion kilometers (9AU). With an average orbital speed of 9.68 km/s,[6]it takes Saturn 10,759 Earth days (or about29+1⁄2years)[86]to finish one revolution around the Sun.[6]As a consequence, it forms a near 5:2mean-motion resonancewith Jupiter.[87]The elliptical orbit of Saturn is inclined 2.48° relative to theorbital planeof the Earth.[6]Theperihelion and apheliondistances are, respectively, 9.195 and 9.957 AU, on average.[6][88]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 Ihas a period of10h14m00s(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 toSystem I.All other Saturnian latitudes, excluding the north and south polar regions, are indicated asSystem IIand have been assigned a rotation period of10h38m25.4s(810.76°/d).System IIIrefers to Saturn's internal rotation rate. Based onradio emissionsfrom the planet detected byVoyager 1andVoyager 2,[89]System III has a rotation period of10h39m22.4s(810.8°/d). System III has largely superseded System II.[90]

A precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004,Cassinifound that the radio rotation period of Saturn had increased appreciably, to approximately10h45m45s±36s.[91][92]An estimate of Saturn's rotation (as an indicated rotation rate for Saturn as a whole) based on a compilation of various measurements from theCassini,Voyager,andPioneerprobes is10h32m35s.[93]Studies of the planet'sC Ringyield a rotation period of10h33m38s+1m52s
−1m19s .[17][18]

In March 2007, it was found that the variation in radio emissions from the planet did not match Saturn's rotation rate. This variance may be caused by geyser activity on Saturn's moonEnceladus.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.[94][95][96]

An apparent oddity for Saturn is that it does not have any knowntrojan asteroids.These are minor planets that orbit the Sun at the stableLagrangian points,designated L4and L5,located at 60° angles to the planet along its orbit. Trojan asteroids have been discovered forMars,Jupiter, Uranus, and Neptune.Orbital resonancemechanisms, includingsecular resonance,are believed to be the cause of the missing Saturnian trojans.[97]

Natural satellites

Main article:Moons of Saturn
Artist conception of Saturn, its rings and major icy moons—from Mimas to Rhea

Saturn has 146 knownmoons,63 of which have formal names.[12][11]It is estimated that there are another100±30outer irregular moons larger than 3 km (2 mi) in diameter.[98]In addition, there is evidence of dozens to hundreds ofmoonletswith diameters of 40–500 meters in Saturn's rings,[99]which are not considered to be true moons.Titan,the largest moon, comprises more than 90% of the mass in orbit around Saturn, including the rings.[100]Saturn's second-largest moon,Rhea,may have a tenuousring system of its own,[101]along with a tenuousatmosphere.[102][103][104]

Many of the other moons are small: 131 are less than 50 km in diameter.[105]Traditionally, most of Saturn's moons have been named afterTitansof Greek mythology. Titan is the only satellite in theSolar Systemwith a majoratmosphere,[106][107]in which a complexorganic chemistryoccurs. It is the only satellite withhydrocarbon lakes.[108][109]

On 6 June 2013, scientists at theIAA-CSICreported the detection ofpolycyclic aromatic hydrocarbonsin theupper atmosphereof Titan, apossible precursor for life.[110]On 23 June 2014, NASA claimed to have strong evidence thatnitrogenin the atmosphere of Titan came from materials in theOort cloud,associated withcomets,and not from the materials that formed Saturn in earlier times.[111]

Saturn's moonEnceladus,which seems similar in chemical makeup to comets,[112]has often been regarded as a potentialhabitatformicrobial life.[113][114][115][116]Evidence of this possibility includes the satellite's salt-rich particles having an "ocean-like" composition that indicates most of Enceladus's expelledicecomes from the evaporation of liquid salt water.[117][118][119]A 2015 flyby byCassinithrough a plume on Enceladus found most of the ingredients to sustain life forms that live bymethanogenesis.[120]

In April 2014, NASA scientists reported the possible beginning of a new moon within theA Ring,which was imaged byCassinion 15 April 2013.[121]

Planetary rings

Main article:Rings of Saturn
Therings of Saturn(imaged here byCassinion October 2004) are the most massive and conspicuous in the Solar System.[48]

Saturn is probably best known for the system ofplanetary ringsthat makes it visually unique.[48]The rings extend from 6,630 to 120,700 kilometers (4,120 to 75,000 mi) outward from Saturn's equator and average approximately 20 meters (66 ft) in thickness. They are composed predominantly of water ice, with trace amounts oftholinimpurities and a peppered coating of approximately 7% amorphouscarbon.[122]The particles that make up the rings range in size from specks of dust up to 10 m.[123]While the othergas giantsalso have ring systems, Saturn's is the largest and most visible.

There is a debate on the age of the rings. One side supports that they are ancient, and werecreated simultaneously with Saturn from the original nebular material(around 4.6 billion years ago),[124]or shortly after theLHB(around 4.1 to 3.8 billion years ago).[125][126]The other side supports that they are much younger, created around 100 million years ago.[127]AnMITresearch team, supporting the latter theory, proposed that the rings are remnant of a destroyed moon of Saturn, named″Chrysalis″.[128]

Beyond the main rings, at a distance of 12 million km (7.5 million mi) from the planet is the sparse Phoebe ring. It is tilted at an angle of 27° to the other rings and, likePhoebe,orbits inretrogradefashion.[129]

Some of the moons of Saturn, includingPandoraandPrometheus,act asshepherd moonsto confine the rings and prevent them from spreading out.[130]PanandAtlascause weak, linear density waves in Saturn's rings that have yielded more reliable calculations of their masses.[131]

In September 2023, astronomers reported studies suggesting that the rings of Saturn may have resulted from the collision of two moons "a few hundred million years ago".[132][133]

Natural-color mosaic ofCassininarrow-angle camera images of the unilluminated side of Saturn's D, C, B, A, and F rings (left to right) taken on 9 May 2007 (distances are to the planet's center).

History of observation and exploration

The observation and exploration of Saturn can be divided into three phases: (1) pre-modern observations with thenaked eye,(2) telescopic observations from Earth beginning in the 17th century, and (3) visitation byspace probes,in orbit or onflyby.In the 21st century, telescopic observations continue from Earth (includingEarth-orbitingobservatorieslike theHubble Space Telescope) and, untilits 2017 retirement,from theCassiniorbiter around Saturn.

Pre-telescopic observation

See also:Saturn (mythology)andPlanets in astrology § Saturn

Saturn has been known since prehistoric times,[134]and in early recorded history it was a major character in various mythologies.Babylonian astronomerssystematically observed and recorded the movements of Saturn.[135]In ancient Greek, the planet was known asΦαίνωνPhainon,[136]and in Roman times it was known as the "star ofSaturn".[137]Inancient Roman mythology,the planet Phainon was sacred to this agricultural god, from which the planet takes its modern name.[138]The Romans considered the god Saturnus the equivalent of theGreek godCronus;in modernGreek,the planet retains the nameCronus—Κρόνος:Kronos.[139]

The Greek scientistPtolemybased his calculations of Saturn's orbit on observations he made while it was inopposition.[140]InHindu astrology,there are nine astrological objects, known asNavagrahas.Saturn is known as "Shani"and judges everyone based on the good and bad deeds performed in life.[138][140]AncientChineseand Japanese culture designated the planet Saturn as the "earth star" (Thổ tinh). This was based onFive Elementswhich were traditionally used to classify natural elements.[141][142][143]

InHebrew,Saturn is calledShabbathai.[144]Its angel isCassiel.Its intelligence or beneficial spirit is'Agȋȇl(Hebrew:אגיאל,romanized:ʿAgyal),[145]and its darker spirit (demon) isZȃzȇl(Hebrew:זאזל,romanized:Zazl).[145][146][147]Zazel has been described as agreat angel,invoked inSolomonic magic,who is "effective inlove conjurations".[148][149]InOttoman Turkish,Urdu,andMalay,the name of Zazel is 'Zuhal', derived from theArabic language(Arabic:زحل,romanized:Zuhal).[146]

Telescopic pre-spaceflight observations

Galileo Galileiobserved the rings of Saturn in 1610, but was unable to determine what they were.
Robert Hookenoted the shadows (aandb) cast by both the globe and the rings on each other in this drawing of Saturn in 1666.

Saturn's rings require at least a 15-mm-diametertelescope[150]to resolve and thus were not known to exist untilChristiaan Huygenssaw them in 1655 and published about this in 1659.Galileo,with his primitive telescope in 1610,[151][152]incorrectly thought of Saturn's appearing not quite round as two moons on Saturn's sides.[153][154]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'smoon Titan;Giovanni Domenico Cassinilater discovered four other moons:Iapetus,Rhea,Tethys,andDione.In 1675, Cassini discovered the gap now known as theCassini Division.[155]

No further discoveries of significance were made until 1789 whenWilliam Herscheldiscovered two further moons,MimasandEnceladus.The irregularly shaped satelliteHyperion,which has aresonancewith Titan, was discovered in 1848 by a British team.[156]

In 1899,William Henry Pickeringdiscovered Phoebe, a highlyirregular satellitethat does not rotate synchronously with Saturn as the larger moons do.[156]Phoebe was the first such satellite found and it took more than a year to orbit Saturn in aretrograde 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.[157]

Spaceflight missions

Main article:Exploration of Saturn

Pioneer 11flyby

Pioneer 11image of Saturn

Pioneer 11made the first flyby of Saturn in September 1979, when it passed within 20,000 km (12,000 mi) 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 a highphase angle(towards the Sun), meaning that they contain fine light-scattering material. In addition,Pioneer 11measured the temperature of Titan.[158]

Voyagerflybys

In November 1980, theVoyager 1probe 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 1performed a close flyby of Titan, increasing knowledge of the atmosphere of the moon. It proved that Titan's atmosphere is impenetrable atvisible wavelengths;therefore no surface details were seen. The flyby changed the spacecraft's trajectory out of the plane of the Solar System.[159]

Almost a year later, in August 1981,Voyager 2continued 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. 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.[159]

The probes discovered and confirmed several new satellites orbiting near or within the planet's rings, as well as the smallMaxwell Gap(a gap within theC Ring) andKeeler gap(a 42 km-wide gap in theA Ring).[160]

Cassini–Huygensspacecraft

Main article:Cassini–Huygens
At Enceladus's south pole geysers spray water from many locations along thetiger stripes.[161]

TheCassini–Huygensspace probeentered orbit around Saturn on 1 July 2004. In June 2004, it conducted a close flyby ofPhoebe,sending back high-resolution images and data.Cassini'sflyby 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 theHuygensprobeon 25 December 2004.Huygensdescended onto the surface of Titan on 14 January 2005.[162]

Starting in early 2005, scientists usedCassinito track lightning on Saturn. The power of the lightning is approximately 1,000 times that of lightning on Earth.[163]

In 2006, NASA reported thatCassinihad found evidence of liquidwater reservoirsno more than tens of meters below the surface that erupt ingeyserson Saturn's moonEnceladus.These jets of icy particles are emitted into orbit around Saturn from vents in the moon's south polar region.[164]Over 100 geysers have been identified on Enceladus.[161]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".[165][166]

Saturn eclipses the Sun, as seen fromCassini.The rings are visible, including theF Ring.

Cassiniphotographs 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 offJanusandEpimetheus.[167]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 theCaspian Sea.[168]In October 2006, the probe detected an 8,000 km (5,000 mi) diameter cyclone-like storm with an eyewall at Saturn's south pole.[169]

From 2004 to 2 November 2009, the probe discovered and confirmed eight new satellites.[170]In April 2013,Cassinisent back images of a hurricane at the planet's north pole 20 times larger than those found on Earth, with winds faster than 530 km/h (330 mph).[171]On 15 September 2017, theCassini–Huygensspacecraft performed the "Grand Finale" of its mission: a number of passes through gaps between Saturn and Saturn's inner rings.[172][173]Theatmospheric entryofCassiniended the mission.

Possible future missions

The continued exploration of Saturn is still considered to be a viable option for NASA as part of their ongoingNew Frontiers programof missions. NASA previously requested for plans to be put forward for a mission to Saturn that included theSaturn Atmospheric Entry Probe,and possible investigations into the habitability and possible discovery of life on Saturn's moons Titan and Enceladus byDragonfly.[174][175]

Observation

Amateur telescopic view of Saturn

Saturn is the most distant of the five planets easily visible to the naked eye from Earth, the other four beingMercury,Venus,Mars, and Jupiter. (Uranus, and occasionally4 Vesta,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 meanapparent magnitudeof Saturn is 0.46 with a standard deviation of 0.34.[23]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 the time 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.[23]It takes approximately 29.4 years for the planet to complete an entire circuit of theeclipticagainst the background constellations of thezodiac.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 a clear resolution is present.[48][150]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.[176]

Saturn and its rings are best seen when the planet is at, or near,opposition,the configuration of a planet when it is at anelongationof 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 the favorableorientation of its ringsrelative to the Earth,[177]even though Saturn was closer to the Earth and Sun in late 2003.[177]

From time to time, Saturn isoccultedby 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-yearnodal precessionperiod of the Moon's orbit influence the periodicity).[178]

See also

Notes

  1. ^The dot in the bottom left isTitan
  2. ^abcdefghRefers to the level of 1 bar atmospheric pressure
  3. ^Based on the volume within the level of 1 bar atmospheric pressure

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