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Saturn

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   CAPTION: Saturn   Astronomical symbol for Saturn

                     The planet Saturn
   Saturn, as seen by Cassini
                  Orbital characteristics
                        Epoch J2000
         Aphelion distance:       1,503,983,449 km
                                  10.053 508 40 AU
                                  934,534,231 miles
        Perihelion distance:      1,349,467,375 km
                                  9.020 632 24 AU
                                  838,522,163 miles
          Semi-major axis:        1,426,725,413 km
                                  9.537 070 32 AU
                                  886,528,196 miles
       Orbital circumference:     8.958 T m
                                  59.879 AU
           Eccentricity:          0.054 150 60
          Sidereal period:        10,756.1995 day
                                  (29.46 yr)
          Synodic period:         378.10 day
        Avg. orbital speed:       9.638 km/s
        Max. orbital speed:       10.183 km/s
        Min. orbital speed:       9.137 km/s
            Inclination:          2.484 46°
                                  (5.51° to Sun's equator)
    Longitude of ascending node:  113.71532811 04°
      Argument of perihelion:     338.716 90°
            Satellites:           56 confirmed
                  Physical characteristics
         Equatorial radius:       60,268 km
                                  (9.4492 Earths)
           Polar radius:          54,364 km
                                  (8.5521 Earths)
            Oblateness:           0.097 96
           Surface area:          4.27×10^10 km²
                                  (83.703 Earths)
              Volume:             8.27×10^14 km³
                                  (763 Earths)
               Mass:              5.6846×10^26 kg
                                  (95.152 Earths)
           Mean density:          0.6873 g/cm³
                                  (less than water)
    Equatorial surface gravity:   8.96 m/s^2
                                  (0.914 g)
          Escape velocity:        35.49 km/s
     Sidereal rotation period:    0.449 375 day
                                  (10 h 47 min 6 s)
   Rotation velocity at equator:  9.87 km/s = 35,500 km/h
                                  (at the equator)
            Axial tilt:           26.73°
   Right ascension of North pole: 40.59° (2 h 42 min 21 s)
     Declination of North pole:   83.54°
              Albedo:             0.47
           Surface temp.:
      Surface
      Cloudtop
                                  min  mean  max
                                  82 K 143 K N/A
                                       93 K
            Adjectives:           Saturnian
                         Atmosphere
         Surface pressure:        140 kPa
            Composition:          >93% hydrogen
                                  >5% helium
                                  0.2% methane
                                  0.1% water vapor
                                  0.01% ammonia
                                  0.0005% ethane
                                  0.0001% phosphine

   Saturn ( IPA: [ˈsæɾɚn], [ˈsætən]) is the sixth planet from the Sun. It
   is a gas giant (also known as a Jovian planet, after the planet
   Jupiter), the second-largest planet in the Solar System after Jupiter.
   Saturn has a prominent system of rings, consisting mostly of ice
   particles with a smaller amount of rocky debris and dust. It was named
   after the Roman God Saturn (the Greek equivalent is Cronos, father of
   Zeus). Its symbol is a stylized representation of the god's sickle (
   Unicode: ♄).

Physical characteristics

   Saturn is an oblate spheroid; that is, it is flattened at the poles and
   bulges at the equator. Its equatorial and polar diameters vary by
   almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid
   rotation and fluid state. The other gas planets are also oblate, but to
   a lesser degree. Saturn is the only planet of the Solar System that is
   less dense than water. Although Saturn's core is considerably more
   dense than water, the average specific density of the planet is 0.69
   due to the gaseous atmosphere.
   Saturn's temperature emissions: The prominent hot spot at the bottom of
   the image is at Saturn's south pole.
   Saturn's temperature emissions: The prominent hot spot at the bottom of
   the image is at Saturn's south pole.

   Saturn's interior is similar to Jupiter's, having a rocky core at the
   centre, a liquid metallic hydrogen layer above that, and a molecular
   hydrogen layer above that. Traces of various ices are also present.
   Saturn has a very hot interior, reaching 12,000 kelvins (11,700 °C) at
   the core, and it radiates 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.

   Saturn's atmosphere exhibits a banded pattern similar to Jupiter's (in
   fact, the nomenclature is the same), but Saturn's bands are much
   fainter and are also much wider near the equator. Saturn's winds are
   among the Solar System's fastest. Voyager data indicates peak easterly
   winds of 500 m/s (1116 mph). 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 Saturnian phenomenon with a
   roughly 30-year periodicity. 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 colour is most likely caused by Rayleigh scattering.
   Visual comparison of Saturn and Earth
   Visual comparison of Saturn and Earth

   Astronomers using infrared imaging have shown that Saturn has a warm
   polar vortex, and is the only such planet known in the solar system.

   An apparently permanent hexagonal wave pattern around the polar vortex
   in the atmosphere at about 78°N was first noted in the Voyager images.
   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 have not 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).

Rotational behaviour

   Since Saturn does not rotate on its axis at a uniform rate, multiple
   rotation periods have been assigned to it (as in Jupiter's case):
   System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses
   the Equatorial Zone, which extends from the northern edge of the South
   Equatorial Belt to the southern edge of the North Equatorial Belt. All
   other Saturnian latitudes have been assigned a rotation period of 10 h
   39 min 24 s (810.76°/d), which is System II. System III, based on radio
   emissions from the planet, has a period of 10 h 39 min 22.4 s
   (810.8°/d); because it is very close to System II, it has largely
   superseded it.

   While approaching Saturn in 2004, the Cassini spacecraft found that the
   radio rotation period of Saturn had increased slightly, to
   approximately 10 h 45 m 45 s (± 36 s). The cause of the change is
   unknown — however, it is thought that this is due to a movement of the
   radio source to a different latitude inside Saturn, with a different
   rotational period, rather than an actual change in Saturn's rotation.

Planetary rings

   The rings of Saturn, as imaged by Cassini in 2007
   The rings of Saturn, as imaged by Cassini in 2007

   Saturn is probably best known for its system of planetary rings, which
   makes it one of the most visually remarkable objects in the solar
   system.

History

   The rings were first observed by Galileo Galilei in 1610 with his
   telescope, but he was unable to identify them as such. He wrote to the
   Duke of Tuscany that "The planet Saturn is not alone, but is composed
   of three, which almost touch one another and never move nor change with
   respect to one another. They are arranged in a line parallel to the
   zodiac, and the middle one (Saturn itself) is about three times the
   size of the lateral ones [the edges of the rings]." He also described
   Saturn as having "ears." In 1612 the plane of the rings was oriented
   directly at the Earth and the rings appeared to vanish, and then in
   1613 they reappeared again, further confusing Galileo.

   In 1655, Christiaan Huygens became the first person to suggest that
   Saturn was surrounded by a ring. Using a telescope that was far
   superior to those available to Galileo, Huygens observed Saturn and
   wrote that "It [Saturn] is surrounded by a thin, flat, ring, nowhere
   touching, inclined to the ecliptic."

   In 1675, Giovanni Domenico Cassini determined that Saturn's ring was
   actually composed of multiple smaller rings with gaps between them; the
   largest of these gaps was later named the Cassini Division.

   In 1859, James Clerk Maxwell demonstrated that the rings could not be
   solid or they would become unstable and break apart. He proposed that
   the rings must be composed of numerous small particles, all
   independently orbiting Saturn. Maxwell's theory was proven correct in
   1895 through spectroscopic studies of the rings carried out by James
   Keeler of Lick Observatory.

Physical characteristics

   The rings can be viewed using a quite modest modern telescope or with
   good binoculars. They extend from 6,630 km to 120,700 km above Saturn's
   equator, average close to one kilometer in thickness and are composed
   of silica rock, iron oxide, and ice particles ranging in size from
   specks of dust to the size of a small automobile. There are two main
   theories regarding the origin of Saturn's rings. One theory, originally
   proposed by Édouard Roche in the 19th century, is that the rings were
   once a moon of Saturn whose orbit decayed until it came close enough to
   be ripped apart by tidal forces (see Roche limit). A variation of this
   theory is that the moon disintegrated after being struck by a large
   comet or asteroid. The second theory is that the rings were never part
   of a moon, but are instead left over from the original nebular material
   from which Saturn formed. This theory is not widely accepted today,
   since Saturn's rings are thought to be unstable over periods of
   millions of years and therefore of relatively recent origin.

   While the largest gaps in the rings, such as the Cassini Division and
   Encke Division, can be seen from Earth, the Voyager spacecrafts
   discovered the rings to have an intricate structure of thousands of
   thin gaps and ringlets. This structure is thought to arise from the
   gravitational pull of Saturn's many moons in several different ways.
   Some gaps are cleared out by the passage of tiny moonlets such as Pan,
   many more of which may yet be discovered, and some ringlets seem to be
   maintained by the gravitational effects of small shepherd satellites
   such as Prometheus and Pandora. Other gaps arise from resonances
   between the orbital period of particles in the gap and that of a more
   massive moon further out; Mimas maintains the Cassini division in this
   manner. Still more structure in the rings actually consists of spiral
   waves raised by the moons' periodic gravitational perturbations.

   Data from the Cassini space probe indicates that the rings of Saturn
   possess their own atmosphere, independent of that of the planet itself.
   The atmosphere is composed of molecular oxygen gas (O[2]) produced when
   ultraviolet light from the Sun disintegrates water ice in the rings.
   Chemical reactions between water molecule fragments and further
   ultraviolet stimulation create and eject, among other things O[2].
   According to models of this atmosphere, H[2] is also present. The O[2]
   and H[2] atmospheres are so sparse that if the entire atmosphere were
   somehow condensed onto the rings, it would be on the order of one atom
   thick. The rings also have a similarly sparse OH (hydroxide)
   atmosphere. Like the O[2], this atmosphere is produced by the
   disintegration of water molecules, though in this case the
   disintegration is done by energetic ions that bombard water molecules
   ejected by Saturn's moon Enceladus. This atmosphere, despite being
   extremely sparse, was detected from Earth by the Hubble Space
   Telescope.

   Saturn shows complex patterns in its brightness. Most of the
   variability is due to the changing aspect of the rings, and this goes
   through two cycles every orbit. However, superimposed on this is
   variability due to the eccentricity of the planet's orbit that causes
   the planet to display brighter oppositions in the northern hemisphere
   than it does in the southern (Henshaw, C., 2003).

   In 1980 Voyager I made a fly-by of Saturn that showed the F-ring to be
   composed of three narrow rings that appeared to be braided in a complex
   structure; it is now known that the outer two rings consist of knobs,
   kinks and lumps that give the illusion of braiding, with the less
   bright third ring lying inside them.

Spokes of the rings

   Spokes in the B ring, imaged by Voyager 2 in 1981
   Spokes in the B ring, imaged by Voyager 2 in 1981

   Until 1980, the structure of the rings of Saturn was explained
   exclusively as the action of gravitational forces. The Voyager
   spacecraft found radial features in the B ring, called spokes, which
   could not be explained in this manner, as their persistence and
   rotation around the rings were not consistent with orbital mechanics.
   The spokes appear dark against the lit side of the rings, and light
   when seen against the unlit side. It is assumed that they are connected
   to electromagnetic interactions, as they rotate almost synchronously
   with the magnetosphere of Saturn. However, the precise mechanism
   generating the spokes is still unknown.
   Spokes imaged by Cassini in 2005
   Spokes imaged by Cassini in 2005

   Twenty-five years later, Cassini observed the spokes again. They appear
   to be a seasonal phenomenon, disappearing in the Saturnian
   midwinter/midsummer and reappearing as Saturn comes closer to equinox.
   The spokes were not visible when Cassini arrived at Saturn in early
   2004. Some scientists speculated that the spokes would not be visible
   again until 2007, based on models attempting to describe spoke
   formation. Nevertheless, the Cassini imaging team kept looking for
   spokes in images of the rings, and the spokes reappeared in images
   taken September 5, 2005.

Natural satellites

   Four of Saturn's moons: Dione, Titan, Prometheus (edge of rings),
   Telesto (top center)
   Four of Saturn's moons: Dione, Titan, Prometheus (edge of rings),
   Telesto (top centre)

   Saturn has a large number of moons. The precise figure is uncertain, as
   the orbiting chunks of ice in Saturn's rings are all technically moons,
   and it is difficult to draw a distinction between a large ring particle
   and a tiny moon. As of 2006, a total of 56 individual moons have been
   identified, along with 3 unconfirmed moons that could be small dusk
   clumps in the rings. Many of the moons are very small: out of 56, 30
   are less than 10 km in diameter, and another 13 less than 50 km. Only
   seven of them are massive enough to have collapsed into spheroids under
   their own gravitation. These are compared with Earth's moon in the
   table below. Saturn's most noteworthy moon is Titan, the only moon in
   the solar system to have a dense atmosphere.
   Saturn's rings cut across an eerie scene that is ruled by Titan's
   luminous crescent and globe-encircling haze, broken by the small moon
   Enceladus, whose icy jets are dimly visible at its south pole. North is
   up.
   Saturn's rings cut across an eerie scene that is ruled by Titan's
   luminous crescent and globe-encircling haze, broken by the small moon
   Enceladus, whose icy jets are dimly visible at its south pole. North is
   up.

   Traditionally, most of Saturn's other moons are named after actual
   Titans of Greek mythology. This started because John Herschel — son of
   William Herschel, discoverer of Mimas and Enceladus — suggested doing
   so in his 1847 publication Results of Astronomical Observations made at
   the Cape of Good Hope, because they were the sisters and brothers of
   Cronos (the Greek Saturn).

   Saturn's major satellites, compared with Earth's Moon.
   Name

   ( Pronunciation key)
   Diameter
   (km) Mass
   (kg) Orbital radius (km) Orbital period (days)
                            Mimas ˈmaɪməs 400
                            (10% Luna) 0.4×10^20
                            (0.05% Luna) 185,000
                               (50% Luna) 0.9
                                  (3% Luna)
                       Enceladus ɛnˈsɛləɾəs 500
                            (15% Luna) 1.1×10^20
                             (0.2% Luna) 238,000
                               (60% Luna) 1.4
                                  (5% Luna)
                           Tethys ˈtʰiθɪs 1060
                            (30% Luna) 6.2×10^20
                             (0.8% Luna) 295,000
                               (80% Luna) 1.9
                                  (7% Luna)
                           Dione daɪˈəʊni 1120
                            (30% Luna) 11×10^20
                             (1.5% Luna) 377,000
                               (100% Luna) 2.7
                                 (10% Luna)
                              Rhea ˈriə 1530
                            (45% Luna) 23×10^20
                              (3% Luna) 527,000
                               (140% Luna) 4.5
                                 (20% Luna)
                          Titan ˈtʰaɪʔən 5150
                           (150% Luna) 1350×10^20
                            (180% Luna) 1,222,000
                               (320% Luna) 16
                                 (60% Luna)
                        Iapetus aɪˈæpəɾəs 1440
                            (40% Luna) 20×10^20
                             (3% Luna) 3,560,000
                               (930% Luna) 79
                                 (290% Luna)

Exploration of Saturn

   A Hubble Space Telescope image, captured in October 1996, shows
   Saturn's rings from just past edge-on. Credit: NASA/ESA.
   A Hubble Space Telescope image, captured in October 1996, shows
   Saturn's rings from just past edge-on. Credit: NASA/ESA.

Pioneer 11 flyby

   Saturn was first visited by Pioneer 11 in September 1979. It flew
   within 20,000 km of the planet's cloud tops. Low-resolution images were
   acquired of the planet and a few of its moons; the resolution of the
   images was not good enough to discern surface features. The spacecraft
   also studied the rings; among the discoveries were the thin F-ring and
   the fact that dark gaps in the rings are bright when viewed towards the
   Sun, or in other words, they are not empty of material. Pioneer 11 also
   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, rings, and
   satellites. Surface features of various moons were seen for the first
   time. Voyager 1 performed a close flyby of Titan, greatly increasing
   our knowledge of the atmosphere of the moon. However, it also proved
   that Titan's atmosphere is impenetrable in visible wavelengths, so no
   surface details were seen. The flyby also 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. They also discovered the small
   Maxwell and Keeler gaps.

Cassini orbiter

   Saturn eclipses the sun, as seen from Cassini.
   Saturn eclipses the sun, as seen from Cassini.

   On July 1, 2004, the Cassini-Huygens spacecraft performed the SOI
   (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn.
   Before the SOI, Cassini had already studied the system extensively. In
   June 2004, it had conducted a close flyby of Phoebe, sending back
   high-resolution images and data.

   The orbiter completed two Titan flybys before releasing the Huygens
   probe on December 25, 2004. Huygens descended onto the surface of Titan
   on January 14, 2005, sending a flood of data during the atmospheric
   descent and after the landing. During 2005 Cassini conducted multiple
   flybys of Titan and icy satellites.

   On March 10, 2006, NASA reported that the Cassini probe found evidence
   of liquid water reservoirs that erupt in geysers on Saturn's moon
   Enceladus.

   On September 20, 2006, a Cassini probe photograph revealed a previously
   undiscovered planetary ring, outside the brighter main rings of Saturn
   and inside the G and E rings.

   As of 2006 the probe has discovered and confirmed 4 new satellites. Its
   primary mission will end in 2008 when the spacecraft will be expected
   to have completed 74 orbits around the planet.
   Picture of a large hurricane-like storm on the south pole of Saturn
   Picture of a large hurricane-like storm on the south pole of Saturn

Best viewing of Saturn

   Saturn Oppositions: 2001-2029
   Saturn Oppositions: 2001-2029

   Saturn has been known since prehistoric times. It is the most distant
   of the five planets easily visible to the naked eye, the other four
   being Mercury, Venus, Mars, and Jupiter (Uranus is visible to the naked
   eye in very dark skies), and was the last planet known to early
   astronomers until Uranus was discovered in 1781. Saturn appears to the
   naked eye in the night sky as a bright, yellowish star varying usually
   between magnitude +1 and 0 and takes approximately 29½ years to make a
   complete circuit of the ecliptic against the background constellations
   of the zodiac. Optical aid (large binoculars or a telescope) magnifying
   at least 20X is required to clearly resolve Saturn's rings for most
   people.

   While it is a rewarding target for observation for most of the time it
   is visible in the sky, 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). In the opposition on January 13, 2005, Saturn appeared at its
   brightest until 2031, mostly due to a favorable orientation of the
   rings relative to the Earth.

   For more details, see Aspects of Saturn.

Saturn in various cultures

   In Hindu astrology, there are nine planets, known as Navagrahas. Saturn
   is known as "Sani" or " Shani", the Judge among all the planets, and
   determines everyone according to their own performed deeds bad or good.

   Chinese and Japanese culture designate the planet Saturn as the earth
   star (土星). This is based on Five Elements which were traditionally used
   to classify natural elements.

   In Hebrew, Saturn is called 'Shabbathai'. Its Angel is Cassiel. Its
   Intelligence, or beneficial spirit, is Agiel (layga), and its spirit
   (darker aspect) is Zazel (lzaz). See: Kabbalah.

   In Ottoman Turkish and in Malay, its name is 'Zuhal', derived from
   Arabic زحل.

   Saturn was also called Φαίνων, Phainon ("shiner") by the Greeks.

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