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Saturn has been known since prehistoric times. Galileo was the first to observe it with
a telescope in 1610. He noted its strange appearance but was confused by it. Early observations of Saturn were complicated
by the fact that the Earth passes through the plane of Saturn's rings every few years as Saturn moves in its orbit. A low
resolution image of Saturn therefore changes drastically. It was not until 1659 that Christiaan Huygens correctly inferred
the geometry of the rings. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were
discovered around Uranus (and shortly thereafter around Jupiter and Neptune).
Saturn is named after the
Roman god Saturn, equated to the Greek Cronus (the Titan father of Zeus) the Babylonian Ninurta and to the Hindu Shani. Saturn's
symbol represents the god's sickle.
Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant.
Together, these four planets are sometimes referred to as the Jovian, meaning "Jupiter-like", planets. Saturn has
an average radius about nine-times larger than Earth's. While only one-eighth the average density of Earth, due to its larger
volume, Saturn's mass is just over ninety-five times greater than Earth's.
Because of Saturn's large mass and resulting
gravitation, the conditions produced on Saturn are extreme. The interior pressures and temperatures are beyond what can be
reproduced experimentally on Earth. The interior of Saturn is likely composed of a core of iron, nickel, silicon and oxygen
compounds, surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium, and
an outer gaseous layer. Electrical current within the metallic-hydrogen layer is thought to give rise to Saturn's planetary
magnetic field, which is slightly weaker than Earth's magnetic field and approximately one-twentieth the strength of the field
around Jupiter. The outer atmosphere is generally bland in appearance, although long-lived features can appear. Wind speeds
on Saturn can reach 1,800 km/h, significantly faster than those on Jupiter.
Saturn has nine rings, consisting mostly
of ice particles with a smaller amount of rocky debris and dust. Sixty-two[16] known moons orbit the planet, fifty-three are
officially named. This is not counting hundreds of "moonlets" within the rings. Titan, Saturn's largest and the
Solar System's second largest moon (after Jupiter's Ganymede), is larger than the planet Mercury and is the only moon in the
Solar System to possess a significant atmosphere.
A rough comparison of the sizes of Saturn and Earth.Due to a
combination of its lower density, rapid rotation, and fluid state, Saturn is an oblate spheroid; that is, it is flattened
at the poles and bulges at the equator. Its equatorial and polar radii differ by almost 10%—60,268 km versus 54,364
km. The other gas planets are also oblate, but to a lesser extent. Saturn is the only planet of the Solar System that is less
dense than water. Although Saturn's core is considerably denser than water, the average specific density of the planet is
0.69 g/cm³ due to the gaseous atmosphere. Saturn is only 95 Earth masses, compared to Jupiter, which is 318 times the
mass of the Earth but only about 20% larger than Saturn.
There is no direct information about Saturn's internal structure, it is thought that its interior
is similar to that of Jupiter, having a small rocky core surrounded mostly by hydrogen and helium. The rocky core is similar
in composition to the Earth, but denser. Above this, there is a thicker liquid metallic hydrogen layer, followed by a layer
of liquid hydrogen and helium, and in the outermost 1000 km a gaseous atmosphere. Traces of various volatiles are also present.
The core region is estimated to be about 9–22 times the mass of the Earth. Saturn has a very hot interior, reaching
11,700 °C at the core, and it radiates 2.5 times more energy into space than it receives from the Sun. Most of the extra
energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient
to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the
"raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction
as they fall down through the lighter hydrogen.
Saturn's temperature emissions: the prominent hot spot at the
bottom of the image is at Saturn's south pole.The outer atmosphere of Saturn consists of 96.3% molecular hydrogen and 3.25%
helium. Trace amounts of ammonia, acetylene, ethane, phosphine, and methane have also been detected. The upper clouds on Saturn
are composed of ammonia crystals, while the lower level clouds appear to be composed of either ammonium hydrosulfide (NH4SH)
or water. The atmosphere of Saturn is significantly deficient in helium relative to the abundance of the elements in the Sun.
Saturn's atmosphere exhibits a banded pattern similar to Jupiter's (the nomenclature is the same), but Saturn's bands
are much fainter and are also much wider near the equator. At depth, extending for 10 km and with a temperature of -23 °C,
is a layer made up of water ice. Above this layer is likely a layer of ammonium hydrosulfide ice, which extends for another
50 km and is approximately -93 °C. Eighty kilometers above that layer are ammonia ice clouds, where the temperatures are
roughly -153 °C. Near the top of the atmosphere, extending for some 200 km to 270 km above the visible ammonia clouds,
are gaseous hydrogen and helium. Saturn's winds are among the Solar System's fastest. Voyager data indicate peak easterly
winds of 500 m/s (1800 km/h). Saturn's finer cloud patterns were not observed until the Voyager flybys. Since then, however,
Earth-based telescopy has improved to the point where regular observations can be made.
Saturn's northern hemisphere,
as seen by Cassini. Note the planet's blue appearance through the ring.Saturn's usually bland atmosphere occasionally exhibits
long-lived ovals and other features common on Jupiter. In 1990, the Hubble Space Telescope observed an enormous white cloud
near Saturn's equator which was not present during the Voyager encounters, and, in 1994, another smaller storm was observed.
The 1990 storm was an example of a Great White Spot, a unique but short-lived phenomenon which occurs once every Saturnian
year, or roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice. Previous Great White
Spots were observed in 1876, 1903, 1933, and 1960, with the 1933 storm being the most famous. If the periodicity is maintained,
another storm will occur in about 2020.
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In recent images from the Cassini spacecraft, Saturn's northern hemisphere
appears a bright blue, similar to Uranus, as can be seen in the image below. This blue color cannot currently be observed
from Earth, because Saturn's rings are currently blocking its northern hemisphere. The color is most likely caused by Rayleigh
scattering.
Infrared imaging has shown that Saturn has a warm polar vortex, the only example of such a phenomenon
known to date in the Solar System. Whereas temperatures on Saturn are normally -185 °C, temperatures on the vortex often
reach as high as -122 °C, believed to be the warmest spot on Saturn.
North polar hexagonal cloud feature,
discovered by Voyager 1 and confirmed in 2006 by Cassini.A persisting hexagonal wave pattern around the north polar vortex
in the atmosphere at about 78°N was first noted in the Voyager images. Unlike the north pole, HST imaging of the south
polar region indicates the presence of a jet stream, but no strong polar vortex nor any hexagonal standing wave. However,
NASA reported in November 2006 that the Cassini spacecraft observed a 'hurricane-like' storm locked to the south pole that
had a clearly defined eyewall. This observation is particularly notable because eyewall clouds had not previously been seen
on any planet other than Earth (including a failure to observe an eyewall in the Great Red Spot of Jupiter by the Galileo
spacecraft).
The straight sides of the northern polar hexagon are each about 13 800 km long. The entire structure
rotates with a period of 10h 39 m 24s, the same period as that of the planet's radio emissions, which is assumed to be equal
to the period of rotation of Saturn's interior. The hexagonal feature does not shift in longitude like the other clouds in
the visible atmosphere.
The average distance between Saturn and the Sun is over 1 400 000 000 km (9 AU). With an
average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days (or about 29½ years), to finish one revolution
around the Sun. The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth. Because of
an eccentricity of 0.056, the distance between Saturn and the Sun varies by approximately 155 000 000 km between perihelion
and aphelion, which are the nearest and most distant points of the planet along its orbital path, respectively.
The
rings of Saturn (imaged here by Cassini in 2007) are the most conspicuous in the Solar System.Main article: Rings of Saturn
Saturn is probably best known for its system of planetary rings, which makes it the most visually remarkable object in the
solar system. The rings extend from 6 630 km to 120 700 km above Saturn's equator, average approximately 20 meters in thickness,
and are composed of 93 percent water ice with a smattering of tholin impurities, and 7 percent amorphous carbon. The particles
that make up the rings range in size from specks of dust to the size of a small automobile. There are two main theories regarding
the origin of the rings. One theory is that the rings are remnants of a destroyed moon of Saturn. The second theory is that
the rings are left over from the original nebular material from which Saturn formed. Some ice in the central rings comes from
the moon Enceladus' ice volcanoes. Beyond the main rings at a distance of 12 million km from the planet is the sparse Phoebe
ring, which is tilted at an angle of 27° to the other rings and, like Phoebe, orbits in retrograde fashion.
Four
of Saturn's moons: Dione, Titan, Prometheus (edge of rings), Telesto (top center)Saturn has at least 62 moons. Titan, the
largest, comprises more than 90 percent 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. Many of the other moons are very small: 34 are less than 10 km in diameter,
and another 14 less than 50 km. Traditionally, most of Saturn's moons have been named after Titans of Greek mythology.
Saturn is the most distant of the five planets easily visible to the naked eye, the other four being Mercury, Venus,
Mars, and Jupiter (Uranus and occasionally 4 Vesta are 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 point of light whose magnitude is usually between +1 and 0 and takes approximately 29½ years to make a complete
circuit of the ecliptic against the background constellations of the zodiac. Most people will require optical aid (large binoculars
or a telescope) magnifying at least 20× to clearly resolve Saturn's rings.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). During
the opposition of December 17, 2002, Saturn appeared at its brightest due to a favorable orientation of its rings relative
to the Earth. But Saturn was closer to the Earth and Sun in late 2003.
orbit: 1,429,400,000 km
(9.54 AU) from Sun
diameter: 120,536 km (equatorial)
mass: 5.68e26 kg
Saturn (Click on photo's
to enlarge)
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