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| Pictured in natural
color approaching equinox, photographed by
Cassini in July 2008; the dot in the bottom left
corner is Titan. |
Saturn
Saturn is the sixth planet from the Sun in the Solar
System. It is the second largest planet in the Solar
System, after Jupiter. Like Jupiter, Uranus and Neptune,
it is a "gas giant".
Inside Saturn is probably a core of iron, nickel,
silicon and oxygen compounds, surrounded by a deep layer
of metallic hydrogen, then a layer of liquid hydrogen
and liquid helium and finally, an outer gaseous layer.
Saturn has 62 known moons orbiting the planet; 53 are
officially named. The largest moon is Titan, which is
larger in volume than the planet Mercury. Titan is the
second-largest moon in the Solar System. The largest
moon is Jupiter's moon, Ganymede. There is also a very
large system of rings around Saturn. These rings are
made of ice with smaller amounts of rocks and dust.
Saturn is about 1,400,000,000 km (869,000,000 mi) from
the Sun. Saturn takes 29.6 years on Earth to revolve
around the Sun.
Saturn was named after the Roman god Saturnus (called
Kronos in Greek mythology). Saturn's symbol is ♄ which
is the symbol of Saturnus' sickle. |
|
 |
| Methane bands circle
Saturn. The moon Dione hangs below the rings to
the right. |
Physical features
Saturn is an oblate spheroid, meaning that it is
flattened at the poles, and it swells out around its
equator. The planet's equatorial diameter is 120,536 km
(74,898 mi), while its polar diameter (the distance from
the north pole to the south pole) is 108,728 km (67,560
mi); a 9% difference. Saturn has a flattened shape; it
is due to its very fast rotation, rotating once every
10.8 hours. Saturn is the only planet in the Solar
System that is less dense than water. Even though the
planet's core is very dense, it has a gaseous
atmosphere, so the average specific density of the
planet is 0.69 g/cm3 (less than the density of water).
This means if Saturn could be placed in a large pool of
water, it would float.
Atmosphere
The outer part of Saturn's atmosphere is made up of
about 96% hydrogen, 3% helium, 0.4% methane and 0.01%
ammonia. There are also very small amounts of acetylene,
ethane and phosphine.
Saturn's clouds show a banded pattern, like the cloud
bands seen on Jupiter. Saturn's clouds are much fainter
and the bands are wider at the equator. Saturn's lowest
cloud layer is made up of water ice, and is about 10 km
(6 mi) thick. The temperature here is quite low, at 250
K (-10°F, -23°C). However scientists do not agree about
this. The layer above, about 77 km (48 mi) thick, is
made up of ammonium hydrosulfide ice, and above that is
a layer of ammonia ice clouds 80 km (50 mi) thick. The
highest layer is made up of hydrogen and helium gases,
which extends between 200 km (124 mi) and 270 km (168
mi) above the water cloud tops. Auroras are also known
to form in Saturn in the mesosphere. The temperature at
Saturn's cloud tops is extremely low, at 98 K (-283°F,
-175°C). The temperatures in the inner layers are much
higher than the outside layers because of the heat
produced by Saturn's interior. Saturn's winds are some
of the fastest in the Solar System, reaching 1,800 km/h
(1,118 mph), ten times faster than winds on Earth. |
 |
| 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. |
Storms and spots
Saturn's atmosphere is also known to form oval shaped
clouds, similar to the clearer spots seen in Jupiter.
These oval spots are cyclonic storms, the same as
cyclones seen on Earth. In 1990, the Hubble Space
Telescope found a very large white cloud near Saturn's
equator. Storms like the one in 1990 were known as Great
White Spots. These unique storms only exist for a short
time and only happen about every 30 Earth years, at the
time of the summer solstice in the Northern Hemisphere.
Great White Spots were also found in 1876, 1903, 1933,
and 1960. If this cycle continues, another storm will
form in about 2020.
The Voyager 1 spacecraft found a hexagonal cloud pattern
near Saturn's north pole at about 78°N. The
Cassini−Huygens probe later confirmed it in 2006. Unlike
the north pole, the south pole does not show any
hexagonal cloud feature. The probe also discovered a
hurricane-like storm locked to the south pole that
clearly showed an eyewall. Until this discovery,
eyewalls had only been seen on Earth.
Interior
Saturn's interior is similar to Jupiter's interior. It
has a small rocky core about the size of the Earth at
its centre. It is very hot; its temperature reaches
15,000 K (26,540 °F (14,727 °C)). Saturn is so hot that
it gives out more heat energy into space than it
receives from the Sun. Above it is a thicker layer of
metallic hydrogen, about 30,000 km (18,641 mi) deep.
Above that layer is a region of liquid hydrogen and
helium. The core is heavy, with about 9 to 22 times more
mass than the Earth's core.
Magnetic field
Saturn has a natural magnetic field that is weaker than
Jupiter's. Like the Earth's, Saturn's field is a
magnetic dipole. Saturn's field is unique in that it is
perfectly symmetrical, unlike any other known planet.
This means the field is exactly in line with the
planet's axis. Saturn generates radio waves, but they
are too weak to be detected from Earth. The moon Titan
orbits in the outer part of Saturn's magnetic field and
gives out plasma to the field from the ionised particles
in Titan's atmosphere. |
|
 |
| Composite image
comparing the sizes of Saturn and Earth. |
Rotation and orbit
Saturn's average distance from the Sun is over
1,400,000,000 km (869,000,000 mi), about nine times the
distance from the Earth to the Sun. It takes 10,759
days, or about 29.7 years, for Saturn to orbit around
the Sun. This is known as Saturn's orbital period.
Voyager 1 measured Saturn's rotation as being 10 hours
14 minutes at the equator, 10 hours 40 minutes closer to
the poles, and 10 hours 39 minutes 24 seconds for the
planet's interior. This is known as its rotational
period.
Cassini measured the rotation of Saturn as being 10
hours 45 minutes 45 seconds ± 36 seconds. That is about
six minutes, or one percent, longer than the radio
rotational period measured by the Voyager 1 and Voyager
2 spacecraft, which flew by Saturn in 1980 and 1981.
Saturn's rotational period is calculated by the rotation
speed of radio waves released by the planet. The
Cassini−Huygens spacecraft discovered that the radio
waves slowed down, suggesting that the rotational period
increased. Since the scientists do not think Saturn's
rotation is actually slowing down, the explanation may
lie in the magnetic field that causes the radio waves. |
 |
| Saturn and rings as
viewed by the Cassini spacecraft (28 October
2016). |
Planetary rings
Saturn is best known for its planetary rings which are
easy to see with a telescope. There are seven named
rings; A, B, C, D, E, F, and G rings. They were named in
the order they were discovered, which is different to
their order from the planet. From the planet the rings
are: D, C, B, A, F, G and E.
Scientists believe that the rings are the material left
after a moon broke apart. A new idea says that it was a
very large moon, most of which crashed into the planet.
This left a large amount of ice to form the rings, and
also some of the moons, like Enceladus, which are
thought to be made of ice.
History
The rings were first discovered by Galileo Galilei in
1610, using his telescope. They did not look like rings
to Galileo, so he called them "handles". He thought that
Saturn was three separate planets that almost touched
one another. In 1612, when the rings were facing edge on
with the Earth, the rings disappeared, then reappeared
again in 1613, further confusing Galileo. In 1655,
Christiaan Huygens was the first person to recognise
Saturn was surrounded by rings. Using a much more
powerful telescope than Galilei's, he noted Saturn "is
surrounded by a thin, flat, ring, nowhere touching...".
In 1675, Giovanni Domenico Cassini discovered that the
planet's rings were in fact made of smaller ringlets
with gaps. The largest ring gap was later named the
Cassini Division. In 1859, James Clerk Maxwell showed
that the rings cannot be solid, but are made of small
particles, each orbiting Saturn on their own, otherwise,
it would become unstable or break apart. James Keeler
studied the rings using a spectroscope in 1895 which
proved Maxwell's theory.
Physical features
The rings range from 6,630 km (4,120 mi) to 120,700 km
(75,000 mi) above the planet's equator. As proved by
Maxwell, even though the rings appear to be solid and
unbroken when viewed from above, the rings are made of
small particles of rock and ice. They are only about 10
m (33 ft) thick; made of silica rock, iron oxide and ice
particles. The smallest particles are only specks of
dust while the largest are the size of a house. The C
and D rings also seem to have a "wave" in them, like
waves in water. These large waves are 500 m (1,640 ft)
high, but only moving slowly at about 250 m (820 ft)
each day. Some scientists believe that the wave is
caused by Saturn's moons. Another idea is the waves were
made by a comet hitting Saturn in 1983 or 1984.
The largest gaps in the rings are the Cassini Division
and the Encke Division, both visible from the Earth. The
Cassini Division is the largest, measuring 4,800 km
(2,983 mi) wide. However, when the Voyager spacecrafts
visited Saturn in 1980, they discovered that the rings
are a complex structure, made out of thousands of thin
gaps and ringlets. Scientists believe this is caused by
the gravitational force of some of Saturn's moons. The
tiny moon Pan orbits inside Saturn's rings, creating a
gap within the rings. Other ringlets keep their
structure due to the gravitational force of shepherd
satellites, such as Prometheus and Pandora. Other gaps
form due to the gravitational force of a large moon
farther away. The moon Mimas is responsible for clearing
away the Cassini gap.
Recent data from the Cassini spacecraft has shown that
the rings have their own atmosphere, free from the
planet's atmosphere. The rings' atmosphere is made of
oxygen gas, and it is produced when the Sun's
ultraviolet light breaks up the water ice in the rings.
Chemical reaction also occurs between the ultraviolet
light and the water molecules, creating hydrogen gas.
The oxygen and hydrogen atmospheres around the rings are
very widely spaced. As well as oxygen and hydrogen gas,
the rings have a thin atmosphere made of hydroxide. This
anion was discovered by the Hubble Space Telescope.
Spokes
The Voyager space probe discovered features shaped like
rays, called spokes. These were also seen later by the
Hubble telescope. The Cassini probe photographed the
spokes in 2005. They are seen as dark when under
sunlight, and appear light when against the unlit side.
At first it was thought the spokes were made of
microscopic dust particles but new evidence shows that
they are made of ice. They rotate at the same time with
the planet's magnetosphere, therefore, it is believed
that they have a connection with electromagnetism.
However, what causes the spokes to form is still
unknown. They appear to be seasonal, disappearing during
solstice and appearing again during equinox. |
 |
| A montage of Saturn
and its principal moons (Dione, Tethys, Mimas,
Enceladus, Rhea and Titan; Iapetus not shown).
This image was created from photographs taken in
November 1980 by the Voyager 1 spacecraft. |
Moons
Saturn has 53 named moons, and another nine which are
still being studied. Many of the moons are very small:
33 are less than 10 km (6 mi) in diameter and 13 moons
are less than 50 km (31 mi). Seven moons are large
enough to be a near perfect sphere caused by their own
gravitation. These moons are Titan, Rhea, Iapetus, Dione,
Tethys, Enceladus and Mimas. Titan is the largest moon,
larger than the planet Mercury, and it is the only moon
in the Solar System to have a thick, dense atmosphere.
Hyperion and Phoebe are the next largest moons, larger
than 200 km (124 mi) in diameter.
In December 2004 and January 2005 a man-made satellite
called the Cassini−Huygens probe took lots of close
photos of Titan. One part of this satellite, known as
the Huygens probe, then landed on Titan. Named after the
Dutch astronomer Christiaan Huygens, it was the first
spacecraft to land in the outer Solar System. The probe
was designed to float in case it landed in liquid.
Enceladus, the sixth largest moon, is about 500 km (311
mi) in diameter. It is one of the few outer solar system
objects that shows volcanic activity. In 2011,
scientists discovered an electric link between Saturn
and Enceladus. This is caused by ionised particles from
volcanos on the small moon interacting with Saturn's
magnetic fields. Similar interactions cause the northern
lights on Earth. |
|
 |
| Pioneer 11 image of
Saturn. |
Exploration
Saturn was first explored by the Pioneer 11 spacecraft
in September 1979. It flew as close as 20,000 km (12,427
mi) above the planet's cloud tops. It took photographs
of the planet and a few of its moons, but were low in
resolution (the quality was not very good). It
discovered a new, thin ring called the F ring. It also
discovered that the dark ring gaps appear bright when
viewed towards the Sun, which shows the gaps are not
empty of material. The spacecraft measured the
temperature of the moon Titan.
In November 1980, Voyager 1 visited Saturn, and took
higher resolution photographs of the planet, rings and
moons. These photos were able to show the surface
features of the moons. Voyager 1 went close to Titan,
and gained much information about its atmosphere. In
August, 1981, Voyager 2 continued to study the planet.
Photos taken by the space probe showed that changes were
happening to the rings and atmosphere. The Voyager
spacecrafts discovered a number of moons orbiting close
to Saturn's rings, as well as discovering new ring gaps.
On July 1, 2004, the Cassini−Huygens probe entered into
orbit around Saturn. Before then, it flew close to
Phoebe, taking very high resolution photos of its
surface and collecting data. On December 25, 2004, the
Huygens probe separated from the Cassini probe before
moving down towards Titan's surface and landed there on
January 14, 2005. It landed on a dry surface, but it
found that large bodies of liquid exist on the moon. The
Cassini probe continued to collect data from Titan and a
number of the icy moons. It found evidence that the moon
Enceladus had water erupting from its geysers. Cassini
also proved, in July 2006, that Titan had hydrocarbon
lakes, located near its north pole. In March 2007, it
discovered a large hydrocarbon lake the size of the
Caspian Sea near its north pole.
Cassini observed lightning occurring in Saturn since
early 2005. The power of the lightning was measured to
be 1,000 times more powerful than lightning on Earth.
Astronomers believe that the lightning observed in
Saturn is the strongest ever seen. |
|
 Kiddle:
Saturn
Wikipedia: Saturn |
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