Saturn

Saturn is the sixth planet from the Sun in the Solar System. 

Saturn is one of the four gas giant planets in the Solar System, with Jupiter, Uranus, and Neptune. It is the second largest planet in the Solar System (Jupiter is larger).

Saturn was named after the Roman god Saturnus. He was the Roman equivalent of the Greek god Kronos. Saturn's symbol is ♄ which is the symbol of Saturnus' sickle.

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 145 known moons orbiting the planet.  The largest moon is Titan. Titan is larger in volume than the planet Mercury. It is the second-largest moon in the Solar System. The largest moon is a moon of Jupiter, Ganymede. There are also many rings around Saturn. These rings are made of ice with some rocks and dust. Some people think that that the rings were made by a moon impact or other event. Saturn is about 1,433,000,000 km (870,000,000 mi) on average from the Sun. Saturn takes 29.6 Earth years to move around the Sun.

Physical features 

Saturn is an oblate spheroid. This means that it is flattened at the poles and wider around the equator. The planet's equatorial diameter is . Its polar diameter (the distance from the north pole to the south pole through the centre) is . This is a 9% difference. Saturn has a flattened shape because of its very fast rotation. It rotates 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. This makes its average density is 0.69 g/cm3. 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 is also some acetylene, ethane and phosphine.

Saturn's clouds show a banded pattern. This is 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. It is about  thick. The temperature there is quite low, at 250 K (-10°F, -23°C). However, scientists do not all agree on this. The layer above is made up of ammonium hydrosulfide ice. It is about  thick. Above it is a layer of ammonia ice clouds which are  thick. The highest layer is made up of hydrogen and helium gases. It goes to between  and  above the water cloud tops. Auroras can be seen in Saturn in the mesosphere. The temperature at Saturn's cloud tops is very 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 made by Saturn's inside. Saturn's winds are some of the fastest in the Solar System. They can reach 1,800 km/h (1,118 mph), ten times faster than winds on Earth.

Storms and spots 
Saturn's atmosphere can make oval shaped clouds. This is like the clearer spots seen on 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.

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 clouds. The probe also found a hurricane-like storm on the south pole that showed an eyewall. Until this finding, eyewalls had only been seen on Earth.

Interior 
Saturn's inside is similar to Jupiter's inside. It has a small rocky core about the size of the Earth at its center. It is very hot. Its temperature reaches 15,000 K (). Saturn is so hot that it gives out more heat energy into space than it gets from the Sun. Above it is a thicker layer of metallic hydrogen, about  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 (it has a North and a South). 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.

Rotation and orbit 
Saturn's average distance from the Sun is over 1,400,000,000 km (869,000,000 mi). This is about nine times the distance from the Earth to the Sun. It takes 10,759 days, or about 29.8 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 inside. 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 longer than the radio rotational period measured by the Voyager 1 and Voyager 2 spacecrafts, which flew by Saturn in 1980 and 1981.

Saturn's rotational period is calculated by the rotation speed of radio waves given off by the planet. The Cassini−Huygens spacecraft found that the radio waves slowed down. This suggested that the rotational period increased. Since scientists do not think Saturn's rotation is actually slowing down, the explanation may be that the magnetic field causes the radio waves.

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. They were named in the order they were found, which is different to their order from the planet. From the planet the rings are ordered: D, C, B, A, F, G and E.

Some scientists think that the rings are 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 some of the moons. This includes Enceladus, which is thought to be made of ice.

History 
The rings were first found by Galileo Galilei in 1610, using his telescope. They did not look like rings to Galileo. He called them "handles". He thought that Saturn was three different planets that were right next to each other. 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 say that Saturn was surrounded by rings. Using a much more powerful telescope than Galilei's, he said that Saturn "is surrounded by a thin, flat, ring, nowhere touching...". In 1675, Giovanni Domenico Cassini found that the planet's rings were in fact made of smaller rings 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  to  above the planet's equator. While the equatorial circumference of Saturn is 378,675 km (235,298 miles). 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  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  high, but only moving slowly at about  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  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 reactions also occur 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 (a combination of oxygen and hydrogen called an anion), which 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 appear dark when lit by sunlight, and appear light against the unlit side of the planet. 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 rate as 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.

Moons 

Saturn has 53 named moons, and another 29 which are still being studied. Many of the moons are very small: 33 are less than  in diameter and 13 moons are less than . Seven moons are large enough to be a near perfect sphere caused by their own gravity. 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  in diameter.

Between 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, landed on Titan, on land. 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. Its batteries lasted about 3 hours. Enceladus, the sixth largest moon, is about  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.

Exploration 

Saturn was first explored by the Pioneer 11 spacecraft in September 1979. It flew as close as  above the planet's cloud tops. It took photographs of the planet and a few of its moons, but were low in resolution. 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. 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 showed some of 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 spacecraft 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 towards Titan's surface and landed 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.

Notes 
Refers to the level of one bar atmospheric pressure
Orbital information is based on the barycentre of the Saturn system, the centre of mass, not the geometric centre. Barycentre measurements are used because they are not changed by the daily movement of the moons.

Related pages 
 List of planets

References 

Notes

Further reading 

 
 
  Theoretical description of the rings of Saturn.
 A WAV file of radio emissions from Saturn.