Saturn

Saturn is the sixth planet from the Sun in our solar system. On average, Saturn is about 1,432,000,000 km (870,000,000 mi) away from the Sun.

Saturn takes about 29.5 Earth years to complete one orbit around the Sun. A day on Saturn is much shorter than a day on Earth, lasting only about 10.7 hours. This means that Saturn spins much faster than Earth, spinning more than twice in the same time it takes Earth to spin once.  

Saturn is one of the four giant planets in our solar system, along with Jupiter, Uranus, and Neptune. Saturn is the second largest planet in the Solar System, after Jupiter.

Saturn was named after the Roman god Saturn, which the Greeks called Kronos. Saturn's symbol is ♄, which is the symbol of the Roman god's sickle.

Inside Saturn are many layers: a core of iron, nickel, silicon and oxygen compounds, a deep layer of metallic hydrogen, a layer of liquid hydrogen and liquid helium, and a gaseous layer on the outside.

There many rings around Saturn, made of ice with some rocks and dust. Some people think that the rings were made by a moon crashing into the planet.

While a final number of Saturnian moons cannot be given, Saturn has the most of any planet in our solar system, 274 having stable and confirmed orbits. The largest of Saturn's moons is Titan, which is larger than the planet Mercury. Titan is the second-largest moon in the Solar System, after Jupiter's moon Ganymede.

 Physical features 

Saturn is a squished sphere. This means that it is flattened at the north and south 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 Earth hours.

Saturn is the only planet in our solar system that is less dense than water. This means if Saturn could be placed in a large pool of water, it would float. Even though the planet's core is very dense, it has a gaseous atmosphere. Saturn's average density is 0.69 g/cm3.

 Atmosphere 
The outer part of Saturn's atmosphere is about 96% hydrogen, 3% helium, 0.4% methane and 0.01% ammonia. There is also some acetylene, ethane and phosphine.

Saturn's clouds are in 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  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 of ammonium hydrosulfide ice. It is about  thick. Above it is a layer of ammonia ice clouds which are  thick. The highest layer of clouds is made up of hydrogen and helium gases and is found between  and  above the lowest layer. Auroras can be seen 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 inside Saturn. Saturn's winds are some of the fastest in our 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. These clouds are like the clearer spots seen on Jupiter. These oval spots are rotating storms, similar to cyclones seen on Earth. In 1990, the Hubble Space Telescope found a very large white cloud near Saturn's equator. Storms like this one in 1990 were known as Great White Spots. These unique storms only exist for a short time and only occur in about every 30 Earth years, in summer solstices 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 Cassini-Huygens 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. At the centre is a small rocky core that is about the size of the Earth. Saturn's core is very hot, with temperatures reaching 15,000 K (). Saturn is so hot that it loses more heat to space than it gets from the Sun. The core is heavy, with about 9 to 22 times more mass than the Earth's core. Above the core is a thicker layer of metallic hydrogen that is about  deep. Above that layer is a region of liquid hydrogen and helium.

 Magnetic field 
Saturn has a natural magnetic field that is weaker than Jupiter's. Like the Earth's magnetic field, Saturn's field is a magnetic dipole (it has north and south poles). The field is perfectly symmetrical, which means it is exactly in line with the planet's axis. It is the only perfectly symmetrical magnetic field of any planet in our solar system. Saturn makes radio waves, but they are too weak to be heard on Earth. Saturn's moon Titan orbits in the outer part of Saturn's magnetic field. The ionised particles in Titan's atmosphere give out plasma to the field.

 Rotation and orbit 
Saturn's distance from the Sun is over 1,400,000,000 km (886,000,000 mi). This is about nine times the distance from the Earth to the Sun. It takes 10,756 days, or about 29.4 years, for Saturn to orbit around the Sun. This length of time 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. These measurements are known as its rotational periods.

The Cassini-Huygens spacecraft 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 the rotation speed of radio waves given off by the planet. The Cassini-Huygens spacecraft found that the radio waves had slowed down since the Voyager spacecrafts flew by the planet. This meant that the rotational period had increased. Since scientists do not think Saturn's rotation is actually slowing down, they think that the magnetic field may be causing 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 newer idea is that it was a very large moon, most of which crashed into the planet. This would have left a large amount of ice to form the rings and some of the moons, including Enceladus, which is 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 Galileo'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 are made of small bits or particles, each orbiting Saturn on their own. He found that if the rings were solid, they would become unstable or break apart. James Keeler studied the rings using a spectroscope in 1895, which proved Maxwell right.

 Physical features 
The rings are found between  to  above the planet's equator. The circumference of Saturn at its equator is 378,675 km (235,298 miles). As found by Maxwell, even though the rings look solid and unbroken when viewed from above, the rings are made of small particles of rock and ice. The particles 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 they move slowly, at about  each day on Saturn. Some scientists believe that the waves are caused by Saturn's moons. Another idea is the waves were made by a comet hitting Saturn.

The largest gaps in the rings are the Cassini Division and the Encke Division which are both visible from Earth. The Cassini Division is the largest of Saturn's ring gaps at  wide. When the Voyager spacecrafts visited Saturn in 1980, they discovered that the rings are in a complex structure made out of thousands of thin gaps and smaller rings. 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 gravity of shepherd satellites, such as Prometheus and Pandora. Other gaps form due to the gravity of a large moon farther away. The moon Mimas is what made the Cassini Division.

Data from the Cassini spacecraft has shown that the rings have their own atmosphere, free from the planet's atmosphere. The rings' atmospheres have oxygen and hydrogen gas. The oxygen gas is made 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), which was discovered by the Hubble Space Telescope.

 Spokes 

The Voyager space probe discovered features shaped like rays, called spokes. These were seen later by the Hubble telescope. The Cassini-Huygens probe photographed the spokes in 2005. They appear dark when lit by sunlight, and appear light against the dark 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 the solstices and appearing again during equinoxes.

 Moons 

Saturn has a total of 274 known moons; 63 of these are named. Most of the moons are very small: 248 are less than  in diameter. 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 of Saturn, larger than the planet Mercury, and it is the only moon in our solar system known to have a thick, dense atmosphere. Hyperion and Phoebe are the largest moons that are not shaped like a sphere; they are each larger than  in diameter.

Between December 2004 and January 2005 a manmade 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. Named after the Dutch astronomer Christiaan Huygens, it was the first spacecraft to land on a planet beyond Mars. The probe was designed to float in case it landed in liquid, but it landed on ground. The probe's batteries lasted about three hours. Enceladus, the sixth largest moon of Saturn, is about  in diameter. It is one of the few solar system objects beyond Mars that shows volcanic activity. In 2011, scientists discovered an electric link between Saturn and Enceladus. This is caused by ionised particles from volcanoes on the moon touching Saturn's magnetic fields. This is like what causes 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 they 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 that the gaps are not empty. The spacecraft measured the temperature of the moon Titan. Mission Descriptions. Retrieved July 5, 2007.

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 visited the planet. Its photos 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 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 landing on Titan on January 14, 2005. It landed on dry ground, but it found that large bodies of liquid exist on the moon. The Cassini probe continued to gather 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 starting in 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.