Saturn (planet)

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Saturn is the sixth planet from the Sun. It is a gas giant, meaning it is mostly made of gas and does not have a solid surface. It is the second largest planet in the Solar System, the largest being Jupiter. The planet was named after the Roman god Saturnus, equivalent to the Greek god Cronos. Saturn's symbol is ♄ which is the symbol of Saturnus' sickle.^[1][2]

Saturn has a very large system of rings made of ice with smaller amounts of rocks and dust. It has 59 moons, plus 3 others that are not confirmed yet as of 2007. The largest is Titan, which is larger than the planet Mercury.

Contents 1 Physical features 1.1 Atmosphere 1.2 Interior 1.3 Magnetic field 2 Rotation and orbit 3 Planetary rings 3.1 History 3.2 Physical features 3.3 Spokes 4 Moons 5 Exploration 6 See also 7 References 8 Other websites

[change] 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 (74,867 mi) whereas its polar diameter (ie. from the north pole to the south pole) is 108,728 km (67,535 mi); a 9% difference. Its flattened shape is due to its very fast rotation, rotating once every 10.8 Earth 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 its average specific density of the planet is 0.69 g/cm³ (less than water). As a result, if Saturn were placed in a large pool of water, it would float on it.

[change] Atmosphere

The outer part of Saturn's atmosphere is made up of 96.7% hydrogen and 3% helium, 0.2% methane and 0.02% of ammonia. There are also very small amounts of acetylene, ethane and phosphine.^[3]

Saturn's clouds show a banded pattern, similar to the cloud bands seen in Jupiter. Unlike 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 thick. The temperature here is quite low, at 250 K (-10°F, -23°C). However there are disagreements among scientists about this. The layer above, about 77 km thick, is made up of ammonium hydrosulfide ice (chemical symbol: NH₄HS), and above that is a layer of ammonia ice clouds 80 km thick. The highest layer is made up of hydrogen and helium gases, which extends between 200 and 270 km above the water cloud tops. Auroras are also known to form in Saturn in the mesosphere.^[3] The temperature at Saturn's cloud tops is extremely low, at 98 K (-283°F, -175°C). The temperature in the inner layers are much higher than the outside layers because of the heat produced by Saturn's interior.^[4] Saturn's winds are some of the fastest in the Solar System, reaching 500 m/s (1,800 km/h, 1,118 mph),^[5] much faster than the fastest winds recorded on Earth.

Saturn's atmosphere is also known to form oval shaped clouds, similar to the more clearer spots seen in Jupiter. These oval spots are cyclonic storms, identical to cyclones seen on Earth. In 1990, the Hubble Space Telescope detected an enormous white cloud near its equator. Storms like the 1990 storm were known as the Great White Spot, unique Saturnian storms that only exist for a short time and only appear about every 30 earth years, around the time of the summer solstice in the Northern Hemisphere.^[6] Great White Spots were also found in 1876, 1903, 1933, and 1960. If this constant cycle continues, it is predicted that another storm will form in about 2020.^[7]

The Voyager 1 spacecraft detected 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. Interestingly, the Cassini probe discovered a hurricane-like storm locked to the south pole that shows a very clear eyewall. This discovery is notable as no eyewalls have been observed in any other planet in the Solar System other than the Earth.

[change] Interior

Saturn's interior is similar to Jupiter's interior. It has a small rocky core at its centre and it is very hot; its temperature reaches 15,000 K (26,540 °F, 14,730 °C). It is so hot that it radiates about two and a half times more heat energy into space than the amount of energy Saturn receives from the Sun.^[4] The core is about the same size as the Earth's, but more dense. Above it is a thicker layer of metallic hydrogen, about 30,000 km (18,600 mi) deep. Above that layer is a region of liquid hydrogen and helium.^[8] The core is heavy, about 9 to 22 times more mass than the Earth's core.^[9]

[change] Magnetic field

Saturn has a natural magnetic field that is much 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 axisymmetrical, unlike any other known planet (ie. 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.^[10]

[change] Rotation and orbit

Saturn's average distance from the Sun is over 1.4 billion km, about 9 times is distance from the Earth to the Sun. It takes 29.5 Earth years for Saturn to orbit around the Sun. This is known as Saturn's orbital period. Saturn takes about 10.8 Earth hours to rotate around its axis. This is known as its rotational period. It is known that its rotational period does not stay the same, therefore, Saturn does not rotate at a constant rate. Its rotational period is determined by the rotation speed of the radio waves released by the planet. The Cassini-Huygens spacecraft discovered that the radio emissions slowed down, thus the rotational period increased. It is unknown what caused the radio waves to slow down.

[change] Planetary rings

[change] History

Saturn is best known for its clearly visible planetary rings which makes it the most visually remarkable object in the solar system. The rings were first discovered by Galileo Galilei in 1610, using his telescope. He did not identify them as rings though, describing them as "ears". He stated that Saturn was actually three separate planets that almost touch 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.^[11] In 1655, Christiaan Huygens was the first person who suggested that Saturn was surrounded by rings. He observed Saturn using a much more powerful telescope than Galilei's. He quoted that "It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic."^[11] 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.^[12] James Keeler studied the rings using a spectroscope in 1895 which proved Maxwell's theory to be correct.

[change] Physical features

The rings range from 6,630 km to 120,700 km 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. The rings are on average up to one kilometre thick and is 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.^[13]

The largest gaps in the rings are the Cassini Division and the Encke Division, both are visible from the Earth. The Cassini Division is the largest, measuring 4,800 km (2,980 mi) wide.^[14] 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. This structure is thought to arise from 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 maintain 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 further away. The moon Mimas is responsible for clearing away the Cassini gap.^[14]

Recently, from data received by the Cassini spacecraft, 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.^[15] In addition to the oxygen and hydrogen gases, the rings have an atmosphere made of hydroxide, but it is also very sparse. This compound was detected from Earth by the Hubble Space Telescope.^[16]

[change] Spokes

The Voyager space probe discovered features shaped like rays, called spokes. They are seen as dark when under sunlight, and appear light when against the unlit side. It is assumed that the spokes are made of microscopic dust particles that are raised above the ring plane. They rotate at the same time with the planet's magnetosphere, therefore, it is assumed that they have a connection with electromagnetism. However, the exact reason explaining why these spokes exist is still unknown.

The Cassini probe detected the spokes too, 25 years later. They appear to be seasonal, disappearing during solstice and appearing again during equinox.

[change] Moons

Saturn has a total of 59 confirmed moons, 48 of these have names. Many of the moons are very small: 33 of the 50 moons are less than 10 km in diameter and a further 13 moons are less than 50 km.^[17] 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 in diameter.

On Titan, the largest moon of Saturn, in December 2004 and January 2005, lots of nearby photos were taken, thanks to a man-made satellite called the Cassini-Huygens probe. One part of this satellite, known as the Huygens probe, eventually landed on Titan.

[change] Exploration

Saturn was first explored by the Pioneer 11 spacecraft on September, 1979. It flew as close as 20,000 km 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, and discovered that the dark ring gaps appear bright when viewed towards the Sun, indicating the gaps are not empty of material. The spacecraft meaured the temperature of the moon Titan.^[18]

On November, 1980, Voyager 1 visited Saturn, and took higher resolution photographs of the planet, rings and moons. Unlike Pioneer 11, the images were good enough to focus on the surface features of the moons. Voyager 1 made a close encounter of Titan, and gained much information about its atmosphere. In August, 1981, Voyager 2 continued to study the planet. Images taken by the space probe indicated 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 made a close flyby of Phoebe, taking very high resolution images of its surface and gaining high amounts of 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 confirmed that large bodies of liquid exist on the moon. The Cassini probe continued to gain data of Titan and a number of the icy moons. It found evidence that the moon Enceladus had water erupting from its geysers.^[19] Cassini also proved in July, 2006 that Titan contained 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.^[20]

Cassini observed the lightning occurring in Saturn since early 2005. The power of the lightning was measured to be 1000 times more powerful than lightning from the Earth. Astronomers believe that the lightning observed in Saturn is the strongest ever seen.^[21]

[change] See also

• List of planets

[change] References

[change] Other websites

• Saturn at Space Wiki
• Saturn at ESA/Hubble
• Saturn Profile by NASA's Solar System Exploration
• Saturn - About Space
• NASA's Saturn fact sheet
• NASA's Cassini mission to Saturn
• Change of seasons on Saturn
• Theoretical description of the rings of Saturn
• Saturn Research News
• A Trip Into Space Description and photos of Saturn
• Planets - Saturn A kid's guide to Saturn.
• A WAV file of radio emissions from Saturn
• NASA's Cassini Images of Hexagon on Saturn

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See also: List of solar system objects