Saturn

Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; however, with its larger volume, Saturn is over 95 times more massive. It was named after the Roman God Saturn (the Greek mythology equivalent is Kronos, father of Zeus). Its symbol is a stylized representation of God's sikhle (Unicode: ♄).

The planet's most famous feature is its prominent ring system, which is composed mostly of ice particles, with a smaller amount of rocky debris and dust.

## Physical Characteristics

Saturn is a gas giant composed predominantly of hydrogen and helium. It lacks a definite surface, though it may have a solid core. Saturn's rotation causes it to have the shape of an oblate spheroid; that is, it is flattened at the poles at its equator. Jupiter, Uranus, and Neptune, the other giant planets in the Solar System, are also oblate but to a lesser extent.

Saturn is the only planet of the Solar System that is less dense than water - about 30% less. Although Saturn's core is considerably denser than water, the density of the planet is 690 kg/m3. Jupiter has 318 times Earth's mass, and Saturn is 95 times Earth's mass. Together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System. In terms of area and volume, the surface area of Saturn is 4.27×1010 km2 or 83 Earths. While, the volume of Saturn is 8.2713×1014 km3 or 7,559 times the Earths.

## Atmosphere

Saturn has hidden similarities to Jupiter, mainly due to its banded atmosphere. But the bands are usually hidden by a haze of ammonia. Every 30 years, a large, white oval appears. These ovals are Great White Spots, large storms with lightning which can cover the whole planet within a matter of weeks, like the 2010/11 storm. Another notable feature is a vast lightning storm in Saturn's southern hemisphere called the Dragon Storm. The Dragon Storm is large enough to engulf the largest of the Earth's continents and produces lightning thousands of times more powerful than storms on Earth. Saturn also contains one of the highest winds in the Solar System, capable of reaching 1,120 mph (1,800 km/h). 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 above. 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. Astronomers using infrared imaging have shown that Saturn has a warm polar vortex, and is the only such planet known in the solar system.

The straight sides of the northern polar hexagon are each about 13,800 kilometers long. The entire structure rotates with a period of 10h 39m 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.

## Planetary rings

*Main article: Rings of Saturn*
Saturn is probably best known for its system of planetary rings, which makes it one of the most visually remarkable objects in the solar system.

### History

The rings were first observed by Galileo Galilei in 1610 with his telescope, but he was unable to identify them as such. He wrote to the Duke of Tuscany that "The planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one (Saturn itself) is about three times the size of the lateral ones [the edges of the rings]." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish, and then in 1613 they reappeared again, further confusing Galileo.

In 1655, Christiaan Huygens became the first person to suggest that Saturn was surrounded by a ring. Using a telescope that was far superior to those available to Galileo, Huygens observed Saturn and wrote that "It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic."

In 1675, Giovanni Domenico Cassini determined that Saturn's ring was actually composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division.

In 1859, James Clerk Maxwell demonstrated that the rings could not be solid or they would become unstable and break apart. He proposed that the rings must be composed of numerous small particles, all independently orbiting Saturn. Maxwell's theory was proven correct in 1895 through spectroscopic studies of the rings carried out by James Keeler of Lick Observatory.

## Natural satellites

Saturn has a large number of moons. The precise figure is uncertain, as the orbiting chunks of ice in Saturn's rings are all technically moons, and it is difficult to draw a distinction between a large ring particle and a tiny moon. As of 2007, a total of 57 individual moons have been identified, plus 3 unconfirmed moons that could be small dust clumps in the rings. Many of the moons are very small: out of 57, 31 are less than 10 km in diameter, and another 13 less than 50 km. Only seven of them are massive enough to have collapsed into spheroids under their own gravitation. These are compared with Earth's moon in the table below. Saturn's most noteworthy moon is Titan, the only moon in the solar system to have a dense atmosphere.

Traditionally, most of Saturn's other moons are named after actual Titans of Greek mythology. This started because John Herschel — son of William Herschel, discoverer of Mimas and Enceladus — suggested doing so in his 1847 publication *Results of Astronomical Observations made at the Cape of Good Hope*, because they were the sisters and brothers of Cronos (the Greek Saturn). The moons (ordered by the largest size) are as follows:

* Titan
* Rhea
* Iapetus
* Dione
* Tethys
* Enceladus
* Mimas

## Exploration of Saturn

### Pioneer 11 flyby

Saturn was first visited by Pioneer 11 in September 1979. It flew within 20,000 km of the planet's cloud tops. Low-resolution images were acquired of the planet and a few of its moons; the resolution of the images was not good enough to discern surface features. The spacecraft also studied the rings; among the discoveries were the thin F-ring and the fact that dark gaps in the rings are bright when viewed towards the Sun, or in other words, they are not empty of material. Pioneer 11 also measured the temperature of Titan.

### Voyager flybys

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, rings, and satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan, greatly increasing our knowledge of the atmosphere of the moon. However, it also proved that Titan's atmosphere is impenetrable in visible wavelengths, so no surface details were seen. The flyby also changed the spacecraft's trajectory out from the plane of the solar system.

Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's turnable camera platform stuck for a couple of days and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus.

The probes discovered and confirmed several new satellites orbiting near or within the planet's rings. They also discovered the small Maxwell and Keeler gaps.

### Cassini orbiter

On July 1 2004, the Cassini-Huygens spacecraft performed the SOI (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn. Before the SOI, Cassini had already studied the system extensively. In June 2004, it had conducted a close flyby of Phoebe, sending back high-resolution images and data.

The orbiter completed two Titan flybys before releasing the Huygens probe on December 25 2004. Huygens descended onto the surface of Titan on January 14 2005, sending a flood of data during the atmospheric descent and after the landing. During 2005 Cassini conducted multiple flybys of Titan and icy satellites.

On March 10 2006, NASA reported that the Cassini probe found evidence of liquid water reservoirs that erupt in geysers on Saturn's moon Enceladus.

On September 20 2006, a Cassini probe photograph revealed a previously undiscovered planetary ring, outside the brighter main rings of Saturn and inside the G and E rings.

In July 2006, Cassini saw the first proof of hydrocarbon lakes near Titan's north pole, which was confirmed in January 2007. In March 2007, additional images near Titan's north pole discovered hydrocarbon "seas", the largest of which is almost the size of the Caspian Sea.

As of 2006 the probe has discovered and confirmed 4 new satellites. Its primary mission will end in 2008 when the spacecraft will be expected to have completed 74 orbits around the planet.

*For the latest information and news releases, see Cassini website.*
## Observation

Saturn has been known since prehistoric times. It is the most distant of the five planets easily visible to the naked eye, the other four being Mercury, Venus, Mars, and Jupiter (Uranus is 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 star varying usually between magnitude +1 and 0 and takes approximately 29½ years to make a complete circuit of the ecliptic against the background constellations of the zodiac. Optical aid (large binoculars or a telescope) magnifying at least 20X is required to clearly resolve Saturn's rings for most people.

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). In the opposition on January 13 2005, Saturn appeared at its brightest until 2031, mostly due to a favorable orientation of the rings relative to the Earth.

## Saturn in various cultures

In Hindu astrology, there are nine planets, known as Navagrahas. Saturn is known as "Sani" or "Shani", the Judge among all the planets, and determines everyone according to their own performed deeds bad or good.

Chinese and Japanese culture designate the planet Saturn as the *earth star* (土星). This is based on Five Elements which were traditionally used to classify natural elements.

In Hebrew, Saturn is called 'Shabbathai'. Its Angel is Cassiel. Its Intelligence, or beneficial spirit, is Agiel (layga), and its spirit (darker aspect) is Zazel (lzaz). See: Kabbalah.

In Ottoman Turkish and in Malay, its name is 'Zuhal', derived from Arabic زحل.

Saturn was also called Φαίνων, *Phainon* ("shiner") by the Greeks.

## External links

* Saturn at ESA/Hubble
* 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