(ESTD. BY GOVERNMENT OF TAMILNADU)


Saturn


In Roman mythology, Saturn was the god of agriculture, and was the father of Jupiter. The Greeks referred to the planet as 'Cronus'. In Indian mythology it is known as Saneeswar.

Saturn is the last planet that is visible with the naked eye from Earth. Saturn is located 143 crore kilometers away from the Sun and has a diameter of over 1,20,000 kilometers, making it the second largest planet in our Solar system. Saturn takes 29.46 Earth years to complete a single revolution around the Sun, however it's day is only 10.25 hours, less than half of Earth's. Saturn is a 'gas giant' planet, like its larger neighbour Jupiter. The atmosphere, mostly hydrogen and helium, with traces of simple compounds, spins around the planet.

As a result of its rapid rotation and gaseous state Saturn is visibly flattened or oblate when viewed through a small telescope; its equatorial and polar diameters vary by almost 10% . Saturn has the most spectacular ring system in the Solar System. These rings aren't solid, but made up of billions of separate chunks. They range from microscopic particles to rocks that are a few metres in diameter.

No probes have ever penetrated the planet's surface. The clouds are thought to hide a layer of metallic liquid hydrogen covering a rocky core. Even so, the planet has the very low density. Lower than the density of water. In fact, Saturn is so light that it would float on water - if there were an ocean large enough!

Saturn's interior is similar to Jupiter's consisting of a rocky core of the size of the Earth, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.

Saturn's interior is hot at about 12000 degree Celsius at the core and Saturn radiates more energy into space than it receives from the Sun. The bands so prominent on Jupiter are much fainter on Saturn. They are also much wider near the equator. Details in the cloud tops are invisible from Earth so it was not until the Voyager encounters that any detail of

Saturn's atmospheric circulation could be studied. In 1990, HST observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters.

When Galileo Galiei first spotted the rings, he was convinced that he was seeing three separate planets moving together.

Though they look continuous from the Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few kilometer-sized objects are also likely. Saturn's rings are extraordinarily thin: though they're 2,50,000 km or more in diameter they're less than one kilometer thick.

Despite their impressive appearance, there's really very little material in the rings -- if the rings were compressed into a single body it would be no more than 100 km across. The ring particles seem to be composed primarily of water ice, but they may also include rocky particles with icy coatings.

Saturn's rings are composed of many hundreds of smaller bands, with gaps in-between. The largest break is called the Cassini Gap. It is visible from Earth through telescopes. Saturn's moons may create some of these gaps as they sail through, clearing debris from their paths.

Two prominent rings A and B and one faint ring C can be seen from the Earth. The gap between the A and B rings is known as the Cassini division. The much fainter gap in the outer part of the A ring is known as the Encke Division Every few years the rings of Saturn seem to disappear as viewed through a small telescope. This is due to a change in the orientation of Saturn compared to Earth. When the rings are edge on, they are very hard to spot being only one kilometre thick.

When Voyager visited Saturn, we realised that there was not one, two, three or even four ring systems as was previously thought. There were literally thousands of rings. The rings are divided into seven main rings. Saturn was first visited by NASA's Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini arrived on July 1, 2004 and will orbit Saturn for at least four years.

There are complex tidal resonances between some of Saturn's moons and the ring System. The “shepherd moons" Atlas, Prometheus and Pandora are important in keeping the rings in place; Mimas seems to be responsible for the insufficiency of material in the Cassini Division. Pan is located inside the Encke Division.

The ring structure is very complex and as yet poorly understood. The origin of the rings of Saturn is unknown. The ring systems are not stable and regenerated by ongoing processes like the breakup of larger satellites. The current set of rings may be only a few hundred million years old.

Like the other jovian planets, Saturn has a significant magnetic field.

Astronomers using the Mauna Kea Observatory and the images from the Cassini mission found several Moons of Saturn in the year 2005. Saturn is currently known to have 48 moons. However, the precise number of Saturn's moons will never be certain 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.

The moons Phoebe and Hyperion rotate synchronously. Janus and Epimetheus, are co-orbital moons. These two moons are of roughly equal size and have orbits with only a few kilometer's difference in diameter, close enough that they would collide if they attempted to pass each other. Instead of colliding, however, their gravitational interaction causes them to swap orbits every four years. The exchange takes place about once every four years; the next closest approach is in Jan/Feb 2006.

This arrangement is unique in the solar system.

Titan the largest Moon of Saturn is bigger than the planet Mercury. Titan is the only Moon in the Solar system with a thick atmosphere. Its atmospheric pressure is greater than Earth’s. To study the atmosphere and the clouds a probe which was named after Christiaan Huygens the Dutch astronomer who discovered Titan in 1655 was sent there.

NASA's Cassini spacecraft carried Huygens to Saturn from Earth. In January 2005 to study Titan, the spaceship Huygens landed on Titan. Parachutes on Huygens lowered it through Titan's atmosphere. The probe is sending useful information on this unique satellite. The surface of Titan is very, very cold. The temperature there is about -178º. Its main gas is nitrogen. Smoggy orange clouds hover high in the atmosphere.