Saturn

The Bringer of Old Age

  In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and the father of Zeus or Jupiter. Saturn is the root of the English word Saturday.

  Saturn is the sixth planet in order of distance from the Sun and the second largest:

  orbit:             1.429.400.000 km (9.54 AU) from Sun
  diameter:      120.536 km equatorial
  mass:            95.2

  Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes through the plane of Saturnīs rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefor changes drastically. It was not until 1659 that Christian Huygens correctly inferred the geometry of the rings. Saturnīs rings remains unique in the known solar system until 1977 when very faint rings were discovered around Uranus and shortly thereafter around Jupiter and Neptune.

  Saturn was first visited by Pioneer 11 in 1979 and later by Voyager 1 in 1980 and Voyager 2 in 1981. Cassini is now on its way to Saturn and it will arrive in 2004.

  The planet is visibly flattened when viewed through a small telescope; its equatorial and polar diameters vary by almost 10% (120.536 km vs. 108.728 km). This is the result if its rapid rotation and fluid state. The other gas planets are also oblate, but not so much so.

  Saturn is the least dense of the planets; its specific gravity, 0.7, is less than that of water. It means that it would float on water.

  Saturn is, like Jupiter, about 75% hydrogen and 25% helium with traces of water, methane, ammonia and "rock", similar to the composition of the primordial Solar Nebula from which the solar system was formed. 

  Its interior is similar to Jupiterīs consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.

  The interior of Saturn is very hot about 15000°C at the core and Saturn radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism as in Jupiter. But this may not be sufficient to explain Saturnīs luminosity; some additional mechanism may be at work, perhaps the "raining out" of helium deep in Saturnīs interior.

  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 Saturnīs atmospheric circulation could be studied. Saturn also exhibits long-lived ovals and one other features common on Jupiter. In 1990, the Hubble Space Telescope, observed an enormous  white cloud near Saturnīs equator which was not present during the Voyager encounters; in 1990 another smaller storm was observed.

  From Earth can be seen two prominent rings on Saturn, A and B, and one faint ring, C. 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. But probably did Encke never see that gap. The Voyager pictures show four additional faint rings. Saturnīs rings, unlike the rings of the other planets, are very bright. They have an albedo of 0.2 - 0.6.

  Although they look continuous from the Earth, the rings are actually composed of innumberable small particles each in an independent orbit. They range the size from a centimeter to several meters. Objects few kilometers in size are also likely to exist.

  Saturnīs rings are extraordinarily thin: though theyīre 250.000 km or more in diameter they are no more than 1.5 kilometres thick. Despite their impressive appearance, there is very little material in the rings. If the rings would be compressed into a single body it would be no more than 100 km across. The thin rings can be very difficult to see or almost invisible when they are edge on to us.

  The ring particles seem to be composed primarily of water ice. They may also include some rocky particles with icy coatings.

  The Voyager probes confirmed the existence of puzzling radial inhomogeneties in the rings called "spokes" which were first reported by amateur astronomers. Their nature remains a mystery, but may have something to do with Saturnīs magnetic field.

  Saturnīs outermost ring, the F ring, is a complex structure made up of several smaller rings along which "knots" are visible. Scientists speculate that the knots may be clumps of ring material, or mini moons. The strange braided appearance visible in the Voyager 1 images in not seen in the Voyager 2 images, perhaps because Voyager 2 imaged regions where the component rings are roughly parallel.

  There are complex tidal resonance's between some of Saturnīs moons ad the ring system: some of the moons, the so called "shepherding satellites" (i.e. Atlas, Prometheus and Pandora) are clearly important in keeping then rings in place; Mimas seems to be responsible for the paucity of material in the Cassini division, which seems to be similar to the Kirkwood gaps in the asteroid belt; Pan is located inside of the Encke Division. The whole system is very complex and as yet as poorly understood.

  The origin of Saturnīs rings, and the jovian planets, is unknown. Though they may have had rings since their formation, the ring systems are not stable and must be regenerated by ongoing processes, probably the break-up of larger satellites.

  Saturn has, like the other jovian planets, significant magnetic field.

  Saturn is easily visible to the naked eye in the nightime sky. It is not nearly as bright as Jupiter, it is easy identify as a planet because it doesnīt "twinkle" like the stars. It is always near to Jupiter. The rings and the moons are one of the most beautiful sight that I have ever seen, and they are easily visible in small telescopes.

  Saturn has eighteen known satellites, the largest is Titan

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