SATURN

Image taken by Voyager 1 while moving away after the encounter with the planet. (Calvin J. Hamilton)

Saturn is the sixth planet of the Solar System and the second as far as the size is concerned, after Jupiter, to which it resembles under many aspects. In Roman Mythology, Saturn was the god of agriculture.   The planet has been known since ancient times and, since it has been observed with the aid of telescopes, it has been known especially for the spectacular system of rings that surrounds it.

Saturn was visited for the first time by the Pioneer probe in 1979, but most of what we know about it was discovered by the Voyager in the years 1980-81. A new space mission, called Cassini, is scheduled in 1997; the Huygens probe will be released from the spacecraft, and it will reach the atmosphere of Titan, the major of its satellites. 

Saturn and two of its moons (Tethys and Dione), photographed by Voyager 1 in 1980, from the distance of 13 million Km. You can notice the Cassini gap, between the rings A and B, and the Ecke division. (NASA/JPL)

Saturn orbits around the Sun at an average distance of 1.429 million Km, with a period of 29.5 terrestrial years. The orbit has an inclination of approximately 2.5 degrees on the ecliptic. It completes a rotation around its axis, which has an inclination of 25.33 degrees with respect to the perpendicular to the orbital plane, in barely 10 hours and 39 minutes.
 

Saturn is the flattest among the planets of the Solar System: its equatorial diameter measures 120,536 Km, the polar diameter measures only 108,728 Km. This is due, as in the case of Jupiter, to the low density and the rapid rotation of the planet, which became oblate under the effect of centrifugal force. 
In fact, Saturn has the smallest density of all the planets: 0.7.
The mass of the planet equals almost 570,000 billion billion tons (5.688 x 10²⁹ g), that is 95 times that of the Earth. 
Its albedo is 0.47. 
 
 

The Rings
 

Saturn photographed by Voyager 2 in 1981, from a distance of 34 million Km.  (NASA/JPL)

The system of rings of Saturn, even though it is not unique in the Solar System, is the most magnificent and luminous. It is situated between 170,000 and 285,000 Km from the centre of the planet, but its thickness is only 1.5 Km.   The temperature in the region of the rings is -200 °C.  Two large rings called A and B and a weaker one called C are visible from the Earth. Between the first two there is the Cassini division, in honour of the astronomer who discovered it in 1675. The A ring on the other hand has the Encke division.

 

Image with intensified colours that shows the structure of the rings. The A ring is the most external band, crossed by the Encke division, while the Cassini division is situated towards the inner zone and divides the A and B rings. (Calvin J. Hamilton)

In image with intensified colours taken by Voyager 2, you can see the variations in the chemical composition of the rings.  (NASA/JPL)

 

The most external ring, called F, is composed by two thin and luminous rings and some "nodes", which could be small unknown satellites.  Actually the Pioneer 11 probe revealed that each one of them is composed by thousands of thinner rings, as in a musical record with innumerable grooves.

The most external ring of Saturn, called F ring, is a complex structure composed of two thin and bright rings. (NASA/JPL)

The rings are made of a large number of solid particles, crystals of ice, silicates and perhaps iron grains of meteoritic origin. Some grains could be covered by dry ice, that is frozen carbon dioxide.
These particles probably derive from the disintegration of small satellites of Saturn, due to impacts of meteorites or something else.
 

Vision of the rings of Saturn edgewise, of August 1995. Saturn is visible edgewise once every 15 years, when the Earth crosses the plane of its orbit. (HST)

Another image of the rings of Saturn, edgewise. They do not disappear completely because the external edge reflects the solar light. (HST)

 
 

The satellites 
 

A photomontage of Saturn and its satellites, composed of images of Voyager 1. You can see Dione in front of the planet; Tethys and Mimas bottom right, Enceladus and Rhea on the left, Titan top right...  (NASA/JPL)

In the region of the rings there are also 18 satellites: Atlas, Calypso, Dione, Enceladus, Epimetheus, Helene, Hyperion, Iapetus, Janus, Mimas, Pan, Pandora, Phoebe, Prometheus, Rhea, Telesto, Tethys and Titan. In 1995, Hubble Space Telescope revealed four objects that could be new satellites.

 

There is a complex interaction between the rings of Saturn and its satellites: the latter have the effect of gravitationally stabilizing the whole of the rings, and create unhomogeneities and radial structures.

The structure of the rings and the satellites of Saturn, in a scale image. (Dave Seal, JPL)

 

Hubble Space Telescope discovered new satellites around Saturn, in 1995. In this sequence of images, they appear as light elongated spots, just outside the F ring. (HST)

 
 

The structure 
 

Rare image of Saturn after the movement of the Sun under the plane of the rings. (HST)

Saturn probably has a nucleus of metal and rock, contained in a mantle of liquid Hydrogen. This element is in the metallic state due to the pressures it is subjected to. The whole is surrounded by a dense atmosphere.  The chemical composition of the planet is similar to that of Jupiter: Hydrogen (75%), Helium (25%), with traces of water, methane, ammonia and rocky matter.

Saturn, as well as Jupiter, seems to have an internal source of energy responsible for the fact that it emits approximately 2.2 times more energy than that received from the Sun. It is probably the slow gravitational contraction of the planet, that causes an increase of its temperature: that of its nucleus reaches approximately 12,000 degrees. 
Another thing in common with Jupiter is the magnetic field with inverse polarity compared to that of the Earth.
 

The atmosphere 
 

The atmosphere of Saturn is essentially composed of Hydrogen (97%), and Helium (3%). The visible surface, of a yellowish colour, is covered with light blue and dark clouds, disposed in bands as those of Jupiter, but paler. They are agitated by whirls, less intense than those that characterize Jupiter. 
On the surface of the planet there are very strong winds, with speeds of up to 1,800 Km/h; the stronger winds are at the equatorial zone. 
 

The first image of a polar aurora on Saturn, taken in ultraviolet light by Hubble Space Telescope (top). The aurora is produced when the charged particles of its magnetosphere enter the atmosphere. (HST)

Equatorial tempest on Saturn. It is visible as a lighter region, formed by crystals of frozen ammonia. (HST)

 
 

ANIMATIONS

The rings of Saturn,  AVI, 1.3 Mb (NASA/JPL)

Tempest on Saturn,  MPEG, 186 Kb (NASA)