Saturn

Like Jupiter, Saturn with its train of satellites is a miniature solar system, but it is above all famous for its fantastic show of rings, discovered in 1610 by Galileo and shown to be rings by Huygens in 1655. Saturn has been explored three times by NASA space probes: Pioneer 11 at the end of August and the beginning of September 1979, Voyager 1 in November 1980 and Voyager2 in August 1981.

Saturn seen by the probe Cassini

1. Structure of the planet

The halo of rings around Saturn gives it a unique appearance in the solar system. However, the planet itself is similar to Jupiter. It has almost the same dimensions: its equatorial radius - 60.268 kilometres - is equal to 9.45 terrestrial radii instead of 11.21 for Jupiter. Its mass is of the order of 95 times that of the Earth, compared with 318 for Jupiter. Its average density is 0.7; in other words, if it were dropped into an imaginary swimming pool, Saturn would float on the surface like a ball. This suggests that Saturn, like Jupiter, is formed of elements that made up the primitive solar nebula, i.e. mainly hydrogen and helium. However, as we shall see, these two components are not uniformly mixed inside the planet, and the internal structure of Saturn is qualitatively different from that of Jupiter. The fact remains that Saturn, like Jupiter, is essentially a giant ball of gas compressed under its own weight, and that what we see are clouds composed of minor elements that condense at the low temperatures at the periphery of the planet.

Like Jupiter, Saturn rotates very fast, its internal rotation period being 10 hours 40 minutes. It is highly flattened, more than Jupiter: its equatorial radius is 10.8% greater than its polar radius, while that of Jupiter is 6.5% of its polar radius.

Saturn also has an internal energy source, i.e. it emits more energy (in the form of radiation) than it absorbs from the Sun. The origin of this energy source is, however, different from that of Jupiter.

Imagine a traveller coming from interplanetary space towards the centre of the planet; what would he discover?

A "cloud" of atomic hydrogen and perhaps molecular hydrogen in the form of a torus centred on Saturn surrounds the planet. The torus is in the equatorial plane and extends between 8 and 25 Saturn radii (i.e. from 480 000 to 1.5 million kilometres) and has a thickness of 14 Saturn radii ( 840 000 km). It is thought that the cloud, which has a density of the order of 20 atoms per cubic centimetre, comes from hydrogen that has escaped from Titan's atmosphere and been attracted around Saturn by its gravitational attraction. The torus may also contain molecular hydrogen and even have a density higher than that of atomic hydrogen.

The exosphere, i.e. the atmosphere external to Saturn above the region where the various gaseous constituents are uniformly mixed under the effect of turbulence, is at a temperature of around 400 kelvins. The density of molecular hydrogen increases rapidly below 61 400 kilometres altitude, measured from the centre of the planet, i.e. at about 1300 kilometres above the 1 atmosphere pressure level. There is probably also methane in this region.

The homopause, i.e. the region below which the non-condensible components, or those not dissociated by radiation, are uniformly mixed, is at about 200 kelvins and at 1150 kilometres above the 1 atmosphere level. Below the homopause, the relative proportions of the two major components, helium and hydrogen, are respectively 7% by volume (14% by mass) and 93%. In Jupiter, the proportions of these same elements are 10 and 90%. In the stratosphere,i.e. between the homopause and the tropopause located at the 0.1 atmosphere level, apart from methane in a proportion of 1 to 2 thousandths, there are various products from the dissociation of the methane under the action of solar ultraviolet radiation: acetylene (C²H²), ethane (C²H⁶) and probably propane (C³H⁸) and methylacetylene (C³H⁴). These compounds are present in very small quantities. Other more complex molecules may also have formed. In fact, phosphine (PH³) has been detected, in a proportion of a few parts per million, up to the level of 5 to 10 hectopascals (0.005 to 0.01 atmospheres). The hydrocarbons formed in the stratosphere should not be present in the troposphere, unlike phosphine, which comes from inside the planet.

The temperature decreases up to the tropopause, where it is only 85 kelvins, then increases again continuously towards the inside of the planet. Ammonia, which condenses at temperature lower than 145 kelvins, is present in proportions of a few ten thousandths below the 1 atmosphere level. It is probably below this level that the coloured clouds that are observed are located. Information on the temperature of the deeper tropospheric layers is given by the fact that the radioelectric frequency radiation emitted by Saturn comes from these layers. At a wavelength of 21 centimetres, the emission comes from the 10-20 atmosphere level, where the temperature is of the order of 230 kelvins.

At greater depths, the structure of Saturn, like that of Jupiter, can only be deduced from theoretical models subjected to the constraints of three types of information: the value of the hydrogen-helium ratio in the external atmosphere, then the intensity of the internal energy source, and finally the departure from symmetry of the gravitational field radiated by the planet around itself. These three quantities were accurately measured, first of all by the Voyager probes.

Measurement of the gravitational field gives information on the distribution of masses inside the planet. We can deduce that Saturn must have a dense, solid core, composed mainly of silicates and metals, and perhaps frozen water, ammonia and methane. However, this core must be small (around 15 000 km radius) and its mass cannot be greater than 10 to 20 Earth masses.

The internal energy source is 1.76 times more intense than the solar radiation absorbed by the planet. A first hypothesis is that this energy is a remnant of the heat stored up by the planet when it was formed. Functioning like an initially heated storage radiator that cools down gradually, Saturn would emit a flow of energy from the centre to the exterior of the planet which, converted into radiant energy, would be the cause of the observed planetary emission. However, evolutionary models indicate that, given its smaller mass than Jupiter, Saturn should have lost its initial heat about two billion years ago. Another, more plausible, hypothesis is as follows: at two or three million atmospheres, hydrogen changes in nature, becoming monatomic while its density and conductivity rise sharply. It has become metallic hydrogen. Now, if the temperature in the region in question is low enough, thermodynamic calculations indicate that helium is no longer soluble in metallic hydrogen; drops of liquid helium form and migrate towards the centre of the planet, thus releasing gravitational energy. This process does indeed account for the internal energy observed for Saturn, and if it is true, there should be less helium in the external atmospheric layer of Saturn than in Jupiter's. This is exactly the result that the Voyager probes provided: as we have seen, the abundance of helium in the troposphere of Saturn is only 7% by volume, whereas it is 10% for Jupiter. Furthermore, because the temperature of Jupiter is higher in the region in question, the mixture is above the threshold of non-miscibility and the helium drop formation process has not been triggered. It will occur when Jupiter has cooled sufficiently.

To sum up, on going from the periphery to the centre of the planet, we successively encounter:

• A layer around 30 000 kilometres thick, containing mainly 93 % of molecular hydrogen and 7 % of helium; at high enough temperatures there are probably all the other minor components that made up the primitive nebula (carbon, nitrogen, oxygen, metals, silicates, etc.), but in proportions that remain to be determined

• An inhomogeneous layer 5000 kilometres thick containing metallic hydrogen, within which drops of helium are formed and fall as "rain" towards the centre of the planet

• A layer 10 000 to 12 000 kilometres thick of metallic hydrogen, in a proportion higher than that found on Jupiter or in the Sun

• Finally, a core of silicates and metals, and perhaps ices, of the order of 15 000 kilometres radius

It should, however, be borne in mind that this scenario is only a model that may be profoundly reworked as our knowledge of the giant planet grows.

The rings

Observed for the first time by Galileo in 1610, the rings of Saturn are probably one of the most beautiful spectacles that can be seen in the sky with a simple pair of binoculars. Observation of the rings by probes and satellites since Voyager in November 1980 has revealed a magnificent system composed of an incalculable number of billions of "pebbles" in orbit around Saturn and forming thousands of astonishing structures. The Voyager probes not only photographed one of the most beautiful objects in the sky, but also one of the most scientifically interesting.

The rings of Saturn, seen in a false colour image

During the summer of 1610, Galileo, who was one of the first to use a telescope to view the sky, made a haul of discoveries. In particular, he discovered "something around Saturn"; he thought at first that he had discovered two large satellites on either side of the planet, but he noticed that these two companions of Saturn showed no apparent movement relative to the planet, and this intrigued him a lot. He was even more stupefied when, two years later, he saw that the two companions had apparently disappeared. For over forty years, astronomers were intrigued by the changing appearance of Saturn's environment; some saw satellites, others a flattened planet, while still others saw complex structures, and observers argued about the quality of their instruments or the visual acuity of their colleagues. It was not until 1654 that Christiaan Huygens found the solution to the problem: Saturn is surrounded by bright rings in the equatorial plane of the planet; over the course of the twenty-eight years of one revolution of Saturn around the Sun, these rings are seen alternately edge-on and then more openly, hence their changing appearance when seen through the crude telescopes of the time (remember that in the 17th century, telescopes were far beneath the quality of a simple pair of mass-produced binoculars today).

Jean-Dominique Cassini, the first director of the Paris observatory that had just been created, discovered a division (which now bears his name), thus showing that the rings are not homogeneous, and he suggested that they were formed from a multitude of small satellites. Many 17th and 18th century astronomers believed, however, that the rings were solid, and it was not until 1785 that Pierre Simon de Laplace proved that a solid ring would be unstable and destroyed by the tidal effects of the planet. Laplace then suggested that the rings were actually made of a series of thin solid concentric rings. In 1857, James Clerk Maxwell proved theoretically that the rings were made of independent "solid" particles rotating differentially around the planet. In 1898, James Edward Keeler obtained the spectrum of Saturn and its rings and by measuring the radial velocity of the rings using the Doppler-Fizeau effect, showed that the rings did indeed rotate around Saturn differentially as would a multitude of small independent satellites obeying Kepler's laws, with the particles closest to Saturn rotating in less than 8 hours (i.e. faster than the planet itself), and the furthest in more than 12 hours. The theoretical study of Maxwell was thus confirmed.

In 1911, Henri Poincaré pointed out the importance of mutual collisions between the particles of the rings and stated that the collision phenomena now at work within the rings must have played a fundamental role at the beginning of the solar system. However, it was not until 1970 and 1980 that quantitative theoretical studies of the role of these collisions were undertaken.

Saturn's rings in figures

Satellites of Saturn

There are 62 confirmed satellites, including Titan, larger than Mercury or Pluto, which could have harboured life and which has been put forward as a candidate for terraforming. Titan has the elements of primitive Earth, but it is too cold for life to be possible. The numerous flights of the Cassini probe in 2005 over Titan showed that there is little hope of discovering forms of life there.

The total number of Saturn’s satellites is actually unknown, because there is an enormous number of objects in orbit around the planet. Eighteen additional satellites have been discovered since the end of 2000 in unusual orbits, probably fragments of larger bodies captured by Saturn. Some have even been discovered recently through Saturn's rings by the Cassini probe. Waves in the rings, photographed by the probe, intrigued scientists and, with new photos, also by Cassini, small dots have turned out to be minute satellites.

All the satellites of which the period of rotation is known, except Phoebe and Hyperion, are synchronous.

The orbits of the three pairs, Mimas-Tethys, Enceladus-Dione and Titan-Hyperion are in resonance: Mimas and Tethys are in 1:2 resonance (the period of revolution of Mimas is exactly half that of Tethys); Enceladus and Dione are also in 1:2 resonance; Titan and Hyperion are in 3:4 resonance.

The satellites of Saturn

Most of Saturn's satellites have been discovered recently. However, the exact number of satellites will probably never be known. This is because the rings contain large lumps of ice which technically are moons and it is difficult to make a distinction between the large particles composing the rings and small moons.

Before the space age, only ten moons were known:

• Titan (discovered in 1655)
• Japet (1671)
• Rhea (1672)
• Tethys (1684)
• Dione (1684)
• Mimas (1789)
• Enceladus (1789)
• Hyperion (1848)
• Phoebe (1899)
• Janus (1966), only confirmed in 1980, confused in some observations with Epimetheus whose orbit it shares).

With the Voyager probes, which flew over the system in 1980, eight more bodies were discovered (Atlas, Prometheus, Pandora, Epimetheus, Helene, Telesto and Calypso in 1980, Pan only in 1990).

With an observation mission in 2000, twelve more moons were discovered in orbit at a great distance from Saturn (Ymir, Paaliaq, Siarnaq, Tarvos, Kiviuq, Ijiraq, Thrymr, Skathi, Mundilfari, Erriapus, Albiorix and Suttungr). It is now thought that these are fragments of larger bodies captured by Saturn's gravitational attraction.

Narvi was discovered in 2003.

With the Cassini mission, which arrived in the Saturn system during the summer of 2004, several more satellites were discovered: Methone and Pallene early June 2004, S/2004 S 3 and S/2004 S 4 late June 2004, Pollux in October 2004, S/2004 S 6 late October 2004 and Daphnis in 2005 . The real nature (stable satellites or temporarily agglomerated pieces of ring) of S/2004 S 3, 4 and 6 is unknown, and these have kept their temporary designation.

In 2004, a team of astronomers from the university of Hawaii discovered twelve external satellites (S/2004 S 7, S/2004 S 8, S/2004 S 9, S/2004 S 10, S/2004 S 11, S/2004 S 12, S/2004 S 13, S/2004 S 14, S/2004 S 15, S/2004 S 16, S/2004 S 17 and S/2004 S 18).

Finally, nine more small outer satellites of Saturn were announced on 26 June 2006; they were discovered by D. C. Jewitt, S. S. Sheppard, and J. Kleyna using the Subaru 8.2 metre telescope: S/2004 S 19, S/2006 S 1, S/2006 S 2, S/2006 S 3, S/2006 S 4, S/2006 S 5, S/2006 S 6, S/2006 S 7 and S/2006 S 8.

Themis, said to have been discovered in 1905, does not in fact exist.

Positions of Saturn's satellites

Characteristics

• Semi-major axis in astronomical units (au): 9.554909
• Semi-major axis in km: 1 429 394 069
• Orbital eccentricity: 0.05555
• Inclination to the ecliptic: 2°,4889
• Sidereal period of revolution: 29 years and 166.98 days
• Period of rotation: 10.66 hours
• Orbital speed: 10 km/s
• Apparent equatorial diameter at the smallest distance from Earth (maximum value): 20",8
• Equatorial diameter (Earth=1): 9.4335
• Equatorial diameter: 120 536 km
• Visual magnitude at opposition: 0.67
• Flattening: 1/10.2
  Volume (Earth=1): 757
• Mass (Sun=1): 1/3498.77
• Mass (Earth=1): 95.16
• Mass Saturn+satellites (Sun=1): 1/3497.90
• Mass Saturn+satellites (Saturn=1): 1.0002
• Density (Earth=1): 0.125
• Relative density (water=1): 0.69
• Gravity at the surface (Earth=1): 1.07
• Escape velocity: 35 490 m/s
• Reflectivity (geometric albedo): 0.47
• Highest summit: 8 000 m
• Deepest depression: 205 000 m
• Temperature of clouds: -125°C
• Atmospheric pressure at cloud level (Earth=1): 1.4
• Atmosphere hydrogen 97%, helium 3%, traces of methane and other gases