Saturn

Saturn was named after Saturnis, the Roman god of agriculture and harvest, who carried a sickle in one hand and a bunch of wheat in the other. He is also responsible for naming Saturday. Saturn is the sixth planet. It appears a pale yellow in colour, and is most famous for its rings, easily recognisable through a good telescope. They look like a wide, flat disc, circulating the planet’s equator. The rings are made of millions of pieces of rock and ice from big lumps to tiny fragments.

Saturn
Picture was taken by Nasa’s Cassini-Huygens

The planet is an oblate spheroid about 760 times the size of Earth, but only has a mass of 95 times that of Earth. Saturn is a gasbag of hydrogen and helium. Being one of the four gas giant’s it has a low mean density—just 0.687g/cm3, and the least dense of all the planets by a wide margin. To put this in perspective the mean density of Earth is well over eight times this. For reference,  water has a density of about 1 gram per cubic centimetre.

Saturn orbits the sun every 29.46 years. A day is just 10 hours 40 minutes. Like Jupiter, its rapid spin creates a bulge at the equator of about 11%, for an equatorial diameter of 120 536 kilometres. The diameter at the pole is 108 728 kilometres. As such it is the second largest of the planets by a long shot (ranked third in obesity is Uranus at a “slim” 51 000 kilometres or so in diameter). Despite its size, the gravity on Saturn, taken at cloud top level, is only 1.07 that of Earth—a reflection of its very low density. It has at least 62 moons (at the last count) with about 34 duly named.

Structure

Saturn accreted from carbon dust to form a solid core several times larger than the earth. This attracted hydrogen and helium. This mass of gas collapsed under gravity to form the planet. The core is made up of rock and ice and is between ten to twenty times the mass of Earth. Core temperature is about 11 700°C. Above this, its structure begins to look a little like that of Jupiter. The core is encased in an inner layer of liquid metallic hydrogen and helium, where the intense pressure has crushed the atoms together and stripped them of their electrons. The outer layer is liquid hydrogen and helium. There is no discernible surface for the planet, and the liquid layer gradually morphs to an atmosphere of (yes, you guessed it), hydrogen and helium.

Atmosphere

Saturn is thought to have three distinct atmospheric levels. The highest visible layer is made up of ammonia ice crystals, hence the planet’s pale-yellow colour. Beneath this is a layer of ammonia hydrosulphide. Water ice clouds form the lowest layer. The overall atmospheric composition is 96% hydrogen, 3% helium with trace gases of methane, ammonia, hydrogen deuteride and ethane. The temperature as measured at the cloud top is -140°C.

It is a very stormy place. Winds blow in the same direction as the planet’s spin, where near the equator speeds reach up to 1 800 kilometres an hour. There is a giant white spot, which is an upwelling of warm gas. It is believed this stream punches a hole through the thick mantle of old dirty ammonia ice in Saturn’s upper atmosphere. As the gas expands it cools, triggering fresh condensation of white ammonia ice. The spot appears about every 30 years and they think it is triggered when Saturn passes nearest the sun at perihelion during its orbit.

The planet has a strange weather phenomenon that is locked around its north pole. It is a giant hurricane that presents with an unusual hexagonal shape. In the eye of the storm, the wind blows at more than four times the speed of hurricane-force winds on Earth.

While Saturn is made up of the same gases as Jupiter it is smaller and colder and receives less energy from the Sun (about 1% of the amount received by Earth). At 1.43 billion kilometres it is twice the distance from the sun as Jupiter. However, this is still enough to generate seasonal smog in the upper atmosphere.

Magnetic field

Electric currents within the liquid metal layer of Saturn generate its magnetic field. Unusually this internal magnetic field is aligned almost exactly with the planet’s spin axis—as discovered towards the end of the Cassini-Huygens mission in 2017. There is yet to be an explanation for this weird arrangement. It was previously thought that magnetic fields can only form around a planet when there is a degree of tilt between the spin axis and the magnetic field axis.

Saturn has the second largest magnetic field behind Jupiter and is about 600 times stronger than that of the earth. This generates a magnetosphere, which is an area of space around a planet where charged particles (protons and electrons) are trapped by its magnetic field. The normal shape of a magnetosphere would look like two massive earlobes either side of the planet, pulled in at its north and south poles. However, the presence of the solar wind, which is a stream of charged particles (plasma) released from the upper atmosphere of the sun, distorts this symmetry. The plasma mostly consists of electrons, protons and alpha particles. It is very dangerous. Earth’s own magnetosphere protects our planet from this solar wind by deflecting the plasma so that is passes around it. The force of the solar wind dramatically changes the shape of the magnetosphere by compressing it sunwards but creating a massive tail on the dark side of the planet. In the case of Saturn the average size of the magnetosphere facing the sun is about 1.2 million kilometres. Between this and the incoming plasma there is a bow shock, similar to the waves caused by the passage of a ship through water. Either side and behind the planet this bow shock ripples out, again as if in the wake of a passing ship.

The magnetosphere and the planet’s powerful gravitational field have an interesting interaction with Saturn’s rings.

Saturn’s rings

Saturn’s rings are about 280 000 kilometres in diameter, and maybe up to only a hundred metres thick. They consist of a collection of separate pieces of dirty water ice, ranging in size from dust grains to boulders several metres across. They orbit Saturn at speeds as fast as 800 000 kilometres an hour. They were possibly created when a moon was destroyed by the impact, or perhaps when two moons collided. Another theory is that a massive meteor crashed into an existing moon, gouging out a huge piece that burst into fragments. A more elaborate idea is that a comet hit a moon and knocked it closer to Saturn. Under gravity, this moon then broke up. In this way, the moon and comet debris formed the rings we see today.

Many of Saturn’s moons are embedded in the rings themselves. Their gravitation distorts the material in the rings, creating perturbations—sometimes shepherding the material together; sometimes breaking it apart. As if wading in a swamp the small inner moons create ripples and waves in the ring material. In some ways, the behaviour of the rings is analogous to protoplanetary dust accreting and breaking up at random. However, there are too many competing forces (gravity of the moons and the planet; solar winds; magnetic fields; and meteorites) to allow for gradual accretion.

It has been suggested that two inner moons, Prometheus and Pandora, are the “shepherd moons” that keep the rings together by gravitational influence.

There are seven ring regions designated by the letters A-G (some papers claim there are as many as 14 major divisions). The rings are tilted at 26.7° in line with the planet’s equator. The visual effect of this is that the rings alternately seem to expand and shrink to a fine line as the planet orbits the sun, as viewed from our perspective.

As mentioned there is a complex soup of forces acting on the dust and ice particles of Saturn’s rings. The materials are electrically charged by ultraviolet light from the sun and this attracts them along the lines of magnetic force, to be sucked in by gravity to rain down onto the planet’s surface. It has been suggested that within, say, 400 million years down the line the rings could well disappear through this process. But there may well be something else going on here too. For it has also been suggested that the larger moons, such as Enceladus, are replenishing the stocks of Saturn’s rings.

The moons of Saturn

As for the moons of Saturn, I think astronomers are beginning to lose count1. The last figure I can come up with is 79. They are now being grouped in terms of their orbital inclination to the planet and whether they orbit in the same direction as Saturn’s spin (anti-clockwise as viewed from above the north pole) or retrograde. While it is suggested the rings of Saturn were caused by a moon or moons that were demolished on impact, in turn, Saturn’s icy moons may have been caused by impacts on the planet itself.

The seven major moons are: Mimas, Enceladus, Tethys, Dione, Rhea, Titan and Iapetus, ranked in order of increasing orbital distance from the planet. There are fourteen inner moons, grouped together because of their irregular shape. Even so, four of them orbit at a greater distance than some of the major moons. There are many smaller outer moons, few of which have been named.

The moons of Saturn 
Inner moonsDiameterDistance from the planet
Prometheus148 kms139 535 kms
Epimetheus138 kms151 422 kms
Janus194 kms151 472 kms
Pallene4 kms211 000 kms
Telesto30 kms294 660 kms
Calypso30 kms294 660 kms
Hyperion370 kms1.48 million kms
Major moons
Mimas418 kms185 520 kms
Enceladus512 kms238 020 kms
Tethys1 072 kms294 660 kms
Dione1 120 kms377 400 kms
Rhea1 528 kms527 040 kms
Titan5 150 kms1.22 million kms
Iapetus1 436 kms3.56 million kms
Outer moon
Phoebe230 kms12.95 million kms
Titan

Titan is the second biggest moon in the solar system, and bigger than planet Mercury. The surface temperature is -185°C. Titan accounts for 90% of all matter in orbit around Saturn in terms of mass. Jupiter’s Ganymede is the largest moon. Titan is, however, the only moon in the solar system which has a dense atmosphere: nitrogen (98.4%) with the balance in methane and hydrogen. Its atmosphere is four times as dense as Earth’s. The atmosphere also has a very fine particle haze, where nitrogen and methane create organic substances. With these, methane and ethane rain down onto the surface to collect into channels and flow toward oceans. One ocean, for example, is called Ontario Lacus, which has a surface as smooth as a mirror.

Titan is the only body in the outer solar system on which a space probe has landed. On 14th January 2005 Cassini sent the Huygens probe down. It revealed a sea of dunes around the equator, deposits of solid hydrocarbons, and seas of a sort of ‘pudding’ – neither fluid nor solid.

It is believed that Titan has a large amount of nitrogen ion plasma, which is released into Saturn’s inner magnetic fields.

Titan orbits at a mean distance from the planet of 1.2 million kilometres with an orbital period of 15.88 earth days.

Enceladus

Enceladus is only 512 kilometres in diameter and yet may hold intriguing possibilities of extra-planetary life—making this little guy an important solar system body. Enceladus has no internal heat source, which instead is generated by Saturn. As the moon orbits the planet it is squeezed and pulled by gravity and this induces the heat that drives geological activity on the icy moon.

It is proposed that Enceladus, like Mars and Europa, has an ocean of water beneath its icy crust. The heat generated by Saturn’s gravity warms the water, which erupts from the surface in huge fountains of sparkling ice crystals. A large amount of water vapour escapes into space, ionizes and rotates with the magnetic field. Some of these materials may well be finding their way into Saturn’s ring system. The surface of the moon is white because some of the geyser material falls back in the form of snow. This gives the moon one of the highest known albedos of any body in the Solar System, reflecting 99% of light. The dwarf planet Eris has an albedo of 0.96. To put that in perspective the overall albedo of our own moon is just 0.12; Mercury is 0.1.

Nasa’s Cassini-Huygens2 spacecraft arrived 1st July 2004 and spent thirteen years orbiting Saturn. As it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. In 2015 the spacecraft made low passes over Enceladus, which allowed it to sample the geyser material. Sensors found it contained organic compounds along with ice and vapour, and that there was salt in the icy particles. The salinity is the same as that of Earth’s oceans. It is suggested that the rocks in the core of the moon could furnish the ocean with the chemical ingredients essential for life. The moon’s atmosphere is water (91%), Nitrogen (4%), carbon dioxide (3.2%) and methane (1.7%).

Enceladus orbits at a mean distance from the planet of 238 000 kilometres with an orbital period of 1.37 days.

Dione

Dione’s surface has hints of past or present activity. It sounds crazy but it could be snowing microbes on the surface of this little world. Scientists have presented evidence that Dione has features resembling tiger stripes and a cryovolcano, which erupts with water-ammonia or methane instead of molten rock. It is unclear whether there is current geological activity at Dione, but, if so, it is almost certainly at a lower level than on Enceladus.

Other moons

Iapetus has an atmosphere of hydrogen (96%), helium (3%) and the rest consists of methane, ammonia, hydrogen deuteride and ethane. A ridge—some 18 000 metres high in places—lies around the equator bisecting the moon’s lighter and darker side. Indeed, the lighter side is ten times brighter than the dark side. Its mean orbital distance from the planet is 3.56 million kilometres.

Mimas is a major moon with a diameter of 400 kilometres. The puzzle about this little chap is how did it survive a massive impact, so vicious it created a crater a third of the diameter of this moon.

Hyperion has an interior of rock and ice, which is filled with cavities. It is unusually shaped, somewhat like a cigar. Indeed, it is one of the largest non-spherical bodies in the solar system. It is prevented from finding a stable orbit and behaves erratically because of the gravitational influence of Titan.

Tethys, another major moon, even has two trojan moons of its own; moons that travel in the same orbit. These are Telesto and Calypso. Others misbehave, such as Pandora and Prometheus. These two inner moons swap orbits. Phoebe is by far the largest of the outer moons. It follows a highly inclined orbit, a typical feature of the outer moons. Phoebe’s orbit is inclined by 175.3° and travels in a retrograde manner. The other outer moons are at most no more than 20 kilometres in diameter.

Footnotes

1 Nasa still carries an article detailing 53 moons plus “a further 9 awaiting official confirmation”. A December 2019 article now suggests the number is 82.

2 Starting on 26th April 2017, the Grand Finale phase of the Cassini mission took the spacecraft through the gap between Saturn’s atmosphere and the inner edge of its innermost ring (the D-ring) 22 times, ending with a final plunge into the atmosphere on 15th September 2017. This was intentional to avoid the craft colliding and therefore contaminating Enceladus.

There have been few missions to Saturn. The first, Pioneer 11, made a flyby in September 1979 to take the first ever close-up pictures of the planet and discovered a previously unknown ring. Voyager 1 and Voyager 2 (also from Nasa) made flybys in 1980 and 1981. Voyager 1 returned close-up images of Titan; Voyager 2 made flybys of Enceladus, Tethys, Hyperion, Iapetus, Phoebe amongst others.

Back to Top

By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.

Last updated: Friday, 14th February 2020