The planet Saturn is a mysterious body cloaked in thick cloud and ringed by an icy disc of extraordinary structure and beauty. This is an ultraviolet image of the rings, taken by NASA's Cassini spacecraft during its orbital insertion in June 2009.
The colours of these rings reveal that there is more ice toward the outer part of the rings than the inner part, and this graduation of ice may reveal clues about their origin.
Although the ongoing Cassini-Huygens mission has revealed an enormous amount of data about Saturn, its rings and its many moons, Saturn remains a world of mystery. This real colour image of Saturn shadowed by its rings shows a gradual polar shift in colour from gold to azure, which is still not fully understood, but it may be related to seasonal temperature changes (Saturn has an axial tilt of about 27°).
Below is an extraordinary panoramic view of Saturn with its rings, published in October 2006 and created by combining 165 Cassini images. Here, Saturn is sheltering Cassini from the Sun's glare, revealing faint outer rings never before seen.
The Composition of Saturn
Saturn is enormous, almost one hundred times the mass of Earth, even though it is only about 1/8 Earth's density. It is composed almost entirely of hydrogen with about 3% helium and some trace elements, some of which contribute to a relatively tiny rocky core (scientists believe) surrounded by a layer of hydrogen that is under so much pressure it exists in a metallic liquid state. This is the result of a phase transition into a degenerate state of matter. It is degenerate because the electrons and protons are unbound to each other at pressures of around 400Gpa (400 billion pascals - standard air pressure on Earth is about 100,000 pascals). At least this is what is believed; liquid metallic hydrogen has yet to be made in the lab although techniques are currently being developed to create pressures of up to 500 Gpa. The creation of liquid metallic hydrogen will be an exciting breakthrough in high-pressure physics. Liquid metallic hydrogen is believed to exist to varying extents within all four of the gas giants, shown as a dark purple layer here.
Surrounding this liquid metallic layer is a thick liquid layer of hydrogen and, to a lesser extent, helium molecules. The outermost 1000 km of Saturn's 60,000 km radius consists of a gaseous atmosphere.
The rocky and liquid metallic core of Saturn is intensely hot, reaching almost 12,000°C. This means that Saturn radiates 2.5 times more energy than it receives from the Sun. This energy is believed to be what fuels massive storms on the planet. We will explore these in a moment. The mechanism for this heat production is not entirely worked out but it is believed by many scientists to be the result of at least two mechanisms: slow gravitational compression as well as friction, as heavier helium droplets “rain out” through hydrogen deep within the liquid interior. This latter helium rain theory is not without its controversy, however.
Saturn's Atmosphere
Like the rest of the planet, Saturn's atmosphere is mostly hydrogen with about 3% helium and trace amounts of other substances including water, nitrogen and methane. Despite its exotic features, Saturn is also known to have water snow, water rain, winds, lightning and storms just as Earth does. Well, not quite like Earth. Saturn's thick bands of clouds are pale orange because there is some trace sulfur in them and its winds are as high as 1800 km/h at the equator. Large white storms lasting up to a few months circle the planet. The outermost clouds, which we can see, are made up of ammonia at a very chilly -250°C. Below this deck resides a band of ammonium hydrosulfide clouds, about 170 km below the top layer. The temperature here is about -70°C. The lowest cloud deck is warmer still and is made up of water clouds. Here, temperatures approach 0°C. As we go further inward pressure and temperature increase and, at some point, hydrogen gas condenses into liquid. Below this level, helium eventually condenses into its liquid state as well. And eventually, of course, we reach a layer of liquid metallic hydrogen.
Saturn's atmosphere does not approach the violence of that of Jupiter but it does support a periodic storm called a Great White Spot, an enormous white oval storm, which tends to form once every Saturn year, equivalent to 30 Earth years, during the northern hemisphere's summer solstice. This false-colour infrared image from Cassini shows large ammonia ice crystals dredged up by the storm (yellow).
During this storm, ammonia gas is dragged more than 50 km upward where it reaches the upper cloud deck and freezes into large crystals
Saturn also supports an eerie hurricane-like storm, about 8000 km wide, locked in place rotating around its south pole. The images below were taken by the visual and infrared mapping spectrometer onboard the Cassini spacecraft. The infrared image, right, shows "leopard spots" blocking the heat radiating form the planet. These are substorms rotating around the eye.
Hurricanes on Earth are fueled by warm water and sunlight, both of which are in very short supply on Saturn. And, yet, this enormous hurricane-like structure complete with a well-defined eye (unlike Jupiter's famous Red Spot and Saturn's White Spot, which are eyeless) and ringed by an eye-wall of clouds up to 5 times higher than any on Earth with a rotation of about 350 km/h, seems to be a semi-permanent feature. It's been howling around the south pole since it was first detected in 2003. No one is quite sure what drives this storm but its period of rotation coincides with that of Saturn's radio emissions. This, along with the fact that the storm seems locked in place at the southern pole, provides a clue that the structure may be caused by a standing wave pattern in the atmosphere. Saturn's south pole is warmer than the rest of the surface of the planet by about 4°C. This may not seem like much but models suggest it is enough to cause the atmosphere across the planet from its north to south poles to sink, compress, heat up, and rise, maintaining this weird hurricane-like structure. The bigger picture of what is ultimately driving this phenomenon is currently of great interest to researchers involved in the ongoing Cassini-Huygens mission. It may ultimately be linked to Saturn's southern summer season, which is occurring right now. Images taken over time will help scientists figure out if it is indeed seasonal in nature.
Saturn's Magnetosphere
Saturn boasts a very powerful magnetosphere, second in strength only to Jupiter. This Hubble photo reveals Saturn's double aurorae (bright rings at the bottom and top of the planet), a consequence of its powerful magnetosphere.
Charged particles streaming from the Sun race along the planet's magnetic field lines into the upper atmosphere where they collide with, excite, and ionize gas atoms, making them glow. This is the same process that occurs on Earth, as explored in my article, Aurora Borealis. Refer to this wikilink for a brief explanation of the auroral mechanism.
Saturn's magnetosphere is filled with plasmas (ionized atoms). These come from both Saturn itself and its moons, especially the moon, Encelades, which ejects incredible volumes of water vapour from huge geysers erupting from its south pole (this moon is explored in detail later on). Saturn's magnetic field is generated by a fluid dynamo within a layer of circulating metallic hydrogen in the outer core. It is a dipole, much like Earth's magnetic field, except that its magnetic poles (like Jupiter's) are opposite that of Earth. Saturn's magnetic field is actually slightly weaker than Earth's magnetic field but its magnetic moment is almost 600 times larger. You may be wondering how this is so. If so, I attempt to explain the general concept in the asterisked (*) aside at the end of this article. A technical explanation of magnetic field and magnetic moment is explained here.
Saturn's rings and moons strongly affect its magnetosphere. The plasma in the magnetosphere co-rotates with the planet so its particles are continuously colliding with, and being absorbed by, slower moving moons and components of the ring system. Three moons, particularly Enceladus, also contribute new plasma particles to the magnetosphere. These interactions create large gaps in the radiation belts that surround Saturn as well as a low-radiation zone close to the planet
The Ring System
What makes Saturn so unique is its intricate ring system. These rings extend from about 7000 km to about 120,000 km from the planet and average only about 20 km in thickness. The particles that comprise the rings consist almost entirely of water ice (93%). The rest is a mixture of mostly carbon and a tiny amount of tholin impurities (tholins are made when UV radiation from the Sun acts on simple organic compounds like methane. They don't exist naturally here on Earth but they are found in abundance on the surfaces of icy bodies in the outer solar system, giving them a reddish brown appearance). The ring particles can range in size from dust-sized specks to the size of a car. The rings may have come from either the destruction of an earlier moon or from uncondensed material leftover from the formation of Saturn itself. In this case, some of the material that formed Saturn's protoplanetary disk was within what is called the Roche limit so it could not coalesce to form moons. The Roche limit in this case is the distance within which a potential Saturnian moon, held together only by its own gravity, will disintegrate when Saturn's tidal forces acting on it exceed its gravitational self-attraction. A third possibility is that a moon that once existed was struck by an impact, not large enough to destroy the moon outright but enough to cause it to exceed the Roche limit. Some researchers believe that planetary rings are inherently unstable and dissipate over time, say tens or hundreds of millions of years. However, new dating techniques suggest that Saturn's rings may be as old as the planet itself. Some ice in the central rings comes from the water expelled from Enceladus' geysers. Strangely, a faint ring far from the planet, about 12 million km away, also exists. It is called the Phoebe ring and it is titled at an angle of 27° to the other rings, which all lie in the same plane, and it orbits in a retrograde fashion, opposite the other rings. Not much is known about the origin and eventual fate of any of these rings or why only some planets have them. Our solar system has four ringed planets: Saturn, Jupiter, Neptune and Uranus. They are all gas giants residing in the outer solar system.
Saturn's Moons
So far I have mentioned Saturn's moon, Enceladus. You may have heard of other Saturnian moons such as Titan and Mimas. Saturn actually boasts a roster of more than 60 moons. Most of these moons are insignificant; some being downright tiny, less than 50 km wide. However, the moon Titan, aptly named, contributes more than 90% of the mass orbiting Saturn, and it is a mystery unto itself.
Titan
Titan, larger than Mercury, shown here in this Cassini composite image along with the tiny moon, Epimetheus with Saturn's rings in the foreground, is especially fascinating for two reasons.
First, it is the only moon known to have a significant atmosphere and, second, it is the only object other than Earth known to have stable surface liquid. This liquid is not water, however, but liquid methane. Methane clouds and nitrogen-rich smog envelope a surface that exhibits many of the same features as those created by liquid water on Earth - rivers, lakes and shorelines, all dominated by seasonal weather patterns. Titan orbits straight above Saturn's equator, so it experiences seasons along with Saturn, due to Saturn's 27° axial tilt. The catch is that all this takes place at a very frigid average temperature of -179°C. The reason Titan is so cold is that Titan's haze has an anti-greenhouse effect, reflecting the sun's energy back into space. This mechanism is similar to that of nuclear winter and it cools Titan even though some methane in its atmosphere contributes to a small greenhouse effect. The greenhouse effect is what keeps Earth warm. Carbon dioxide, methane and water vapour absorb the sun's energy and re-radiate it in all directions. Part of this re-radiation is back to the surface and it is this energy that contributes to atmospheric warming. Infrared images of Titan gathered by Hawaii's Keck observatory suggest that methane rains down to the surface in huge drops, 1000 times larger than water drops, from giant clouds, and collects in lakes that eventually evaporate back into the atmosphere, creating a methanological cycle, similar to Earth's hydrological cycle. As part of the Cassini/Huygens Mission, NASA sent the Huygens Probe to the surface of Titan, shown here in this artist's rendition based on images sent back from Huygens shortly after it landed.
To see what its cameras observed as it descended and landed on a methane-moisted sandy riverbed in 2005 (and much more!), watch the 25-minute video at the end of this article
Titan has no magnetic field to protect its atmosphere from being stripped away by solar wind. However it is almost always enveloped within Saturn's powerful magnetosphere and during brief periods when it is exposed it retains protective remnants of Saturn's magnetic field. Even with powerful protection from Saturn's magnetosphere, solar radiation should have converted all of the methane in Titan's atmosphere into more complex hydrocarbons within as brief a period as 50 million years. So how is this moon replenishing its methane? Most researchers believe it is replenished through eruptions of volcanoes, which spew water and ammonia into the atmosphere, and which are perhaps fueled by internal radioactive heat or tidal forces exerted on the moon by Saturn. No volcanoes have yet been positively identified on Titan (except possibly very recently), however, leading some researchers to wonder if Titan is, in fact, a dead moon and that the methane is replenished simply through slow diffusion from its cold interior. A few bold researchers have questioned whether there might be a biological source for the methane. Titan lacks liquid water, an organic solvent thought by many researchers to be essential to life. However, its atmosphere is rich in organic compounds and it is known to be chemically active (which seems to me to be an argument against the dead moon theory). As of June 2010, scientists analyzing data from the Cassini/Huygens mission have found anomalies in Titan's atmosphere, which could be the products of methane-producing organisms. But they could also be due to abiotic chemical or meteorological processes as well. There is significant debate in the scientific community over whether methane could function as a biological solvent, making life, even on frigid Titan, possible.
If you enjoy imagining what it might be like to personally visit Titan, as I do, consider this intriguing tidbit: Titan's extremely weak gravity (0.0113 g at the surface) combined with its thick atmosphere means that you could (protected in a spacesuit with air supply of course) simply attach some fabricated wings, flap your arms, and fly! Perhaps you could carry with you instruments to investigate the moon's suspected methanological cycle, setting up base camp in a laboratory-equipped research balloon similar to the one proposed for the now-delayed Saturn System Mission, shown here.
Or, perhaps as suggested in an interview with Dr. Ralph Lorenz at NASA's Jet Propulsion Laboratory, you could set off exploring Titan's unique atmosphere and surface in a helicopter!
An interesting future career possibility don't you think?
Enceladus
Now that we've explored what we know (and don't know) about Titan, let's focus now on another Saturnian moon I have mentioned in this article. Enceladus is the sixth largest moon in the Saturn system, only a tenth the size of Titan. Both Voyager 1 and Voyager 2 flew by this moon but neither spacecraft was designed to slow down and achieve orbit like the Cassini/Huygens craft did. The two earlier spacecraft determined that Enceladus is water ice-covered, highly reflective and covered with smooth regions suggesting a youthful, and therefore active, surface. It appears as a bright white sphere deeply embedded in Saturn's E-ring, shown here.
Scientists had to wait for data from the current Cassini mission to answer why this moon was so active (it shouldn't be – based on its tiny size it should have cooled off and frozen solid long ago) and whether it might be the source of Saturn's E-ring (this is the outermost diffuse ring in the composite image of Saturn – the fourth image in this article).
Cassini performed four close flybys of Enceladus and found out some extraordinary things (curious mission specialists have now planned even more flybys, some as close as 25 km from the surface). The moon's south pole ejects massive amounts of water ice from what are called cryovolcanoes into space. As well, these geyser-like jets contribute to a faint ionized water exosphere concentrated at the south pole where the plumes are located. This 2005 Cassini image dramatizes these incredible plumes.
Cassini was able to fly through Enceladus' enveloping gas cloud and confirm that it has the same chemical composition of minute trace elements as the E-ring, confirming that it does in fact contribute to it.
Close inspection of the southern surface of Enceladus reveals blue ice “tiger stripes,” shown here.
Scientists believe that the blue walls of fracture lines crossing the region expose outcrops of coarse-grained ice that appear blue compared to powdery surface ice covering the flat terrain. The coarse crystals are young (less than a thousand years old) and have recently been thermally altered. There are also some simple organic compounds present in them. Scientists think that water plumes from this fractured region come from pressurized subsurface chambers, and there is recent infrared spectrometer evidence from Cassini that these chambers may be as warm as -116°C. This finding suggests that the interior of the moon is a heat source, and this heat must come not only from the radioactive decay of heavy elements within the core but also from the continuous strain of tidal forces from Saturn. Enceladus is cyclically stretched and deformed as it orbits the planet, creating frictional forces acting all through it.
Warm low-density material rises from within the moon to the surface, within its icy shell (yellow) and/or its rocky core (red). Somehow, Enceladus rolled or rotated to place this area of low density at the south pole, where heat escapes within water plumes. No one is sure what caused this area of low density, perhaps an earlier impact.
Some scientists wonder if the escaping water comes from a subterranean water ocean, and whether this ocean could harbor life.
The highly successful Cassini/Huygens mission, emphasizing Saturn's mysterious moons, Titan and Enceladus, as well as some of Jupiter's moons, is explored in this excellent 25-minute video.
*A Brief Aside To Explore Magnetic Fields and Magnetic Moments
This is how it works: The magnetic moment of a magnet (in this case Saturn is a gigantic magnet) is the torque exerted on that magnet in a magnetic field. It's a vector force, just like the magnetic field force, but in this case it is the product of the strength of the magnet and the distance between its poles, in this case the gigantic diameter of Saturn itself. Perhaps it will help to think of it this way: Each pole of a magnet is a source of magnetic force. Like electrical force and gravity, it weakens with distance. Magnetic poles, however, are unique – they always come in pairs, equal in strength and opposite in type. Their forces interfere with each other so that while one pole attracts the other one repels. Intuitively you might guess that this interference is greatest when the poles are close to each other, for example, when the magnet is short, and you would be right. The magnetic force produced by a magnet at any given point in space depends on two factors – on the strength of its poles and on the distance separating them. This force is, in fact, proportional to the product of these two factors, and this is how we get to the magnetic moment. This is why Saturn's large diameter can mean a very large magnetic moment and a relatively small magnetic field.
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