IT HAS NOW been just over four years since you left home and you’ve no doubt thought many times about all those comforts you left behind: home-cooked meals, a decent hot shower, a nice pot of tea or pint of beer, fresh air. Mealtimes are a vitally important part of any space mission as they are a familiar routine that has both physical and psychological benefits. Of course we all recognize that we need to eat and drink in order to function efficiently, and to that end there are two important elements: nutritional value and calorific value. The latter is the energy you get from food and drink so sufficient calories are required for that purpose, but if there are insufficient nutrients in meals then your ability to focus on tasks will be affected, as will your general performance and, ultimately on a long voyage like this, your health.
Having food that provides all the goodness required by the human body is one thing, but the food must also be acceptable if morale is to be kept high. The acceptability of food is based on the experience of eating, so it’s all about the way it interacts with our senses. Taste, texture, smell and appearance are of almost equal importance to its nutritional and calorific value.
So, having regular good-quality meals is essential for space travellers such as yourself, even though in the confines of a spacecraft this is no mean feat. Pre-packaged food has been used almost exclusively on board the International Space Station, and with a shelf life of a year and a half it serves the purpose well. Unfortunately a huge amount of waste is produced this way: many of the space shuttle missions reported that 80 to 90% of their waste products was food packaging. When there are regular visits from supply ships this is not a problem as they can transport waste back to Earth for responsible disposal, but on a long space journey such as yours, waste must be kept to a minimum. In fact, your journey around the Solar System will take nearly fifty years to complete so it has not been possible to load on board enough pre-packaged and dried food to sustain you. Not only would it be unlikely to have a long enough shelf life, that amount of food and its waste products in storage would add far too much mass to the Kaldi. The Apollo missions to the Moon in the 1970s allowed 1.1 kilograms of food per person per day. In order to keep just one person fed on a fifty-year voyage would therefore require 20 tonnes of pre-packaged food – about as much as the weight of ten average family cars.
Clearly long-term space exploration can only happen with some type of system to facilitate the production and growth of food. There has been extensive research into systems like these, and by far the most promising is known as hydroponics. This method relies on growing plants in a liquid nutrient solution without the use of soil. It has many benefits, the greatest among them that watering is not required as the water stays within the system, and the nutrient levels can be completely controlled within the solution allowing for far more efficient cultivation. There is a fully functioning computerized hydroponics nursery on board the Kaldi which has been providing you since launch with a range of fruit, vegetables and herbs, and it’ll continue to do so for the duration of the journey. The only way to include meat in the diet would be to keep some sort of farm on board, which for obvious reasons is wildly impractical. For the Kaldi’s trip, then, we have settled for a vegetarian diet with food supplements to provide whatever additional nutrients you require.
This doesn’t quite satisfy the requirement mentioned earlier – about the importance of taste, texture, smell and appearance – but don’t forget, we chose you to go on this mission because you were made of hardy stuff. You’re certainly able to prepare and cook it all in much the same way as you did back on Earth. Because of the simulated gravity environment, conventional appliances such as a microwave can be used in a conventional way – though electric or gas ovens have been avoided because they increase the risk of fire, and one thing you do not want in a spacecraft is a fire. There are of course smoke detectors on board, but best they don’t go off in the first place.
Water flows freely out of taps, too, though to provide enough water for a single person for just fifty years would mean carrying 220 tonnes of the stuff in tanks – clearly not an efficient way to remain hydrated. Fortunately there is a water recycling system in place. Living creatures, anything that lives and breathes, drink water and then recycle it by exhaling it, sweating and urinating. All this water can be collected, cleaned and purified by a recycling system like the one on the Kaldi, so you have always had fresh drinking water. Not a drop is wasted. There should be no need to top things up, but if this was necessary there are plenty of places in the Solar System where water supplies can be replenished – the polar caps on Mars, the deep craters of the Moon, inside the gas giants, the many icy moons of the outer planets, and even comets. Having sufficient water on board the Kaldi has of course allowed you to indulge daily in one of your luxury items – coffee.
As you sip your latest cup and munch on your most recent culinary creation – a carrot, fennel and broccoli stir-fry – you stare out for the umpteenth time at what has been for a long while now a largely uninspiring view. It’s easy to understand why the good old Starship Enterprise had a holodeck on board to give its occupants some means of escaping the monotony of long voyages. Unfortunately holodeck technology is a little too far-fetched for us, but thanks to the increasing capacity of digital storage there is a vast library of your favourite television shows and movies for you to choose from and watch on the huge plasma television with surround sound. There is plenty of music to listen to as well, and the folks on Earth have been sending regular audio news bulletins to keep you up to date on what is going on back on your home planet.
You have been dipping into this library a little more often of late because this section from Jupiter to Saturn has been the longest leg of the journey so far – but there are longer ones ahead. The ‘sky’ is looking blacker than the blackest nights you will remember on Earth, the stars are looking brighter than ever before, and Jupiter is getting ever smaller … but being slowly replaced by a growing Saturn. As you approach the planet, it starts to appear slightly oval in shape, but essentially it is the view you may have seen through a telescope from Earth. The rings are finally visible in their full glory, but you will not be able to make out any moons just yet.
Saturn is the second largest planet in the Solar System with a diameter of 120,000 kilometres across the equator and 108,000 kilometres across the poles. It rotates on its axis around once every ten hours and thirty-four minutes (this is about thirty-nine minutes slower than Jupiter, which rotates once every nine hours and fifty-five minutes on average), but that very much depends on the latitude of features which rotate faster around the poles than at the equator. The fast rotation means it bulges out at the equator, taking on the appearance of a squashed ball – an appearance shared by most of the other planets but to a lesser degree than Saturn. There is a wonderful fact about Saturn: regardless of its monstrous size, its average density is low enough that if you could find a body of water large enough, Saturn would float. Although it is thought that there is a rocky core, it is the extensive atmosphere that reduces its average density to 30% lower than that of water.
On arrival at the sixth planet from the centre of our Solar System, the Sun appears only as a very bright star. It now lies 1.4 billion kilometres away, which is nine and a half times further than the Earth is from the Sun. It takes sunlight seventy-nine minutes to reach Saturn and seventy-one minutes for that light to reflect back from Saturn to Earth. One of the things you will have noticed is the increasing difficulty of communicating with Earth. Radio waves propagate through space at the speed of light so any message you now send to Earth, even just saying ‘hello’ through an interplanetary telephone, takes seventy-one minutes to get there; assuming an immediate response, it’ll be the best part of two and a half hours before you hear a ‘hello’ coming back. The further away you travel, the fainter the signal is getting too, and that will be making feelings of isolation grow stronger every day.
But it is easy momentarily to forget such things when you’re close to a planet like Saturn. Being the first human to see it like this will be an amazing and emotional experience. It’s easy to imagine what the Apollo 8 astronauts must have felt when they became the first people to leave Earth orbit and fly around the Moon. To be the first to visit this alien world, to be the first pair of human eyes to gaze upon Saturn and its beautiful ring system … words alone will not be able to describe the feeling.
From seeing Saturn through a telescope from Earth you may recall how large the ring system looks, and also that beyond their boundaries was a tiny speck of light – the planet’s largest moon, Titan. There are now sixty-two officially identified moons in orbit around Saturn of varying sizes, some measuring just a few kilometres across. Titan, however, is well named: it is larger than the planet Mercury. They are all divided into ten groups based on their orbital properties, from ring shepherds to co-orbitals and ring moonlets to large outer moons. As their name suggests, ring moonlets and ring shepherds orbit in the vicinity of the planet’s ring system. The moonlets differ from the shepherds owing to their impact on the rings, the latter creating very well-defined gaps in the rings – the Encke Gap is one of them – while the moonlets create only partial gaps. Co-orbitals share broadly the same orbit so that they gravitationally interact with each other. There are actually only two co-orbital moons, the larger Janus and the slightly smaller Epimetheus, and their orbits are 151,460 kilometres and 151,410 kilometres from Saturn respectively. Another of the groups is imaginatively known as the inner large moons and includes Mimas, Enceladus, Tethys and Dione, all of which orbit in the so-called E Ring of Saturn (there’s more on these rings later). Orbiting between Mimas and Enceladus are three moons that fall into a different group known as the Alkyonides. There are then four other groups that chiefly cover small, irregular-shaped moons.
The remaining two groups have some interesting properties. The Trojan group of moons are unique to Saturn and are special because of the orbital relationship they share with the planet and two other moons. There are four Trojan moons: Telesto and Calypso form a system with the inner large moon Tethys, while Helene and Polydeuces are bound to Dione. It is the gravitational relationship with their parent moon which sets them apart from the other small satellites of Saturn. In any orbital system where one object orbits another there are five places where a third object, which is affected only by the force of gravity, can maintain its relative position with respect to the other two objects. These places are known as the Lagrangian points, after the French mathematician Joseph Louis Lagrange who in the eighteenth century made his reputation in the field of celestial mechanics. Three of the Lagrangian points are found on a line between the two objects: L1 is directly between the two objects, L2 is along the line but beyond the smaller of the two, while L3 is along the same line but behind the more massive of them. The final two points, L4 and L5, are at a position where they make up the third point in an equilateral triangle with the line between the other two objects as its base. L4 is the point that lies ahead of the moon and L5 is at a point behind the moon. Saturn’s Trojan moons sit at L4 and L5 of the two larger moons.
Just like any other body in the Solar System, the Earth’s system has Lagrangian points too and they have been used over the years with some success for certain space missions. The Lagrangian point between the Earth and the Sun at position L1 is ideal for space observatories for solar study because from this point the Sun is never obscured by the Earth or Moon whereas a conventional satellite in Earth orbit would suffer periods of time when the Sun was out of view. The L2 position is a great place to put a more general space telescope, as from here the Sun, Moon and Earth are all relatively closely placed, leaving a huge area of permanently unobscured sky.
The most famous of Saturn’s moons are found within the group known as the outer large satellites. The smallest of them is Hyperion with an average diameter of just 270 kilometres – small enough to be able to sit it on the United Kingdom and have room to walk around its perimeter. It is the largest irregular-shaped object in the Solar System and resembles a badly misshapen rugby ball; but perhaps its most intriguing feature is its sponge-like appearance, which comes from the high quantity of deep, high-walled and jagged-edged craters that cover the surface. The largest of these craters is 121 kilometres in diameter and an incredible 10.2 kilometres deep – and bear in mind that the entire moon is just 270 kilometres in diameter. At the bottom of the majority of the craters is a dark, reddish-coloured sediment that is thought to contain carbon and hydrogen compounds – a combination commonly found on other Saturnian moons. Taking the appearance of the moon into consideration, it is very likely that Hyperion was once part of a larger satellite which was broken apart by an impact event. The satellite we now see has a low density which suggests it is likely to be composed mostly of water ice with only small amounts of rock and takes on the structure of a pile of rubble rather than a solid object.
Not only is the shape and appearance of Hyperion a little unusual, so too is its rotation. Unlike all other naturally occurring planetary satellites, it is not tidally locked to Saturn. This means it does not always present the same face to the planet, as does our own Moon. Instead, Hyperion rotates in a rather chaotic and almost random fashion so that its axis of rotation rarely points in the same direction for long. In contrast, Earth’s axis of rotation does point to different positions in space but it moves so slowly that it seems to be stably pointing in the same direction for thousands of years.
Iapetus is over five times larger than Hyperion with a diameter of around 1,468 kilometres. Unlike its smaller cousin, it has a much more uniform shape and structure. One of the most impressive features of Iapetus, which orbits Saturn at a distance of 3.5 million kilometres, is its rather strange two-tone colouration. This discovery was made back in the seventeenth century by Giovanni Cassini who noticed that he could see the moon when it was on the western side of the planet but never when it was on the eastern side. This rather curious observation led Cassini to the correct conclusion that it must be tidally locked to Saturn and have one hemisphere that was darker than the other, so when that hemisphere was presented to Earth it became difficult if not impossible to see visually. ‘Magnitude’ is a term astronomers use to describe brightness as they see it, represented by a number on a logarithmic scale, brighter objects having a negative number. The dark hemisphere, which is the leading face of Iapetus, has an apparent magnitude in the sky of 11.9 but the trailing edge is brighter with an apparent magnitude of 10.2. It just happens that the best telescopes of the time could detect the moon when it was shining at 10.2 but not when it presented its darker face.
The darker region was named Cassini Regio after its discoverer and the lighter area was separated into two areas: to the north of the equator is Roncevaux and to the south is Saragossa Terra. The darker region, which has a reddish-brown hue to it, is thought to have originally looked the same as the lighter regions, which are rich in ice, but the low pressures and temperatures at the surface will have allowed the ice to sublimate – a process we have considered already, where a solid turns straight into a gas but in doing so it leaves behind the rock and dust it was once mixed with. It is now believed that the darker regions are ‘lag’ or deposits from the sublimation process. Observations from spacecraft and from Earth-based telescopes have shown that the darker deposits, which are no more than a few tens of centimetres at their deepest, are carbon-based but also contain significant quantities of the extremely poisonous element hydrogen cyanide.
Interestingly, because the darker material absorbs more energy from the Sun, the darker regions experience higher daytime temperatures, on average 16 degrees warmer than the lighter regions. This higher temperature around the Cassini Regio means that ice sublimation occurs at a much faster rate while ice deposits are more likely in the colder regions, which ultimately leads to the dark regions becoming darker with more lag and the lighter regions becoming lighter with all exposed ice in Cassini Regio eventually disappearing. For this thermal runaway process to start, there must have been some kind of catalyst, and it is likely that this came from debris falling on to Iapetus, probably from nearby Phoebe. The colour of Phoebe closely resembles the brighter regions of Iapetus, although it would only take a very slight difference in reflectivity for the temperature difference to be sufficient to start the process.
Another strange feature of Iapetus makes it somewhat resemble a walnut. Running for almost 1,300 kilometres (over a quarter of the circumference of the moon) is an equatorial ridge that cuts through Cassini Regio and rises up to 20 kilometres higher than the surrounding plain. This makes some of its peaks the tallest mountains in the Solar System, and the presence of cratering along its length suggests it is very old. There are a number of possible explanations for the formation of the ridge but none of them satisfactorily accounts for its appearance, not least the accuracy with which it seems to hug the equator of Iapetus. One possible cause rests with the moon’s rotation period, which may have been a lot higher in previous millennia. If this were the case, then Iapetus may have cooled fast enough to retain a more plastic viscosity, allowing the pull of Saturn’s gravity to maintain the height of the ridge. Another competing theory suggests that Iapetus may have retained a ring system soon after its formation which slowly accreted on to the surface around the equator. It remains just another of those outstanding mysteries of the outer Solar System.
Rhea is the second largest moon of Saturn with an equatorial diameter of approximately 1,526 kilometres. It is a pretty typical moon of the outer Solar System with a heavily cratered surface and a makeup of about 75% water ice and 25% rock. One unusual feature was discovered back in 2008 by the Cassini spacecraft as it flew past. It detected a change in the flow of electrons that were trapped by the magnetic field of Saturn and noted a higher concentration of dust and debris around Rhea. There is a region around all astronomical bodies known as the Hill Sphere, and within this sphere the gravity of the more massive body is dominant. Saturn will have a Hill Sphere which has shaped its family of moons and the wonderful ring system, but the moons too will have such a region surrounding them. In the case of Rhea, an increase in density of electrons and dust and debris suggests that it too has a tenuous ring system. The important word here is ‘suggests’ as there has so far been no direct observation of ring system particles, although ultraviolet observations revealed bright flashes around the equator which may have been the result of ring debris crashing into the surface. If Rhea does indeed have a ring system then it will be the first discovery of a system of rings in orbit around a moon.
The largest moon of the Saturnian system we have met already, and Titan is without doubt one of the most intriguing. An approach to Titan will give away its most unique feature – or rather lack of them. Nearly every other moon in the Solar System will reveal craters as you draw closer but Titan appears as a featureless world because of a dense atmosphere. Not only does it have a thick atmosphere which sets it apart from all other moons, it is also the only body other than Earth in the Solar System which shows evidence of bodies of surface liquid. Not a lot was known about Titan until the advent of space exploration and certainly its atmosphere makes remote observation of the surface pretty tricky. Radar can penetrate the atmosphere and has been used with great success by the Cassini probe, but the most we have ever learned about this strange world came from a lander which Cassini deployed. In 2004, the Huygens probe descended through the atmosphere, landed on the surface and gave us our first real glimpse of this almost prehistoric Earth-like environment.
Of all the places you are flying by on your journey around the Solar System, Titan is most definitely one that is worth stopping off to see, to take a walk on its foreign shores. Such an excursion is subject to many of the challenges faced by spacecraft on return to planet Earth – for example, the thick atmosphere would lead to overheating of any spacecraft trying to penetrate it unless the angle of attack was just right. And the approach angle has to be just right, not only to avoid overheating: if it were too shallow then the craft would skip off the atmosphere like a stone skimming across the surface of a pond; too steep and it would just burn up. The atmosphere is nitrogen-rich, much like Earth’s, with hydrogen and methane the other main constituents. The atmosphere is denser than our own so by the time you reach the surface and start to walk around, your body will be exerted to 1.45 times the surface pressure you experience on Earth. The temperature at the surface is about minus 180 degrees but this would be a whole lot lower if it were not for the methane creating a greenhouse effect and warming the climate. Taking that low surface temperature and lack of oxygen into consideration, you will most definitely need to wear your space suit.
The surface is lit by an eerie orange glow not too dissimilar to the surface of Mars, although an awful lot darker. On Titan, the Sun is giving you just 1% of the light it provides back on Earth; add that to the dense atmosphere, and the surface is only very gently illuminated, even in daylight. You have landed along the southern coastline of Ligeia Mare, the second largest of the seas on Titan, which is found in the north polar region. You have landed in this particular location owing to data collected by the Cassini spacecraft. Radar was used on Titan just as it was on Venus because of the obscuring effects of the atmosphere, and you will remember that bouncing a signal off the surface and analysing the echo allows us to interpret the terrain below. When Cassini passed over Ligeia Mare it discovered a sea that was roughly 500 kilometres along its longest axis and surprisingly calm. The resolution of the radar technology on board Cassini would have allowed it to detect waves as small as a millimetre in height, but it could not even detect those.
Looking out over the sea now, you can see it is almost as smooth as glass, reflecting the sky beautifully. There is hardly any wind, which goes some way to explaining the absence of waves, but there may be other causes. The sea is mostly methane liquid with a little ethane and other elements but it looks like it may be covered by some other form of liquid that is suppressing the waves in just the same way that oil-spills back on Earth reduce waves to tiny ripples. If only you had a boat that could set sail on this alien sea that has probably sat undisturbed for millennia …
The sea’s southern coastline is characterized by a rolling landscape that has been sculpted by millions of years of erosion. Look in the other direction, to the south, away from the sea, and you’ll see distant hills with what look like dark rivers running down them. These rivers are actually just dark channels which are the result of organic compounds being created high up in the atmosphere of Titan from the interaction of the gases with ultraviolet radiation from the Sun. A methane-based rain then helps to bring these compounds out of the atmosphere and they wash down the mountains to be deposited in the channels and on the plains of the hills. Rain on Titan, concluded the Huygens probe which landed there in 2004, is probably a fairly rare occurrence.
The surface underfoot is covered in a soil-like material that has an almost bouncy feel to it, a little like compressed snow. Step carefully and you can walk across the surface; tread too hard and you’ll probably sink down into softer material. It may even have a wetter consistency further down – a little like a crème brûlée, with a crusty surface and a more soggy substance underneath. There seems to be little evidence of impact cratering from where you’re standing but that is consistent for a body with a dense atmosphere which causes all incoming chunks of rock to burn up, letting through only the largest pieces. Those that are present are quite young; the rest of the surface is no more than a billion years old. What there does seem to be plenty of evidence of is volcanism.
On the journey in the RSU back up to the Kaldi, analysis of the atmosphere reveals the high quantities of methane in it, and when studied over a period of time the level seems to be fairly sustained. Certainly the bodies of methane liquid on the surface will produce a certain amount through evaporation, but one likely explanation is that volcanic eruptions are injecting further amounts of methane into the atmosphere. The presence has also been detected of an element known as argon-40, which indicates the existence of cryovolcanoes, or volcanoes that erupt lava of water and ammonia.
Despite all the evidence in the atmosphere, there seems to be a distinct lack of direct evidence for volcanic activity on the surface. Among those features that have aroused suspicion are some bright spots that appeared in Titan’s atmosphere in 2008. These could easily have been explained by meteorological phenomena but they seemed to last far too long to be weather events. A change in brightness was also detected on the surface in a region known as Hotei Arcus in the southern hemisphere, which was attributed to possible lava flow.
Perhaps the most likely candidate for a cryovolcano on Titan was announced by NASA’s Cassini team in 2010. The area, known as Sotra Patera, is a chain of three mountains that rise 1.5 kilometres above the surface and they all seem to have a large crater at the top. The chain nature of the mountains and the craters strongly suggests that some sort of geological process led to their formation, and radar detection revealed what looks like frozen lava flows around their bases.
Perhaps one of the most tantalizing things about Titan is its similarity to Earth when it was in a more primitive state. The moon seems to harbour some really quite complex organic compounds – an environment that is said to be ‘prebiotic’. As we have seen, though, the surface temperature is far lower than the average temperature on Earth, at least by a couple of hundred degrees. This low temperature and the lack of surface liquid water lead many to rule Titan out as a possible location for finding primitive alien life, but the conditions could perhaps be tolerable for non-water-based life. It may just be that Titan is another moon that has a sub-surface ocean of water which could support life that would not necessarily be dependent on the Sun for its energy. The oceans of methane could support life too: where life on Earth takes on oxygen and produces carbon dioxide, on Titan it may start with hydrogen, which is plentiful, and produce methane.
If even primitive life like this existed, we might find evidence within the atmosphere of the moon. Levels of atmospheric hydrogen would be reduced within the troposphere, along with a reduction in the levels of acetylene which would form part of the biological process. A report published in 2010 by Darrell Strobel of the Johns Hopkins University in the United States announced exactly that. The study showed how the levels of hydrogen and acetylene reduced with altitude in Titan’s atmosphere, giving credibility to the possibility of organic activity. I choose these words carefully since this is not evidence of alien life, just a suggestion that there might be some form of organic chemistry taking place on Titan. More research is needed before we get to the bottom of the true nature of the processes.
Whether life has or will ever evolve on Titan is still one for scientific debate, but one thing that is more certain is that the conditions on this Saturnian moon will evolve as the Sun moves along its evolutionary path. In a few billion years’ time the Sun will start to swell as it becomes a red giant star, increasing in size so much that life on Earth may become impossible. There is hope for us, though, because by that time Titan may well offer an alternative location for humanity to evacuate to. When the Sun turns into a red giant, the surface temperature on Titan is likely to increase by about 100 degrees, taking its highest temperature to around minus 70 degrees which is almost 20 degrees higher than the lowest temperature ever recorded at the surface of the Earth. At temperatures like this it may just be possible for oceans of water and ammonia to exist on the surface. Looking into the future, then, Titan is one of the best locations for us to keep a close eye on for long-term exploration and maybe even habitation.
If we were ever to set up a human outpost on Titan, or any of the Saturnian moons, its inhabitants would have the most incredible view. To gaze up at night, and even in the daytime, and see Saturn with its beautiful ring system spanning the sky would leave most people awestruck. Without doubt Saturn is famous for its ring system, which is composed almost entirely of water ice particles with a few rocky components made of carbon-based elements. The particles range in size from a millimetre or less up to about 10 metres, yet from a distance they give the appearance of a stunning system of rings encircling the planet. It is very difficult to give dimensions for the system because some of the outer rings are faint and quite diffuse, but it is generally accepted that the main rings extend for about 270,000 kilometres. Taking into account the fainter outer rings then the entire system spans nearly 26 million kilometres.
The average thickness of the rings is no more than about 100 metres so the whole thing looks somewhat like a giant yet very thin celestial disc. Here’s one way to visualize just how thin they are: if the whole ring system were shrunk down to the thickness of a piece of paper and the proportions kept the same, then the diameter would still be 26 kilometres.
The rings were discovered by Galileo in 1610 when he turned one of the first telescopes to the sky, although he did not realize he had discovered them. Now bear in mind that it was a pretty primitive piece of kit he was using, so the views he got were far from the best. He noted that ‘Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another’. Galileo became confused in 1612, however, because the two ‘ears of Saturn’, as he sometimes referred to them, vanished from view, only to return again in 1613. What he had observed was a ring plane crossing. Like a child’s spinning top, the planet Saturn wobbles on its axis so that every fourteen to fifteen years the rings are presented to us edge-on. As we have seen, they are very thin, so when appearing edge-on from Earth they seem to vanish from view, only to return some months later. In 1655, the Dutch physicist Christiaan Huygens used an improved telescope he had made with a magnifying power of around 50 to observe Saturn and became the first person to note that it ‘was surrounded by a thin flat ring, nowhere touching the planet’.
Observation of the rings with modern Earth-based telescopes reveal that they are not a solid structure. Indeed, if they were, then they would be unstable and soon break apart. Instead they can be seen to be made of hundreds of smaller rings that are broken up by gaps. The main rings are named alphabetically in the order they were discovered. Starting from the planet, the main rings are D, C and B, then there is a gap known as the Cassini Division, followed by the A ring. Within the A ring is another gap known as the Encke Gap, and beyond are the F, G and E rings.
There are two competing theories for the origin of the rings. The least popular relates to the ring particles being leftover material from the formation of the Solar System. The proportions of ice and rock particles within the rings suggest a different origin. A more popular theory has it that the ring particles are actually some of the remnants of a moon that wandered a little too close to Saturn, close enough to reach the Roche limit and get destroyed by the immense tidal forces. The high ice content of the rings suggests that the moon may well have been massive enough to become either partially or completely differentiated, possibly even as large as Titan. The icy outer layers of the moon were stripped off as the moon got slowly ripped apart with the majority of the inner rocky material being engulfed by the mighty planet itself, while the icy debris from such a tidal encounter spread out in an orbit around the planet. It is very likely that the rings would have been more massive soon after their formation but that much of the ring material would slowly have coalesced under the force of gravity to form some of the moons we see today.
The rings are not only made of ice but also of gas. Data from the Cassini probe shows that the ring system has its own atmosphere composed of molecular oxygen. The gas comes from ultraviolet energy from the Sun striking the water ice particles and ejecting oxygen and hydrogen molecules from them. In another process, energetic ions from the moon Enceladus also strike the ice particles and release different molecules of oxygen and hydrogen. The result of both processes is a very rarefied atmosphere that surrounds the ring system.
Earlier in our journey around the Solar System we looked at flying through the asteroid belt. We took the risk and went for it. But then the distribution of material in the asteroid belt was really quite rarefied. What about the rings of Saturn? Would it be possible to fly through them safely? If you had to fly through them then the best route to take would be one of the many gaps like the Cassini Division or the Encke Gap. This was the approach taken by the Cassini spacecraft before entering into orbit around Saturn; it was directed through the gap between the F and G rings. The particles in the ring system vary from pieces smaller than a grain of sand to some that are as large as a house, so if a spacecraft were to try and fly through one of the rings there would be a pretty good chance of being hit. If one of the smaller particles were to impact then in all likelihood some damage would be sustained, though it shouldn’t be catastrophic; a collision with one of the larger pieces would spell an end to the journey, however. It would take a brave and perhaps foolhardy adventurer to ignore the gaps and try to fly through one of the rings.
With all these particles of varying size in orbit around Saturn you might think the ring system will slowly disappear over time, but we have already seen how the shepherd moons keep the system intact. The Cassini Division is a 4,800-kilometre-wide region between the A and B rings that seems devoid of ring particles, although images from Voyager revealed particles within the gap but with a much lower density. The inner edge of the Division is maintained by Mimas, one of Saturn’s many shepherd moons. Ring particles at the inner edge of the Division which are part of the B ring experience a 2:1 resonance with Mimas, so for every one orbit of Mimas, the ring particles orbit twice. This means that the pull of gravity from Mimas on the ring particles slowly builds, making their orbits unstable and resulting in a sharp cut-off of ring particles. In the rest of the Division are many thinner ringlets of less dense material, and among these are more gaps, such as the Huygens gap on the inner edge of the Division.
The Encke Gap can be found within the A ring and measures just 325 kilometres wide. This gap is the result of the small moon known as Pan which is on average 28 kilometres in diameter and which orbits in the gap. Any ring particles that drift into the ring in front of Pan will have their velocity slowed as the moon’s gravity tugs on them, causing them to fall into the inner edge of the gap, whereas those that drift into the gap behind Pan will be accelerated, causing them to be ejected out of the gap towards the outer edge.
There are many other gaps and divisions within the rings. Some have explanations, others do not. But for the most part they are kept in place by gravitational effects from Saturn’s many moons.
There are many wonderful structural features that can be seen in the rings that are also the result of the moons. Studies of the A ring revealed strange propeller-shaped disturbances along its outer edge, and it was soon discovered that tiny moonlets no more than 100 metres in diameter were orbiting within the ring debris and creating the disturbances. The fainter and more distant F ring was discovered by Pioneer 11 and is another great example where tiny moons have a big impact on the appearance of the ring. The ring is only about 300 kilometres wide in places and appears as two rings, with one spiralling around the other. It is kept in check by the two shepherd moons Prometheus (on the inside edge) and Pandora (on the outer edge), and it is their gravitational influence that seems to have shaped the beautifully dynamic nature of the ring. As Prometheus orbits Saturn, its most distant point takes it into the ring, causing it to steal some of the ring material and in the process induce disturbances in the ring that appear as knots and kinks. Pandora too seems gravitationally to disturb the ring particles, which are thought to be no larger than smoke particles, so that between the two moons and other possible unseen moonlets within the ring they have taken on a dynamic nature that has not been seen in a ring system anywhere else in the Solar System.
Not all ring features are driven by moons, however. Around the periphery of the B ring, for example, are vertical structures, some extending as high as 2.5 kilometres above the ring’s main plane. Images from Cassini detected shadows cast by the structures when the angle from the sunlight was low, in just the same way that long shadows are cast on Earth when the Sun is low in the sky. There is no moon that seems to be creating these features so for now their origin remains a mystery.
Another feature whose origin seems to be eluding scientists is the surreal spokes that were first discovered by the Voyager probes in 1980. Until then it was assumed the ring system was a purely gravitational phenomenon, but this discovery of spokes which radiated out from the planet like the spokes on a bike wheel could not be explained. They appeared as dark patches when the Sun was behind the camera yet bright when illuminated from behind. What really perplexed scientists was the way they seemed to stretch across a number of rings yet retain their broad shape as they moved around the rings. Orbital mechanics should make this impossible: dust or ice particles that orbit nearer to Saturn would move faster than those further away so that any shape formed by them would disappear as soon as it had formed.
The particles that make up the spokes are now thought to be microscopic ice particles no more than a micron (one millionth of a metre) or so in diameter that have been elevated above the ring plane and suspended there by electrostatic repulsion. This happens because the particles in the spokes and the ring particles are thought to have the same polarity, which produces a repulsive effect. Because the particles are electrically charged they are also subject to the conditions and movements within Saturn’s magnetic field, so as the field rotates with the planet, the particles in the spokes get moved too. The spokes seem to be much more common when sunlight is striking the rings from edge-on, so around the time equivalent to Saturn’s spring and autumn equinoxes. During the summer and winter months they are almost absent, which suggests some link to seasonal variation. But for now the mystery of the ghostly spokes remains.
The Saturnian magnetic field that seems to drive the spokes is about twenty times weaker than the magnetic field on Jupiter but operates in the same way. Like Jupiter, Saturn has a liquid hydrogen core and it is currents within this core that seem to generate the field. As with all planets with a magnetic field, it deflects the solar wind from the Sun towards the poles where it accelerates particles already existing within the field causing them to produce beautiful aurora displays. They have been seen around Saturn’s north and south polar regions.
Other features can be seen in the polar regions, such as the hexagonal cloud pattern at the north pole which is driven in part by a polar vortex which is itself the effect of solar heating and the movement of gas in the atmosphere. The atmosphere of Saturn is made up almost entirely of molecular hydrogen, a little over 96%, and just over 3% helium; the rest consists of various different elements including ammonia, methane and ethane, which are the main components of the clouds in the upper atmosphere. Visually, the atmosphere of Saturn has an appearance not too dissimilar from Jupiter’s, although the banding and other features are much more subtle. The clouds responsible for the detailing in the upper atmosphere are mainly made of ammonia crystals, while the clouds lower down are thought to be made up of water and a chemical compound of ammonia, hydrogen and sulphur known as ammonium hydrosulfide. Ultraviolet radiation from the Sun is partly responsible for the creation of some of these chemical compounds, which are transferred around the planet through circulation cells like the high and low systems found in the weather on Earth.
The bands of clouds on Saturn were only discovered in the latter stages of the twentieth century with the advent of space exploration. Earth-based telescopes were simply not up to the job of detecting them. But now, with advances in optics, image processing and manipulation, the features and structures can be observed from Earth. The subtle system of bands is occasionally punctuated by storms that appear as white ovals against the pale cream background. A major storm seems to erupt every thirty years or so and, because their appearance resembles the Great Red Spot on Jupiter, they have been given the rather imaginative title of Great White Spots. They only seem to occur in the northern hemisphere of the planet and generally appear around the time of the summer solstice. It is quite likely that a similar event takes place in the southern hemisphere summer solstice (which is the winter solstice in the northern hemisphere), but owing to the orientation of the rings at this point, observation from Earth is challenging at best. The correspondence to the summer solstice suggests that the storms are related in some way to the amount of sunlight incident on the planet. The incoming solar radiation warms gas lower in the atmosphere, making it expand and causing its density to decrease, and because its density is lower than the surrounding gas, it rises. The white spots that we see are the huge atmospheric upwellings that start as moderately sized spots before increasing in size, particularly along the longitudinal axis. The last ones that were seen, in 1990 and 2010, stretched out so far that they encircled the entire planet.
Strong winds on Saturn are responsible for driving the storms around the planet, and they’re also responsible for the generation of the belts. When Voyager visited Saturn in 1980 it detected wind speeds in excess of 1,800 kilometres per hour, which is over 1,000 kilometres per hour faster than winds on Jupiter. We have already looked at one of the effects of the strong winds on Saturn, the hexagonal cloud pattern at the north pole. This strange shape, which was discovered by Voyager in 1981, measures just under 14,000 kilometres along each side making the whole structure larger than the Earth. The real driving force behind the cloud pattern is not the wind speed itself but a sharp change in wind speed with latitude. As Saturn revolves on its axis, the gases in the atmosphere around the poles move at significantly different speeds. This not only produces a turbulent flow, but vortices seem to form on the southern side of these boundaries. The vortices, which are spinning columns of gas, interact with each other and evenly space themselves out around the pole, leading to the generation of the hexagon shape. There seems to be a very specific set of requirements in terms of wind speed and other atmospheric conditions before polar hexagons will form, which is why there is no corresponding one at the south pole. Studies in laboratories have attempted to recreate the process by rotating a fluid in a container, with the outer portions rotating slower than the central region. Similar results are seen in these experiments: shapes form around the axis of rotation, though they have not always just been six-sided. Sometimes they appeared with four sides, five, seven, or even eight.
Instead of a hexagonal cloud feature, the south pole is marked by a very well-developed hurricane about 8,000 kilometres across which is much like those seen on Earth – apart from its size, of course. Saturn’s south polar storm is the only storm (other than those on Earth) that has a significant eye that is encircled by rings of huge towering clouds estimated to be 50 kilometres taller than the surrounding clouds. These so-called ‘eye-wall clouds’ are formed in terrestrial hurricanes when moist air travels in towards the centre of the storm, usually as it forms over an ocean, before rising, condensing into clouds and then releasing significant quantities of rain around a central column of descending air which forms the eye of the storm.
What makes the Saturnian hurricane unique is that it does not move or drift around. It seems somehow to have anchored itself to the polar region. Nor has it formed above an ocean, and on Earth, the moisture from bodies of water like that are a key ingredient for their formation. Wind speeds around the periphery of the eye have been measured at around 550 kilometres per hour, but within the eye, things are pretty calm. The eye appears dark because it is devoid of high-level clouds, which means that by studying the eye of the storm in detail we can peer deep down into Saturn’s atmosphere. Using thermal imaging techniques that record infrared radiation it is possible to see clouds which are much lower in the atmosphere, giving us an unprecedented view of the conditions there.
The conditions in the atmosphere that surround the features at the north and south poles preclude the formation of water ice crystals. With pressures at around 50,000 pascals (100,000 pascals is roughly equivalent to atmospheric pressure at sea level on Earth) and temperatures between minus 170 and minus 110 degrees, the conditions are right for the formation of ammonia ice crystals. These crystals make up the majority of the clouds at this altitude, much like high-level cirrus clouds on Earth, although these are made of water ice crystals; but as altitude decreases, the composition of the clouds changes. A descent through the atmosphere will result in an increase in atmospheric pressure, and once the pressure reaches around a quarter of a million pascals and the temperature reaches minus 50 degrees, water ice crystals start to form. At the top of this region there will be a mixture of water ice and ammonium hydrosulfide which turns to a more pure water ice composition lower down. Clouds formed even lower in the atmosphere are composed of water and ammonia in their liquid form – more analogous to the fluffy cumulus or rain-bearing nimbostratus clouds on Earth.
Further down in the atmosphere it becomes clear that Saturn is a gas giant made almost entirely of gas with no solid surface to land upon. As we saw earlier, the gas is primarily hydrogen with quantities of helium and other elements like ammonia. Eventually a liquid metallic helium layer would be reached, just like Jupiter, then a liquid metallic hydrogen layer, then finally the planet’s core. From studies of the way Saturn interacts with its moons, ring particles and passing spacecraft, it has been possible to deduce that the core is around twenty times more massive than the Earth with a diameter of about 25,000 kilometres.
Temperatures in the core are thought to be a little under 12,000 degrees, but the planet radiates out into space about two and a half times more energy than it receives from the Sun. The extra energy is thought to come from a process known as the Kelvin-Helmholtz mechanism. The mechanism is not unique to Saturn and is one that is thought to drive heat generation on gas giant planets and on brown dwarf stars. These are stars whose mass is insufficient for nuclear fusion to have started in their core. Instead, as the Kelvin-Helmholtz mechanism explains, the surface of a brown dwarf star or planet slowly cools after formation resulting in a gradual decrease in pressure, causing it to shrink. This compression then causes the core to heat up, generating extra energy. But in Saturn’s case the Kelvin-Helmholtz mechanism is insufficient to explain its energy output so there must be some other form of heat generation. One possibility is droplets of helium rain that fall in the lower levels of the atmosphere, and as they fall through denser layers they generate heat through friction. This theory also explains the lower than expected levels of helium in Saturn’s upper atmosphere.
The gravitational kick from Saturn means there is no need to fly through its rings in order to continue your journey deeper into the Solar System. Instead, a glancing fly-by will alter your trajectory enough to send you on to the next planet of the mission, Uranus. But before you leave the Saturnian system behind there is one more thing to take a look at. One image taken by Cassini in 2013 captured a stunning solar eclipse, with Saturn blocking out the Sun. The light from the Sun illuminating Saturn from behind makes it look beautiful, while the silhouette of Saturn’s disc is surrounded by a rim of light being refracted through its upper atmosphere. The rings look absolutely breathtaking, taking on a whole new appearance in the shot. While the inner rings look fabulous, it is the faint outer E ring highlighted by light coming from the Sun that frames the entire planet like a halo. But what really makes it a special photograph is one tiny, rather insignificant speck of light piercing through just on the inside edge of the E ring. That speck of light is home, planet Earth. So, before you leave, you should certainly try to catch a glimpse of that sight for yourself.