The outermost of the planets known in ancient times, Saturn was traditionally considered the limit of the Solar System, a symbolism reinforced by the fact that it has a restrictive-looking set of rings around it. Oddly, Saturnine herbs are partly distinguished by having prominent rings, among other things, even though the association with the planet pre-dates their discovery.
Saturn is a couple of things. It’s the most squashed planet. It’s like it’s been “sat on”. Geddit? Seriously though, it’s flattened to the extent that its polar diameter is 9.8% less than its equatorial. This isn’t as obvious as Jupiter’s because the ring obscures its shape and seems to cause an optical illusion that it’s rounder than it really is. There are two reasons for its oblateness. One is that it spins very fast, with a day of roughly ten and a half hours. It’s difficult to be precise because like Jupiter it doesn’t rotate as a solid object would but has several “systems”. The other is that it’s also the least dense planet, also making it the softest. It’s actually less dense than water. If it were possible to put a tiny version of Saturn in the bath, it would float like a rubber duck. Its average density is only 69% that of water, which is lower than any solid element except lithium. It even looks like it’d make a good pool toy or floatation aid.
According to the Ætherius Society, Saturn is where the Interplanetary Parliament, er, sits. The beings who rule over this Solar System are said to be enlightened golden spheres twelve metres in diameter. However, not many people agree with the tenets of that religion and reject the idea outright. I have no idea why they think this, but in general the religion is a lot less harmful as some other “flying saucer religions”, so to speak.
Before Voyager’s time, Saturn’s rings were divided into three, with actual gaps as well. There was the A ring, on the outside, split by Encke’s Gap which is about 325 kilometres wide, but the most obvious gap is the Cassini Division, 4 800 kilometres in width. An Atlantic-sized gap. The area this gap surrounds is the B ring. Both of these rings are opaque, but an inner ring, known as the “Crêpe Ring” is partly transparent and objects can be glimpsed through it. When the Voyagers got there, unsurprisingly the rings turned out to be a lot more complex than that, and in fact they look more like the grooves on a record, not in terms of spirals but because there are hundreds of concentric rings. There was previously a plan to send the Voyager spacecraft through the Cassini division but it turned out to have plenty of rings within it itself. Encke’s Gap contains a braided ring and a moon which has been called Pan.
Saturn is one of four planets known to have rings, but until the late 1970s CE it was considered unique in this way. This changed when a star in front of which Uranus was passing appeared to blink on and off at the same intervals on either side of the planet, and within a couple of years the Voyagers were able to photograph those rings while the spacecraft were near Saturn. Even still, Saturn’s rings are by far the most spectacular and brightest, the cleanest in fact. Saturn is in general positively gleaming, bearing in mind it only gets one percent of the sunlight Earth does per square metre. This isn’t as dingy as it sounds because the human eye would adjust easily to that without there being any obvious difference after a while. Speaking of dinginess, like the rest of the system Saturn is overshadowed by Jupiter. It’s smaller and further out, and as far as we’re concerned also further away. Thus before anyone was able to point a telescope at it, apart from being on the edge of the system it was relatively dim and insignificant. It’s still brighter than first magnitude and doesn’t vary much on account of it being ten times our own distance from the Sun, meaning we observe it as between nine and eleven AU away, making a difference of only around a third, and because it’s a superior planet we never see it as a crescent and it’s nearly full most of the time.
The rings are extremely thin compared to their width at around fifty metres, and since Saturn’s axis and orbit are both tilted with respect to Earth, they are sometimes more visible than at others. This confused the first astronomers to observe the planet through telescopes because it meant the features they appeared to be able to see changed shape and size and even completely disappeared. The earliest such observer, Galileo, thought he saw two spheres accompanying it on either side, which incidentally he referred to as “planets” (in Italian or Latin presumably), showing how the concept of planet changes over the centuries. This was in 1610. Soon after, others were able to see the rings but were baffled by their sudden disappearance until they realised it was because we were seeing them edge on. This range of angles would also apply to the moons, and rather annoyingly to anyone who might be visiting, all the larger closer moons orbit close to the plane of the rings and you wouldn’t really be able to see them. Only Iapetus, whose orbital inclination is 15°, has a good viewing angle and unfortunately it’s also quite far out, so Saturn would look nice but it wouldn’t dominate the sky like it does closer in. While we’re on the subject, Saturn is likely to be invisible from Titan due to constant thick cloud cover, but it would show the rings a little. Maybe if you were there you could set up a sightseeing service to take tourists above the clouds and look at the ringed planet.
In a sense, Saturn’s rings extend all the way down to the atmosphere, meaning that there must be constant meteor showers at the equator. I don’t know how this would be replenished. Maybe it can’t be and that’s why the Crêpe Ring looks like that. They reflect more light than the cloud tops and are edge-on to us at alternate intervals of thirteen and three-quarters and fifteen and three-quarters years due to the eccentricity of the planet’s orbit, which is 5.2%, thrice ours. The Crêpe Ring is also known as the C Ring and there are a number of others, although many would best be thought of as groups of much smaller rings nowadays. There’s the even fainter D RIng, which is inside the Crêpe Ring and ends around seven thousand kilometres above the cloud tops. The outer edge of the A Ring, beyond the Encke Division, is split into more widely separated narrower rings and there are three moons orbiting near them. The largest, or rather least small, of these is the F Ring, near another moon. All of these are called “shepherd moons”, which of course is also the name of an Eithne album, and they keep the particles in place in the rings. There are also coörbital moons, which swap orbits regularly.
The G Ring starts 2.8 radii from the centre of Saturn, which places it beyond the Roche Limit of 2.44, within which large objects would be unable to hold together. The main part of the rings is somewhat within the limit, but doesn’t extend right up to it. D and G can only be seen from forward-scattering light, and D is also drowned out from here by the planet’s glare. From the other side of Saturn both of them are easier to spot. In fact the progress of the four Pioneer and Voyager probes beyond the planet made it possible to see the rings from the other side for the first time, and also send signals through them to see how they were altered by and interacted with the ring materials, like shining a light through fabric to inspect the weave. This enabled scientists to determine that A, B and the Crêpe Ring are all water ice and that the range of particle sizes was between micrometres (able to scatter visible light) and decametres (the size of a double decker bus or so, able to scatter RADAR frequencies), but are mainly at least a few centimetres in diameter. Thus the material consists substantially of roughly snowball-sized chunks of water ice, although it can be much larger or smaller.
D may go all the way down to the cloud tops, although presumably this would make it unstable. It’s more of a region than a ring. The Crêpe Ring has grooves like the rest of the ring system, but they don’t correspond to gravitational resonances as might be expected. It also has two gaps, one 270 kilometres, or about the distance between Inverness and Dumfries, and another variable gap, more elliptical, between thirty-five and ninety kilometres wide.
Either side of the rings for about sixty thousand kilometres is a very thin cloud of hydrogen at a density of about six hundred thousand particles per litre. This is probably liberated from the ice in the rings by radiation.
The B Ring is redder and it’s been guessed that this is due to iron oxide, but I can’t help thinking it’s more likely to be tholins, but maybe it’s just me. It just seems like Saturn isn’t dense enough to have loads of iron available to do something like that, although that might depend on where the rings came from in the first place. But then, I’m not a scientist and iron does turn up in odd places sometimes, such as in Martian soil even though Mars is the least dense rocky planet. What do I know, eh?
B and the Crêpe Ring have a sharp boundary. There’s no gradual attenuation into the translucence. It just happens. The
The Cassini probe detected spiral ripples in the inner rings which are attributed to currents in the interior of the planet having a gravitational influence on the particles. Interestingly, these clumps and sparse areas are reminiscent of the arms of a spiral galaxy for me, which amount to “traffic jams” and are more like sound waves moving through the rings than permanent structures. Hence there’s a disc with spiral grooves associated with sound waves. Remind you of anything?
There are also spokes, which are harder to explain. These are dark radial features stretching across the rings upwards from Saturn, which maintain their integrity as they move around the planet. I may or may not have mentioned them in connection with plasma at some point. The reason this is odd is that one would expect them to smear out along the rings’ circumference because objects orbiting further out should be moving more slowly, hence the words “move around” rather than “orbit”. It’s thought that they’re held together by electrostatic charges. They persist for twenty to thirty hours and seem to be subject to the rotation of the magnetic field, as they rotate with the planet, unlike the rings generally. After this period they start rotating with the rings orbitally, which causes them to disperse. They’re found in the B Ring. Their relatively small size when they form suggests the fluctuations in the magnetic field are local and short-lived, lasting no more than a few minutes.
Attempting to write about the rings raises another issue. Looking at Saturn with a pole at the top tempts one to believe they’re horizontal, once again like a record sitting on a turntable (now I’m wondering if there are vertical record players), but in fact the A ring is above the Crêpe Ring rather than beside it. The spokes might be thought of as clouds of particles hovering above the rings but they are actually north or south of them. This would mean that when they return to orbiting around the planet, they will tend to move away or towards the equator, which is tantamount to moving away from or towards the rings, and all would move towards them within a period of around six hours and become lost among the fragments.
They also fluctuate in thickness over a period of hours, which can be seen in time lapse films of the rings in close up. This seems to be caused by the presence of satellites within the rings, or within other rings, and is possibly tidal.
The biggest apparent gap, visible from Earth through a good telescope, is the Cassini Division. Although it was thought to be empty before probes were sent there, it turns out to have about the same density of material as the Crêpe Ring, so the plan to send a probe through it would’ve led to the spacecraft being destroyed. It’s slightly elliptical and the width of North America, so like Galileo Regio on Ganymede it emphasises the sheer scale of the system that we can barely see it from here. Although it’s elliptical, varying by 140 kilometres in width, it’s centred on the centre of Saturn rather than being at one focus. I should probably explain this. According to Kepler, planetary, and in fact satellite, orbits are elliptical with the Sun at one focus. There was a notorious mistake on the last English pound note where one of the orbits shows the Sun at the centre rather than a focus, which will illustrate what it means:
It can be clearly seen that the largest orbit is centred on the Sun whereas the smallest is off-centre, as it should be. Then again, maybe the kind of people who forge notes are really obsessed with astronomy and would accidentally correct it! If you draw an ellipse using a loop of string secured to paper with two drawing pins and a pencil to draw the outline, the pins will be at the foci. The reason the Cassini Division doesn’t show this, I think, is related to emergent effects related to the collision of particles within the rings, but this is my guess. The spokes, as I mentioned, also don’t conform to Kepler’s Laws. All that said, the actual position of the Cassini Division does seem to be determined by the orbit of Mimas, the closest large moon, as the outer edge of the B Ring, which is where the Division starts, has a period of exactly half of that body’s.
The other gap visible from Earth is the Encke Division, which is somewhat further out and seems to be part of a general breakup in the integrity of the rings at the outer edge. It’s towards the edge of Ring A. When Voyager 2 was leaving for Uranus, the star Dschubba passed behind (i.e. in our direction) the rings and was eclipsed several times as the Encke DIvision passed in front of it, so there are several ringlets within the gap, and also some are eccentric.
Due to the grooved appearance of the rings and the fact that the gaps are not actually empty, the idea of orbital resonances causing them doesn’t quite work because whereas there’s a threshold from Earth observation which assigns some parts of the rings to gaps and others to, well, “ring”, this is not the situation observed near Saturn itself, and there are too many rings for orbital resonance to be the only explanation for this. My personal feeling is that the rings seem to have their own special case of physics in a similar way to how Earth’s land surface has. Here on Earth, we expect moving objects to slow down and stop on most flat surfaces and for heavier objects to fall faster than light ones, among other things, and some people tend to generalise that to the Universe in general, where it won’t work. Likewise, the presence of multitudinous ring particles, colliding with each other and becoming statically charged and repelled, among many other things, seems to lead to a special environment not at all like Earth but also unlike an ordinary orbital environment such as is found around Jupiter. Although there are other ring systems, they are nowhere near as dense and spectacular as Saturn’s. This doesn’t answer the questions in detail, but in a way it would be surprising if it did behave intuitively like a load of bodies obeying Kelper’s Laws because there are just so many of them. One idea is that there are density waves triggered initially by orbital resonances, but which then ripple outwards under their own momentum, creating the LP-style pattern we see.
As I recall it, there used to be two theories about what the rings were composed of. One was that it was ice, the other that it was rock. I tried to come up with a compromise where they were rocks coated in ice. This was when I was about six, and little was known about the place because no spacecraft had ever visited. It’s an example of my attempt to resolve an issue by finding a compromise between two opposing viewpoints. I’m not sure I would do that today, but my aversion to conflict often drives me in this direction. Another personal take on this is that it’s been so long since the Voyager probes discovered the detailed appearance of the rings that it’s hard to imagine things being any other way, but before they got there, the Pioneers’ cameras not being good enough to reveal that structure, everyone assumed they were smooth apart from the broad divisions we were familiar with and the Encke and Cassini divisions. It’s hard to remember what everyone used to think they were like. The question arises of whether there actually are smooth ring systems out there. Jupiter’s probably is, but it’s also quite insubstantial. Around another star system, or perhaps long ago in the history of this one, there may be or might have been extensive smoother rings, such as around the moonless Venus
This raises the question of how they got there in the first place. One relevant aspect here is that they seem to be temporary and in fact even the features which have been mentioned may be more transient than permanent fixtures. The rings themselves could be gone within a hundred million years, and since it’s fairly unlikely that we’d be around that close to their demise, the chances are they weren’t there soon after the planet formed, although another set of rings may well have been. The current set is probably less than 200 million years old, which is younger than the first dinosaurs and mammals. The chances are that the rings never formed part of a single larger object but are instead a collection of comets and asteroids which were captured by the gravity of the planet, although I don’t see how this makes sense because if they’re temporary it sounds much more like they were a single object which was broken apart. Asteroids are often rubble piles, so it does make sense that there was never a single object.
The whole subject of the rings is so involved and extensive that it’s almost like I’m talking about a different entity than the planet, but they’re also such an essential part of how we think of Saturn that it can’t really be mentioned without mentioning the rings themselves. Even so, we happen to be in a period of less than five percent of the Solar System’s age so far when Saturn has these rings. Maybe at another time Jupiter’s rings were much more obvious.
Moving on to the atmosphere, which in Saturn’s case is basically the whole planet, being so tenuous, the situation isn’t as simple as Jupiter’s because unlike the giant planet, Saturn is tilted. Whereas Jupiter is almost a model of simplicity, Saturn has an axial inclination of 27°, and since its years last almost thirty of ours it has seasons lasting more than seven years each. This leads to the same sort of “blowiness” as we get in spring and autumn, but on a far larger scale and a much longer period of time. Saturn’s cloud tops are also considerably colder than Jupiter’s, but like Jupiter it emits about 60% more heat than it absorbs. This is also less straightforward than the other planet because it can easily be accounted for there by it being so huge that it’s taken this long to cool down, but in Saturn’s case this is not so.
While I’m at it, this would probably be a good place to talk about the consequences of Saturn’s size and tilt. I’m personally guessing that shadows cast by the rings influence the weather. Twenty-seven degrees of inclination is slightly more pronounced than our own 23°.4 and all other things being equal the seasons will be somewhat more pronounced than ours, but also, the ring shadow reaches 48°from the equator, which creates a large colder area in darkness for long periods at a time, most pronounced during mid-summer and mid-winter. For the former situation there will be a particularly big temperature difference between the mid-latitudes under the Sun and those under the shadow. This would cause powerful winds into the area which would be weaker but still exist during the winter. It also has photochemical effects because the influence of ultraviolet light from the Sun is absent under the shadows. And it is “shadows” because of the various gaps such as the Cassini and Encke divisions.
Another markèd aspect of Saturn is, well, its aspect as the most “squashed” planet. It’s twelve thousand kilometres wider at the equator than the poles, giving it a gravitational pull almost 23% less there. Furthermore, since it takes only ten and a half hours to rotate on its axis, the centrifugal effect is quite large, though not so much as it on Jupiter. The average surface gravity at cloud top level is about the same as ours at sea level. Wind speed is as high as 1 800 kph, which is fifteen times hurricane force on the Beaufort Scale. Cloud top temperature is between -185 and -122°C.
Saturn has a rather blank appearance as a whole and is easily upstaged by its own rings, but it has some similarities to Jupiter in that it’s banded and has oval storms on its “surface”. The comedian Will Hay was also an astronomer and his chief claim to fame in that area is that he discovered one such storm, the Great White Spot, in 1933 CE. As an astronomer he made himself known as W T Hay in order to separate the two parts of his public life. He once said that if everyone was an astronomer there would be no more war because everyone would have life on this planet in perspective. On 3rd August 1933, he observed the spot on Saturn while Cynthia was quite bright and Saturn quite low in the sky, so conditions were far from ideal. Other astronomers were able to confirm its presence at about the same time. He made these sketches of the phenomenon:
His finding was published in the British Astronomical Circular on 4th August 1933. In a reference to the rise of Hitler, the ‘London Evening News’ published a cartoon of Hay standing on the rings and observing dark trouble spots on Earth, which actually chimes really well with his own attitude of getting perspective on human affairs by realising a sense of their relative scale. As a slight aside, I know I’m typing this with Russian manœuvres and Western posturing over the Ukraine, and it might look like I’m just ignoring it, but what I’m trying to do is provide the “Overview Effect”. When I make the observation, for example, that the Cassini Division is the width of North America but not even visible through a mediocre telescope from here, that’s meant to indicate how petty our squabbles are and the ultimate unity of this planet. If Hay’s drawing of his Great White Spot is proportionately accurate, it had a diameter about five times Earth’s, and this is important. Our own problems are of course major, but this makes the planet seem all the more precious because it’s a tiny oasis of life lost in the vastness of the Cosmos, even just of the Solar System. If the orbit of Neptune was scaled down to the circumference of Earth, Earth on that scale would be about the size of a double-decker bus or large tree, or perhaps a medium-sized back garden. That’s not insignificant but it’s still a lot smaller than the world, and that’s just the bit with the planets in it. I have seen a couple of Will Hay films by the way but didn’t get an enormously clear impression of what his cinematic work was like. I would expect it to be rather dated, and the same might be said about his astronomy but it still has the same effect.
Great White Spots are of course named after Jupiter’s Great Red Spot, but they’re harder to, well, spot from here because they aren’t red and Saturn is about twice as far away and somewhat smaller than the next planet in. Also known as Great White Ovals, they appear in the northern summer every twenty-eight and a half years. In 1876, Asaph Hall, who discovered the Martian moons, used one to time Saturn’s rotation period, although that assumes they don’t move relative to whatever counts as stationary for Saturn, which like Jupiter and the Sun is hard to define. Oddly, none were seen before that one even though telescopes had been good enough for a very long time, and it’s thought that before that, Saturn was undergoing a quiescent period similar to the one which has sometimes made the Great Red Spot (GRS) disappear, so there would’ve been some before the telescopic era but nobody would’ve been able to see them. They also appear alternately in the northern temperate zone (NTZ) and at the equator. This makes them similar to the GRS in that they occur in one hemisphere but not the other, in this case the opposite one. They differ in that they leave long trails and have lightning. They also don’t have “eyes”, unlike Earth’s hurricanes, but are active all the way to the centres. It appears that Saturn’s atmosphere is more humid than Jupiter’s and when it cools, rain or snow takes heat away from it, being proportionately much heavier than the air, which is mainly hydrogen and helium, than Earth’s nitrogen-oxygen atmosphere. This cooling effect means there are weaker air currents in the upper atmosphere, which results in a colder and very stable condition only disturbed in the summer when the Sun heats it up again and gives rise to storms. An individual storm can be larger than Earth, as was Will Hay’s for example.
Like Jupiter, Saturn is divided into zones and belts, like this:
The polar regions are of special interest and I’ll be returning to them, but for now the northern region is much bigger than the southern, reaching down to 55° whereas the Southern Polar Region reaches down to only 70°. The brightest part of the planet is the Equatorial Zone, bisected by the narrow Equatorial Belt, which could be constantly in receipt of D Ring fragments. It’s the EZ which has the ovals along with the NTZ. Many of these are hard to see from here due to being covered by the rings much of the time, although they’re so thin that everything is visible when the planet is edge-on to us.
The whole planet is rather bland-looking and therefore differences in the clouds are harder to see, if there are many. Features over a thousand kilometres across are only about a tenth as common as they are on Jupiter. All the way through this bit, I feel like I have to compare to Jupiter and that seems quite unfair. Why can’t Saturn just be considered in its own right? Nonetheless it is also the planet most like Jupiter. There is not much helium at six percent. This is thought to be because by about two æons ago, the planet had cooled enough for helium to rain out of its lower atmosphere onto the core. This was very deep down and under enormous pressure. It doesn’t mean the planet cooled down to the extreme low temperatures required for helium to become liquid at sea level pressure on Earth. Helium, incidentally, wouldn’t behave like it does in our atmosphere. Because Saturn has such a low density, and also so much hydrogen in its atmosphere, helium is twice as dense as its air and would tend to sink. This process took the heat from the then warmer upper atmosphere into the depths, which is thought to be why the centre of the planet is hotter than might otherwise be expected.
The internal heat is a factor in driving the weather systems. On Earth, most of the heat comes from the Sun although some is trapped by greenhouse gases and volcanoes would sometimes make a very minor contribution. On Saturn, most of it comes from below, and given that it’s further from the Sun than Jupiter, proportionately more than on that planet. Most heat is lost from the poles and the least from the equator, meaning that the poles can be the warmest parts of the planet. I’d expect the oblateness to contribute to this as at the poles there are twelve thousand fewer kilometres for the heat to make its way through than at the equator, meaning that the atmosphere forms an uneven insulating blanket wrapped around the interior.
There are the usual problems of defining the surface of a gaseous body. In this case it’s fairly clear, because the cloud tops are also the point at which the temperature reaches a minimum at around -183°C and is higher both above and below it. This does, however, mean that the troposphere, i.e. the layer of atmosphere immediately above Earth’s surface, is actually below the surface on Saturn. The top layer of clouds is one of several, the top being ammonia, beneath which is ammonium hydrosulphide. This is one of the chemicals used in “stink bombs”, so the planet might look beautiful but it actually smells revolting. Its boiling point is 56.6°C, so there is adequate range for the existence of these clouds in aerosol form. Below them are water vapour clouds like we have here. These are getting on for two hundred and fifty kilometres down, where the pressure is about ten times that at sea level on Earth. Saturn’s clouds tend to be similar colours and are thicker than Jupiter’s, with fewer gaps, all of which contribute to the planet’s uniform appearance from space.
Because Saturn is the least dense planet, and in connection with that has lower surface gravity, the pressure increases more slowly with depth. The atmosphere is both less dense and lighter. Coincidentally, it’s also lighter in the sense of not being as dark, although in another sense there’s only a quarter of the sunlight present at Jupiter’s orbit, but it does reflect more sunlight.
Saturn has a diameter of 116 460 kilometres, which is nine and a half times ours. This makes it something like seven hundred times Earth’s volume although the oblateness makes this complicated to calculate. However, its density only being 68.7% that of water, a “Saturn” the size of Earth would have lower gravity than Cynthia’s at the surface. It also wouldn’t hold together very long for that reason. It also gives it eighty times our surface area, which means that Earth is to it roughly as Australia is to Earth. Hence Earth is actually a somewhat respectable size compared to the planet, being equivalent to a small continent, although that does also include all the ocean. In terms of land, Earth is analogous to the Sudan on this scale. As far as Jupiter is concerned, it’s feasible to be more exact due to the oblateness of both planets. It’s 83% of its diameter and has 69% of its surface area and 57% of its volume. However, its mass is considerably smaller at only 29%, which illustrates a tendency found among exoplanets that on the whole they don’t get much larger than Jupiter because beyond that mass the interior just gets increasingly compressed. While I’m at it, there is also a big gap between the largest planets and smallest stars which remains unexplained, and there are also “puffy planets” and large planets which are in the process of forming and contracting.
A layer of haze above the clouds might be hiding some of the cloud activity further down. The temperature also contributes to the light appearance as many of the clouds are made of frozen white or pale yellow crystals. There are a number of jet streams. The equatorial one has a velocity of 1 800 kph, which is two-thirds of the speed of sound in that region but supersonic for our atmosphere. Then there are three easterly jets in each hemisphere with latitudes of forty, fifty-eight and seventy degrees. All of these are quite stable and durable. However, unlike Jupiter the winds don’t correspond to the stripes. Surprisingly, the winds are symmetrical with respect to the equator, which they “shouldn’t” be because the planet is tilted and has seasons which are in some ways more distinct than even ours. This suggests that the winds extend deep into the planet and that it rotates as a series of nested cylinders, because the heat from the Sun doesn’t seem to be the main influence on the winds. If it were, there would be more seasonal variation. This also means that some of the cylinders actually reach the core, and these are more likely to be different in the different hemispheres, meaning that the polar regions further than 65° from the equator are likely to differ more than the temperate and equatorial ones.
If the visual contrast is ignored, there are many similar structures in Saturn’s and Jupiter’s atmospheres. The jet streams in both are thought to be powered by eddies. However, on Saturn they’re four times stronger, can be twice to four times the width and don’t relate to the banded cloud structure.
No account of Saturn’s atmosphere would be complete without a reference to the Hexagon. Jupiter has its Great Red Spot, Saturn its Hexagon. This is a hexagon (really?) in the north polar region whose sides are 14 500 kilometres wide. It was first detected by Voyager 1 when it passed over the north pole. However, it took another six or seven years before anyone noticed it because there was so much information available. It was initially thought to be the result of a storm happening on the edge of the northern polar region but when Cassini visited more than twenty years later, but less than an entire orbit of Saturn later, it was still there, tough at that point the north pole was in darkness so it was imaged in infrared. It has now been seen from Earth. Also puzzling is the complete absence of a similar shape at the south pole. Jupiter doesn’t have a hexagon, but it does have a polar vortex surrounded by eight other equidistant storms. Other planets with atmospheres also have them, including Venus, Earth and Mars. Mathematical models were able to produce triangles but not hexagons. After some time, it was suggested that the shape emerges from a wave passing around the northern polar circle of the planet of a certain length which interacts with itself to produce a kind of interference pattern. It rotates once every Saturnian day of ten and a half hours. What we see is quite like active noise cancellation, where a wave of reverse phase (troughs and peaks in opposite places) is used to reduce sound level.
Another aspect of the Hexagon is that it has certain things in common with the former Antarctic ozone hole. Both are atmospheric regions sealed by a rotary jet stream, whose atmospheric composition differs markèdly from their surroundings. Over Antarctica, the jet stream prevents ozone from entering from outside and concentrates CFCs inside, then the winter conditions exacerbate it. On Saturn, large droplets cannot pass into the polar region, again due to the jet stream, and again winter conditions strengthen this effect. Also, the Antarctic ozone hole was worse than the situation in the Arctic, so both structures exist over only one pole.
The central vortex is about four dozen times the size of a typical hurricane eye on Earth. The colour of the Hexagon changes – it can be blue or red. Presumably the blue is due to the same effect which makes the cloudless sky blue here and is connected to the size of the aerosol droplets, which are smaller inside the shape. It spins counterclockwise, though quite slowly compared to the rotation of the planet as a whole, insofar as it even does rotate in one piece, but some of the vortices within it spin clockwise. To the human eye, the area would look like this:
The central hurricane, PIA14947, unlike the Hexagon, does have its counterpart at the south pole. It’s a little under two thousand kilometres in diameter, and takes only six hours to rotate, so unlike Earth, whose poles are stationary and polar regions rotate slowly on account of it being a solid object, Saturn’s poles rotate faster in terms of revolutions per minute than the rest of the planet, and the inner ring moves even faster, at the same speed as sound on Earth at sea level although it doesn’t break the sound barrier for that part of Saturn.
The south polar vortex is eight thousand kilometres across. Although I’ve heard that “conditions” mean there is no hexagon there, that doesn’t really explain it to me and I haven’t managed to find out why there’s this asymmetry. With Earth, the Arctic and Antarctic regions are very different due to the presence of an ocean at the North Pole and a continent at the South, but this doesn’t apply to Saturn. Nor does it have anything to do with spacecraft visiting it at particular seasons, as Cassini was there for quite some time and the Voyager probes flew by during different seasons compared to Cassini. The south pole is 60°C hotter than the equator, which has been likened to discovering Antarctica is hotter than the Sahara, and given that Saturn as a whole is so much colder at cloud level, the contrast is even more dramatic. Clouds around the area are thirty to six dozen kilometres higher than their environs. This is known as an “eyewall” and has only otherwise been seen in hurricanes on Earth.
I haven’t mentioned the interior of the planet in detail yet. The scale of the interior is somewhat different than Jupiter’s due to the fact that Saturn is smaller, has much weaker gravity and is less dense. Both planets’ magnetic fields are generated by liquid metallic hydrogen near the centre, but in Saturn’s case the amount is proportionately smaller at forty-six percent of its diameter as opposed to Jupiter’s seventy percent. The interior of Saturn has relatively little helium compared to Jupiter’s. The rocky core is about the size of our own planet, but also has three times our mass. These differences relate to those between the weathers of Jupiter and Saturn. The magnetic field is around a thousand times stronger than Earth’s. Like Jupiter, the “true” rotation of the planet can be found by monitoring its radio waves, which have a period of ten hours, thirty-nine minutes and twenty-four seconds, the peak in strength being defined as local noon. The centre of the magnetic field is 2 400 kilometres north of the centre of the planet itself, but it’s also the only planet whose magnetic field is almost perfectly aligned with the axis of rotation. A compass on Saturn would actually point to geographic north.
That, then, is it for Saturn. I was rather surprised how long this one took me although I did also write about twenty thousand words of fiction while also writing this. Even so, Saturn is quite an involved planet, mainly because the rings are so prominent and important but also because I didn’t want to neglect the planet itself, which is as interesting in its own right. And you might think that now I’ve got to Saturn, I’m half way through my coverage of the Solar System. Not a bit of it! Saturn has so many moons that this is still the first half of my “trip”, as do Uranus and Neptune, although Saturn’s moons are much better known, and although Jupiter has four large moons and Saturn just ones, some of its smaller moons are large enough to be thought of worlds in their own right and this skews the half way point way down the line.
Next time: Mimas, the Death Star moon.
A Box Of Chocolates 