In Our Time - Saturn

Episode Date: January 14, 2016

Melvyn Bragg and guests discuss the planet Saturn with its rings of ice and rock and over 60 moons. In 1610, Galileo used an early telescope to observe Saturn, one of the brightest points in the night sky, but could not make sense of what he saw: perhaps two large moons on either side. When he looked a few years later, those supposed moons had disappeared. It was another forty years before Dutch scientist Christiaan Huygens solved the mystery, realizing the moons were really a system of rings. Successive astronomers added more detail, with the greatest leaps forward in the last forty years. The Pioneer 11 spacecraft and two Voyager missions have flown by, sending back the first close-up images, and Cassini is still there, in orbit, confirming Saturn, with its rings and many moons, as one of the most intriguing and beautiful planets in our Solar System. WithCarolin Crawford Public Astronomer at the Institute of Astronomy and Fellow of Emmanuel College, University of CambridgeMichele Dougherty Professor of Space Physics at Imperial College LondonAndAndrew Coates Deputy Director in charge of the Solar System at the Mullard Space Science Laboratory at UCL.

Thank you for downloading this episode of In Our Time, for more details about In Our Time, and for our terms of use, please go to BBC.co.com.uk slash Radio 4. I hope you enjoy the program. Hello, in 1610, Galileo, using a rather primitive telescope, observed Saturn, one of the brightest points in the night sky. He couldn't make sense of what he saw. Perhaps two large moons on either side? When he looked a few years later, those supposed moons had disappeared. He was mystified. It was another 40 years before the Dutch scientist, Christian Higgins solved the mystery.

He realized that Galileo's moons were a system of rings. Successive astronomers added more detail with the greatest leaps forward made in the last 40 years. The Pioneer 11 spacecraft and two voyage emissions have flown by, sending back the first close-up images of Saturn, and Cassini is still out there in orbit, confirming Saturn with its rings and many moons as one of the most intriguing and beautiful planets in our solar system. With me to discuss Saturn are Carolyn Crawford,

Public astronomer at the Institute of Astronomy and Fellow of Emmanuel College University of Cambridge. Michel Doherty, Professor of Space Physics at Imperial College London. And Andrew Coates, Deputy Director in charge of the Solar System at the Mallard Space Science Laboratory at UCL. The latter two are working closely on the Cassini Space Mission, which is still out there. To Karen Claufford, first, what does Saturn look like to someone on Earth at the Powerful Telescope? Well, through a powerful telescope, the first thing you notice is that Saturn is surrounded by this enormous set of rings. This is what gave Saturn this curious, elongated shape when Galileo was looking at it through. It's a very small telescope.

And as you said, they present a different aspect to us through over a period of seven years. Sometimes you see them edge on and they just about disappear. But then they'll open up and the whole system will become brighter and you see them looping around the planet, sort of inclined to our view. And the planet itself is a lovely kind of orange colour. When you look through a really powerful telescope, you can see a sort of mix of whites and creams and sort of almost like warm butterscotch colours.

And again, you can see it's surrounded by a retinue of moons. It's got one big moon and there are lots of smaller ones as well. And can you make easy early conclusions from that first sighting? From the first sighting... From what you've said, what would Galileo, what would a person, what would they say first of all about this planet? Well, just with the inaded eye, it doesn't look like anything special apart from the orange colour, but with the very small telescope,

you would see there was something weird about it. It was kind of elongated, but you need the big telescope to make out the rings and to see perhaps they break up into different bands, that there may be gaps in the rings, that it's not just one big structure around the planet. and you might also be able to see some of the mix of colours on the surface. How many rings are there? How long is a piece of string?

Again, through a small telescope, you might think there are three main bands. We now know there's a whole system of rings, and in fact, each ring breaks up into millions of ringlets. So it is a very complicated structure. But that's the only thing you see, you need to fly past with a spacecraft to see that level detail from the earth. They just look like solid bands of rings. What lies, we've got these clouds that surround the planet. What lies beneath the clouds,

beyond the clouds? When you look at Saturn, you're not actually seeing the surface as such. You're just seeing the tops of the clouds in the atmosphere. We see down to about 100 kilometres. Down at the centre, there's probably a rocky core, well, there will be a rocky core about 10 Earth masses, but that's enveloped in this enormous atmosphere and it's an atmosphere of molecular, well it starts up being molecular, but a hydrogen atmosphere and the clouds we see are just in the top layers.

And then as you go down, the gas only for that first layer, it starts to get squashed and it gets more and more squeezed. And we recognise this atmosphere of Saturn as air or molecular hydrogen gaseous in the normal sense, only in the outer layers, and then it gets more and more squashed.

It becomes almost like a liquid. It gets really dense and incompressible. And then right down around the rocky core, you've perhaps got a very weird state of hydrogen, followed by a sort of soup of ice. So you're talking methane, ammonia, water ice surrounding that central core. So there's no sort of solid surfaces such that you're seeing. It's just seeing the tops of these clouds in the atmosphere.

Can you give the listeners an idea of the size of it compared with Earth would be the easiest way? Yeah, it is, well, it's 120,000 kilometres across, but that's just over nine times the width of Earth. And even that doesn't really do Saturn justice. If you looked at the volume, you would have to stuff over 760 Earths inside it to do it justice. And the other thing we should point out is that it is so far out from the sun.

Again, it's about nine times further out from the sun than the Earth is, that it's really cold there. It's about minus 150 degrees at the cloud tops And at that distance from the sun It takes 29 and a half years Earth years to go once around the sun So it is very cold It's a very different kind

Very different region of the solar system To where the Earth is Andra Coates, can you tell us when Saturn Do we know when Saturn was formed? Well, Saturn was formed along with the rest of the planets About 4.6 billion years ago So that's 1,000 million years ago Same time as us?

Same time as us, yeah. So everything was formed. wonder about the same time. So the formation of Saturn involved lots of things banging together in the outer solar system region, basically comets and things like that, planetesimals as they're called, the building blocks of planets. And those form this large core, which Caroline was talking about, about 10 times the Earth mass. And that starts pulling in material from the forming solar nebula. So this is all in the spinning disk of gas and dust, which is eventually to become our solar system. So Saturn is forming, it has the core, it has the material being brought in from

outside. And there are different theories of exactly what happens next, because it's not quite clear whether Saturn and Jupiter and the other outer planets formed in the place that they are at the moment. There are some indications that they actually formed a slightly different place and then moved over time and then came back again. But the basic formation progress process is taking about 10 million years. So this is a very small amount of time compared to the 4.6 billion years since this all happened.

But it seems to pull in a lot of what you could call the particle space debris almost, doesn't it? Yes, that's right. So space debris really, for a start, is exactly what's happening. And so bits of the forming material, the cold material in the outer solar system.

So we're beyond the region, the ice line or frost line, which happens at about the orbit of Jupiter. about five times the Earth's sun distance. That's where you can get hydrogen and water, well, water condensing. And so these are icy planetesimals, really, but they also carry some rock and material from supernova explosions before this. And so this is building up the core and then eventually the atmosphere of Saturn.

You end up with the hydrogen, helium atmosphere of Saturn, some other heavier stuff as well, but it's really a very primitive type. type of composition, much the early solar nebula. Sorry, is there a sense in which it's going on rapid? Is it settling down to be something else, or is it as it is and will stay like that as long as you can predict? Well, that's a good question, because, of course, Saturn is not only the Saturn itself,

but it's the rings and the moons, so all that is not happening at the same time. There are some indications from Cassini that... The spacecraft. The spacecraft, Cassini, that the rings actually formed perhaps at very similar time to the formation of Saturn itself. How did they form? Well, there are indications that, I mean, some people had thought, before Cassini,

people had thought it's a relatively transient phenomenon of maybe the last 100 million years, something like that, that the rings formed. But now there are new indications, based on the temperature of the material in some of the rings, that actually they may have been there since the formation of Saturn, and there's a constant reforming, regeneration, collisions forming in that region, which is sort of regenerating the rings all the time. Is the rings just a way to get the description out of the way, or does it signify something?

The word rings? I mean, they're rings, yes. I mean, they're running. But how broad are they? How big are they? Right, oh yes, the rings themselves. So it's something like three Saturn radii, 2.7 Saturn radii, the main rings that we can see in a telescope from Earth.

Which means what? it's about 180,000 kilometres or something like that. So this gives you the scale of the rings and this compared to the radius of the earth, which is 6,370 metres, it's very large, so it's extremely big. And they're not very broad, are they? That's right, they're very thin.

100 metres, so they ask the hoops going around this great planet. Yes, and in fact if you shrunk the rings down to be the size of the M25 or something like that, they would be about two millimeters thick, something like that. So that gives you an idea of how thin these rings are. But they're separate rings, and we have the rings, as Karen was saying, which were named in order of their formations. So there's A, B, C, D, E, F, and G rings.

It's a bit uninventive, considering all the rest of the stuff. That's right. So the most visible ones are A-B-N-C-B-C-B-C-E. Who is responsible for that? Yes, we're rather good at names, you guys. Yes, yeah, normally. Pitch, a lot of Greek, but they're still good at names, but A-B-C-C-D, come on. Yes, I know. you know, what can we do?

But we can't rename them, but that's a sort of simple way of thinking of them. But of course, we're much more imaginative with naming of the moons, and that is another topic. Michelle Docherty, can you take us back to Galileo and just tell listeners, why the moons were there, then they weren't there,

and what's going on? We're still on these rings, aren't we? What effect they're having on the eye? That's correct, yes. Well, when Galileo first looked through a very small telescope at Saturn, this was back in 1610, what he saw over a period of about 10 years was that Saturn seemed to change. It looked as if it had a moon on either side, and the next time he looked a couple of years later, it looked as if those moons had somehow merged with the planet.

He looked again in four or five years' time, and then it looked as if there was a, there was a, band around Saturn and so it was very clear that something was changing and it took I think it was about 45 years after Galileo first saw Saturn that Huygens looked through a slightly better telescope and what he realized was actually a large ring system around the planet and he thought it was a solid ring and it was another 20 years after that before the Cassini scientist before an Italian scientist called Cassini, after which the Cassini spacecraft is called, realized there was actually a gap between the rings.

And this is happening because of the fact that the seasons at Saturn change the orientation of Saturn. So Saturn tilts towards the sun and away from the sun, depending on whether it's summer or winter. And so that means that if it's tilting towards the sun and therefore towards the Earth, the rings are going to look different. And so sometimes we see them side on

and you can't see them at all. And sometimes they're oriented towards us and so you can see them in all their glory. Can we again talk about how the magnetic field works there? Because that isn't quite resolved, is it? No, it's not. Can you tell us why it isn't resolved?

What's the problem? Absolutely. Because we don't think the magnetic field we're measuring at the moment is actually coming from the deep interior. we think the measurements that we're taking with the Cassini spacecraft are actually measuring changes in the magnetic field due to the atmosphere. And so what we really need to do is get much closer than we've got.

And we're going to do that at the end of the mission. Why is the magnetic field not working the way magnetic field should work according to your current observations? We don't know. What is it about your current observations that say to you they are not working? Okay. The magnetic field of a planet, for it to be able to be generated in the deep interior, you need there to be a tilt between the rotation axis and the axis of the field.

And that's what we know from the Earth, and that's what we know from Jupiter as well. The field that we're measuring at Saturn so far is telling us that those two axes are lying on top of each other. and all the theories that we understand to date say that the field therefore can't be generated in the interior it must be dying, it must be decaying but we don't think that's true we think the field is being generated in the interior and the effects of it are being masked

by the atmosphere. So something is going on nearer the core of the planet about which yet you don't know but you hope to find out towards the end of Cassini which we will come to towards the end of this programme if it doesn't fruzzle out. That's correct. Colin,

the Huygens identified the moon Titan in 1655. Great excitement. Can you tell us more about it? Well, it's the largest moon around Saturn, so a very appropriate name. And again, just to give you the scale of these things, it's 2,400 kilometres across. That's bigger than the planet Mercury.

So it's big enough to be a planet in its own ride. But it's very exciting because when you look at it, it is quite blank. And that's because it's the only moon in our atmosphere. in our whole solar system, which is enveloped by a really rich atmosphere, a deep, complex atmosphere. And Cassini, the spacecraft, has done numerous flypasts of the planet, and it's also when it arrived in 2004, it dropped a probe, named after Hoygens, which parachuted down through the atmosphere and landed on the surface.

And it's been fascinating to discover more about what lies under that atmosphere, because it is a mainly nitrogen atmosphere. It's got a bit of methane in. But right at the top level of the air, you've got a sort of smog where the ultraviolet radiation from the sun and the electrons trapped in magnetic fields that Michelle were talking about,

they break down the molecules and they reform this kind of haze of complex hydrocarbons and that just envelopes the moon and you can't see what's on the surface. So when you do a fly-past with the spacecraft, you can bounce radar signals down, collect the echoes and see what lies underneath the surface. And the moon, it's, you know, a rocky body like most of the other moons underneath all this atmosphere. And it shows a wealth of features on the surface.

You've got the standard things like your craters, you've got hills, you've got planes, you even got sort of rippling sand dunes. Well, not quite sand, but dunes around the centre of the planet. But the thing that's really exciting is that it has... lakes and rivers and tributaries. And we can see these again from the radio signals the way they're echoed off the planet. You've got flat expanses of liquid. But here, this is not liquid water. This is methane and ethane. The temperature on the surface of Titan is minus 180 degrees. Any water is going to be completely frozen. But it's about what we call the triple point of

methane. And the triple points where you can have a substance, you know, either as ice or snow, as well as liquid, as well as vapour. And so on the surface of Titan, you have a methane cycle. So underneath this atmosphere, you'll have methane clouds, which form from the vapour, that will rain or snow onto the surface and collecting rivers that channel down to small lakes. And we can see this active weather cycle beneath the clouds. So you get this triple thing that we have here, which is cloud, liquid and then solid. Yeah, so on Earth we have the water cycle. So the equivalent on Titan is you have the methane cycle.

I think also, just to follow up on something that you said, Carolyn, it's one of the really interesting things about Titan is for the first four years that Cassini spacecraft was in orbit around Saturn, we didn't see any liquid on the surface at all. And so it was very clear that the seasonal cycle on Titan sometimes gives you liquid on the surface, and sometimes it doesn't. And so we saw that change over a very short period of time.

Andrew, do you want to come in on this question? I would ask you if not. Yeah, and I think actually with Hoygens, it happens to be 11 years ago today, actually that Hoygens landed on the surface of Titan. So, I mean, just to think how that time has flown really quickly. But, yes, the discoveries which it made were fantastic. But yes, the whole idea of, you know, a really alien world

with methane shaping the topography. but actually looking at the images from Huygens as it went into land, it looked a little bit like the coast of the south of England, you know, because the processes are very similar, but it's actually methane which is carving those structures. Another interesting thing about Titan is that sort of in the interior of Titan, we think there's a subsurface ocean. And not only do we think there's a subsurface ocean,

but it seems to be over ten times the amount of water which is here on Earth. So, you know, a really large amount of water, There are some little boulders and little rocks on the surface which seem to be made of water. Carolyn and I'll come back to Andrew. The really exciting thing about Titan from the scientist's point of view is that it's got this nitrogen-rich atmosphere. And it's a very good analogue for what perhaps the early Earth looked like. Before we have living things, creating, you know, changing the atmosphere, introducing oxygen. Obviously, it's been held in the deep freeze of space.

So the chemistry, the same chemistry hasn't happened, but you've got very, very, complex hydrocarbons on the surface. And it's a very good example of what the Earth might have looked like, four and a half billion years ago or so. Can I have a few numbers here? How far away is Saturn? How long did it take

to get there? Well, it's about 10 times the distance that the Earth is from the sun. It took Cassini from the launch in 2007. It took till, sorry, in 1997 it took till 2004 to get there, so seven years.

And that involved flybys of Venus twice, then the Earth, then Jupiter. These are gravity assist to get the spacecraft actually there. Still on numbers. Why are you still counting the moons? I suggested 65 because it's in the notes of one of you. But I think it's still counting, isn't it? Still counting, yes. I mean, 62, 65, whatever one thinks.

And actually, if you think about the number of individual particles, which are actually in orbit around Saturn, it's a huge number. There's a question of definition, what does one call a moon? What does one call a moon let? There are a moon lets actually within the rings of Saturn, within the A ring in particular, that outermost ring which can be seen. And those are sort of of order 100 metres, something like that large. And they cause very interesting structures in that ring, propeller structures.

So they look like propellers, you know, in terrestrial. aeroplanes or whatever and these were discovered by the Cassini mission because we were not able to see the detail in the rings that we were able to see before and so this gives these whole new population of moonlets which Cassini has discovered

well to your colleague on the Cassini mission Michelle what's it revealed about the moon what's the mission revealed about the moon Enceladus and why is it important to concentrate on that what we have seen with the Cassini spacecraft at Enceladus is it

has an atmosphere. It's a very strange atmosphere. It's focused at the South Pole. And what it consists of is a vast water vapor plume, which is emanating from cracks, which are called Tiger Stripes at the South Pole. And within this plume, not only do we have water vapor, but we've got organic material, we've got dust as well. And this is a real surprise because Enceladus is small. its diameter is 500 kilometers. And so we thought before we saw the observations with the Cassini spacecraft that it was long since dead. Its internal heat source had died away.

But it's very clear that's not the case. And so what we have in Celadus, we have an internal heat source. We now know we have a liquid water ocean under the surface, but it seems to mainly be focused at the South Pole. And we also have organic material as well. And so now everyone's really excited about Enceladus, because you need four things for life to form. You need liquid water.

You need a heat source. You need organic material. And we have those three things at Enceladus. The fourth thing you need is for the system to be stable over a relatively long period of time. I never know how long that period is. But you need it to be stable over time so that life can form. And that's what we're not sure about it, Enceladus. Given that, what...

Does anybody have any views of the potential, what we might call life there? Well, yes, I mean... What do you call life? I mean, is it slugs? No, really... Maybe we are slugs? I mean, of course, at the moment, Earth is the only place where we know that there is life anywhere in the universe. So to discover anywhere else would be fantastic.

I mean, life, it would be very simple types of life at somewhere like in Selvus. But there are other solar system targets as well, which are on the sort of prime list for life. And so Mars is one of those. 3.8 billion years ago, Mars was a very different place. We have the Exo Mars rover going there in a few years' time. We're leading an instrument on that to look at Mars to drill underneath the surface for the first time. There's also Europa, one of the moons of Jupiter, which seems to be another sort of prime place where we have the four ingredients that Michelle was talking about.

And also Enceldus. So Enceladus is now in this class of really potential places where we should be doing exploration and looking for science of life. And I'm sure it's going to be there. It's just a matter of finding it. Michelle? One of the things about Enceladus, though, is what we'd really like to do is go into orbit. Because to be able to understand a system or a moon or a planet properly, you need to spend as much time there as you can. What's as much time as you can?

You need to be there during at least one season. and so the amount of time it takes for Enceladus to orbit around Saturn, which is about 16 days. So you would at least want to be there for 16 days. But it's really difficult to get into orbit around Enceladus because its gravitational field is so small. So you need a huge amount of fuel to be able to get into orbit. And that's the real stopping point at the moment, I think,

is how are we going to be able to do that. Let's think, you want to say something. For me, yes, both Titan and Enceladus are really good places to go. back to. I mean at Titan, we have these really complex organic molecules which we talked about. So we discovered these in the very high atmosphere, the ionosphere of Titan. These

sort of coalesce to become larger and larger. We think they fall through the atmosphere to go down onto the surface. So that's another great place to look for potential signs of life as well. What about the other moon? Phoebe Phidavis, I'm in mid-Summer-night's stream here.

Well, Phoebe's is another example of the odd-ball moons you seem to get around Sassan. It's the outermost moon. And we think it's actually a fairly late-comer. We think it's an interloper to the whole system, and there are various clues that lead you in this direction. First of all, when you look at it, it's not sort of bright, white and shiny like so many of the other moons.

It's sort of dark, it's heavily crated, it's small, it's regular, it's only about 200 kilometres across. But the key thing about it is that, first of all, it orbits Saturn the wrong way around. You've got the Saturn's turning on its axis, and the rings are turning in the same direction, and all the other moons orbit Saturn above the rings, you know, above the equator of sand in the same direction. And then you've got Phoebe that's going the other way round, and it's not going in that flat level plane aligned with the rings and the other moons,

but it's following an orbit that's tilted relative to the others. Do we know why? Well, presumably it hasn't been incorporated into the system, settled down to the system, or we think all that sort of in that rotational sense is inherited from the formation of Saturn, the rings and the moons. They all form at the same time from the same little sort of chunk of solar system. It kind of you build up that, they share that rotation that was inherent in the original system. Phoebe comes along later from another direction.

It's captured into orbit around Saturn. And it hasn't had so long to settle down and kind of conform with the rest of them. It's also right on the edge of the system. It's like four times further out than the next nearest moon. And the other weird thing about it, it is enveloped within the last ring of Saturn that was discovered. I mean, it's relatively recent in 2011 because it's a very dark ring.

It's made of dark dust particles. And we think this little moon is actually generating that ring. It's right on the outskirts of the system. It's really exposed to meteorite impacts, micrometre impacts from the outer wider environment and as they pulverise the surface it trails in its wake

a kind of debris that builds up this ring if you like it's like pig pen from Charlie Brown it's just leaving this trail of dust in its wake and it's building this outermost ring Andrew Coates is there any reaction between the moons and the rings well certainly I mean this moon Phoebe which it produces all this stuff

that has left the trail of material which Carolyn was talking about and that actually ends up on the surface of another moon, Iappitus. So that is another moon in the outer region of Saturn's environment there. Saturn's almost a mini solar system in itself with all these moons and so on. And so Iappitus is, you know, it was a mystery for a long time as to why one of the surface of Iapetus was relatively dark, the other one relatively light. the sort of leading edge was dark, the trailing edge was light.

And it seems that it's this material from Phoebe, which has done that. But there are lots of other interactions between the moons and the rings as well. There's very tiny shepherd moons near the main rings of Saturn, the main rings of Saturn, which can kick material out of the rings of Saturn. That's what the shepherd's heard of things in. Well, yes, yes, it's sort of moving things around at least. and yeah, sometimes you lose things, right? But so you get this whole idea of material will be moved around.

And so in particular in the F-ring, there's a particular moon where you can see the repeated interactions, gravitational interactions between the moon and this ring. And in Saturn's rings, there's some of the activity, which is actually called stuff to go out of the plane and cast shadows onto the ring itself, which you can actually see during the time when the rings are edge on, as we were talking about before.

Michelle Doherty, why don't we know precisely at the length of a day on Saturn? It's a bit embarrassing, actually. We've been in orbit for quite a long time, and we don't know. And there are a couple of... How do you have been in orbit, just to remind us? We went into orbit 1st of July 2004, so almost 12 years now.

It's difficult at a gas planet, to be able to exactly know what its rotation rate is. It is a big gas tank really, isn't it? It is, yes. Yes, and it's not as if like you can on the earth, you can watch a part of the surface go round and measure it. And so what we usually do is we use proxies. At Jupiter we do this.

We use the repetition of radio signals. We use measurements of the magnetic field to actually work out what the rotation rate is. But at Saturn, it's much more complicated because of the fact that these radio signals, and the magnetic field, depending on whether you're looking at them or measuring them in the northern hemisphere or the southern hemisphere, depending on what season you're measuring them in, they seem to be different.

And so you can be tracking a radio signal in the northern hemisphere and in the southern hemisphere at the same time, and they're giving you different rotation rates. And so it's very clear what we're measuring at the moment is not the interior rotation rate, but it's something that's going on in the atmosphere. Carolyn, why is the visible surface of Saturn so different from Jupiter, which is another gas giant? It's mainly because it's colder. Saturn has got all the same activity in its atmosphere that we see in Jupiter.

You've got those clouds that are stretched by intense winds in the atmosphere to form kind of rings and bands around the planet and between those bands you get vortices start swelling up. So you've got hurricanes, you've got storms. However, Saturn's just that bit colder and you've got a higher level of smog which obscures the lower levels. Jupiter's just a bit warmer and it's more transparent. It doesn't have that.

So when you look at Saturn, and particularly when the Cassini spacecraft looks at Saturn, you don't see this level of detail, as you say in the visible. You need to use ultraviolet infrared to see down through that haze, down through that smog. And then you can track all the wonderful and weird hurricanes and storms that are going on beneath that haze. And Saturn's incredibly active. You've got, like Jupiter's giant red spot, Saturn has transient giant white spots where you've got fresh ammonia ice crystals

well up from underneath and break out on the cloud tops to form a big white spot. You've got hurricanes within the clouds. You've got winds ripping around the whole system about 500 meters per second, phenomenal speeds. And Saturn, again, we keep saying this, it has unique things on our solar system. It has two enormous hurricanes above each of the poles, one at the North Pole, one at the South Pole,

with, you know, well-developed eyewalls down above the pole. And the amazing thing about the northern hurricane, the northern storm, is it is hexagonal. It is a perfect hexagonal shape. It was discovered when Voyager flew past the 1980s, and it has remained stable all the time, up to all the time that Cassini's been watching it. And we think it's to do with a faster-moving jet stream, that's kind of zipping round faster than the surrounding atmosphere and you can build up eddies that retain the shape

but you wouldn't necessarily expect it to be stable and it is surprising if it is that easy to form that we haven't seen it in any other system so Saturn has its own peculiarities Andrew goes space exploration has actually depended entirely on the developments of technology in some ways it's almost technology led isn't it really

what sort of equipment do you have on Cassini Well, the Cassini spacecraft. I mean briefly, we don't want to know we're a bit in peace, but broadly speaking. Okay, broadly speaking. There's 12 different instruments, and they look at imaging in various different ways, so it's looking in the visible, the infrared and the ultraviolet. And these are variations on telescopes? Yes, I mean, to measure some of those wavelengths,

you have to go above the Earth's atmosphere because the Earth's atmosphere actually absorbs those. So there's the imaging experiment like that. There's the in-situ measurements like the magnetometer, the plasma instrument which we're involved in and other instruments to look at the magnetic field and the plasma environment, the particle environment of Saturn.

There's also instruments to look at dust. So altogether 12 instruments on the Cassini spacecraft and then there were six on the Hoygens probe as well. So each of these looking at their particular speciality in terms of trying to put the picture together of what it's really like at Saturn. Michelle, you built one of these instruments on the Cassini about 20 years ago,

what information does that bring you and what more information do you want a better version of it to bring you? The instrument that I'm responsible for, I didn't build it, but I'm now responsible for it, is the magnetometer. So what it does is it measures the magnetic field in the vicinity of the spacecraft.

And it measures the three components of the field and the magnitude as well. And what I find so interesting is if we built it now, it would be much smaller, it would use a lot less power and it would be much faster and so you know when you think about it

when you're involved in missions to the outer planets you start designing and building instruments 20, 25 years before you're going to be taking the data so you need a lot of patience to be involved I'm actually building a similar instrument for the Jupiter mission called Juice and the instrument is much smaller and it's going to take exactly the same type of data

And so you're right, the technology changes over time, but you're still focusing on taking the data that you need. Cassini comes to an end quite soon, and you have a most dramatic ending in mind for it, this kind of this kamikaze plunge to the centre, which is going to take just a few minutes, and somehow you've got to catch it. What are the problems, and what's the point?

Well, before we dive into the atmosphere and burn up, we're going to have 22 very close flybys to Saturn. We're going to be inside of the rings. We're going to be about 3,000 kilometers from the surface, from the atmosphere. And that's going to allow us to measure the internal planetary field, which we don't think we've been able to measure before, and the gravity field as well. And then right at the end of the mission,

we're hoping we're going to be able to take data as we dive into the atmosphere and burn up. So we're going to point the high-gain antenna, which is the way we get data back from the spacecraft at the Earth, and we're going to be taking data. as we go in. You don't look to, you don't look 100% confident about this, Michelle.

I'm pleased to say. I mean, I'm pleased to say it makes it exciting. When this thing goes, you don't get messages back forth. It's only going to take half an hour before it burns out, as I understand. That's correct. But you get messages every 40 minutes,

so have we got a problem? Well, no, it takes us 40 minutes to get the data back. And so hopefully the spacecraft would have sent the final data back. It would have died, and we will then receive the data 14 minutes off to that. How far do you think it gets before?

Is it going to hit the rocks, the rocky centre? No, it would have burnt up by then. Really? No, I would expect it will burn up somewhere in the top part of the atmosphere. Yeah, so probably about the first 100 kilometres or something like that. I mean, there was a similar end of the mission to the Galileo mission at Jupiter, and that did a similar type of thing. So it's a natural end of the mission, but this unique data at the end is very important.

And it's a return to the rings, because very, very, early on in the Cassini mission, when we first went into orbit, we flew just over the rings. One of the things we discovered with our instrument was the formation of the ring atmosphere. The atmosphere has a ring, an atmosphere itself involving oxygen. So that was one of the early interesting discoveries. But a return to the rings, going inside the rings, looking at the radiation belt, which we think is there as well. And then this last plunge, which will help to look at the interior structure, is fantastic. Sorry, but there is great number of moons.

as well Caroline and we've said at least 60 or Janus and Epimetheus are they important? What are you learning for them? It's a rotten question. Is each of the moon teaching you something different and important? Well, important is the wrong way. It's just really intriguing. You've got a couple of moons

Epimetheus and Janus, they're tiny moons, about 110 kilometres across. They're probably formed from the same original body that broke apart. And they almost share an orbit and their orbit separated by any 50 kilometres and that's much you know that's less than the diameter of the moons and you would think that after all these billions of years of in being an orbit around Saturn they would have collided but what they do is the well the the moon on the inside is moving slightly faster than them on the outside and it will lap it every four years and instead of just colliding when they draw close they do the sort of neat little sort of

pirouette where they swap gravitational energy. And the faster moving moon on the inside gets slowed down by the other and moves out to the outer radius and the other moon gets spared up and moves to the inner radius. So they just swap orbits and then they'll drift apart. You're like a couple of horses on the rice truck. Exactly. And they'll do this. So how does that happen?

Well, we wish we knew. This is the own, you know, this is why Saturn is exciting, especially for numerical dynamists, trying to work at how a large number of gravitational bodies. interact with each other. This is the only pair of co-orbiting moons we know about co-orbiting moons sharing an orbit that do this. It'll sort of little share.

There's another thing which is unique about the Saturn system because it has associated with some of the larger moons. There are moons at what's called the Lagrange point, so it's in the same orbit as that moon, but just a little bit further round in that orbit at a stable point. And this is

the only moon which has that only planet which has that type of moon with these Lagrange point moons as well. So it's a really intriguing system and full of detail and we're just beginning to understand it. Are we talking, Michelle, are we talking about, with this mission, this Cassini mission, having found out an enormous amount that is to be digested

and put together on her surface, are we talking about, oh, we've just touched the surface and there's masses more to find out? I mean, what do you feel? Yes. What Cassini has done for us is it has shown us some of the highlights of the Saturn system. It's allowed us to make discoveries that we really weren't expecting to make. I mean, if I can focus on Enceladus, what the Cassini spacecraft has done is it shown us that it's active.

It has liquid water under the surface and potentially an environment where habitability could, form. And so what we need to do is we'd like to go back and understand it better. And so it's almost, it's like a stepping stone onto a better understanding. What do we learn about this planet from the study of Saturn, Carolyn? Well, it's just so different from Earth that it just makes a tremendous comparison about how different planets can be. And when you scale things up, how it can deviate from the Earth. The Earth is not the norm. And certainly when you look beyond the earth to around other stars, many planets around other stars are giant planets,

not necessarily with rings and moons like Saturn, but resembling these gas giants. So perhaps we can understand them a bit better by understanding our gas giants closer to us. But it shows us, you know, what the weather's like on a planet, which is all atmosphere without any mountains or seas or anything to affect it, the storms within the atmosphere. It shows us what it would be like if you have this whole retinue of moons.

It just gives us a completely different object to study, completely different environment. There's almost a sort of a tick in listeners, including myself, so I'm listening to you, that says, well, do we learn anything that is useful for us here now? Well, I mean, we learn, for example, we can test terrestrial meteorology using the atmosphere and dynamics of Saturn's atmosphere. But I think there's another thing which is really important. Titan is sitting there. It's got all these very complicated organic compounds.

It's got an atmosphere which is very similar to the Earth's early atmosphere. In four billion years, when the sun explodes, it could be another Earth waiting to happen. Well, thank you very much. Thank you very much. Andrew Coates, Michelle Docherty, Carolyn Crawford. Next week we'll be talking about the great Thomas Payne and his revolutionary pamphlet, common sense. I wonder why we haven't done it before, but we haven't. So there's a treat in store. Thanks for listening. And the In Our Time podcast gets some extra time now

with a few minutes of bonus material from Melvin and his guests. Well, the inevitable question, what did we miss out that was important that would have made with that? Caroline, I'm going to step out now. You've got this bit of the programme completely. We didn't point in fact there are not of normal moons around Saturn. No. Out of those 60 moons, I mean,

Most of them are tiny. You've only kind of got 13 or so that are big enough to be around. But a lot of those like Ria and Dionne and Tethys they're just your bog standard rocky, icy moon full of craters. So there are ordinary, well, nothing's ordinary around Saturn, but there are very

standard moons around Saturn as well. And four of those discovered by Cassini himself. So, you know, it's true. It really sort of takes us back to those early times. And when we were talking about the rings and how the moons and the rings interreg, what we didn't say was the fact that Enceladus actually generates the

E-ring. Yes, the E-ring, yeah. These plumes that spray out the sort of microscopic ice particles. Some of them fall back onto the surface of Enceladus, but a lot of them disperse, and they form this sort of very wide, fluffy earring. That's correct, yeah. And every once in a while I get asked

the question, how long can Enceladus continue to form the earring? And I don't know the answer to that. You know, how much longer will it be able to produce this water-vapor plume? Well, yeah, I mean, if you calculate it, the amount of, you know, the amount of time which it could potentially be there doing that is the age of the solar system. I mean, based on the amount of material inside it.

But presumably the e-rings quite easily, you know, pushed away because it is so light and fluffy. Do you not need to sort of be continually replenishing it? I mean, is it quite a transient feature? Yes, I mean, not all the rings are being replenished or more time. And so then the earring absolutely no exception. Of course, it got Encelda's the smoking gun which is causing all this stuff to be pushed into the e-ring and so that's happening.

So material is coming out both neutral, a lot of neutrals coming out, but also charged dust grains coming out as well, which we were able to see with our instrument. And those are amazing because they move, they deflect in the magnetic field of Saturn. So it's like a huge mass spectrometer. It's really nice, you know, just like a school experiment.

I think the final thing on Enchilla is, is we didn't talk about how the activity changes. You have these cracks on the surface that are opening and closing. And so the amount of material that you're seeing changes depending on Enceladus's orbit around Saturn. When it's closer to Saturn, so the tidal forces are stronger, there's more material. The geese is in Iceland. Yes, they are.

But there are really neat things that Cassini, the spacecrafts, have been able to actually fly through the plumes and sample. It was really quite cute. you know, we had a close flyby, 25 kilometre flyby, and my instrument is on a long boom that sticks off from the side of the spacecraft. And the problem was the mission controller said they're never going to do that close to flyby again because they almost felt that the spacecraft was about to tumble, because the atmosphere of the plume caused it to get a little bit wobbly on its motion,

and so they're never going to do that again. And I suppose with Cassini, actually today, I mean, even, you know, the mission is going on. And, of course, there's a pass through the ring plane again today. That's right. And there's a, you know, there's a possibility of looking at some of these rings, which are affecting the, or some of these moons which are affecting the rings. Close flybys of that picture has been taken and so on just as we speak.

And of course, you know, as we said, the end of the mission is going to be spectacular. But there's still some flybys to do before that. And I think what we didn't touch on as far as the end of the mission is concerned. We talked about the really close flybys right at the end. But prior to that, there's a six-month. period where we're going to be going very close to the edge, outer edge of the rings. And so we're going to be flying right over the rings and taking images that we've only ever dreamed of being able to take.

Yeah. So will we sort of be going past like the F ring, that sort of very thin braided ring? Or is it just more a general view over the rings? I think it's inside of the F ring. It's between the F and the main rings. Oh, fabulous. And actually the going through the ring system today is in that region. So, no, sorry, it's just outside the F ring.

But afterwards it's going to be inside the F ring, I think, yes. And then sort of doing those types of orbits and then going inside of the rings as well. So those last orbits actually inside the rings. Just amazing. I think it's about... I'm sorry about this. I'm sorry about this all you're listening to this. I think we're going to see...

Back to Earth, okay? Well, here's the producer Simon Tillotson with BBC tea or BBC coffee in small quantities. There are many more science and discussion programs from Radio 4 to download for free. Find these on the website at BBC.co.com.uk slash radio 4.