There’s so much we know about Saturn’s beautiful rings, and yet, there’s so much we don’t know. Morgan Rehnberg, a PhD student at the University of Colorado, Boulder and works with the Cassini mission. Morgan joins Fraser to talk about Saturn’s amazing rings, and how they might have formed.
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Fraser: Astronomy Cast episode 344: Saturn’s Rings.
Hey everyone, Fraser here. So this is another episode of Astronomy Cast that we recorded during the 2014 Hangout-a-thon. And this episode was with Morgan Rehnberg, who is a PhD student at the University of Colorado Boulder, and works on the Cassini mission. And Morgan answered all my questions about Saturn’s rings. Now remember, if you haven’t already, take a second and donate to the Hangout-a-thon. Go to Cosmoquest.org/hangoutathon. All right, enjoy the interview.
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Fraser: All right, so why don’t you tell us who you are and what you do, Morgan.
Morgan: All right. I am a grad student a the University of Colorado Boulder, where I am attached to the Cassini Project, the mission to Saturn, where I study the rings of Saturn, using observations from the Cassini spacecraft. In particular, I look for irregularities within the rings, so clumps and waves and things that you wouldn’t necessarily expect to be there just from the basic physics that we know. And hopefully in the future, we’ll be able to connect these things to the formation of things like moons, and then extrapolate that out to talk about how things like planets form; how did the Earth form? Probably formed in a disk that looked somewhat like the rings of Saturn do today.
Fraser: So take us back sort of the history of Saturn and Saturn’s rings. It wasn’t until Galileo first turned his telescope onto Saturn that we had any inkling that there was at kind of funny about Saturn. So what did Galileo see when he first turned his rudimentary telescope on the rings/
Morgan: That’s right. Even though the rings make – if they’re tilted the right way, they make Saturn look about three times bigger than it would look otherwise. They’re so far away, that people in Antiquity couldn’t tell the difference. Saturn of course is one of the classical planets. It’s been known for virtually all of human history. But we had no reason to think that it was any different than Jupiter or Mars or Venus, because they’re so far away that they look just like pinpricks of light; they look just like stars. And so it was Galileo who sort of got the first peek at the true nature of Saturn. Of course, Galileo got the first peek at the true nature of practically everything.
Fraser: Right, of the moon and Venus.
Morgan: Yeah, the all time kind of low hanging fruit grabber, there. But when he looked at Saturn, he saw immediately that it didn’t look like Jupiter. That was the most important thing. But his telescope was weak. And he wasn’t able to resolve the rings as an actual disk. What he saw – and the rings were pretty much tipped towards the Earth, kind of like they are now. When Galileo saw them, he saw it as the circle of Saturn with two disks next to it. So he called them basically companion bodies to Saturn. But he noticed that unlike the companion bodies that he discovered at Jupiter – of course we call those the Galilean satellites today – that these companion bodies to Saturn didn’t move.
They didn’t go around and move back and forth with respect to the planet. They were always there. And that’s basically all he could discern. And you can see that in his 1612 pictures – those drawings – that show the moons of Jupiter. You can also see this kind of lumpy shape that he found Saturn to be.
Fraser: Right. He did his best – he thought they were maybe ears, or as you say companion bodies. And so it really took like Christiaan Huygens, with a better instrument, to get a better sense of what was going on there.
Morgan: Right. It was about 50 years later that Christiaan Huygens made the first observations with a much more powerful telescope. Still in the era where he built his own telescope, but much more powerful than what Galileo had. And he was the first to resolve the rings as an actual disk. And he saw this disk going around Saturn. And today, of course, when we dropped the probe on Titan at the beginning of the Cassini Mission back in 2005, we named that probe Huygens after Christiaan.
Fraser: Right. And then Cassini sort of took the observations to the next level, right?
Morgan: Right. So Giovanni Cassini was the first person to see the rings more or less as we see them today. He saw the rings weren’t just a uniform disk, but there were variations in them. In particular, he saw what we now call the Cassini Division for the first time, which was this dark region between the main rings of Saturn. And he assumed that it was empty. And today, we know it’s not actually empty; it’s just a lot less full. But he was the first to really see the rings in the same way you might today if you pulled out a backyard telescope.
Fraser: The observations of Saturn didn’t really get – we had bigger and bigger telescopes, we had better views of the planet. But it really wasn’t until we started to send some spacecraft out to this plane that we really got the chance to see it up close and personal, and our view of the planet really changed. So what was the first time that we saw Saturn up close?
Morgan: Right. So before we get there, I will throw in one more milestone, which happened in the mid-1800s. We’d finally started building up math and physics to support astronomical observations. And it was at this point that we realized that the ring of Saturn couldn’t be a hard, rigid disk. Because if it was a hard, rigid disk, it would become unstable like a top and would eventually either crash into Saturn or break apart. So up until the mid-1800s, it basically could be like a top going around Saturn. But at that point we realized that this ring must be made of smaller particles.
And that was sort of the first step to understanding the rings as we understand them today. But you’re right; we didn’t really learn that much more about the rings until we started getting in there with spacecraft. We saw variations here or there, a gap here or there, but we didn’t see world changing information about the rings until we started arriving. And the first spacecraft that really made good observations was Pioneer 11. And this was in the late 1970s . And Pioneer 11 got up close and personal as it flew by Saturn. And it made observations of the rings, the moon, and the planet itself.
And for the first time, we saw new rings. So Huygens and Cassini saw – and what you might see if you look out with your own telescope – saw what astronomers today call the A and the B rings. And we’ll talk a little bit more about what those mean in a bit. But once we got there with spacecraft, we realized that there were a lot more rings that we couldn’t see otherwise. And one of the ones that was discovered and is of great interest today is what we now call the F ring.
And this is a very narrow ring outside of the main rings that isn’t really visible from observations from the earth even from today. But by getting there close with the spacecraft, we were able to see it and today it’s one of the most interesting places in the rings, and one of the most active areas in the whole solar system.
Fraser: Yeah, so we had Pioneer and then what came after Pioneer 11?
Morgan: So then we had Voyager. And we had Voyager 1 and 2 in the early 1980s; ’81 and ’82. And I can never remember which one launched first, which one arrived at the planets first. It was within weeks of each other. We had the two Voyager flybys of Saturn. And these made the bulk of the measurements then that scientists would spend the next 20, 25 years analyzing.
Fraser: Yeah, all the pictures that we’ve seen of Saturn until just recent memory, they’re all the Voyager pictures.
Fraser: All the pictures that we’ve seen, everyone is familiar with these pictures. They’re all the pictures that were captured by those Voyager spacecraft.
Morgan: Right. And Voyager made additional observations outside of pictures that helped characterize the rings as well, and the same thing with the moons and the same thing with the planet. And that was it. Once Voyagers left, there wasn’t a lot more we could do. And we didn’t really get any new data until the launch of the Hubble space telescope. And we often think of Hubble as looking at galaxies, and the Hubble ultra deep field and things like that. But Hubble’s had an important role in the solar system, as well. And one of the things it’s made is observations of Saturn and Saturn’s rings.
And that allowed us to sort of extend the timeline of observations that we have. We had 1978 with Pioneer 11, we had the early ‘80s with Voyagers 1 and 2. But now we’re able to extend out into the ‘90s. And this showed us that the rings were changing. The rings – we might have thought they were rigid back in the 1600s, and in the 1800s you might have felt they were static. And after Voyager and after Hubble, we knew that the rings were an exceptionally dynamic place. And this kind of built up to the need to send another mission to Saturn.
We’d sent Galileo – the Galileo mission – to Jupiter, and that arrived in the early 1990s. But we hadn’t had any sort of follow up observations of Saturn. And so in the early ‘80s, actually, they started putting together the Cassini Mission. That was led by NASA. And then eventually ESA, the European Space Agency, added on the Huygens Lander. And the Huygens Lander was designed directly to probe Titan. And this mission was put together and eventually launched in the late 1990s. And then we waited. It’s a long time to get places in the outer solar system. If the earth is one astronomical unit away from the sun, Jupiter’s five, and Saturn is ten. So it takes –
Fraser: They launched Cassini – I think I was – I hadn’t even started Universe Today. I started Universe Today in 1999 and Cassini had been launched at that point.
Morgan: I think it was ’97 that it was launched.
Fraser: Yeah. And then we reported on it going past Jupiter, and we reported on it spending a lot of time. And it – what – arrived 2003?
Morgan: ’04. July of 2004.
Fraser: Yeah. 2004 to Saturn. Cassini changed everything.
Morgan: Everything. Absolutely everything. There’s really no aspect of Saturn – the Saturn system – that wasn’t completely revolutionized by Cassini. This is the story of planetary science. Is we have a mission – Voyager, for example. Voyager is the default mission for practically everywhere in the solar system. And from the observations we make, we construct theories and hypotheses about how various things work in the system.
And then we go back again and we find out how terribly wrong we were. And I don’t mean that in a bad way. Because we’re not wrong in as if like we completely misinterpreted things. But the picture is always more subtle and always more intricate than what we previously imagined. And 20 extra years, basically, of technological advancement meant that Cassini was able to reveal things that we could never, ever see before and just fascinated us.
Fraser: Yeah. It took great observations of the planet and great observations of the moons, which we’re not going to go into in detail for this episode. But just the observations that it made of the rings itself. And some of the interactions between the moons and the rings and how these shepherd moons are causing these gravity waves moving through the rings, the differentiation of it, the source of some of the particles of some of the rings; a lot of stuff was all brand new.
Morgan: It’s hard to talk about the rings without talking about the moons, but we’re gonna try to stay away from that. I will highlight one episode that Cassini changed everything for. And that was when Voyagers got to Saturn, they discovered this ring, the E ring. It’s the outermost of the really important rings of Saturn. It’s really fluffy. And people sat down with pads of paper and pencils and figured out that because the ring was so fluffy – if it was in the room – if my office here was sitting in the middle of the E ring, I could be breathing it in and out and not even know it. And they realized that it’s so tenuous that it ought to just blow away, and that the ring shouldn’t be there.
It shouldn’t last more than a few years; maybe a few decades. But there it was. So something must be replenishing it, but they couldn’t figure out what that was. And then Cassini arrives and it spots these geysers coming off of the south pole of Enceladus. And this changes everything. Now we have a moon active in the solar system sustaining an entire planetary ring. And of course today, Enceladus is one of the most exciting and intense fields of research in planetary science.
Fraser: Let’s go back and sort of bring all of this research and knowledge that people – that scientists have gathered over all this time, and sort of like let’s just talk about the physical characteristics of the rings. First, like how big are they?
Morgan: So in size, they’re about equal in size – a little bit bigger – than the radius of Saturn. So the radius of Saturn is about 60,000 kilometers. And the rings extend from about 67, 000 kilometers from the center – so about 7,000 kilometers from the cloud top of Saturn – out to about 140,000 kilometers. That’s the edge of the main ring. So that’s where the F ring ends. And then you go out about the same distance again to get out to a ring like the E ring. In terms of mass – so we can talk about size; we can talk about mass. Size is easy to measure. We basically just look at it and make a measurement.
Mass is a lot more challenging to measure? And as we’ll talk about, it’s one of the big open questions that we still haven’t solved. But we think that the rings are about as massive as one of the medium sized moons of Saturn. And the one we often draw comparisons to is Mimas. Mimas is a few hundred kilometers across. And if you were to break Mimas up and distribute it about the planet, you’d have about the mass of the main rings. The vast majority of the mass is in the A ring.
Fraser: It just looks like there’s so much there. It just looks like there’s a mountain of ice and dust surrounding the planet. But really it’s just [inaudible]
Morgan: It’s really not. And that’s because the third dimension we can measure is the height. And this is the most shocking aspect of the rings. Is these rings are more than 100,000 kilometers across, yet they’re only about 10 meters high.
Fraser: And it’s funny. I did an article at one point and I put in accidently 10 kilometers: and they’re only 10 kilometers thick. And someone was like: uh, Fraser no, it’s 10 meters. So hundreds of thousands of kilometers across; 10 meters thick. You literally could – your house, from the floor to the top of your house; that’s the thickness of Saturn’s rings.
Morgan: Yes. Saturn’s rings are the flattest thing that we’ve observed in the universe. If you go out there and you divide 10 by 100,000 times 1,000 for kilometers to meters, you get a very, very, very small number. And the rings are almost impossibly flat. And it’s just remarkable when you look at them. They literally disappear when they face the earth.
Fraser: Did you ever get that Our Universe book, and there were these space aliens that were skating around on Saturn’s rings? Did you ever see that?
Morgan: No, I don’t think I ever saw that.
Fraser: No? That was like the most influential book for me, was this book called Our Universe. It was published by National Geographic. And oh, man. It had some crazy aliens inside of it, like these Martian creatures that had these big ears that would hide in the middle of the night, and then it would open them up and get some – during the day. And then it also had these creatures that would skate around – I think it was on Saturn’s rings, or maybe it was on Titan’s icy seas or something. But yeah.
Okay, so 10 meters thick, hundreds of kilometers – hundreds of thousands of kilometers across. But they’re broken up into these separate rings. And this is one of the things that Cassini saw. So what’s going on here, and how many of them are there?
Morgan: Right. So this is how the story of the rings cannot be extracted from the story of the moons. Because the gravitational influence of Saturn’s moons – and there are a lot of moons; more than 60, in fact. The gravitational influence of them affects the rings. So the rings at one point – way, way back – may have been one big disk. But then at certain locations, particles can enter what we call resonant orbits with the moon. And this is basically – you could find a location where a ring particle goes around two times for exactly every one time a moon does.
And that means they line up every two times the ring particle goes around. And when they line up, it feels a little gravitational tug. And this kind of pulls them outwards a little bit. It clears out this gap at that distance where you have this effect of going around twice for every one time the moon goes around. And it’s that effect that created this Cassini division that separated the main rings.
Fraser: So there is a moon in the Cassini division?
Morgan: No. There is a moon outside of the rings; in fact it is Mimas. And that moon is a certain distance away from Saturn. And because it’s a certain distance away, it has a certain velocity. And if you go inwards, there’s gonna be ring particles at a certain distance that also have a certain velocity. And the closer you get, the faster you go. And so a certain distance in, you’re going to be going fast enough to go around Saturn exactly twice for every one trip that Mimas makes. And when you do this, you line up very frequently every two trips.
And this allows a regular gravitational tug to kind of move you around. And so it tends to scoot particles of the ring from that location outwards a little bit, and they pile up at the edge. And what you’re left with behind is this empty area. And this empty area is what we see as the Cassini division.
Fraser: Right. And so we kind of always imagined when you see this gap in the rings, you imagine that some moon is pushing through that area and clearing out this path. But the reality is, as you say, it’s this resonance. That moon is outside of the rings. But because of the way the gravity and the resonance is working, it’s pulling that portion of the ring which is at the right balance point out a little bit and so you get this gap. It’s really amazing.
Morgan: There are gaps farther out in the rings that do have moons in them. So two that come to mind are the Encke gap and the Keeler gap. And these have tiny little moons in them: Pan and Daphnis. And Daphnis is about 10 kilometers across, or maybe the size of your town. And Pan’s about 30 kilometers across. And they do do exactly what you just described, which is sweep through the rings and push material out of the way. And they clear these very narrow, very fine gaps.
Fraser: And they’re some of the most amazing pictures that have ever been taken by Cassini, in my opinion, are the ones of these moons with these gravitational interactions with the rings, where you can see this trail of material that’s threading down towards the moon as the moon is sort of zipping around the ring.
Morgan: You can see these sinusoidal wakes that look a lot like the boat wakes you see if you ever go water skiing or something like that. And it’s not physical collisions that are causing those; it’s gravitational interactions. And that’s what’s so amazing about the rings, is they are really a pristine laboratory for studying gravitational interactions on a whole bunch of different scales.
Fraser: Yeah, just absolutely phenomenal. Okay, so how many rings are there, then?
Morgan: So it kind of depends on how you count. But there are, I would say, six major rings that astronomers talk about. If you were to pull out your home telescope and look at Saturn, you would see what we call the A ring and the B ring. And the A ring is the outer of those two rings. Then you have that dark Cassini division. And then closer to Saturn, you have the B ring. And those contain the vast majority of the mass of the ring system. If you move in closer to Saturn, you get the C ring, and then the D ring. Those are progressively less dense; more diffuse.
In fact, particles from the D ring are very close to the clouds of Saturn and are falling directly onto Saturn. And so you can kind of catch this ice ring; it’s kind of hailing onto the surface of Saturn from the innermost D ring.
Fraser: Right, okay. So outermost ring, A ring; Cassini division and then B ring; and then C ring and the D ring is getting you closest to Saturn. But then the categorization gets a little weird again.
Morgan: Then it goes crazy. Because they were basically labeled in their order of discovery, not in their order of sanity. So right outside the edge of the A ring is the F ring. And we’ll talk more about the F ring, I think, because that’s the coolest ring. And then outside of the F ring we have this very tenuous ring called the G ring. And for some reason, we don’t really care about the G ring. I’m sure there’s someone out there studying it, but it’s not a particularly heavily studied ring. And then outside of the G ring is the E ring. And this is the ring that’s created by the little moon Enceladus, and it’s very different from the other rings.
The other rings are made up of particles that could be the size of boulders all the way down to the size of snowflakes. The E ring is made up of microscopic ice particles that are just smaller than grains of dust. And that’s because of the way the volcanic activity on Enceladus is spraying them out in space.
Fraser: But doesn’t it have even more sort of tenuous particles that go out a lot further?
Morgan: Yeah. One thing Cassini has discovered is this ring-like structure that some people call the Phoebe ring, that’s out close to the orbit of the moon Phoebe. And like I said, depending on how you categorize things as rings, there’s also things we call arcs, which are kind of like rings but they don’t go the whole way around. And there’s particles all over the place. And a lot of those, especially as you start getting out, come from Enceladus.
And Enceladus, just like in the Jupiter system with the moon Io, Io’s volcanoes spread ash and dust all over Jupiter, Enceladus is spreading ice grains all over the outside of the Saturn system. And at that point, it kind of just becomes semantics in terms of what you call a ring and what you don’t.
Fraser: Okay. And here I’m gonna hit you with the big question, the one that everybody wants to know. Which is where did Saturn’s rings come from?
Morgan: Ah, yes. This is a fascinating question. And it’s tied in very closely to another question that I’m sure you’re going to ask, so let’s kill two birds with one stone. And that’s how old are the rings. Because this highlights another one of those advances we get by sending another mission to a planet. When Voyager got to Saturn and made observations of the rings, it was concluded that the rings couldn’t be that old; only a few hundred million years at most. And that’s because of how clean the rings look.
There’s dust and dirt basically floating through space. And over billions of years, the argument went, the rings should basically act like a giant Swiffer and just stick to that dust and get dirty. And so therefore, the rings couldn’t be more than a few hundred million years old. The problem was that every time someone tried to come up with a formation model – and we’ll talk about what those might be – those processes took at least a billion years to form the rings. So we have this ring that would take a billion years to form, but at most could last for only a few hundred million.
That just doesn’t add up. And so it actually wasn’t until last year that Cassini was able to make observations of that space dust and basically show that there’s a lot less of it near Saturn than we thought there was. And this is the effect of Jupiter. So Jupiter has this enormous magnetic field that stretches hundreds of times farther out from Jupiter than the size of the planet. And that kind of acts as this dust collector. You can kind of think of it as a broom and a dustpan, sweeping up dust, moving outwards from the sun and collecting it in the Jupiter system, while leaving the dust environment at Saturn a lot cleaner.
And so the rings didn’t seem dirty because they weren’t dirty. The buildup rate of dust was far less than we thought. And that helped us kind of align our idea of how old the rings looked with how old we thought they should be. And we now think that they’re billions of years old.
Fraser: Like possibly even formed at the – early on in the solar system?
Morgan: Yes. Early on but probably didn’t form with Saturn. So it wasn’t as if – when we form planets, we have this big, swirling disk of gas and dust we call the protoplanetary disk. And from that, planets like Saturn and Jupiter and Earth formed. And then some moons, like Europa or Ganymede at Jupiter, they formed in that same disk of gas and dust. That’s probably how Titan formed around Saturn. But it’s probably not how the rings formed. Because at some point, the sun turns on and the solar wind blasts out. And it pushes all that gas and all that dust out of the system.
And had the rings been there at that time, they probably would have been pushed out, too. So it didn’t happen then, but it probably happened right about that time. Because we still need that gas and the dust for how we think they actually formed. Which is the breakup of a moon. And these theories have actually been around since the late 1800s. But it wasn’t until we got good observations from Cassini about the mass and the composition and things like that, that we started to figure out how this might work.
Fraser: Yeah, and that makes sense because like Enceladus, for example, is almost pure waterized. There’s a lot of ice orbiting around Saturn. So it makes sense that one of those moons could have gotten broken up and went into orbit around the planet.
Morgan: Right. So the theory that I think that I would say – and this is by no means settled. But the theory that I would say probably has the most credence right now was put forth by Robin Canup, who works at the Southwest Research Institute. And she was actually, incidentally, the first person to simulate the formation of the earth’s moon from a giant impact. So she likes breaking things apart. And the idea that she’s put forth is back at the start of the solar system, Saturn had more than one giant moon, just like Jupiter does today. Jupiter has Europa and Callisto and Ganymede and Io; four.
So it’s always been kind of unusual that Saturn, which is pretty similar in size and composition to Jupiter, only had one in Titan. And so the argument goes that at one point, there was more than one of these big guys. And then somehow they interacted together, and one of these big Titan-sized moons started spiraling in towards Saturn. And as it spiraled inwards, the tidal forces started increasing. And the tidal forces worked just like tidal forces do here on the earth, in that the moon lifts up the oceans when the ocean faces the moon, and then lets the ocean go back down when the ocean falls away – or turns away from the moon.
And the same thing happens between Saturn and this other moon, except Saturn’s a lot more massive than our moon is. And so it’s not just lifting up water on the surface; it’s lifting the surface up. And as it spirals inwards, this becomes more and more violent and it starts to rip the moon apart. And what happens is it rips these outer layers, these outer icy layers of the moon off, leaving the rings. And the big solid metal rock core falls into Saturn. And this is key because the rings are observed to be almost 100 percent pure water ice. And this is unlike anything else we see in the solar system.
And it’s unlike any other moons that we see, which suggests that the early composition of the solar system wasn’t really conducive to forming things that were 100 percent pure ice. But with the spiraling in moon idea, we can strip off those outer icy layers and plunge that rocky metallic core deep into Saturn.
Fraser: Right. And there’s a situation that’s kind of similar to this, which is the Mars moon Phobos, which is the same thing; it’s sort of within the Roche limit, or it’s gonna get within the Roche limit. Because it orbits the planet faster than a day is on Mars, it’s doomed. The tidal forces that are pushing our moon away from the earth are working in the opposite direction or pulling Phobos into Mars. And within the next few million years, Phobos is going to get torn apart and then it’s gonna rain down on the planet.
You can imagine there might be a similar situation where it’s going to separate out the moon a little bit into a ring, briefly, before it all crashes down onto the planet. And so you can imagine the situation I guess with Saturn where it pulls off that water ice, separates the whole thing, but I guess enough of that stuff is outside of that limit that it remains as a ring.
Morgan: So two things. One is that the main rings of Saturn today are within the Roche limit, which is why they are there at all. Because if they were outside the Roche limit, they would have collected together and formed moons. So that’s happy for the rings. But another thing to think about is when this disruption event happened 4 billion years ago, the rings were way more massive than they are today; maybe hundreds or even a thousand times more massive than they are today. And Saturn didn’t have these 60 moons. It only had one or two; one of them being Titan.
And so this really massive ring is going to start to spread out. It’s just like when you pour sand or sugar or something into a pan, it’s gonna start spreading outwards as it gets more massive. And as it does this, it’s able to clump up and form all of these different moons that we see; moons like Enceladus, moons like Tethys and Mimas. And so over the next few billion years, it sort of squirts these moons out past the Roche limit and creates all of these super icy moons that we see today. The moons of Saturn have densities that are very similar to ice. And this is not found in moons of any other body in the solar system.
And so we have to have somehow created them out of stock that was nearly pure water ice to begin with. And so the idea is – it’s kind of a poetic idea – is that this moon is disrupted to form the rings. And then the rings form the next generation of moons. And now this next generation of moons is once again disrupting the rings. And so this is why I always say that you can’t tell the story of the rings without telling the story of the moons. And it’s because not only are they linked today, but their history is linked and their histories have been linked all the way back to the beginning of the solar system.
Fraser: Fantastic. Well Morgan, thank you very much for joining us for Astronomy Cast. We really appreciate it.
Morgan: It was great to be here.
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