Planetary Radio • Jun 21, 2023

The disappearing act of Saturn's young rings

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Richard Durisen

Professor Emeritus of Astronomy from Indiana University Bloomington

Paul estrada portrait

Paul Estrada

Research Scientist at NASA Ames Research Center

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

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Sarah Al-Ahmed

Planetary Radio Host and Producer for The Planetary Society

Saturn's rings are so young that they may have formed when dinosaurs walked the Earth. Richard Durisen, a Professor Emeritus of Astronomy from Indiana University Bloomington, and Paul Estrada, a Research Scientist at NASA Ames Research Center, join Planetary Radio to discuss their research on the surprisingly recent formation of Saturn's rings and why they are disappearing over time. Then Bruce Betts and host Sarah Al-Ahmed share what's in the upcoming night sky and chat about creepy-crawly constellations.

In Saturn's Shadow (2006)
In Saturn's Shadow (2006) With giant Saturn hanging in the blackness and sheltering Cassini from the sun's blinding glare, the spacecraft viewed the rings as never before, revealing previously unknown faint rings and even glimpsing its home world.Image: NASA / JPL-Caltech / SSI
Endless rings of ribbon
Endless rings of ribbon Saturn's rings look like ribbon candy in this stunning mosaic, composed of images taken by NASA's Cassini spacecraft on April 25, 2007.Image: NASA/JPL-Caltech/Space Science Institute
Curiosity's 'Postcard' of 'Marker Band Valley'
Curiosity's 'Postcard' of 'Marker Band Valley' Through the lens of NASA's Curiosity Mars rover's black-and-white navigation camera, the red planet robotic explorer captured breathtaking panoramas of the "Marker Band Valley" at two different times on April 8, 2023. The photos were subsequently merged and enhanced with color, infusing an artistic touch into the scene: the morning panorama adorned with vibrant yellow hues, while the afternoon panorama embraced soothing blues.Image: NASA / JPL-Caltech

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Sarah Al-Ahmed: Saturn's rings are younger than humans initially thought. This week on Planetary Radio. I'm Sarah Al-Ahmed of The Planetary Society with more of the human adventure across our solar system and beyond. Richard Durisen and Paul Estrada joined me this week to talk about their research about Saturn's rings. Their findings show that the ring system didn't form with the planet. The event that created them happened much more recently, cosmically speaking. Not just that, but the rings are disappearing over time. We'll get into all of the details then Bruce Betts and I will share what's going to happen in the upcoming night sky and talk about bug constellations and if you enjoy shiny space nail polishes and fun facts, you'll want to stick around for our space trivia contest. We got some cool findings from the James Webb Space Telescope last week, but that thing just keeps cranking out the hits. JWST has imaged the faintest galaxy ever detected. The ultra faint galaxy, which is called JD1, is thought to be one of the universe's earliest galaxies. It was formed during a time called the epoch of reionization. That's when light first began to permeate through the fog of hydrogen in the early universe. Back on earth, the United States Federal Aviation Administration and the Commerce Department are working to manage space traffic and debris. A new bill from the US House of Representatives tasked the FAA with tracking objects in orbit that might reenter the atmosphere and could pose a threat to aircraft. The Commerce Department is also responsible for managing orbital traffic so they could potentially duplicate the FAA's efforts. And if you enjoy pictures from Mars rovers, we've got a new and beautiful one that you'll want to check out. Day in and day out, the Curiosity rover lives on Mars and while it was stationary for a day, the NASA rover captured two views of Marker Band Valley in the foothills of Mount Sharp. One of the pictures was taken in the morning and the other one was taken in the afternoon. The photos were originally captured in black and white, but by merging and enhancing the colors, this picture gives the morning view on the left side in a yellowish tint and the afternoon view on the right in a blueish view. It's really beautiful. We show the photo and more stories from space in the June 16th edition of our weekly newsletter, the Downlink. Read it or subscribe to have it sent to your inbox for free every Friday at You'll also find a link in it to the web version of our quarterly magazine called The Planetary Report. The latest issue focuses on the OSIRIS-REx mission, which is going to be returning samples from the asteroid Bennu to earth in September. We explore why the mission matters and what we hope to learn from the samples it returns. Planetary Society members are going to get a physical copy of the magazine sent to them in the mail, but everyone else can read it for free online. Today, we'll be diving into the fascinating history of Saturn's rings. We have two special guests with decades of experience studying Saturn and so many other topics. Paul Estrada is a research scientist from NASA Ames Research Center and Richard Durisen is a professor emeritus of astronomy at Indiana University Bloomington. You may hear his friend Paul call him Dick during the interview. They've recently published two studies on Saturn's rings and the Scientific journal, Icarus. We'll discuss a major finding in our understanding of Saturn's iconic rings. The development was only made possible, thanks to the wealth of data returned by NASA's Cassini spacecraft. During the missions grand finale in 2017, the spacecraft made 22 orbits through the gap between Saturn and its rings, which left us with a trove of invaluable information. After that, Cassini plunged into the planet and ended its mission, RIP Cassini, Richard, Paul and their colleagues have delved into this data, combining it with computer modeling. They've uncovered some surprisingly cool things about the ongoing drama around Saturn's rings. Their findings not only show that the rings are much younger than the planet, but that micrometeoroids from the distant Kuiper Belt out where Pluto lives are smashing into Saturn's rings and causing these iconic structures to dissipate over time. Let's learn more. Hi, Richard and Paul. Thanks for joining me.

Paul Estrada: Hi.

Richard Durisen: Oh, thanks for having us.

Sarah Al-Ahmed: I'm so glad to have you both here to talk about this, because let's face it, if you walk up to a random person on the street and you ask them what their favorite planet is, I mean, other than Earth obviously, because that is obviously the best planet, but they're all going to say Saturn and they're going to say it's Saturn because of the rings, and you both spent decades studying a wide variety of subjects, but you both keep kind of coming back to Saturn's rings at some point. What led you both to conduct this research?

Richard Durisen: Well, I can blame Jeff Cuzzi who's also a research scientist at NASA Ames. When I was there as a postdoctoral fellow many years ago before Indiana University, he came into my office and he said, you're a dynamicist, how is it that Saturn has an oblique rotation axis just like the earth and the planet precesses, so why do the ring stay in the equatorial plane? So that was the first scientific question I was posed about Saturn and we worked on that for a few years, dragged in other people. And then in part because of Jeff, then that connection made the Voyager data in particular excited me and I found out about the idea of erosion producing gas and dust particles flying around as a halo kind of around the rings. I just decided to work on the transport of those particles, not the gas so much, but the particles that are blasted off the rings by meteoroid impacts, and I worked on that with Jeff in the 80s and 90s and then Paul joined the effort and didn't leave.

Paul Estrada: Dick has been working on this problem for quite some time, and the reason why I got dragged in is because I came in as an undergrad working with Jeff Cuzzi here at NASA Ames and the first project that he put me on was more looking at Photometry of the Rings, looking at trying to determine what the spectral color of the rings were. We knew they were mostly icy. That goes way back to 1990 even. But I got dragged into this problem and eventually it just sort of escalated as I moved into grad school studying the physics of this process and the throwing round of material and polluting the rings from these bombarding meteoroids and doing models, complimentary models to what Dick had been doing for years of this process. But Dick was doing it more from a dynamical standpoint of how the rings were evolving structurally. But we, Jeff and I did it from a compositional standpoint, so we were actually looking at how the composition of the rings changes over time and how material can get thrown around. Then Cassini came along and there was this always underpinning idea that the rings are young and that goes back to Voyager. We really never thought about specifically the long-term evolution of the rings and what it means for their age and how long they last. There was enough exciting work just in the compositional evolution and the structural evolution, but I think Cassini changed that picture and so we started to realize that, well, the rings are young. We had said that before in the 90s, but now, we had this sort of added extra detail that the rings are going away. So that sort of led us to this point where we are now.

Richard Durisen: There were some crucial pieces of information that were missing and one of them was the meteoroid influx to Saturn. Cassini helped clarify that, the mass of the rings was somewhat uncertain. It was information like that and confirmation that the rings were icy all the way through, not just on the surface, that really started fleshing out the story. I mean, I had worried about structural features in the rings. Even Voyager had shown that there were these weird structures at the inner edges of the A ring and the B ring and there's no other explanation for them, but ballistic transport, and I'm convinced that that's what causes them, and that implied that the meteoroid bombardment was significant and actually it could produce those features on a short time. That alone made me think the rings were young and then the pollution work that Paul and Jeff did, that was pretty suggested that the rings are young, but Cassini really helped us nail it down.

Sarah Al-Ahmed: Yeah, it's a testament to what one dedicated mission can really do for our understanding of a planet, because having Voyager fly by, that's awesome, but it's not until you can really get up close and personal and particularly with those ring dives, get enough information to do this kind of science. Well, let's start at the beginning. How old do we think Saturn's rings are now and how long are they still going to be with us?

Richard Durisen: After teaching 100 level astronomy for decades, I like to keep round numbers rather than the kind of specific numbers you'll find in papers because there's still some uncertainty, but it looks like the rings are less than a few hundred million years old and will only live less than a few hundred million years more. Few hundred million years sounds like a long time. It's enough to take us back to the dinosaurs if you go back in time, but that's relatively recent when you think about the planet itself and the planet and solar System being basically four and a half billion years old. Most people decades ago thought the rings formed when the planet formed.

Sarah Al-Ahmed: I feel like what's interesting about this is there have been so many little clues here and there over the years that Saturn's rings are young, but the first time that I saw an article that actually said, not only are they young, but they're disappearing. I had this influx of mixed emotions because that's really cool to know, but also it's kind of sad to think that Saturn's rings won't always be around with us, and I'm curious, do you have any kind of words of wisdom or any kind of happiness that you can share with people that might be filling a little melancholy about the fact that Saturn's rings are going to go away?

Paul Estrada: My feeling is is that if we look at other planetary ring systems like the Uranium or Neptunian ring systems, they're still around, but they're kind of very sparse. They're not going to completely go away because I think eventually the process of the bombardment as you get punier and end up in these annual eye of rings is that it becomes less efficient and then other processes can probably hold the rings in place. So I don't think they'll completely go away, but I think it's just because they're so majestic right now and so massive, and I think that's what captures everybody, but I think that that is just maybe not a normal circumstance. I think it's possible that all of those planets, even Jupiter, Jupiter's ring is nothing but a bunch of small micron sized dust now, they were maybe much more massive when the planets formed. And there's reason to think that that would be the case because these moons might have formed from those massive rings, because they can't be too massive because then they will go away and they kind of spread viscously like molasses, so you could actually make all these moons out of these rings. The idea that they're constantly being impacted by this dirt coming from in this particular case mostly is coming from Kuiper Belt, but leftovers of formation of the Solar System, it's just kind of a natural that they're just going to get eroded. I think maybe we were all sort of had the blinders on. On the surface, when you step back and think about it, it kind of makes sense.

Richard Durisen: A slightly different way of looking at it. One, we're impermanent, but we enjoy being here. Okay, so Saturn's rings may be impermanent, but everything's impermanent on some level and I think the wonderful thing is that it's a great lesson in how dynamic the whole universe is, that when you have a complicated system of interacting stuff, you get unique behaviors, unusual behaviors, and a system can sit there rather quietly for billions of years and then do something dramatic. We have the Cambrian explosion of life evolution on the earth happened in a very short period of time half a billion years ago, but life was around for billions of years before that. These things happened in complex systems and it's one of the beautiful things about the place we live.

Sarah Al-Ahmed: Maybe we are in fact very lucky to live during a time when Saturn's rings are so big and beautiful, but you're right, things change over time, and to me, that makes it even more special that they're as awesome as they are now.

Richard Durisen: And one more thing, think about eclipses.

Sarah Al-Ahmed: Yes.

Richard Durisen: Solar eclipses, that's another coincidence. We happen to be around at a time when the moon just about covers the face of the sun, so we get to see the chromosphere, prominences and the corona every once in a while, and Bloomington, Indiana where I live is going to be right in the middle of totality next April.

Sarah Al-Ahmed: That's so lucky.

Paul Estrada: I might even come up there, Dick, check it out.

Sarah Al-Ahmed: Really though, I remember after seeing the last major solar eclipse in the United States, I believe that was 2017, I remember thinking that if I had the capacity to go from planet to planet across the universe, the thing I would chase would be solar eclipses because that's just such a wild awesome situation to have the size of the sun and the moon just happen to be just right for that to happen. That's really special and I don't think people fully understand or comprehend what that's all about. I'm sure we'll get into talking a lot about that in the future when we let everyone know how awesome this next solar eclipse is going to be. If the rings around Saturn were much older than they are now, how would that present itself? How would we know that they were older? What would they look like and how would they be different from what we see?

Richard Durisen: The thing that we showed is they can't be, rings like that have an asymptotic lifetime. They can only be so old before meteoroid bombardment cuts them off and makes them disappear. That's the work on the paper that Paul is the first author on. That's where we do these solar system age kind of evolutions. They cannot be as old as the planet, not a bright massive ring system like Saturn. It doesn't work. If they were old, they might look like the rings of Uranus and Neptune.

Paul Estrada: In that paper, I mean, we do these models where we just look at this sort of traditional view, as I mentioned earlier about how the rings evolve, viscously, they spread. If they start massive, they spread and the mass goes into the planet and some of it goes outside of the Roche limit, which is just that radial distance where bodies can accrete into moons and it won't be broken up by Saturn's tides. If you just sort of did things that way and then just let the micrometeoroids pollute the rings, they would look much darker. I mean, that's kind of the implication for the Uranian or Neptunian rings. They would look darker and I think in that paper, I do some models like that where it's considerably darker than they look now, so that isn't the case. And then on top of that, there's this dynamical evolutionary aspect of this process of bombarding the rings and the impacts throwing out all of this ejected material from, you could think of them as a little cratering impacts from the ring particles and that's the ballistic transport part where all of those little grains get transported to different locations in the ring, and ultimately, it's all this kind of an angular momentum, transferring angular momentum all over the place. The rings, it's almost like classic accretion disk material, angular momentum outward, then the mass flows inwards, so eventually you would just lose the material. I guess you could say, well, maybe this bombarding material doesn't always happen. It's not a constant flux of material. And say, if you turned it off, then in such a situation, sure, the ring could be as old as the solar system, even though there are other processes going on in the rings that could change their structure, but it's actually not a very good argument. As you go back in time, it's going to be worse, the amount of material flowing around and if you sort of go with that idea, it's like, well, that time when the solar system formed and all the planets formed and the rings maybe were massive, there's all this leftover debris and material and things are colliding and the flux of material coming into the Saturn system is going to be much higher. And so if you take that into account, then the rings would last even less time because there's such a stronger erosion rate going on that they would just go away. So I go back to what Dick says, we just find that it's not really possible. That would be the conclusion. It's just not that they can't be that old, they just can't.

Richard Durisen: We have to have a very clean planetary system. We'd have to have not a lot of leftover junk to produce interplanetary meteoroids, but we do, and probably other solar systems will too, but surely there's a clean solar system out there somewhere.

Paul Estrada: I'm not sure. I think it's probably pretty common just that that's just the leftover stuff. Other systems have Kuiper belts and Oort cloud, comets and even the irregular satellites in these planetary systems like Saturn and Jupiter, there's collisions and impacts and there's just dust gets produced one way or another. It's there.

Sarah Al-Ahmed: Are we assuming that these micrometeoroids are coming from the Kuiper Belt just because that's where there's so much leftover material or is there something about their composition that suggests their origin?

Richard Durisen: Well, there's a measurement and that's the key thing. There was a measurement of the influx of interplanetary meteoroids into the Saturn system, that was really key, that actually held up our papers for a few years. That's the third paper in this triumvirate that's being talked about and that's the paper on the Cosmic Dust Analyzer, and Paul is actually on that paper, I'm not, so maybe he wants to address that a little bit. So we know that number and that really helped.

Sarah Al-Ahmed: And the Cosmic Dust Analyzer is an instrument aboard Cassini, correct?

Paul Estrada: Yes, that's correct. That's some work we did with Sascha Kempf at UC Boulder. He was the PI on the CDA instrument. So the CDA instrument basically is just a dust collector and it measures the impact on a sensor and tabulates all of these impactors, as it's flying around, it's got an open dish and occasionally a particle will come in and boom, it hits and it gives off a charge and we can measure. Depending on the orientation, you can determine its dynamical origin and also a sense of what the material is. So just to put it into perspective, let me just back up. So this instrument was on for over 12 years of the Cassini mission. That was its job was to measure these dust particles and so it logged, I think, well, over 2 million impacts over the 12 plus years of observations, but only 163 of those 2 million are considered micrometeoroids of extrinsic origin coming from outside the system. So that just shows you most of them are actually little particles from the E ring due to Enceladus. You have to filter all of those things out. In fact, if you flew through the E ring, we didn't even bother to check whether any of those particles were, there's just too much going on there that we just wouldn't even look for. But there's only 163 and we had the trajectories and we did models and you sort of back up their trajectories and you come up with a dynamical character that is consistent with the Kuiper Belt. So these are basically things that are making their way for millions of years from the Kuiper Belt due to collisions and eventually come into the Saturn system at relatively low speeds as they enter the gravitational influence of this Saturnian system and then they get sped up, they get focused onto the rings because they come in so slow and then my focus system on the rings and then this just enhances this effect of. Because the surface area of the rings are so huge, I mean, the number is actually very small when you look at it, what that micrometeoroid flux is, you look at the number and it's like 10 of the minus 16 kilograms per square meter per second. It's a small number, but it adds up and the surface area of the rings is so huge if you were to put it all into a moon smaller than Mimas, the surface area of the range is 10 to a 100,000 times the surface area of that moon. So that's how you can appreciate that even though that number is so small, it really is consequential for this process.

Sarah Al-Ahmed: I'm not sure a lot of people really understand the fact that these rings are so bright and shiny does not indicate that there's like a bunch of material in the rings. In fact, they're actually kind of thin and we didn't really know how thin they were until Cassini got there and particularly during those ring dives to see just how thin they are. Can you give people an understanding of the structure of Saturn's rings and how surprisingly thin they are?

Richard Durisen: I've got an analogy I used in my 100 level classes and that's if you made a scale model of Saturn's ring and plop them down, say made them the size of my local county or central Indiana, they would be as thin as a sheet of paper. That's how thin they are. In fact, one of the puzzles for a long time was given that they are so thin and that it took time for us to realize that. I was involved a little bit with Jeff Cuzzi again on how thick or thin the rings are. One of the neat things that Cassini clarified is why the rings don't disappear completely when they're edge on, twice a Saturn orbit we see the rings edge on orientation and they have a residual brightness. It turns out that there are bending waves that some of the external moons produce bending waves in the rings that flip them up like folding the piece of paper up and they fold it up kilometers in size in a very narrow region. And so when you look at the ring's edge on, you're seeing the light reflected from this twisted surface of the rings and there are wonderful Cassini images of that, of some of those edges. One of the wonderful things about Saturn's rings is that it's a system that lives on the edge of so many different processes. It's marginally gravitationally unstable. It's got rings, it's got waves being produced in it by external moons, it's borderline collision all, I mean, some parts of the rings, you get multiple collisions per orbit. Sometimes you get about one per orbit or less between the particles themselves. It lives in a place where most physical systems we study are extremely to one side or the other, but in Saturn's rings, it's always on the edge. It's always in a marginal condition, which makes them very hard to model actually. It's most theoretical problem.

Sarah Al-Ahmed: That brings up a question for me because the way that you did this research was by taking data from Cassini and then putting it through computer models that allowed you to understand more about their age and how they're going away over time. But did this computer model just spit out a really clear answer or was there kind of a range of answers that could explain this and how does that help us narrow down how old the rings actually are? We can't actually say they were created on this date by this thing, but how close can we get to knowing that answer?

Paul Estrada: Well, I mean, of course, any model is only good as what you put into it. I should point out that that's process that Dick and I have modeled for many years is actually pretty complicated. The models that actually take into account, the throwing around of material and redistribution of material, it's actually quite complicated, was not possible to do that for these models because the overhead is too high. And so, we, Dick has a paper that I'm the co-author on where we sort of distill things down into the inflow rates and basically the transport part of this equation, not the structural evolution parts. So there's a magnitude of these things that depends on the micrometeoroid flux and it depends on the mass of the rings and the velocity at which things might get ejected and whatnot. From that standpoint, I'd say that the models are a little bit simpler, but the model is complicated just doing the viscous evolution of the rings and you're just adding these features to the rings. And so there of course can be some variation in parameters, but it's not in such that unless the flux is completely wrong, which we measured, there's not really a whole lot of wiggle room. It's giving you not an exact answer of course, because there can be some variation, but I think it's roughly consistent with this idea that they can only be a few hundred million years old, but the main point is just not four and a half billion years old and it's kind of a diminishing return. I don't know if that's the right expression here, because if they are as old as the solar system, that's one thing. If it's a 100 million years old or 500 million years old, you are in trouble because there's no model really out there that applies to rings that young, it's just they're all kind of fit into this category that requires lots of stuff, big stuff floating around the solar system, which no longer is the case anymore. So that's all that stuff's been cleared out.

Richard Durisen: Where you're getting to now is the idea that, well, how did the rings form? Now, that we know they're young, how did they form? And people have tried to work the numbers on how likely it is that say some big object, big enough to create the rings came into the Saturn system from outside and was broken apart by tidal forces of Saturn and settled into an equatorial orbit around Saturn, and the numbers don't work. There are people, at least as far as we know, there aren't enough big objects. The probability of seeing a rain system like that is pretty low based on an object coming in from the outside. That leaves us to what I was saying before about complex systems. Saturnian moon satellite system is actually pretty complicated. And if you look at it closely, they're things that are a little hard to understand. I mean, some of these moons have been melted, some of them have hot oceans underneath the surface. So what's that all about? I mean, they're in orbit resonances now, some of them, but it's hard to make that effect strong enough to produce the kind of surfaces on these moons that we see or explain and tell this is hot ocean. But if something dramatic happened, some sort of billiard game happened due to a simmering instability say of the orbits of this complicated satellite system, all hell might have broken loose a few hundred million years ago. And that's why it looks the way it does now. And as part of that game, somebody got shoved into too close to Saturn and broke apart. That is pure speculation on my part, but there are some people who are thinking about how that might have happened. They're kind of subtle dynamics and I like it because it's an example of what I was saying before, that this complicated system can sit around for billions of years and rather abruptly do something really kind of extravagant and amazing, that's happening all the time in the universe on different scales.

Sarah Al-Ahmed: We'll be right back with the rest of my interview with Richard Durisen and Paul Estrada after this short break.

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Sarah Al-Ahmed: We know that at least the E ring is just mostly water that's just being cranked out by Enceladus, but the rest of Saturn's rings, do we think that it comes from this one single common origin, one object that might have broken up or might it be more than one? And how could we be able to tell the differences there between those scenarios?

Paul Estrada: Going back to this idea that these kind of systems remember are teetering on the edge of stability in many ways. You've got all these moons that are deep in the gravitational well. And if it were early on in the solar system, they would be really in parallel being hit by stuff coming in. They could have been blown apart and remade early on, but not anymore as Dick's saying there's not really much stuff flying around. The implication from the rings being young is there has to be some other mechanism to make them that, it's not like the traditional origin scenario. And that's why I'm pointing to what something happened in this system. And there's lots of evidence I think that says, like Enceladus for example, how is it have such a high heat flow? Why does it have an ocean? It's spewing out all this stuff. It doesn't look old. And there are other things in the system that look strange. For example, the moons, I don't know how much you know about cratering, but typically if you look at the Uranian moons for example, or even the Jovian ones, the cratering is associated with bodies that come from orbiting the sun. They have these populations and they fit it to them and they say, well, all these things look predominantly what we call from heliocentric bodies. But in the Saturnian systems, strangely enough, you have a different population of impactors on many of these moons and some of the surfaces are a little bit saturated, but they are consistent with planeto-centric origin. So it's almost like, so this is another piece is saying, well, there must have been all of this stuff floating around in the system that impacted all these moons equatorially. This is actually a very hot subject with the cratering people. And then there are things that with Titan, it's like why does Titan have an atmosphere? It's kind of weird because there are physical reasons to think that it shouldn't.

Sarah Al-Ahmed: Doesn't even have a global magnetic field, what is even going on there?

Paul Estrada: Exactly. So it's like, well, why is that the case? And so going back to the rings, it's like, well, can one body do it? I don't know. Maybe not. So we have a paper in review right now where we present our model and we as present as the smoking gun of something happening. This is something, it's based on some work of a colleague at SETI, Matija Ćuk, he's a dynamicist and studied the system, and basically, he discovered some things that basically implied that the system doesn't appear to be as old as the age of the solar system. And one of these is this idea that Rhea, which is the outermost moon of the sort of quintet of icy moons from Mimas out to Rhea. Rhea is the outermost one. These moons tidally evolve outwards just like our moon is tidally evolving outwards. So Rhea had to cross, there's a what's called an evection resonance just inside Rhea's orbit. Now, evection resonance is basically, it's where if you were to put a moon at that location and it's precessing, its precessional period around the planet is the same as the orbit period of Saturn around the sun. So the sun is perturbing that body. So what happens, well, that moon had to tidally be evolving outwards and it encounters this resonance and through all these sophisticated dynamical modeling showed that this thing gets excited and will eventually collide with a moon, say proto Dione. That's evolving outwards as well. And so they collide and they get disrupted and this actually can lead to a cascade because then they get disrupted and then there's all this material in large chunks and things from the one body that is kind of on an eccentric orbit and then could hit the moon interior to it. But the smoking gun is, is that, well, did this moon survive crossing this resonance? And because it's now located outside of the resonance, so it must have crossed it and you can estimate with the last a 100, 200 million years, it had to have crossed it. So the issue is, is that if it survives crossing it, its inclination gets perturbed as well as its ease of electricity, and so all of a sudden, we'll become much more inclined than it currently is now and it's really hard to damp inclination. Once it has that high inclination, it's not going to lose it in that time period. So right now, Rhea's inclination is very small, so that suggests that it must have formed outside this resonance so it got disrupted and then the material got repelled. It's a complicated process. I won't go into the details, but the material gets repelled and we can reform a new moon. But this whole process could lead to this chain reaction. But in this paper, we showed that even from that one impact, depending on the impact parameter and how they hit each other and whatnot, I mean, there's some variation. You can deliver a significant amount of material with [inaudible 00:37:16] inside the Roche limit, very close to the planet. But they're also, even though they're on very eccentric orbits, if once they sort of damp into circular orbits, much of that material actually have circular orbits inside the Roche limit, so they could form rings. It's not the entire mass of the rings, it's hard to get the entire mass from that one impact. But interestingly, it's all ice because it comes from the mantle of the moons. So there's the mantle material that gets thrown around the most, right? The rocky material sticks around and stays in larger chunks and could impact other moons or recreate into new moons. So I would say at least based on that, probably it's going to take more than one event, but all falls within this kind of billiard ball scenario that Dick is talking about. It does look like it could work. Of course, it's going to require much more sophisticated modeling than we've always done is modeled. It's more like a proof of concept. It's like here, we've done this very sophisticated collisional simulation to show that wow, all of this stuff gets thrown out, not just inwards but also outwards. So that also might have implications for Titan. Because a lot of this stuff is probably going to impact Titan as well. And so what does that mean for the surface of Titan? What does that mean for how much mass, how much does that stir up the surface of Titan?

Richard Durisen: That is the kind of thing that I can't help but wonder, given what we know and given how perturbed the Saturn system looks, some of the moons, they're not cratered as heavily as they should be. It's just weird. I think if something like that happened, there were probably a lot of events that could have thrown material into near enough to Saturn to produce rings. Even maybe one moon that's not there anymore, got thrown in.

Paul Estrada: Yeah, I mean, that's true because the implication, of course, isn't that what the system we see now was not the system before. So there was a preexisting system. I mean, I think there's good reason to think that it wasn't totally unlike, it probably has to be similar mass more or less, maybe a little more, just the idea of how we understand how moons form, whether it's from massive ring or if they formed in the disk of gas around the planet itself, they tend to form multiple moons and there probably was some other system there and now it's a new system.

Richard Durisen: It may have happened more than once. Saturn may have originated with a tight enough satellite that 200 million years ago wasn't the first time that things went crazy. That's something for science fiction, I guess.

Sarah Al-Ahmed: Well, there's so many satellites in that system. I mean, at this point, what? We think there's 124 moons of Saturn that we know of. There's got to be all kinds of interesting shenanigans going on there. This makes me wonder something about Enceladus, which has been kind of sitting in the back of my brain for a while, that thing's just been geysering water into space for what seems like a while, but at some point, you would imagine if it had been doing it for billions of years, it would run out of water to do so, which always made me wonder if it's only a recent development that it's been kind of geysering this much material into space and this would explain that for me. Maybe there was some kind of event that finally allowed Enceladus to get to this point. And that's wild to think about all on its own.

Paul Estrada: It's not inconsistent. I mean, the state of Enceladus is strange. There's clearly really high heat flow in the southern equator. I mean, it's like, yeah, something knocked the crap out of that thing or reformed. Other moons like Tethys, they're almost pure ice. It's almost like you've redistributed material and it's like why is Tethys pure ice for the most part? Because eventually when things settle down, all this stuff is going to crater the surfaces of these moons as you clean it out. There are a lot of strange things in the Saturn. And Saturn itself, its interior is very strange. So teetering on stability, I think, is a good way to think about it, these kind of systems. Any little thing can push it one way or the other and something happens and all hell breaks loose. There's so many questions. I mean, why does Titan have this large eccentricity? I mean it has a relatively large eccentricity. It's like there's so many things. So of course, we would love to go back to Saturn.

Sarah Al-Ahmed: Yeah, I mean, thankfully maybe we'll get the Enceladus Life Finder mission at some point, fingers crossed. But there are so many mysteries that remain here that really just beg for more observation. We've kind of established that there's all these interesting things going on with all these moons and these micrometeorites that are crashing through the rings, but how much material is actually raining down from this ring onto Saturn? What is the sheer volume of this?

Richard Durisen: Well, that's actually a measurement that was made by Cassini as it was diving between Saturn's atmosphere in the ring, the grand finale, there's a paper that claims that there's a lot of stuff falling into Saturn and it's been detected by different methods, but the deep dive grand finale orbits that people have estimated that about a few to dozens of metric tons of material fall onto Saturn every second from a region at latitudes that are sort of concentrated toward the equator, presumably coming from the rings. That's part of the story of these two papers that Paul and I wrote. I was sitting at the last Cassini conference, which was held in Colorado in 2018. On the last day, admit that like a lot of people, I don't always go to the last session, but I decided to stay along with the scattering of other people and this talk mentioned this flow rate. I was like, wait a minute, I can explain that. And that's what led me to write the one paper showing that when the meteoroids hit the ring particles, they tend to hit the front of the ring particle. And so when there's a little cratering event that produces a whole bunch of ejecta coming out of that crater, they fly forward. So most of the ejected material from an impact by a meteoroid onto a ring particle, by the way they think snowballs or the big balls that a snowman is made out of, that's kind of what a ring particle looks like. Growing material forward like that, it causes everybody the whole ring system to drift inward over time, angular momentum gets moved outward for the physics people in your audience and mass gets moved inward. And if I know the rate of mass coming in for meteoroids, put all that in, you get a flow of material through the B ring and the C ring onto Saturn, that's, well, guess what? It's a few metric tons per second or maybe larger. Obviously, you can see that the measurement was not very precise. I mean it's uncertain by a factor of 10 at least. So within plausible ranges of parameters, that's the amount of material you would expect the inflow of meteoroids and the meteoroid bombardment to cause to flow into Saturn. Unfortunately, we didn't predict that, but we could have. It was explicit in the papers that we were writing in the 90s, but we didn't ever say that in particular that, well, if you put a spacecraft between the rings and Saturn, you would see this flow of material.

Sarah Al-Ahmed: It's unfortunate, you could claim that that was your idea.

Richard Durisen: Yeah, we missed a chance there.

Sarah Al-Ahmed: It's funny because this likes sparks this very futuristic sci-fi thought in my brain, but maybe someday hundreds of years in the future when we have the ability to just go around the solar system, we might have people going to Saturn just to go see the ring rain. Here on earth, we get these beautiful meteor showers anytime earth plows through the trail left behind by a comet, something like that. I wonder if they're just beautiful meteor showers underneath the rings of Saturn at all times.

Richard Durisen: That's unclear. I mean, Cassini did make it through, so there can't be a lot of big particles.

Paul Estrada: Yeah, they're really on the order of nanometers and they're pretty tiny, but it's hard to say because we can say that there's a mass inflow going in, but something is transporting it into the planet, it's causing it to go in as ions and nanoparticles and it's a whole slew of things. And the ring rain in particular could be a combination of nanoparticles and vapor, and as Dick said, there are multiple measurements that were sort of not just in the equatorial plane that in the context of our papers would be basically everything that's like that's telling you how much inflow is being caused by this bombardment. But there were other measurements that higher latitudes that shows that there are probably some charged particles that are coming from the rings and those are also ring rain and those are ending up in the atmosphere too. And clearly there's water in these things because there's emission from the atmosphere that requires a certain amount of water per second, kilograms per second to be deposited there to keep these patterns of emission that you see in Saturn's atmosphere. So it's all gone in there. I'm not sure, it would be great if it was some beautiful process here, but I mean, I don't know. I think they're probably a little bit too small and fortunately because that was one of the worries about sending the Cassini spacecraft through that region between the D-ring and the atmosphere, which is only a couple of thousand kilometers, it's like, is it really empty? What happens? That's why we save it for the last part.

Sarah Al-Ahmed: Yeah, you don't want to take Cassini out before it's time.

Paul Estrada: No, no.

Sarah Al-Ahmed: Were you able to actually go to one of the parties for the end of the Cassini mission?

Paul Estrada: Oh, yeah. I was there for the end at Caltech. It was, well, how can I forget it? September 15th, 2017. So yep, we were there, we watched it. They had a big screen set up with JPL command center and just waiting for the signal to go away as it entered the atmosphere, tears were shed, and hugs were given. It was an emotional moment. I still remember that day and that date and will live it in for me, I guess, is what they say. But I mean, it was hard to see it finally go.

Sarah Al-Ahmed: It was, I'm glad that the people that got to work on that mission were at least able to be with other people who cared so deeply. I was one of those people that literally got out of bed early and just sat at my computer by myself, just feeling all the feelings.

Paul Estrada: It was yeah. Missions of that length, people are working together for so long, it just becomes sort of a part of the daily ritual and routine. That's the amazing thing though about Cassini was that it's amazing that you keep getting surprised all the way to the end, blows your mind. And even to the last, Cassini left us with this amazing realization that all this stuff is flowing into the planet. Just astounding. And so yes, I don't want to get too emotional.

Sarah Al-Ahmed: It's okay. I think a lot of people in the audience get emotional over Cassini, so you're in good company. If we were going to design another mission to go to Saturn specifically to learn more about the age of the rings and their origin, what would you want on that mission and what would you hope it would do while it was at Saturn in order to actually get these readings?

Richard Durisen: One idea that's been kicked around is a ring skimmer, a mission where probe would go into orbit above the rings. Now, you might, if you think about it, say, wait a minute, it's got to orbit at the center of Saturn, it's not... So yeah, it would've thrusters that would keep it up above the rings and it could look down and also even change its orbit radius, that would tell us an awful lot about the rings. And there are a lot of features in the rings that we don't understand. There are these funny ways that the rings look at the resolution that we can get from a mission like Cassini that it's hard to know why do we see those kind of striations in the rings or why is that region bright and a neighboring band is not? There's just a lot of funny structure there that is not explained by these bending waves and spiral waves produced by the moons or they're the plateaus, Paul and I have banged our heads on the plateaus a few times, these funny looking bands in the C ring, and right now, we don't have an explanation for those. I have an idea, but I'm retired. I like that, the idea of a mission like that. But Paul may know more being at NASA and a little more attached to the observers.

Paul Estrada: A lot of these missions would be challenging, but you could just think right off how cool something hovering above the rings would be because as Dick said, at the resolution of Cassini, you're never close enough to the rings to see little individual ring particles. I mean, even the ones that are as big as a truck or a house, we're just never that close. It still looks like a flat plane of material or it's hard to distinguish, but those kind of missions where you could essentially even sit above a particular point in the ring and just watch the particles flow by and interact with each other. To me, that would be very cool. There is also a skimmer which actually flies above the rings repeatedly and can also get that kind of resolution of imagery to see, wow, look, we got all of this data from Cassini and Occultations. Look at these textures in the rings and why do they look that way? But of course, if you could get close enough, then it might all start to make sense. It's like, well, that's what's going on there. You want to be close enough to the rings to be in that layer of ejecta that is getting thrown around all over the place from these impacts. Because now once you embed yourself in that layer, now you're collecting these particles that are coming essentially from the source. Impact, it just got thrown off. Maybe you could determine exactly what part of the ring it came from. You could look at what they're made of, you're directly sampling the ring material. Those kind of things I think would help to really nail home this process. But right now, it's a pipe dream. I mean, we'd have to get another mission that'd be willing to do that. I think from a public standpoint, I mean, if you've got really cool close pictures of the rings, I mean, that would be very cool stuff.

Richard Durisen: However, I think even I would say that going to Enceladus or going to Titan would be higher priority. I'd like a mission that put a drone boat or a drone submarine on one of the mare on Titan, one of the methane seas and just find out what's going on there. There are a lot of interesting possibilities.

Sarah Al-Ahmed: There are, and I'm so looking forward to the Dragonfly mission to Titan. It's not going to do something cool like Bob around in the seas, but having a quadcopter just to tell us anything about it. The Huygens probe showed us that clearly we need to know more about this. I can't tell you how excited I am about Dragonfly. I could go off.

Paul Estrada: No, it's fair. Yeah, I totally agree. I mean, that's just warranting another flagship mission, but of course, NASA's going to have its priorities and there's only a limited pot of money.

Sarah Al-Ahmed: We can't always get what we want, but if we all work together and advocate for these missions, maybe we can increase NASA's budget enough that we can do all this amazing science because I don't want to wait until it's decades and decades and then I'm gone and we won't know anything about these questions I want to know right now, but that's just me being selfish and impatient. Well, thank you both for joining me and for sharing so much about Saturn. I've actually learned a lot just listening to all of the interesting details about moons and rings that I hadn't even considered, and I'm looking forward to that paper that's not out yet. I'm definitely going to read that.

Paul Estrada: Hopefully soon. Yes.

Richard Durisen: I want to thank the audience for being interested in this and I want to thank The Planetary Society and you, Sarah, personally for following through and making this happen.

Sarah Al-Ahmed: Happy to do it and always happy to share more Saturnian science.

Paul Estrada: Same sentiment for me as well. It's been a lot of fun.

Sarah Al-Ahmed: Thanks so much. Thankfully, now is a really great time of year to go out and spot Saturn in the night sky. And if you have a telescope, 10 out of 10 recommend checking out its rings. Let's check in with Bruce Betts, the chief scientist of The Planetary Society for What's Up? Hey, Bruce.

Bruce Betts: Hey Sarah, how you doing?

Sarah Al-Ahmed: Space stuff.

Bruce Betts: Space stuff. There's still space stuff going on.

Sarah Al-Ahmed: Always.

Bruce Betts: Did you know there's still things in the sky. There are a lot of stars you can look at and the night sky, you can check out Venus, super bright over in the evening in the west, still looking very cool. Mars getting closer to it through July 1st and then there'll be a few three and a half degrees apart. So a little ways, but still fairly close. Mars, much dimmer in reddish. If you're picking this up when it comes out, you may be able to go out and look in the evening of the 21st and check out the Crescent Moon hanging out with Venus and Mars. Otherwise, moon, it'll still be there, but it'll be somewhere different the next night. The way that pesky moon does things and the predawn sky, Saturn is cooking its way up higher in the sky, so it's coming up a couple of hours before dawn, and right before dawn is high in the east or southeast, then you got Jupiter looking really bright down low, but getting higher and higher as we go along. So all in all, a good collection of planets still.

Sarah Al-Ahmed: Yeah, I was out the other night and I got one of my neighbors just staring up at Venus in the sky, kind of like, what is that? I never know whether or not to stop and answer the question I see in their eyes like, that's Venus.

Bruce Betts: Just yell Venus at them and keep going.

Sarah Al-Ahmed: That's a great way to make friends. Just Venus.

Bruce Betts: Okay, I didn't know you want to make friends, then yeah, probably not the right way. We move on. Speaking of not jokes, but one of the greatest missions in the history of the world launched this week, four years ago, LightSail 2, The Planetary Society's demonstration of CubeSat Solar sailing for the first time in history. Launched on SpaceX Falcon Heavy four years ago now, and we were up for about three and a half years, then dragged down by the atmosphere and burned up and we're still analyzing and working on the data, and that happened.

Sarah Al-Ahmed: Gosh, I can't believe it was four years ago, things have changed, but I'm sure that's even weirder for you because you were so deeply involved in that project. It's like your spaceship child.

Bruce Betts: Yeah, I mean, I adopted it a fairway and although I was there for the... Anyway, that analogy's getting too weird too fast. I was just talking about LightSail remotely. We're a Solar sailing conference, so it's fresh in my head and we're working on science papers to report the technical aspects of the mission. So it's still living pretty strongly in my life at the moment, even if it's not flying around.

Sarah Al-Ahmed: Yeah, and LightSail will always be in our hearts. I literally have a necklace with little LightSail on it than I wear when I think about little LightSail.

Bruce Betts: Little LightSail.

Sarah Al-Ahmed: This is a great minute to pitch too, because if anybody doesn't know anything about the LightSail mission, well, we could talk about how awesome it is, but we have a whole documentary, so if anybody wants to know more, you can go to The Planetary Society YouTube channel and look up sailing the light. Just make sure you have tissues or something because I can't get through it without crying. I know that's me, but I think that might be everyone.

Bruce Betts: You can also get our general webpage and there are links to pictures and history and more on that page. And if you're really into it, there are also links to the currently existing technical papers and presentations. So all sorts of good stuff on the web there and there will continue to be more as we churn out more of what we learned. All right, let's move on to [inaudible 00:59:16]. So I was thinking we just had the NBA basketball championship and the Stanley Cup NHL Hockey championship. So what if you combine those two in some kind of fact? Well, I did. If Saturn were the size of a basketball, Earth's diameter would be about the height of a hockey puck. So picture basketball with a hockey puck sitting next to it and earth's about the height of the hockey puck. There you go. That's not counting the rings. All right, let's slam dunk and check into the next segment of trivia. Yeah, that was smooth. I asked you to name only all the constellations named for insects and that was to use the IAU officially approved 88 constellations. How'd we do?

Sarah Al-Ahmed: This is actually really funny because I've never seen so many people get a space trivia question wrong. You managed to completely throw people for a loop on this one.

Bruce Betts: Should have done some biology classification beforehand.

Sarah Al-Ahmed: Exactly, right? I mean, if you're one of the people that got this question wrong, don't feel bad, a lot of people, the majority were there with you, which is fun. We're all going to learn something today. So first off, the answer is not Scorpio the scorpion. Scorpions are arachnids like spiders, so they're not insects. Too many legs. And the answer is also not cancer the crab. Because crabs are crustaceans. They are pinchy, but they are not insects.

Bruce Betts: Insects not defined by pinchyness.

Sarah Al-Ahmed: But I get it. That totally makes sense in my brain that scorpions and crustaceans would pop up in people's minds here, but...

Bruce Betts: Oh yeah, definitely, six legs, got to be looking for six legs, no more, no less.

Sarah Al-Ahmed: And the answer is the constellation Musca, the fly. And I think too, that it makes sense that our winner this week is from Australia.

Bruce Betts: Oh, yeah.

Sarah Al-Ahmed: Because you have to be in the Southern Hemisphere in order to see this thing. And I think of all the groups on earth, the ones that are most likely to know their bugs are probably the Aussies.

Bruce Betts: Well, congratulations. And who is that winner?

Sarah Al-Ahmed: Our winner this week is John Guidon from Samford Valley, Australia.

Bruce Betts: Cool.

Sarah Al-Ahmed: And you'll be winning a goodnight epithermal mug. It's one of my last ones, but it'll be a good opportunity to think about the rover and drink some nice warm tea and consider how many bugs are nearby you, John, at any given moment. This was cool. We always get some beautiful poetry in. And forgive me because this one's a little long, but I loved it. This is from Gene Lewin who wrote in, "Amidst the lofty company of constellations, far and wide, four stars make up a creature, which navigators used a guide. Observed by two explorers, the East Indies was their aim. And being from the Netherlands, Dev League became the name. At one time, which was known as Appis, but with a chameleon right nearby, it was later changed to Musca. What in English is the fly? There's also a scorpion that may evoke a shrug, and it's not classified as an insect, and it's definitely not a bug. So if insects are the goal here, and depending on your source, there is only one insect unless you count a flying horse."

Bruce Betts: And we don't, by the way.

Sarah Al-Ahmed: And we don't. That was super clever.

Bruce Betts: That was. That was great.

Sarah Al-Ahmed: And this other comment was actually about your trivia question from last week, which was about who was the first person to fall asleep in space? And Joseph Kelly Butrey from New Jersey, USA wrote in to ask what you would count in space when you're trying to go to sleep? Would it be space sheep or its beef?

Bruce Betts: It's beef.

Sarah Al-Ahmed: All right, what's our next trivia question?

Bruce Betts: Yeah, short and sweet. What is the closest nebula to earth? Go to

Sarah Al-Ahmed: And you have until Wednesday, June 28th at 8:00 AM Pacific time to get us your answer. I don't think I know the answer to this one off the top of my head. That's really fun.

Bruce Betts: Yeah, I thought so. I didn't know myself. I thought, what is the closest Nebula to earth? So we'll see. Hopefully, there's not too much disagreement. I think it's a clear answer, but I off. I usually do.

Sarah Al-Ahmed: Yeah. And I like this prize this week because I was really lucky I got to do a collaboration with this nail polish company called Orly that teamed up with NASA to make a whole line of NASA nail polishes and the JWST nail stickers. I loved it. So we were filming recently and I got a lot of cool extra nail polishes and stuff. So this week, whoever gets this question correct, you're going to be getting a copy of their JWST nail polish called The View From L2. And I'll send some cool Korean nebula stickers along with it. And for those that don't wear nail polish, this makes an excellent gift for people who love space and sparkly stuff. So highly recommend. I am obsessed.

Bruce Betts: Cool.

Sarah Al-Ahmed: Let me do your nails all sparkly. It looks like space. I'm wearing the nail polish right now, actually.

Bruce Betts: Did you just offer to do my nails?

Sarah Al-Ahmed: Yeah. Fun bonding activities, Bruce, I'll do your dog's nails. They'd love it.

Bruce Betts: Yeah. Well, the big guy's got nails like about the size of a bear's claws.

Sarah Al-Ahmed: Perfect.

Bruce Betts: They're black.

Sarah Al-Ahmed: So no base coat. It's perfect.

Bruce Betts: All right, everybody, go out there, look up the night sky and think about the fact that you should never paint the nails of a dog. Thank you. And good night.

Sarah Al-Ahmed: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week with some awesome results about the sun from the Parker Solar Probe. Planetary Radio is produced by The Planetary Society in Pasadena, California, and it's made possible by our Saturn loving members. If you want to help make a future where we have even more missions to the Ring Planet, you can join us at Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor, Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. And until next week, ad astra.