Saturn Put A Ring On It Only Relatively Recently

JOHN DANKOSKY: This is Science Friday. I’m John Dankosky. Ira Flatow is away. 

Later in the hour, we’re talking about conflicts of interest and researchers who aren’t disclosing them to journals. If you’re a medical professional, we’d like to hear from you. How do you approach conflicts of interest? Give us a call. Our number’s 844-724-8255. That’s 844-SCI-TALK. Or you can always tweet us @scifri. 

But first, in our solar system, there’s one planet that is immediately identifiable, Saturn, with its iconic rings. Saturn just wouldn’t be Saturn without that cosmic halo, but those rings weren’t always there. The planet didn’t put a ring on it until relatively recently. They’re estimated to have formed 10 to 100 million years ago, billions of years after the planet formed. Researchers were able to give an age to the rings using data from the final dive of the Cassini mission. Their results were published in the journal Science. 

My next guest is here to fill us in. Burkhard Militzer is an author on that study. He’s a professor of Earth and Planetary Science and of Astronomy at the University of California at Berkeley. Welcome to Science Friday. Thanks for being here. 

BURKHARD MILITZER: Hello. It’s great to be on the show. 

JOHN DANKOSKY: Saturn has four groups of rings around it. Let’s start there. What are these rings made of? 

BURKHARD MILITZER: So we can judge them from the color. We’ve never actually taken a sample of it, so we have to tell from remote observations what they’re made of. And from the color, we can tell what the ratio between ice and the rocky component is. They’re mostly ice, so we also think they started out as almost purely icy objects. 

JOHN DANKOSKY: Started out as mostly ice, and they’ve gotten more rocky over time? 

BURKHARD MILITZER: That’s exactly right. So we can estimate how many meteorites hit these rings over time, and they progressively come slightly more darker every millions of years or so. So there’s an influx of meteorites. The same way that meteorites hit the Earth, and we know how many are hitting the Earth every year, we know also how many hit Saturn every year, but nothing so often. 

JOHN DANKOSKY: So what are some ideas about how these rings were formed? What are the theories that are out there? 

BURKHARD MILITZER: So to be honest, the most fundamental question was actually– originally, we thought, and I thought, that the rings are as old as the planet. And that is now what we are now refuting. Now, we think the rings formed very recently and are left to come up with another sort of mechanism. 

So the two ideas concerning what could have happened involve something totally drastic. The rings are billions of icy particles, so you have to have some drastic mechanism to get them there in place. And what really happened is some bigger object got blown into pieces. 

And the two scenarios– we’re not sure which one’s right– but one is that there was a big comet from the Kuiper belt that somehow got gathered, and it got in the gravity field of Saturn. And it got closer and closer, and that got disrupted by Saturn. And then you have so many ring particles left. So if it comes from the Kuiper belt, that is one of the driving arguments, but some bigger object got disrupted by Saturn’s gravity. 

JOHN DANKOSKY: So most planets don’t have rings. What’s special about Saturn that has allowed the planet to form these rings around it and maintain them over these years? 

BURKHARD MILITZER: So if you look very, very carefully, you’ll find faint rings around all of the giant planets. So Jupiter has one, and Uranus and Neptune have very faint rings as well, but Saturn has one that is so prominent. That makes it so special. 

So there’s quite a lot of mass, a lot of particles. And the way of maintaining them is not that difficult because the ring particles follow the same trajectories like a moon, so they’re relatively stable over time. But the question is, how do you get so many particles at just an exquisite location, because if you are slightly out of the orbit with the other ring particles, you’re bound to hit the other ring particles at some point when you go around Saturn, and then you get kicked out of the orbit. 

So you have to have many particles all perfectly aligned. And we think we have many, many more particles, and those who are not in line just get kicked out of the orbits around Saturn. It’s just that many of them will find a line, and they are still there today. 

JOHN DANKOSKY: Well, what else is special about Saturn though? I understand it spins very fast for such a large planet, so the gravity around Saturn must be interesting. Does that have anything to do with the rings? 

BURKHARD MILITZER: Well, that’s– exactly. What the Cassini spacecraft did in its last 25 orbits, before it actually burned up in the atmosphere of Saturn, it measured the gravity field. And I was on the team anticipating these measurements, and what– I’m a theorist– so what we do is, before our experimental colleagues make the observations, we give them some numbers, what they should find. 

So we did lots of calculations in the year before these measurements were made, and we told our Italian colleague that this– we mapped the gravity field– told him this number should be between minus 9 and minus 8.5, and we were sure it would be in between these numbers. And then they measured minus 14. 

So at that point, we were totally puzzled because Saturn’s gravity field was totally different from all the models we constructed before, and the reason was that Saturn has very deep winds. So we left them out of the models. Now, we know that the winds that you see from the clouds that we’ve seen before, they’re not just very thin surface clouds like we see on our planet. 

They were super deep, and they go about 9,000, or possibly more, kilometers into the planet. But the winds are moving slightly faster than the rest of Saturn, and that changes the gravity field. They’re so massive, and that’s what the spacecraft picked up. That was the one remarkable measurement, and the other home was, it measured the mass of the rings. And that was equally important. 

JOHN DANKOSKY: Yeah. And tell us more about that, about the mass of the rings. 

BURKHARD MILITZER: So this was the first time that the mass of the rings was determined from gravity. And you needed to go in between the rings so that the rings are pulling you out while the massive planet pulls you the other way. Then you can really nicely detect the signal from the rings. 

And they are not that heavy. They are about– I don’t know– one four thousandth of the mass of our moon. But now we have this number very accurately, and we can really say the rings are not massive. 

And from that mass measurement, and the color of rings that tell us the rock to ice ratio, and the information regarding how many rocky components are added to the rings every year, we can extrapolate back in time, and we can say when the rings have formed. And that’s not 4.5 billion years ago. This is almost yesterday. This is like 100 million years ago, so there must have been something really drastic happening around Saturn relatively recent that produced the rings. 

JOHN DANKOSKY: Does that suggest to you, though, that these rings are somewhat ephemeral, that as quickly as they were formed not too terribly long ago, that they could disappear again someday relatively soon? 

BURKHARD MILITZER: Well, I think not tomorrow. I think it’s good. If you want to have a look at Saturn, you can go tomorrow, but next week it will still be there, and when you retire, you’re safe. 

But they do degrade. So the ring particles– you think the rings are all perfectly stable, but as we get better and better spacecraft data, we see how dynamic they are. And the best analogy is actually the Earth’s ocean. If you look from far away, it’s just a smooth surface, but if you’re actually on it, you see the waves and sometimes the bigger waves. 

And similarly, that happens in the Saturnian ring system. You lose particles because they drift in and they’re swallowed up by Saturn. And sometimes, there’s a moon embedded in the rings, and then when that comes by, it leaves a big wake in the ring particles nearby. 

So there’s some loss. So over, we think now, 100 million years, maybe they’re gonna be very, very faint or almost gone in 100 million years. We think they were more massive when they started out, and they’re losing mass over time gradually. 

JOHN DANKOSKY: I have to ask you quickly– because this is based on this amazing trip of Cassini, that crashes into Saturn and gives you all this information on the way down– if you could just describe the path that Cassini took as it was gathering this data for you, flying between the rings and Saturn. 

BURKHARD MILITZER: So the Cassini spacecraft has been in orbit around Saturn [INAUDIBLE] for 13 years. And they flew by the moons. And they made very intricate orbit calculations for everything, but they saved the best for last. And if you look at these orbits– the rings, they look big– but if you’re the Cassini spacecraft, you orbit was far outside the ring system, so they had to brake a little bit to do the dive on the inside. 

And that is– it can all be done and calculated, and you can do it with high precision, but you’re never quite sure that you wouldn’t run into one of those errant ring particles that just happened to drift in your trajectory. Nothing happened, but there was– the risk was not zero. So they calculated it and they went inside the rings, which meant they had to come really close to the surface, go through the gap of the innermost ring and the surface of the planet, and then go back out. 

And the orbits are very elliptical. They have the size of 15 times the diameter of Saturn. So they only come for a short moment in this small gap– relatively small– and then they go back out. They’re very elliptical and come back and [INAUDIBLE] 

They measured the gravity field, which made these two observations possible, the deep winds and the mass of the rings. And then they came closer, and they got a little bit of a snippet from the atmosphere. They looked at what molecules there are– there was hydrogen and helium– and what their ratio is, and so on. 

JOHN DANKOSKY: So what does all this ring data tell us about a picture of how Saturn was formed and how, honestly, the entire solar system was formed? What can you learn from this? 

BURKHARD MILITZER: So we get the– first of all, the fact that the rings are so young, it tells us our solar system, even today, is not perfectly stable as we think it is. There’s still collisions. So 100 million years ago, something happened there. We know there was a big impact 65 million years ago on our planet, so in the million year timescale, there’s still sizable impacts or drastic events. That’s the first thing. 

So the other thing, which is a little indirect, that’s why we actually fly these missions. We want to understand how the solar system formed. And how many rocks and how much ice was there available, and how were they distributed? 

And the rocks and the ice, these particles collided, and they made the four rocky planets in the inner part of the solar system. And there was more ice, and it made the giant planets in the outer part of the solar system. We want to understand this better. We have an idea of what happened, but the details are really murky. 

JOHN DANKOSKY: The details are murky, and hopefully, we’ll find out more details over time, but this is fascinating. Burkhard Militzer is a professor of Earth and Planetary Science and of Astronomy at the University of California at Berkeley. Thank you so much for joining us. I really appreciate it. 

BURKHARD MILITZER: It was a pleasure. Thank you.

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