Hal Levison on Saturn’s weird moon Iapetus
“Iapetus is one of the weirdest things in the solar system, said Hal Levision, “and as we study it more and more, it gets weirder and weirder.”
Hal Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, is talking about Iapetus, the the third largest of the more than 60 moons of the planet Saturn. One of the moon’s unusual features is a chain of mountains about 15 kilometers, or nine miles high, that almost completely encircle Iapetus at its equator. Dr. Levison presented a paper at an April 2011 meeting of the American Astronomical Society Division on Dynamical Astronomy, held in Austin, Texas, where he showed evidence that the moon Iapetus might have had a ring of space rocks, like it’s parent planet Saturn, that collapsed onto the moon and formed the mountainous ridge. Dr. Levison spoke with EarthSky’s Jorge Salazar.
What’s happening on Saturn’s moon, Iapetus?
Iapetus is one of the weirdest things in the solar system. Of course, weird things are cool. It was even known to be weird at the time of discovery, when Cassini, who first saw it, realized he only saw it on one side of Saturn. The reason for that is because of a large albedo difference. That is, a brightness difference between each side of Iapetus. One side is almost totally jet black. The other is white. As we study it more and more, it gets weirder and weirder.
The thing that I’m interested in are two characteristics of Iapetus. The first is, if you’ve ever seen a picture of it from Cassini, the first thing people said when pictures started coming down is, “it looks like a walnut.” That is, it’s squished at the poles compared to its radius, and it looks like an object that’s spinning at 16 hours. But actually it’s spinning at 79 days. So it’s got this fossilized bulge that seems to come from a time when it was spinning much faster than it is today.
Also, just along the equator is a ridge that is about 15 kilometers high, and 50 kilometers wide, that we can see, at least in circles at least 110 degrees of the satellite. Because of incomplete coverage, it may actually go all the way around. That’s why the analogy of a walnut is a good one.
We’re talking about a ridge of mountains 6 miles, or 15 kilometers high, that span the equator?
Not necessarily, because it’s almost continuous in places. It looks like a lip. The first thing I thought of when I saw it was a manufacturing flaw. When I was a kid, I used to play with these balls that were made from two rubber halves that were glued together. And there’s a little ridge along the equator. That’s what this thing looks like, except that they’re sort of off-center. So I thought, ahh, it’s one of these balls that was mis-manufactured.
The issue is trying to understand how such a weird thing can come about. There are people who have been working on the idea that it was all internal, that there was shrinkage due to cooling, and the internal heating due to Aluminum-26 could occur. And that could maybe give you that weird shape.
We’ve been working on another idea. And that is, that it was actually a ring around Iapetus that collapsed onto its surface, making a ridge. There’s another problem. We’ve been talking about the ridge, but remember, the first thing I said was the shape. It looks like it’s spinning at 16 hours, really fast, but it’s actually spinning very slowly. And one of the problems for people who try to understand this de-spinning have is that, in the classical ideas, it’s Saturn that’s doing all the de-spinning. Iapetus is spinning real fast. There’s a tidal bulge raised on Iapetus due to Saturn. That’s offset a little bit from pointing directly at Saturn, that cause Iapetus to spin.
The same thing is happening on the Earth-moon system. As the Earth spins, there’s the tidal bulge, which is offset a little bit from the moon. As a result, the Earth is slowing down a little bit. And the Earth is moving away from the moon, slowly. The same argument can be made for Iapetus.
The problem is, that if you look at this, at least naively, is that in order to get this to work and get a lot of energy dissipated in Iapetus as it spins, Iapetus has to be nice and squishy and maleable. If it’s rigid, it won’t de-spin very quickly. But in order to keep its original shape of 16 hours, it has to be rigid. So you have this inconsistency. To get the satellite to de-spin in the age of the solar system, you need something that’s sort of squishy. But in order to preserve the bulge, you need something that’s rigid. And it’s been very difficult to try and solve both of those constraints at the same time. We’ve been able to do it, but it requires some fancy footwork when it comes to setting up the models.
How does one study this moon of Saturn, beneath the surface, without actually going there?
We do that by running numerical experiments. This is the kind of game I play all the time. You see the solar system as it is today. And you try to figure out how it could get there. The way we do that is we tend to build a computer model that has all the physics that you think may have been important; make up initial conditions in many ways; stick them in your code; have them grind for a while; and out pops an answer that we think the solar system should look like; we compare it to what we see and say yes, this is good, this is bad; and try to decide whether it was your initial conditions that were wrong, or your physics was wrong. And you keep iterating until you get something like what we see.
People might be familiar with the rings of planet Saturn, seen through a telescope. But what you’re talking about are rings around a moon of Saturn?
Yes. Our theory, that we’re suggesting for the de-spinning of Iapetus, is that Iapetus underwent a collision with another satellite, which was about 10 percent its own mass.
This is again, another analogy to the Earth-moon. Our favorite ideas concerning the formation of the moon is that something about the size of Mars hit the Earth and created a disk of material around the Earth in that collision, which accreted to form the moon.
We’re suggesting almost the same scenario on Iapetus, that it got hit by something about a tenth of its mass, which is again, what we think happened to the Earth in forming the moon. That created a ring of material around Iapetus. On the outside of that ring, material accreted to form a satellite. And the interior of that ring, stuff rained down into a disk, just on the equator of Iapetus, forming the ridge.
Now earlier, I said that it’s hard to come up with a model involving just that will both de-spin Iapetus and keep its bulge. Adding that satellite, which, since it’s much closer than Iapetus, tidally de-spins it much more effectively and allows us to de-spin it very quickly and still keep it rigid enough that it can keep its shape.
How well is this theory holding up, that Saturn’s moon Iapetus collided with a large space rock to give it the features we’ve been talking about today?
This idea is in its infancy. There’s a way to come up with the broad idea, and write a paper, and say to your colleagues, which is what I’m doing at this meeting, “Here’s this weird idea. And it seems, to first order, to work.” You can write down simple equations that define what we think this behavior is going to be and show that you can sort of get the right answer.
And then, over the next year or two, or even longer, we’ll build more and more sophisticated models, some involving how the ridge will grow, and see if the ridge that we can build on Iapetus from this ring looks like the ridge we see. We’ve got some preliminary results suggesting that may be right. More complicated models on the evolution, the tidal evolution evolution of Iapetus using more and more sophisticated models for the interior of the satellite. Now we’re doing something very simple. And at every stage, we take what the computer models are saying and compare it to what we see, as best we can, and see if we can rule out an idea or not.
Why do scientists study something like a ridge on a moon of Saturn, why the scientific interest?
My interest entirely is trying to figure out how the Earth formed — why it’s where it is, why it has the mass it does and has the chemical composition it does, why it has a moon that we see. That’s my main interest. So when I look around to the rest of the solar system, what I’m looking for is ways of studying process — the physics that goes into building planets. And when we see something that we don’t understand, like this very strange satellite of Saturn. when we see something that we don’t understand, it’s an excellent laboratory for testing the processes that we think were important here. And looking at Iapetus, for example, and this ridge, gives us constraints on how its interior evolved, which tells us something about its chemical composition. Or, if our model is right, how impacts generate rings and discs, which could be important for example, on the moon-forming impact. So these are analogies. All these worlds are analogies to the Earth and what we see here. And the only way we can be sure that our models for the formation of the Earth are right is by looking at these other bodies of the solar system.
One thing that strikes me about Iapetus is just how unusual it is – its ‘yin yang’ colors where one side is bright white and the other dark as coal; this huge ridge of mountains going across its “seam”; and its 79 Earth day rotation.
Iapetus is strange on so many levels. And again, it’s the exception to the rules that help us learn. And looking at Iapetus, you have to take a step back. It’s not my normal thing to study satellites of the giant planets. My main thing is trying to do planet formation, in a way. But when you see something like that, you’re compelled to understand it, again, because it puts it in context. There’s always exceptions. But it’s hard to find exceptions, in science that aren’t telling you something about what you think is true. And so you have to go after these weird things to understand them.
I’m reminded of an interview EarthSky had with you several years ago, when you described the early solar system as a “demolition derby,” a time that astronomers still don’t quite understand that well.
I think you’re going to see a revolution — this is independent of Iapetus — I think we’re going to see a revolution in our understanding of how planets form in the next three or four years. I think that the models that existed previously have not been sophisticated enough to capture some of the physical processes that drove things. And there’s a lot going on, when we look at the solar system, that we don’t understand.
To me, the most fundamental of these is Mars. I know there’s a lot of attention to Mars these days. But what I’m talking about is really a fundamental issue concerning Mars, and that is its size. Our theories of planet formation predict that the planets got larger the farther away you are from the sun. And Mars is an exception to that. It’s probably ten times smaller in mass that it should be, based on any of our models. And there’s been a concerted effort from the community to try and understand that. And we have a new model coming out, concerning that, which I think is going to solve the problem. If that’s true, it illustrates that we’ve ben leaving out an important process in the understanding of terrestrial planets that I think, in the long run, could change a lot of what we think. We’re in the process of writing the paper right now. Come and talk to us when we get it published.
What’s the most important thing you want people today to know about Saturn’s moon Iapetus?
I guess what I want people to take home from this is that the exploration of the solar system that we’ve been doing is continuously giving us surprises, and those surprises help us understand the general picture better. And so I think that the more exploration that we do, the more likely it will be that we’ll be able to figure out the formation process of the planets. And that goes not just for Iapetus, but to Kepler and the extrasolar planetary systems, all these things are linked, in my mind, in an attempt to understand what happens.
Hal Levison of the Southwest Research Institute in Boulder, Colorado, spoke about Saturn’s moon Iapetus, the third largest of the more than 60 moons orbiting the ringed planet.