NOVA; To the Moon; Interview with Robin M. Canup, Astrophysicist

There's something else, declines with time. But that said, small things are also produced when things collide. Like when asteroids collide, they'll produce more asteroids and fragments from collisions. So even today, there are a lot of small impacts going on. We don't have any Mars-sized bodies that are flying around about to collide with the Earth. It's in the case of the giant impact, but of course we have one meter-sized bodies. And the occasional kilometer or ten kilometer-sized body, which even though it's tiny compared to the giant impact, is fundamentally significant if you're living on the planet at the time. What's the minimum size to not burn up and actually hit the ground? I believe it's something like a hundred meters and I like that.

As far as the Tunguska type of fireball, where you have something that nearly hits the ground and sends a shock wave of energy at the impact site and does significant damage. So what happens if a one meter chunk of something encounters our atmosphere? Does it make it down to the ground or does it just burn up a shooting star? I'm not really sure exactly how much of that makes it down, to be honest with you. It certainly becomes a meteorite in the sky, and you see it. Oh, okay. Very good. Okay, so I'm going to ask you about... Well, I don't know. What would you rather describe first? What happens after a giant impact and work or how you got into working on that? Probably that direction.

Yeah, let's start there. How are we doing? We're rolling. Go ahead. Oh, I'm sorry. I didn't know you were rolling. Yep. Okay. How did you get started? Well, I was developing dynamical models to describe the evolution of planetary ring systems that orbit around all of the gas giant planets. And I was presenting my results at a conference in Bill Hartman, who of course is one of the originators of the giant impact theory, heard my talk and talked to me afterwards and said, have you thought about applying your models for how these little satellites in the ring systems accumulate to the moon? Because I don't think anybody's actually looked at how the moon accretes after the impact recently, and it's an important point. And I looked into that, and sure enough, this was about 1993, and at that time the giant impact theory was very well accepted as the mechanism for forming the moon. And sure enough, while there had been a large number of simulations by all camera and others that modeled the actual collision and the ejection of debris into orbit around the earth, no one had looked at or modeled the accumulation of that debris to form a moon or moons. And it turns out that it was a very interesting dynamical problem, and it's placed new constraints on the type of impact that's needed to form the moon.

The debris that's placed into orbit by the impact actually orbits in a very similar spot as, say, the rings of Saturn. And in both cases, the gravity of the central planet actually frustrates the accumulation of the material into satellites. That's why Saturn's rings don't accumulate into a satellite. And this same process means that accumulation from an impact generated proto-lunar disk is actually quite inefficient, and a whole series of simulations that we've been doing in the past couple years all suggest that you never end up incorporating more than half of the debris that you initially put up into orbit into the moon. In fact, usually, the majority of the debris ends up being scattered or raining back down onto the earth. Well, what this meant was that the impacts that were placing one lunar mass worth of material into orbit quite effectively were now no longer large enough or sufficient to form a moon. And it's turned out that even this factor of two difference, the fact that we now need two lunar masses in orbit instead of one lunar mass, has been incredibly difficult to reconcile with the dynamical models.

How long do you think it would take? Once this impact happens and the debris is out there, how long does it take till we end up with a moon? Well, the interesting thing is, is it can take very different times depending on what the material looks like when you eject it. If it's an extremely energetic impact, then you can have most of the material is actually in the vapor state. You have a silicate vapor cloud surrounding the earth. And if that cloud is optically thick, it could take a long time to cool, a long time being maybe 100 years. If most of the material is instead thrown out in the form of intact chunks or magma pieces, then it would cool much more rapidly on the time scale of about a year. Once the debris cools and the particles and the bodies and the disk are colliding, the moon accumulates extraordinarily quickly in about a year.

And this is just a function of the fact that the particles in orbit so close to the earth make one orbit every four or five hours. So the moon accumulates once things cool in about a year, which has very important implications for how hot the moon was initially. Because when you form the moon that quickly, it really seems that it should have been completely melted. And it's not clear whether we see geochemical evidence for that. Very interesting. So I know it would be physically impossible. But if I were here on earth, what would I have seen? Let's say the moon formed in a year. What would it be like? Well, it would be a really beautiful sight because the moon is currently very far from the earth. But when it formed, it was in very close. It was ten times closer than it is now. And so it would be ten times larger in the night sky than it is now.

And it might even provide enough reflected light to have nighttime actually feel more like dusk. So it would have been a very extraordinary sight. No way. It's a shame. It's moved outwards so quickly. Well, is it moving outward gradually steadily? Or did it sort of quickly move out and then stay? Where is it now in relation and how fast did it change? Well, the mechanism that causes it to move outwards is its tidal interaction with the earth. It's the same mechanism that gives rise to the twice a day oceanic tides. That mechanism transfers angular momentum to the earth's orbit. It happens very quickly when the moon is close in and then slows down rapidly the farther out the moon gets. And so when the moon was close in, it was moving out very rapidly at hundreds of kilometers per year, say. Whereas now the moon is only moving out at about a centimeter or so per year. Great. Do we have another magnum to find around the table about how would we be if we didn't have the moon?

You were talking about the effect of rays and Jupiter and stuff. Are we rolling? Yes. Okay. The earth moon system and how is the moon helping us? Well, one of the things we've realized in the last decade is that not only is the moon a beautiful object intrinsically, but it's had very significant effects on the evolution of the earth and probably on the evolution of life. One example of this is that the moon is massive enough relative to the earth that it actually acts to stabilize the variation of the earth's north pole. With the moon, our axis tilt, which of course gives us our seasonal variation, varies by about a degree and a half. Now, if you remove the moon, that effects due to the giant planets and the sun lead to a chaotic variation of the earth's north pole that is estimated to have amplitudes of like 60 degrees. So you can imagine that if the earth were subject to that kind of variation, it would be impossible to maintain a stable life supporting environment. You would have periods where the earth would be completely frozen over and you would certainly greatly frustrate the development of life.

And so now, coupled in with the question of what does it take to make a terrestrial planet? We've added on another condition. What does it take to form a terrestrial planet with a moon like ours so that that system appears to be required in order to have a habitable planet? Are we a very special planet in that sense? The kind of moon we have and the fact that we do have a moon? Well, we're very special relative to the other terrestrial planets. There's one other planet in our system that looks a lot like us dynamically, even though it's very far away. And that's Pluto. Pluto actually has a moon that is very large relative to it also. And not coincidentally, we think that a planet moon system formed as the result of a giant impact as well. Great. Good. This is fantastic.

Okay. How do you meet Garwee as a planet? Well, we're finding many other planetary systems now and we've really only just begun our searches. So it's looking more and more like the formation of planets is a natural consequence of stellar formation in many cases. Maybe planetary systems that are very different from our own with giant planets orbiting very close to the star that might preclude having Earth-like planets always, but it certainly looks like generally planets are common. Great. Okay. We should... Would someone have been able to do your work, say, 14, 15 years ago? Well, the advances in computers and computer speed has, of course, been central to the incredible detail in the modeling that we're seeing particularly at this conference. But, you know, usually what happens is that somebody has the initial idea and it's just a creative new way of looking at something. And so, typically, even with a simple model, you can make preliminary estimates to see whether an idea is really going to work or not.

Like the estimates that were made at the Kona conference that suggested that perhaps the giant impact might just work. And then after that, when it comes to working out the details, you're often dependent on your computer resources. For example, in the case of looking at how the moon forms, I really had one fundamental idea. And that was the recognition that the debris after the impact was orbiting in the same place that the Saturn's ring system is located now. And I thought, well, in the Saturn system we have rings and lots of little moons. And they're saying that from this debris cloud that looks like that system, we should get one big moon. It doesn't seem quite right. And that was literally all it took was just the comparison of my models developed for the outer planetary systems and the extrapolation of those to this system. And that, you know, that got me half the way there. 15 years ago, giant impact comes out. Let me describe the lay of the land in your field then and now and how much it's changed.

Well, there were basically three other theories for the origin of the moon at the time. It was capture, coformation, and fission. I'm sorry, I won't ask you to start again. Yes, yes, I know how long ago it was, because they won't hear my question. Oh, okay, I'm sorry. I always forget this. That's right, that's right. So 15 years ago, when the giant impact theory was first being evaluated, there were three other main theories for lunar origin. There was the capture theory, the fission theory, and the coformation theory. And in fact, one of the scientific goals of the Apollo program had been to distinguish between those theories and decide which one was correct. And what ended up happening, of course, was that the results that we got from the moon rocks complicated the picture. We had problems with all three of those theories and a new theory, the impact theory was proposed. At first, it was seen as something that was at hawk, possibly ridiculous, probably unlikely. And in the last 15 years, it has now evolved into the accepted theory for the origin of the earth moon system.

And in that time, our models of how planets form have also developed. And we now believe from that independent line of research that the end stage of terrestrial planet formation will naturally be dominated by these large impacts between planet size bodies. And so now this giant impact event, which seemed so at hawk and unlikely to us a decade ago, we now see as a natural consequence of planet formation. Is this an exciting time in your field? I mean, some people think it all ended in 72 with the last Apollo flight, but it sounds like it's getting bigger and more exciting than ever. This is the best time to be a planetary scientist. Why does one become a planetary scientist? It's because at some point you look up in the sky when you're a kid and you wonder, are we alone? Are there other planets? Are there other earths? And we're in the generation where we are just beginning to find other planetary systems. And there is fundamentally no more important finding, in my opinion, than that.

And I need you to tell me, is the moon a part of that process, the hour moon, our earth moon, understanding that, does that help in that future thinking? The planets that we're able to detect, of course, are not the Earth-like planets. They're the giant planets, the Jupiter's and the Saturn's. And so what our theoretical understanding gives us is a way to look at a system where we can only see the Jupiter and answer the question, does that system have an Earth? And that's really the long-term goals of our models to be able to extrapolate them to other systems with very different architectures of giant planets to determine if those systems might have Earth-like planets and moons. So, studying our moon is important. Studying our moon is fundamentally important, both in understanding the evolution of the history of the Earth, the evolution and development of life, and the likelihood of Earth-moon systems in other solar systems.

15 years ago, when giant impact was one of several, what were you doing? Where were you? How old were you? What was happening? Oh, let me see. I'm just turned 30. So, 15 years ago, I was a high school student, and the Voyager results from the Voyager 1 and 2 encounters with Jupiter, Saturn, and Uranus were coming back. And so we were getting these incredible pictures of the giant planets for the first time. And I have to say, that was very inspirational to me at that time. We've seen some fairly young looking grad students here. Is it possible that when Jeff and I are in our rocking chairs, these people are going to be working on the moon again? I hope so. I hope that some of the young people you see here will be looking at new lunar samples by the time I'm retiring and learning more about the history of the moon and how it formed and ultimately through that the history of the Earth. Great. How important was Apollo? Would any of this be happening if it were not for Apollo?

Well, Apollo was the primary driver, of course, of all of the initial discoveries that depend on the knowledge we got from analyzing the lunar samples. And the origin of the moon was, of course, a big scientific issue before Apollo. And once we had the lunar samples from Apollo, we had all sorts of new geochemical and timing constraints on the origin of the moon, which led to the proposal of this new impact theory. So in terms of going back and these younger students here and continuing, what you said you'd hope that there would be new samples, is it important to have manned missions, humans going? Well, fundamentally, humans are able to distinguish between things in a way that it's very difficult to imagine robotic missions ever being able to do. And when it comes to picking out certain types of rocks, for example, rocks that might look like they were produced from an impact event versus rocks that are basaltic in nature, then the ability to distinguish on the spot gives you an amazing advantage.

And of course, humans are the best at distinguishing on the spot. Great. Are you going to keep working in this field? Because space have a future in general, and does it have a future for you in your career? Well, I certainly hope so. I hope to keep working in this field. It's something that, as a child, I found innately fascinating. I grew up thinking that by the time I was an adult, we would be going to Mars, and that was actually one of my dreams as a child was to be an astronaut and to go to Mars. And of course, a lot's happened since then, and now I'm doing just scientific research. I'm not an astronaut, but the same fundamental scientific questions that motivated me then still motivate me now. Are we alone? Are there other Earths?

Could you imagine, let's say you could go back in time, from what you know of the history of science, would if you were back in the 40s or 50s, would it have been even conceivable to be doing this kind of work, or even taking the moon seriously? Well, the moon fundamentally has been an object of wonder throughout human civilization, I'm sure. I mean, since humans walked on Earth, they have to have looked up and wondered where the moon came from and what it was doing. But it's really only been in the last decade or so that we've realized the extent to which the moon has directly influenced Earth. And then that coupled with these new discoveries of all of these systems of planets around nearby stars, it's hard to imagine that 30 or 40 years ago we could have known of the number of planetary systems we would find of their diversity. That many of them would look very different from our own with large giant planets on its non-circular, close-in orbits.

So we live in a special time scientifically now. How do you feel about the work, the science that worked that was done during Apollo? It was not done mostly by scientists. I mean, those astronauts were pilots. Did they do a good job? Well, you know, I actually saw a talk by Dave Scott at one of our planetary science meetings. And he described the process that they went through, which involved many hours of direct interactions with geologists, where they took the astronauts down into meteor crater and they taught them what the various rock types were and what they needed to look for. And so they were actually extraordinarily well-trained. And so a lot of the initial science is still being built upon today. So those initial rocks are still teaching us things? Oh, of course, yes. And the initial rocks that we have now are, you know, there are only clues.

There are only remnants from an age that it's very difficult to find things preserved on the earth. And there are only direct evidence in any great quantity of material from another planetary body. Great. Cut. Run. No. Oh, cut. Are you all changed now? All right. If we're going to continue. Estimates of whether you can actually get material placed into orbit. And a few very primitive simulations. I've got the whole indoor in just a little bit now. And of course, you know, we're quibbling over what kind of impact, whether we form a big enough moon, but then we didn't even know if you could even get anything close to a lunar mass worth a material into orbit, with anything close to the right spin of the earth. And so that was really a remarkable finding right off the bat. Do you remember you were there? No, no.

And I graduated high school in 86. That's cool. But you've heard about it from some of the older years here. Bill Hartman was actually the person who got me started working on the moon. I was doing work modeling planetary ring dynamics in Saturn's rings and Neptune's rings, the Uranian ring system. And he heard me give a talk at a conference and came up to me afterwards and said, you know, have you ever thought about taking some of these models and applying them to the moon? Because I really don't think anybody has formed the moon after the giant impact yet. And this was back in 93. I thought, hmm. And then I looked into it and I've been working on that ever since. Oh, that's great. Yeah, that's great. Yeah. This is all wild. Head left a little. We're going to see our back. This is Cameroly 217 coming up.

217. This is Sounder 1103. I'll ask you about it. Nice impactors to compare them with what it looks like we need to form the moon. But the main, to be honest, the main contribution I've made so far is the first thing you described. Because that was the first person to do that. Great. Let me ask you about all these impacts going on. Do you think you collectively, is this a steady state rate of impacts going on in our solar system over time? Or was there a particular blip where things really peak at a certain point? Well, certainly things peak when the planets were forming. And then there's more stuff. That's right. That's right. If you take everything that's in the planets now and divide it up into even 100 pieces, you have a lot more big impacts, right? And then the number of surviving things as objects can be eliminated as they collide with something else declines with time.

And that said, small things are also produced when things collide. Like when asteroids collide, they'll produce more asteroids and fragments from collisions. And so even today, there are a lot of small impacts going on. We don't have any Mars side.