NOVA; To the Moon; Footage of a roundtable conversation among lunar scientists, part 2 of 2

either completely prevent or preclude the formation of terrestrial planets or allow for the formation. To have perhaps an ocean under a layer of ice, how do you make the argument to go back to the moon when we've been there, done that, and we haven't been to Europe? I would say we haven't been there, done that, we've already mentioned that we've only been to six Apollo sites, very little time was spent there, there were three other lunar sites, which you don't really understand a planet by doing that sort of thing. We don't even understand our own earth that well, and we're sitting on it. But as he says, if people come to you and say, well, the moon's very dull compared to Mars or Europa, then what's the answer to that? That the moon isn't very dull, and that exploring Europa in a way that would also advance our knowledge of it sufficiently to say anything about life is going to be far more expensive than studying the moon to get the answers we want about that. Let me actually follow that, because I think Graham's got a really good point here. Back when archaeologists were trying to interpret ancient hieroglyphs in Egypt, they were stumped until they came across the Rosetta Stone.

In a sense, what the moon does for us is serve as a Rosetta Stone, it preserves that early history of the solar system is eradicated on our planet, and it's difficult to actually get a handle on other planets like Mercury, Venus and Mars and into the outer solar system because it's hard to get there and it's hard to stay there and make very sophisticated measurements. In a sense, the moon has the potential to let us unlock the secrets of how our solar system formed and evolved, and that's probably what makes it unique, not that we've been there and done that. Besides which, many of us are very interested in the Earth, and it's now the Earth Moon system that we're looking at, not just the Earth or the Moon. And even way before the giant impact idea, I mean, in the days of Apollo, it was the moon that gave us the first idea that such a thing as an ocean of magma could envelop a body, a liquid rock. I'm a astonishing idea that it wasn't. Well, it didn't, people didn't immediately think it was applicable to the Earth at all, it took many, many years. Essentially. And now we think it's almost inevitable. Yeah, I wrote a much ignored paper in 1981 where I did an analysis of that problem,

and you know, if the moon got melted by whatever mechanism, what were the consequences for the other terrestrial planets? And the answer is they all had to have magma oceans, but because everybody knew the Earth didn't have a magma ocean at that time, nobody paid attention to it. But it's surprising that it's 20 years after the recognition of a lunar magma ocean that it finally became mainstream that the Earth must have gone through a very similar event. And even more interesting, it took so long to connect the two, that its causality, probably a large impact into the Earth, caused the terrestrial magma ocean and made the moon and made the moon a magma ocean, but you got to figure out whether that's true or not. But is it a mainstream idea that the Earth had a magma ocean? I seem to be hearing lots of argument that it didn't or couldn't or may not have. But isn't that the way science works, I mean, you know, and the same reason I was very skeptical about Bill's ideas 20 years ago, coming up on 25 years ago, now, just we were young then. Right.

You could have made it back something 25 years ago, it doesn't need to say that somebody else is right about something. No, no, no, but the way you made progress and one of the reasons I've come around on the giant impact is as you poke at it and try to prove it wrong, what you find is you can never prove it right, but all the other hypothesis look worse. And so you end up with something that, the stubbornly persists, that's terrific. It makes sense. We don't understand everything about it, and there's undoubtedly, you know, a number of elements of it that are not correct. But the self-consistent picture from physics, from chemistry, from geology, you know, I find that compelling that it says there had to be big impacts, and that the moon had to be a consequence of that. You call the moon, and Rosetta Stone, we have a colleague Dale Crookshank who has tabulated a list of all the objects in the solar system that have been called the Rosetta Stone of the Solar System. This includes the moon and Mercury and Mars and Hector and Pluto and Triton and so on and so on. But on the other hand, the moon is the only body that records the early history of the Earth moon system as a system, and so it's also accessible, it's nearby.

Well, it has a direct, dynamical effect on the Earth. Yep, yeah, I don't know. Tides, various human rhythms. You know, you talked about where your models of the accretion of the moon and the planets might be 20 years from now. We're never, people need to understand, at least I think this is right, that we're never going to have a model that you put in the initial conditions and you turn on the computer and presto outcomes, the Earth with one Earth mass and a moon of one moon mass going around it, and Mars of one Mars mass, because what we're learning about the solar system is there's this chance, this random chance element in this whole story, and every time you turn on the computer with these initial conditions you're going to get a different outcome, you're going to get a different model, because in one case these two bodies hit, in another case they didn't hit, and it started off differently, and another nuclei turned into

the Earth rather than this nuclei over here, nucleus over here, and so on. That's true. A lot of the aspects of that science is, geological science, that's true of the evolution of life, you start the tape all over as Steve Gould says, you don't get the same playback the next time. People tend to have, particularly lay people, tend to have kind of a Newtonian model that you start with these conditions, and it always ends up this way, but there's a certain chance, chances in all kinds of things can happen, but specific things are unique. You get a solar system with certain characteristics of planets are all in relatively circular orbits and they're all in a disk, and there's a few of them, but you don't always get the precise spacing, you don't always get the same masses as you go out from the sun, and so on. But we're really shocked people, and the first big extra solar planets were discovered, some of them have very eccentric orbits, and so that was not anticipated, although people said what if, but I don't think anybody took that seriously. And those planets, some of them were very close in, at their closest approach. But we're all this modeling is going, is it's going to tell us whether the planets that

we discover around other stars, ten years from now, and five years from now, are created in the same way that our solar system was created, because if the other solar systems are mostly turning out to be very non-circular orbits, and non-copainer orbits and so forth, then it's going to turn out that our solar system is relatively unique, and most of us are just going to go back to the moon. Oh, absolutely. Do we need to send humans? No, absolutely. I mean, where is humanity going? I mean, my own guess is, while we haven't figured out what the physics is yet, we're probably going to go to the stars. And to get to the stars, you've got to start with a tiny step, and that tiny step, which Neil Armstrong described when he got the spacecraft in 1969, was to the moon. I think you go to the moon and learn how to live there in a self-sustaining way, then we probably hop to Mars. And then the question is, where next? And it is certainly true that we cannot sustain human life, even through generations of human life, to another star right now.

But remember, 100 years ago, we didn't know about radioactivity. And I'm not so arrogant as to believe that 100 years in the future, that there won't be physics that's discovered that simply isn't known today, no matter how clever everybody thinks they are. I think you're looking too far into the future, though, by saying go to the stars. I don't think any of us have a particular goal of going to the stars. We think in rather shorter time frame than that, we deal with our own lifetimes, or maybe our grandchildren's lifetimes, and we take it a step at a time. So we think of them going back to the moon more in terms of a rather more local thing than going to the stars, I would think. And I see that as moon, asteroid, Mars. And a lot of people talk, and even President Bush's program was moon, Mars. But it seems to me asteroids are going to be the logical intermediate step, because there's asteroids that are easier to get to and come back from that Mars, or even the moon for that matter. So the moon becomes not only a site for its own sake that's interesting, but as a stepping stone to learn how we can have a capability to operate in space and go explore all these

other bodies. Let's consider the moon itself, and why would it go back to the moon? What is it that we want to know about the moon, which would help us understand the origin of the moon, the evolution of the moon, what happened there, what it's made up? I think one thing is a cratering record, just the impact record, over the history of the Earth and the moon system, and do these impact comes in waves, I mean, do they come every 30 million years, like some people talk about? We can collect rocks and date them, and we'd actually know, but we'll start the moon as well. That's a very important question. But I actually think that the main reason we want to go to the moon is not simply to do science, but as part of a long-term human-yoning to distribute our species. And maybe we don't go further than Mars. I'm sure we will go as far as Mars with humans and set up colonies. And I don't think the public would sustain an expensive space program simply so we could figure out whether some dinosaurs were wiped out in a swarm of asteroids on Earth. But it is important to know whether other stray bodies would come and threaten us. In the future, I agree with that. Yeah, I agree with that. We are the first species capable of protecting ourselves against the entire biosphere of our sort of duty.

And we'll tell you something about whether the universe is full of other civilizations, because the question is, how long is the stable period for life to exist on a planet? And if the characteristic in a planetary system is that you get hit by a huge body every hundred million years that wipes out 70 to get an asteroid. And how does that change? If you move the position of where the giant planet is, do you get a lot more impacts? Do you not allow for life to develop? That's not actually one of the astonishing ideas, I think, recently. And that is if you don't have a Jupiter, something massive, what the possible types of planetary systems are. I think it'll go a long way to talking about the possibility that living things are on planets around other stars. Of course, the moon gets tied into that. Because remember that the moon plays this very important dynamical role in stabilizing the tilt of our North Pole, our rotational axis. And without the moon's gravitational presence, that would have a very large variation, something like 60 degrees.

And with the moon, it only varies by about a degree and a half. And of course, even with that small variation, we still have very large climate fluctuations that resolve from that. Yeah, but we would actually have total catastrophes if you could change the us spin axis by 60 degrees. I mean, you'd literally go into global ice ages on a very frequent basis and possibly never come out of them. But here's a curious thought that connects with this conference that if the production, if the moon is really what makes the Earth stable for life, which some people say, by regulating the seasons, she says. And if the moon is only produced by a chance impact, then our system only has our grand civilization, and they were pleased to call ourselves so intelligent, because of this random impact. And, I mean, is this the nature of the universe that we just have civilizations when random little things like accidents have happened? There's no reason to. That's enough.