NOVA; To the Moon; Interview with William K. Hartmann, Senior Scientist Emeritus at the Planetary Science Institute, part 3 of 3

the cratering history of the Earth because that would give us a better clue as to whether these big impacts that wipe out dinosaurs, wipe out species are periodic. Are they happening all down through history? Do they just come at random? What's going on there? I want to ask you that again. We just hold out. We left off. If we go back, what's to happen if we go back? What's the game going back? What do we get out of it? I think we're... I think that if we want to understand our relation as human beings to the larger cosmic environment, going back to the moon is one of the best things we can do because the moon has been recording the results of living in that cosmic environment and I'm particularly thinking of impactors coming in. And certainly in the last ten years, we've all become aware as a culture of impacts, of the impacts that's been talked about of dinosaurs being wiped out by impacts 65 million years ago,

of the possibility of asteroids in the space hitting the Earth, of even the impact opportunity or the asteroid opportunity, which is that asteroids have resources and we may want to go out and fly out to them and get those resources. So what can we do on the moon about that? Well, one of the things I think would be quite exciting is to go to different places on the moon and actually pick up rock samples, date the ages of impact creators over large regions of the moon, a sample of creators. And look to see, was there some big peak in the impact rate 65 million years ago? Was there an episode of impacts at that time? Are these episodes periodic? Some scientists have looked at creators on the Earth and said, we're seeing a 30 million year periodicity every 32 million years or so, there's a wave of impacts.

Other people say, no, no, no, no, the statistics are no good. This is just a serious result. Well, the moon is the only place where we have enough creators to measure that. So I think a great project on the moon would be a moon base where we go out and date a thousand creators and really reconstruct that cratering record and find out, are we just getting hit one at a time by occasional random events? Or is there really something going on that waves of impact events down through the history of the Earth? We've got to understand that question if we claim to understand the system that we live in. What about your theory? It accepted largely in 84, got people excited, more papers came in about that. Are we seeing it being questioned, challenged now? When you do science, you don't prove new theories, you disprove theories. In other words, you're always trying to find a test. If we perform this test, could we disprove this idea?

When you perform a test and you get a positive answer, it's just consistent with the theory. And I think the situation we're in is that people have begun to perform tests, the computer modeling, looking at Earth rocks, looking at certain chemistries, isotope ratios. And a lot of things are consistent with the giant impact hypothesis, but they don't prove it. And it's a question of building up more and more evidence that establishes that this theory may be right. People are correctly here at this conference, for instance, saying, well, we've got to find more tests. We've got to find some tests that would disprove it. Can anybody think of, if we look in the Earth's mantle rocks, is there some signature there that would tell us whether this impact happened or not? Are there those kinds of tests that we can do? But so far, certainly, this is the theory that's driving a lot of the research. People are making the computer models trying to understand the impact, trying to understand what happens to the debris after the impact.

Does it really assemble into a moon? Would that moon be hot? Would it be cold? Would it match the properties that we see in the existing moon? And all of that work is being done. And that's what a good hypothesis does. Even when it turns out to be wrong, ultimately, it drives a lot of new research. Nature leads us down this road. These are not ideas that scientists just conjure up and get a grant and go study. It's nature's leading us forward. And this is where the data seemed to point. It's driving research down that road to see whether the hypothesis fits the available data. Different subject now. The prospector data coming back. Possibility of water. How do you feel about that? The idea of ice on the poles of the moon has been something kicking around for a long time. And the prospector data seemed to at least be consistent with that in picking up some evidence of more hydrogen concentrations at the poles.

And the big question now is that, do we really know whether that hydrogen is in water or is it some other weird process that's accumulating solar wind hydrogen at the poles of the moon? So I think, again, it's one of these things that we're now that we have this step in the data. We've got to go back with a better set of instruments. And personally, I'd like to see those instruments down on the surface. There are lots of plans for orbiters going around the moon. But it's hard to imagine any set of results from an orbiter that isn't just going to say, you know, lead us to say, well, we really got to get down on the surface. So I'm, in my own mind, I'm saying, why two or three more orbiters? Why don't we just go for it and get down there and find out what really is in those craters on the poles of the moon? Great. Last question. Take me back in the 40s and the 50s from your knowledge of the field. If you were back then practicing, how would you have been received? What was the climate like? Did anybody seriously study the moon or was someone who did that taken to be a little bit off?

Studying the moon in general, you mean, or the origin or impact origin? Was it a serious subject? In the first half of this century, there was very little planetary astronomy as we understand it today. There were a couple of astronomers who specialized in the solar system like my professor, Gerard Coiper. There was the geochemist heraldurie that thought a lot about the origin of planetary bodies and what did the Earth's chemistry tell us. But virtually no field. There was no degree in planetary science. There was no concept of planetary exploration as we understand it today. And I can remember as a little kid, about 10 or 12, my parents would take me to a bookstore in Pittsburgh. We went on shopping expeditions to Pittsburgh. And in this bookstore, when I found an obscure book, University of Chicago Press marked down to $1.49, for some reason I still remember the price.

And it was called The Face of the Moon by Ralph Baldwin. And I bought this book and it was way beyond me at the time. But I poured over this book. And it was a book about the craters on the moon. And there was this controversy going on, where these craters volcanoes, were they volcanic craters, or were they impact craters. And it turns out that that book was the milestone because Baldwin, who had a degree in astrophysics, but went into the war and studied bomb craters in the Second World War, he writes this whole book on the proportions, the depth, the diameters, the characteristics of these craters on the moon, and showed that they all matched explosion craters. And he had all these data from bomb craters all over England and Europe. And that was the pivotal book that really convinced people that the moon had been covered by scars of impact events, asteroids falling out of the sky and hitting the surface of the moon.

And it was such an obscure book. I mean, it was marked down as I said. It didn't have much impact at the time, because it wasn't a field. There wasn't an audience of readers for it. And now we look back and see that this guy, Baldwin, was the one who really made the big step. And not only giving us an answer about craters, but treating the moon seriously as a scientific body to write books about and ask scientific questions about and do geology on. I mean, nobody trained in geology was doing geology about the moon in the 1940s or 50s. And Gene Schumacher was probably the first to come along and really take his geological training and say, let's apply these ideas to the moon. Let's treat the moon like a place. Not like some silvery object up in the sky, but like a real place. And so Baldwin, Shoemaker, Coiper, these people in the 50s were the first just handful of people, three, four, five people that began to create that field.

We went stories, I got stories.