NOVA; To the Moon; Interview with William K. Hartmann, Senior Scientist Emeritus at the Planetary Science Institute, part 2 of 3

............................................................................................................................................................................................................................... So it's 72 ideas of percolating, you're hearing papers, you're reading papers coming in from the Russians, data's coming back from Apollo, pick me up there.

And in 72, 73, 74, there were, let's say, several lines of ideas converging. We knew there was heavy cratering on the moon, that's something I had worked on in my dissertation. The Russian literature that was beginning to appear in the West was painting a picture where not only the earth was growing, but simultaneously many bodies were growing. The asteroid belt today is a picture of that where you have several big bodies, lots of middle-sized bodies, lots and lots and lots of little bodies. So you're in that environment. And the lunar rocks are just coming back with the astronauts and telling us that the lunar material looks in many ways like the rocky material in the mantle of the earth. And so at some point in 72 or 73, I was working with my colleague Don Davis at the Planetary Science Institute and Tucson.

And realizing that if one of these large bodies that was growing, the second largest body that was in the earth's orbit, in the earth's distance from the sun, the second largest or the third largest body, what happened to those? Because one of those collided with the earth and blue rocky material off the outer layers out of the mantle of the earth, if that stuff reassembled into the moon, wouldn't that look like the moon? Talked to some geochemistic meetings and the first reaction was, oh no, no, no, we've got this detail, these isotopes look different, these isotopes look different. And actually for the first year or two, I was dissuaded from doing anything with this idea because the geochemistol convinced me oh no, there's too many details that are different. But I now think, looking back on it, that it's very important when you're trying to come up with new ideas not to get into the details, but to pull way back and look at the big picture,

look at the environment, look at this environment and the solar system when there must have been big things running around and colliding with each other to build planets. And think about what happened during that environment and think about the big similarities and don't get hung up on the details because the details, the detailed differences may come from other processes. And that's what I think has turned out to be correct, at least from our perspective now. So Don Davis and I did some calculations of the rate of growth of second and third largest bodies. As you get the earth up to its size, what would be the second biggest body, 6,000 kilometers across, or 3,000, or only 1,000, or how big? And we concluded that there could be things up to the size of Mars or moon-sized objects that were not orbiting around the earth, but were orbiting around the sun with the earth and then would catch up and crash into the earth. And so we concluded yes, it's plausible, it fits with the Soviet calculations, it fits with the lunar rocks, it fits with the fact that the oxygen isotopes and the lunar rocks

are precisely like the earth's oxygen isotopes. And different from oxygen isotopes that we see anywhere else in the solar system, they're not like meteorite oxygen isotopes, they're like the earth's isotopes. So there were many lines of evidence, again, in a kind of a broad, big picture or first or order way that made it look like the moon could have been formed from material blown out of the earth. One more important line of thinking about this was that we knew since long before Apollo that the moon doesn't have much iron in it, it doesn't have an iron core. The moon's density is 3.3 grams per cubic centimeter, that's like the density of a rock. The earth's global density is 5.5 grams per cubic centimeter because it's got this big iron core in the middle. So the moon's low density tells you that it doesn't have an iron core, now how can that be?

That was the big hang up on other theories because it was very hard to imagine growing a moon right next to the earth, the two bodies growing together, but somehow the earth magically gets all the stuff with iron and the moon doesn't get any iron. And the big impact idea was, no, the earth forms, it's hot, the iron drains to the center, you got iron core hidden down here in the middle, you got rocky mantle, then an impact or comes in, blows off some of that mantle material, and that's what you get the moon made out of. In the modern version of this idea, the impactor itself has an iron core, so it's really the two mantles hitting together and splashing all this silicate rocky stuff out, and the rest of it mushes together and forms the final earth, and you're left with all this debris that was blown off the surface contact layers when the collision happened. Do you recall the moment when you said, hey wait a second, I think this impact really could be it, are you in the bathtub or are you taking a walk in the desert? I can't recall a specific moment, Eureka moment, although I think there may have been one, but I do recall moments like that when I was thinking about the cratering process and

the fact, the size distribution of the objects that were hitting the earth, because that size distribution is the crucial thing, there's lots and lots of small bodies, they make shooting stars, you go outside, you see those, you see one every ten minutes, little pea-sized things falling into the atmosphere from space. There's lots of middle-sized bodies, there's meteorite rocks fall out of the sky every few months, meteorites hit cars, meteorites have gone through roofs, those fall on the earth all the time, there's still bigger bodies that once a century hit and make atom-bombed-sized explosions, we had a huge explosion over Siberia in 1908. Every hundred million years, let's go to a larger-sized body, there's something on that order of several miles across that devastates whole regions of the earth and actually extinguishes

species, this is what we learned during the 1980s, 65 million years ago, there was a big impact that temporarily altered the climate and wiped out the dinosaurs. And if we extrapolate that onto the next larger size, you start thinking about big objects like the biggest asteroids in the asteroid belt today, and still bigger objects, and once in the earth's history, back at the beginning, when there were lots of those big objects still in the solar system, one of those hit. So you arrive at, you think about all that, and you think, yes, these things are all tied together, there could have been a big violent impact. At the time that we proposed that idea in 74 and 75, the scientific climate was very different because there was an instinctive feeling on the part of scientists that these processes

of forming the earth were slow and evolutionary and a little bit at a time and lots of little tiny particles. And here we suddenly come and say, you know, maybe there was this enormous catastrophe where you blew off material in the outer part of the earth. And the first reaction was, oh, no, no, no, no, no, no, we can't think about an idea like that because it's too catastrophic, there was a kind of a social climate that we all, as scientists, were taught to think in terms of long, slow, evolutionary processes, even in geology, the principle of uniformitarianism that the landscapes, the planetary surfaces that we see today are the results of long, slow processes, one grain of sand at a time washing down through the rivers. So there was a whole instinct to be against anything that looked like a big violent explosion. As I come to this conference today, it's fascinating to see a room full of the leading

experts in the world living in this new picture of the early solar system with these violent events happening superimposed on all the small, slow events. And what do I mean by that? A few big objects hitting planets at the same time that all the millions and millions of pea-sized objects were hitting planets. And you can't get the one without the other. You've got this size distribution. If you're going to have a big object, you've got to have lots of small objects. Or, conversely, if you're going to build a planet out of lots of small objects, you're occasionally going to get a big one. So it's not a pure slow, evolutionary process. It's not only a violent catastrophic process, it's both things happening at the same time. And so the whole picture we have of the solar system today is that the planets grew out of a lot of small bodies.

And that formed some of the regularities. Most planets spin the same direction. Most planets have their North Pole pointed more or less perpendicular to the plane of the solar system. You know, we're tilted at 23.5 degrees. Those kinds of properties were regularized by countless small pea-sized, rock-sized, basketball-sized, house-sized objects coming in and hitting establishing certain spin properties and so on. But superimposed on that regular system were these occasional big whoppers that came in. And those are what tipped planets a few degrees off of straight up and down, spun some planets a little faster, some of them hit in the opposite direction and spun the planets a little bit slower. So you impose individual personalities on the planets by hitting them with these big objects. And that's a very different world than the world of 1972 or 74 when everything was supposed to be done in this small, small-scale, slow, regular way.

Great. OK. Let's make it a lot more fun to come to these, huh? Okay. Let's be. OK. So do you-coming to Starbucks? Ok. Welcome. Hello. Can you give me a hand? Yeah. It's me? Okay. So, you're-you're published in a series of guitars. in 1974 and this is sort of in this contrary to the general climate in the field. Tell me about the coma conference, it all comes together, what did that feel like? The papers in 74 and 75 and 76 had languished somewhat because they were these odd little papers about big impact but people didn't really like the idea of a big impact, too catastrophic.

And so nothing really happened until the organization of that conference in 84 and I was asked to be on the organizing committee of the coma conference and so for me the big moment really came not so much at the conference but as we had our organizing committee meetings and we saw the abstracts of papers being submitted and lo and behold here were all these papers coming in from leading lights in the field who hadn't been publishing on this but were beginning to think about it for the sake of the conference and beginning to focus on this big impact idea. And so I could tell going into the conference that that idea which had been languishing as I say for 10 years or something was going to certainly get a lot of discussion, certainly be treated as a major contender and perhaps even come out as the leading idea. And that's basically what happened during that week in coma that this was the first real

meeting of experts to digest 10 years worth of thinking about lunar data, lunar rock data and orbital theory and all of these aspects of the problem. And what happened by the end of that week was that the other theories all seemed to have fatal objections and the giant impact hypothesis was coming out a winner. John Wood one of our most articulate lunar scientists gave a wrap up talk at that conference and he invented what he called his report card and he had each theory and he gave each theory a grade on does it explain the chemistry of the moon does it explain the orbit of the moon while this theory gets a B this theory gets an F and then he added up all the grades and gave a end of the semester grade for each hypothesis and there was the lunar giant impact hypothesis getting the top grade in the course.

So we came out of that conference with that being the leading idea and that's what really I think instigated a lot of the computer modeling a lot of much more sophisticated chemical and geological thinking about the idea and here we are having a large, another large conference now where everybody is living in that world of the giant impact happened and what are the consequences and how can we really make a moon out of that event and so on. So professor Wood gave you an A huh? Yeah we got an A from John Wood. Okay I understand that there was a prior to the Apollo 11 rocks being returned there was an enormous amount of debate and discussion about estimating the age of the moon. Now I told that your estimate was I believe 3.6 billion and when the rocks came you were proven right.

The debate that you're talking about on ages wasn't so much the age of the moon itself but the age of the surface features and the moon has large lava flow planes, the dark spots that we can see from the earth and naked eye that make the features of the man in the moon. Those are big lava flow planes and the question was how long ago were those lava flows and I had in the 60s taken the number of impact impacts on the earth for instance in Canada where there's still a lot of impact craters. You look at the age of that surface and you look at the number of craters you can figure out how many craters per billion years are forming and that gave an answer on the moon that these planes had to be 3.6 billion years old something like that. From the number of impact craters. At the last minute in the last year or two before 1969 before the first landing and odd little quirk happened that Jean Schumacher who had a tremendous career in influence and

planetary science got on to some semi secret defense department data on impacts into the atmosphere. They were picking up pressure waves from impacts in the atmosphere. He knew how to calibrate the pressure waves into figuring out how big an impact there was but Jean thought that he could do that or he was using data that other people thought applied and came up with a much younger age for the surface features of the moon. In other words he came up for a much different impact rate and then if you count up craters on the moon and use that impact rate you get a much younger age for surface features. Just in the last year before Apollo suddenly there was this bandwagon and all the features on the moon are much younger than we thought. Maybe there's still volcanism and then the astronauts went to the moon and brought back these rocks that were 3.6 billion years and the whole crater counting field got a black eye basically because of the spurious atmospheric results but the basic method turned out to

be true and is still used today of counting up craters on Mars or the moon or some other surface that we visit in the solar system and trying to use that to estimate ages. The flights to the moon were the big breakthrough because they gave us the actual rocks so that we knew how old the surface was. The moon can be used to calibrate everything else now if you've got so many craters on the moon and a 3 billion year old surface that tells you how fast the craters are forming and then it's a fairly simple step to go from the moon to Mars and say well we've got so many craters on Mars how old is that surface on Mars or so many craters on Venus. But the moon was the key, the moon was the calibration to help us understand how fast the craters are forming in the inner solar system. And you were the guy that didn't recant?

I didn't recant but I didn't recant on saying the 3.6 billion year age which turned out to be right so I'm glad I didn't recant but on the other hand I was small potatoes because I was this fledgling young PhD and he was the great gene shoemaker had a different answer so it was funny you know I sit in the back of the hall and listen and think well gee maybe genes right maybe I'm wrong but then it turned out that it turned out no. In retrospect the Canadian craters and the craters in the Midwest of the United States and other parts of the earth on old surfaces I mean they do tell you how fast craters are forming and we should have known something was wrong with the atmospheric data that gene was looking at someone once remarked I guess Don Wilhelms in his book about the moon remarked either that either I was smarter very lucky I'm probably lucky. Okay that's great.

The later landings on the moon in the Apollo program were longer stay times and gave more data but from my perspective each each landing was of equal value because we landed in a certain plane or certain surface and got an age for that locale that province on the moon and then that enabled us to count craters on that province and from six different landing sites and you built up some statistics on how was the cratering rate changing through time and so forth it all tends to reveal that there were many more impacts again in the first few hundred million years the first one percent ten percent of the history of the earth moon system the impact rate was greater and it makes sense because that's when the planets were sweeping up all the debris that was left over from planet formation. What's the legacy of Apollo and what did it all mean?

Apollo itself tied together I think for the first time the history of the moon with the history of the earth so that it made us aware that we live in this system we're not just living on an isolated planet but we have this neighbor and the neighbor records the early history of the system because there's virtually no erosion there and the earth records the last part of the history of the system but if you put the two together we get the whole story we get the opening chapters we get those violent events at the beginning when that cratering was very high we get the formation of land forms as the planet surface is cooled down on the earth we get an atmosphere and then it evolves into the last half of the earth's history when when plants have emerged changed the composition of the atmosphere finally the animals and plants came out of the seas onto the continents it's been a whole progression and classical geology even today geology textbooks deal mostly with

just the last fifteen percent or so of the earth's history because that's when the stratigraphic record is best and is revealed by fossils in different strata layers but the moon gave us a much vaster perspective all the way back to the beginning of the system should we go back should we be going back we humans if we want to understand our relation to the cosmic environment the larger the real environment we'll have to go back to the moon and on into the solar system I think in the immediate in the next decade or so one of the interesting things we could do by going back to the moon is study the old cratered terrain and younger less cratered terrain.