 Good afternoon. I would like to introduce to you this afternoon my colleague, Professor Mark Johnson. Dr. Johnson has been an integral part of our geology department since 1984 and has won a number of awards for his efforts in research and teaching, including last year's Edgar M. Carlson Award for Distinguished Teaching. Professor Johnson will introduce our first speaker this afternoon, Dr. David Stevenson. Thank you, Chuck. Like the music we just listened to, the solar system is a creative mix of order and chaos, some structure peppered with splendid accidents and chance, and the result is beautiful. David Stevenson, I suspect, knows this. Dr. Stevenson knows the physics, the properties and processes of planetary stuff. In research over 25 years, he has applied his knowledge of the physical properties of the materials found in the cosmos to answering questions about the nature and behavior of the planets in our solar system. His research is that wonderful wedding between observation and theory that produces compelling science and the realization that we can say something about, for example, the nature and origin of the magnetic field of Mercury or about the seasons on Titan or the nature of convection in the sulfur fields of Io or the origin of life on Earth and elsewhere, all problems that he has worked on. Dr. Stevenson is particularly fond of the giant planets. He says Jupiter is his second favorite planet. He knows what Jupiter, Saturn, Uranus, and Neptune are like on their insides, what they are made of, what their stratigraphy is, and if they have a solid core or not, how they flow, convect, evolve, what their magnetohydrodynamics are. Born and raised in New Zealand, Dr. Stevenson received his BS and MS from Victoria University Wellington. His PhD is in theoretical physics from Cornell. He is taught at Cornell, Australian National University, UCLA, but predominantly Caltech, where he has been since 1980 teaching geology, geophysics, and planetary science, and where he is now George Van Osdahl, professor of planetary science. He is a fellow of the Royal Society of London and the American Geophysical Union. He is authored well over 100 journal articles. He has served on numerous NASA working groups in planetary science and on numerous boards and panels in professional societies. And he is a great teacher. At Caltech, he recently won Excellence in Teaching awards from the grad students in 1993 and from the undergrads last year. Dr. Stevenson enjoyed a special relationship with both Carl Sagan and Gene Shoemaker, to whom this Nobel conference is dedicated. One of the first courses Dr. Stevenson took at Cornell was a course in planetary science from Sagan, and he credits Sagan with influencing his decision to go into planetary science. Gene Shoemaker was a friend and colleague at Caltech. David observed with Gene and his wife Caroline at Mount Palomar, and they both shared strong interest in impact processes and impact structures. At the suggestion of the Shoemakers, an asteroid discovered at Palomar in 1989 was named Asteroid 5211 Stevenson. It is a great pleasure today to welcome to this conference Dr. David J. Stevenson, who will talk on the formation of the Earth and the origin of life. I am here because Gene Shoemaker is not, and so this wonderful occasion is for me tinged with an element of sadness. Gene Shoemaker was a great scientist. He was also a wonderful person. He had an infectious enthusiasm for science, which inspired students, inspired everyone around him. I had the good fortune to observe with him on one occasion at Palomar, and I also taught a course with him at Caltech for two years back in the early 1980s. He died in an automobile accident in Australia while on the way to a field area to look for evidence of impact craters on the Earth. So as field geologists would say, he died with his boots on, but tragically early. Had he been here, he would have told you a story about impacts on planets and about the influence of impacts on the history of the Earth. I have a story to tell also, a story that has some elements in common with what Gene would have told you, and other elements that are somewhat different. There are, of course, many different kinds of stories that you could imagine telling. You could imagine creating a myth, a story that tells you as much about human nature as it does about the universe, about us. Or you could imagine creating a story that is a recitation of facts for some event that passed, and for which you have lots of data. For example, you could make a story about how yesterday the Dow Jones Index varied, or you could make a story about the sequence of scoring at some football match last weekend. Notice that in both cases, the sequence of events may be indisputable, and the description of the event may be mundane, but the event itself is not predictable. You can't, in general, predict the Dow Jones in advance. You can't, in general, predict a football game, the score for the outcome of that game in advance. Something to remember as I get further into my story. But actually, my story has more in common with a detective story, with forensic science. A detective or an investigator, upon arriving at the accident or murder, will be looking for clues as to what happened. A speck of blood over here, a strand of DNA there, a little patch of paint left on the bumper of the automobile that has been destroyed. These are clues that the investigator or detective would use to reconstruct what took place. And of course, that's what we try and do in geology, because geology is the reading of rocks. You take a rock, you try and figure out how it formed. You try and figure out the structure in which you find it. You read the rocks. The rocks are like a manuscript, crumpled to be sure, overwritten to be sure, incomplete, fragmentary, but nonetheless a story about the Earth. And the tremendous success of geology lies, of course, in the ability to reconstruct that story. The story I wish to tell is about how the Earth came to be. And you might well imagine that we would go about constructing that story by using the information in the rocks. What we have displayed here is geologic time and a linear sequence from the beginning, extending back to the origin of the Earth at 4.5 to 4.6 billion years ago. And of course, the great success of geology lies in this tiny bit of time that we call the Phanerozoic, the last 500 or so million years. And geologists have put a tremendous amount of effort into that and have divided that period up into various sub-periods. And of course, from the fossil record, which is by far the best in this period of time, we get a story about what happened to the Earth. As you go back earlier in time, however, through what we call the Proterozoic or the Precambrian, back to earlier epochs, the rock record becomes fragmentary, even more so than in this earlier period of time. And in fact, if you go back early enough, there are no rocks at all. We have evidence of the earliest forms of life on the Earth already quite well developed at about 3.6 to 3.8 billion years ago, back here in the Achaean. And prior to about 4,000 million years ago, 4 billion years ago, there are no rocks surviving at all. The Earth is a marvelous recycler of material. Material is recycled within the Earth. We lose the record. We have to use indirect methods of figuring out what happened back here. It is still possible to do it to some extent, and I will describe the extent to which we do it. But in fact, my story will be more than just a detective story. It will be a story that brings together elements of various other aspects of science, bits of astronomy, bits of what we learned from other planets to build a story about how the Earth came to be. The thing that we're trying to explain is, of course, this, the structure of the Earth, this accident, if you will. For indeed, I think the Earth is an accident. We would like to understand what the Earth is made of, how those materials are distributed in this color-coded diagram showing the major layers of the Earth, the crust and lithosphere and the outer regions, the mantle here in yellow, in orange, the liquid outer core of the Earth where the magnetic field is generated, and at the center of the Earth, a solid ball of crystalline iron, a structure which we can look at right now through the aid of the science of seismology, displayed at the bottom of the slide where the various color-coded regions at the top represent velocities of propagation of waves through the Earth. So this is the accident that we want to explain. And of course, parts of the accident, very important to us, are in a region that is so thin I cannot even display it on the slide, us and the oceans and the air. And the reason, again, why it is so difficult to create this story that I want to tell you is because the Earth is so dynamic. It is dynamic now, it was dynamic in the past. So here is another representation, similar color-coding, but now with an attempt to show in cartoon form the notion that the Earth is dynamic. The surface of the Earth participates in plate tectonics and it is this process that is rewriting the story in the rocks, destroying the rocks, creating new rocks so that as one goes back in time, the evidence is diminished. Just as you as a detective or investigator would have difficulty going back to the scene of the crime decades later, especially if there have been other crimes in the succeeding period of time. So here we have a structure that is very dynamic, the material continuously reprocessed. And for that reason, the story I wish to tell you is one that will bring together many elements of science and not just that of geology. There are no rocks as old as the age of the Earth. The basic question I wish to ask is the extent to which we can talk about the Earth as being a natural outcome of the processes that we understand lead to the formation of planets, to what extent it's a matter of chance. In other words, how much of Earth behavior is determined by initial conditions. And as specific issues, one would like to know the origin and nature of the Earth's core, the existence and nature of the layers above the core, the mantle of the Earth, the origin of the atmosphere and hydrosphere and the origin of life. A modest program. Actually, for those of you who are waiting for me to explain the origin of life, I'm going to disappoint you. I do not know the origin of life. Neither does anyone else. But what I can at least tell you about is the conditions in which life may have formed and set some boundaries on the timescale for that process. So let's begin with our story. And I'm going to begin with the astrophysical astronomical aspect of it. And therefore, the next few slides will have a great deal of overlap with what Alan Boss showed you yesterday. But it's such a wonderful story that I don't think it matters that it gets repeated. The story for us, for our solar system, begins, of course, in the region between the stars. Because out there in space, between the stars, there is gas and there is dust. Dust that obscures the sunlight from more distant stars. Dust that contains the heavier elements. Actually, carbon, oxygen, almost everything heavier than hydrogen and helium. And of course, it is from this medium that through a process of gravitational collapse, one forms a disk of material that is the birthplace for the planets in our solar system. This is exactly the same slide that Alan showed yesterday, in the center of which is a concentration of mass that will become our sun and extending outward. There is the gas and the dust and the particulate matter which is already aggregating into progressively larger objects that will form the building blocks for making the terrestrial planets in particular. And of course, the disk owes its existence to angular momentum. The fact that this cloud upon collapse has to spin and so it has to flatten out like a pancake. And it is indeed angular momentum that is responsible for our existence. If there were not angular momentum, there would not be planets, there would not be life. Here, shown in cartoon form, somewhat imprecisely but sufficient for my story, is a description of the process whereby you go from small solid objects to large solid objects such as the earth. And the basic idea is that you start out with a very large number of small bodies in orbit around the concentration of mass that will become the sun. I keep describing it that way because it's not yet an object that is deriving energy from the burning of hydrogen. And in this process, what happens is that once the objects are moon-sized or so, they're mutual gravitational encounters, just the scattering from each other in the presence of the gravity of each other will cause their orbits to change and will allow the bodies to collide with each other, building up progressively bigger bodies. So what you should see as you go down from the top of the figure towards the bottom is the growth of eccentricity and inclinations of the orbits of the bodies, allowing their orbits to cross so that you go from a very large number of small bodies to a very small number of large bodies. And we think this process took place over a period of tens of millions of years, which is a tiny period of time compared to the total age of the solar system, which remember is about 4.5 to 4.6 billion years. So we're talking about a whole sequence of events in this stage of the talk that is compressed into a period of only a few tens of millions of years at most. And so what you have to imagine at this stage is objects colliding with each other somewhat sometimes quite energetically as shown here and in fact in these kinds of collisions in this artist's representation, you would not be building planets, you would be busting things up. But fortunately the big guys do grow and they grow because of their own gravity. What happens is that when you have impacts between very big objects, the velocity of impact is high, but the gravity is also high so that the debris from the impact is often captured, is kept within the gravity field of the merging bodies. These are collisions that are highly anelastic, collisions in which a lot of energy is converted from energy of motion, kinetic energy into heat. As Alan Boss mentioned yesterday, a particular scientist, George Webrow, has carried out an extensive number of investigations on the accumulation of planets to see whether this kind of scenario can produce planets like the Earth. And success is represented in the following examples. What is shown in this diagram is a sequence of outcomes for a set of planets displayed with heliocentric distance in units where the Earth is at one, one astronomical unit. And at the top you see displayed our real solar system, the real planets, Mercury, Venus, Earth, and Mars with the numbers representing the fraction of the total mass in those planets and the little line representing the eccentricity of the orbit. The details are not important, but what I want you to notice is that in these other examples displayed below which are outcomes of computer calculations, George Webrow is able to get a solar system which in many respects is like our own. That is to say that one often gets a planet that is roughly Earth-like and at roughly the Earth's distance from the Sun. But please notice that there is a great deal of fluctuation in outcome nonetheless in the size of the planet and the precise orbit. And of course that's important for thinking about one aspect of the element of chance in our solar system, because if you were to play God, throw the dice, run the experiment again, and make our terrestrial planetary system, then the outcome would be different. Even with the same material, the same angular momentum, same initial conditions, it's one of those properties of physical systems that we call chaos. The system will behave differently if you just run the same experiment. Even if you get very close to the same initial conditions, the outcome will be different. Not different in a profound way, but different nonetheless and different from the point of view of the conditions on the planet that you have made. Now one of the very important parts of my story is that in this process of aggregating the Earth or any of the terrestrial planets, the building blocks that you use are not very much different in size from the planet that you are making. It is a hierarchical process. You must not think of building the Earth by adding together lots of small things. That is the wrong picture. You should think instead of building the Earth by hitting it with very large things that previously aggregated from small things. And this is represented in a histogram here, which is the accumulated outcome of five simulations each of which made an Earth-like planet. And the histogram shows the distribution of projectile masses in Earth units for the objects hitting the Earth as the Earth formed. Now for reference, you should think about the mass of Mars, which is right here, one-tenth the mass of the Earth, sitting right here in this box. So what this histogram is telling you is that in most of the simulations, an object at least as massive as Mars hit the Earth as the Earth was forming. In some of the most recent work on the formation of the Moon, there is even a suggestion that perhaps one needed still larger projectiles than that. I'm going to dare to show you some equations. Let's face it, science is quantitative and you have to understand the numbers, otherwise you won't appreciate what's going on. These numbers are very important, equations are quite simple, so bear with me. What I've shown up here is a guide to the amount of energy associated with making the Earth. Capital G is the Newtonian gravitational constant, M is the mass of the Earth, R is the radius of the Earth, C is the specific heat of the material from which the Earth is formed. GM upon R is the energy available per unit mass due to the accumulation of the Earth. This is just an approximation, but good enough for our purpose. So if I divide by the specific heat, I get a measure of how much temperature increase I can get from the accumulation process. And the answer is about 40,000 degrees. What does that mean? What that means is that the process of making the Earth was so staggeringly energetic, just the material falling in to the gravity field of the growing Earth was so great that you could heat the material from which the Earth formed by this enormous temperature. Actually, it's not really quite true, because if you were to try and heat a rock by tens of thousands of degrees, it would vaporize. And so the second equation is the one that really matters. The second equation tells us how to compare the gravitational energy of formation with the latent heat of vaporization of rock represented here by capital L, and the Earth just happens to be of a size that the energy released as the Earth formed is enough to vaporize it. It's a staggering thought. We, perhaps, many of you have some vague notion of the amount of energy required to melt a rock. That's already a lot. I can assure you, trying to vaporize a rock is a great deal harder, but the Earth had that much energy as part of its accumulation process. And if you understand that, you will appreciate many of the other points that I wish to make about the early Earth. It was a nasty, hellish place. There is a more complicated issue, which I won't attempt to explain in detail, but I'll state the outcome of the calculation. You can also ask whether radiation from the surface of the Earth to space, just as the Earth currently reemits sunlight as infrared, as Sherry Rowland talked about this morning, you can ask whether this energy formation could be radiated away, and if so, at what temperature over what period of time? And the answer is that yes, you can. Even though this energy up here is so enormous, you can re-radiate it at 400 Kelvin, which is really not that much higher than the temperature in this room, which is about 300 Kelvin. And you can do it over a timescale of 100 million years. But, a very big but, if the Earth is made by the aggregation of a small number of big objects, hitting infrequently over an extended period of time, then this result that I've just described is of no relevance, because although you can eliminate the energy by radiation if it is accumulated gradually, you can't eliminate that energy by radiation efficiently if it arrives all at once, as it will in giant impacts. So giant impacts deliver a lot of heat. Quickly, the energy cannot be radiated quickly and the unavoidable consequences complete melting of the Earth. Another way to appreciate how staggeringly big the energy associated with Earth formation is, is to ask how does the gravitational energy compare with the amount of thermal energy in the Earth now? The inside of the Earth is quite hot, by the way. The center of the Earth is about the same as in temperature as the temperature at the surface of the Sun, about 6,000 Kelvin. It's pretty hot. Nonetheless, the energy available from the formation of the Earth is 10 times larger. And what that's telling you is that the energy that the Earth had to expel at some point in its history is far, far greater than what it has now. And indeed, the total radioactive heat produced during the history of the Earth is only comparable to the thermal energy that now resides in the Earth, emphasizing yet again how important gravity is in this process. So to summarize what I'm saying here so far, when you made the Earth a very important part of that formation process involved the arrival of really big things, delivering staggering amounts of energy, energy in amounts so great that the Earth was transformed into a molten sphere. The energy radiating into space had an effective temperature of 1,500 to 2,000 Kelvin. So it behaved like a small infrared star for a hundred years or so. Incidentally, in passing, one of these giant impact events very likely did lead to the formation of the moon. I'm not gonna talk about the formation of the moon, but it is a byproduct of this process. I can now say a little bit about some aspects of the structure of the Earth as we see it now. It turns out that the Earth's core, which is mostly iron, cannot aggregate easily unless you lubricate the overlying material. Let me put it this way. If I took a mixture of solid rocks, the rocks immediately beneath us here in the Earth's mantle and solid iron or even liquid iron it turns out in the form of droplets and just mixed it up, you might say, wow, the iron's heavy, it'll settle down. Actually, that's not true. It turns out it can't do so. You have to melt the whole thing. And then because the iron doesn't like to mix with the molten rock, just in the same way that water and oil do not like to mix, you get a separation of the materials. The details of it are quite complicated. I'm not going to talk about those details, but the basic idea is that the early Earth following one of these giant impact events had what we call a magma ocean so that you can think of the outermost thousand kilometers of the Earth as being liquid rock. And the iron was settling down, perhaps forming ponds at greater depth and then large blobs, tens of kilometers across that then do in fact migrate down to the core. But you do need this extensively molten region and that can come about from the giant impacts. So that's how we think the main structure of the Earth is formed. We think that the formation of the Earth's core was contemporaneous with these events during the actual formation of the Earth during these giant collisions, which remember, all happened way back at 4.45, 4.5 billion years ago. Everything I told you so far is very, very early on. So the picture one has at this early stage is of an Earth that is being bombarded, is hot, perhaps has a dense atmosphere, outgassing, the gas being released from the magma. The rocks arrive with some gas within them. The gas then gets expelled, partly goes into the atmosphere, partly goes into space. A great deal of nebulosity perhaps still, although this is probably at a period where the original nebula that formed the solar system has already escaped. So the features of the earliest Earth in the first 100 million years are first that we had a small number of severe impact traumas which melted everything, or almost everything. A separation of components of the Earth during a short period following these large impact events. What this means is that atypical conditions are important. It's not useful to talk about the average conditions on the Earth's surface. You have to think about the conditions immediately following one of these large impact events. It's very important to think about the last giant impact. When did that take place? What were the consequences of that event? And so chance was important. Chance was important both in the location of the planet that you make, the size of the planet, but also in the way that it was put together, the amount of gas that was made available, how much material that could subsequently be used to form the Earth's oceans would be highly variable if you played God and ran the experiment again. A great deal of chance is involved in this process. In that sense, the Earth, as we know it, is an accident. Of course, I've only come partway through the story. And here we have God saying, something tells me that this thing's only half-baked and he's got the Earth there. And what we're talking about here is the fact that in this first stage, which is very short-lived, we have been using gravity as our means of cooking the Earth. But other things happen. There is heat stored within the Earth that comes ultimately from gravity but must escape and can drive dynamic processes. There's also radioactivity and there is further differentiation. And at some point, you would like to have life. And so those are the things that I want to talk about in the rest of my talk. In fact, what happens if you think about the heat flow out of the Earth as a function of time is that very early on, accretion was dominating. Accretion is the word we use to describe this process of things bashing into each other. There was then a period of heat leaking out that had been stored during accretion. And then finally, quite a bit later, radiogenic heat. From the point of view of the internal dynamics of the Earth, it really is gravity that will dominate. There are impacts later on, but the stirring of the inside of the Earth will be driven by the heat that has been stored within it. On the other hand, if we look at the surface of the Earth, that will still be affected to some extent by subsequent impacts. Let's talk about this period between about 4.0 and 4.4 billion years ago. And we can ask ourselves some questions. What about trust? What about the stuff that makes up the land that we live on? The answer would seem to be that you make lots of basalt which is the stuff that one finds on the ocean floor. It is the dominant volcanic rock on the Earth. It is not the stuff from which continents are made. That requires recycling. There may have been some solicit crust of the type that makes up the continents. The indications are that that was also recycled. This gets back, of course, to my point that we don't have rocks from this period. And just the absence of a rock record, a rock that you can hold in your hand and read is telling you, of course, that this had to have been a period when the Earth, by virtue of being so dynamic because of this gravitational energy that I've been describing, that the Earth recycled efficiently all of this material. What about water? About oceans? There's an interesting issue here. There surely was water in the rocks out of which the Earth formed, although water is actually a minor constituent on the Earth, water is very important to us, but it's only a very small fraction of the Earth's mass, unlike many of the objects you find in the outer solar system. Water, of course, is the most abundant condensate in the universe. The Earth, by comparison, is very dry, cosmically speaking. But a lot of the water may, in fact, have been destroyed. It does react with metallic iron, and metallic iron is part of the material from which the Earth formed. So there is an interesting question here, and people have suggested that some of the water that makes up the Earth's oceans now came later in the form of comets, comets, bodies that formed in the outer part of the solar system, icy bodies, that could have collided with the Earth over a period of hundreds of millions of years and added to the oceans. The most recent evidence on this issue would suggest, actually, that comets were not a particularly important part of the Earth's oceans. This evidence comes from looking at the amount of deuterium, which is heavy hydrogen, and the ratio of deuterium to light hydrogen for comets turns out to be a factor of two different, higher than what we see in the Earth's oceans, whereas the deuterium we see in meteorites is about the right value to explain the Earth's oceans. So the oceans probably were present even early on and very likely completely covered the Earth. Very likely there was no dry land at this stage. What about atmosphere? Yes, and it was probably dense and CO2 rich because to the extent we understand it, it does seem that the amount of CO2 in the atmosphere is a balance between sources and sinks and the sources at that time were stronger. Volcanic activity, extremely high because the Earth was hotter, because it was churning over faster, and those two things together will lead to immense amounts of volcanic activity. And in fact, the CO2 rich atmosphere and the greenhouse effect that Sherry Rowland talked about this morning would have been a much more important part of the early Earth. We believe, in fact, that it's needed to prevent the Earth from freezing over because the early sun was less luminous. A more substantial greenhouse effect would have prevented the Earth from becoming frozen over. I want now to turn finally to the origin of life and remember again my warning, I'm not gonna tell you how life formed. I'm only going to tell you about the circumstances in which life may have formed. Of course, the origin of life is a fundamental scientific question. We see here in this quotation from Louis Pasteur, there is a question about which perhaps I can dare speak to you because it is accessible to experiment and because I have made it an object of serious and conscientious studies. It is the question of generation, so-called spontaneous. Can matter organize itself? In other words, can beings come into the world without parents, without ancestors? Here is the question to resolve. And of course, we believe that indeed it is possible in some sense at the molecular level, the precise pathway is not yet determined, but let me tell you about the constraints that we can put on this. What I have been telling you so far, of course, is that the origin of the Earth sits back here at 4.6 billion years before present. There was then a period of nastiness that I have been describing to you. The Hadean Earth, a very nasty place indeed because you're being bashed on the head continuously, material coming from space, vaporizing the surface, melting the surface, obliterating anything that you create at the surface over and over and over. But we also know that there's life here at 3.6 and it's already quite well developed in the form of algal mats, stromatolites, microfossils, things that we think already take some time to develop. We're not sure exactly how long but some people think several hundred million years perhaps. So perhaps you have to push the origin of life back to 3.8 or 3.9 billion years from now and so we have this window of opportunity, if you will. This is where life had to begin, somewhere in here because you can't do it earlier, you'll be destroyed. Even if you get started, you'll have to start again. You can't do it later because then you have no means of explaining what you see in the rock record at 3.6 so it's got to be done in here. Some of the ideas for what is needed were already pointed out by two famous American scientists at the University of Chicago, Chamberlain, Barbara and Son team during the critical stages of growth talking about the formation of the Earth. The infalling of planetesimals is supposed to have continued to be active, the planetesimals are assumed to have contained carbon, sulfur, phosphorus and all other elements found in organic matter and as they impinged more or less violently upon the surface formed of previous sessions of similar matter, there should have been generated various compounds of these elements. So this is a statement about the material that arrived and the consequences of its arrival. In a very real sense, you could use the words of Charles Dickens and say this was the best of times and the worst of times, the best of times because the stuff that you need to do the prebiotic synthesis to start life was falling out of the sky. At the same time, it was arriving rather to energetically if you will for the survival of anything that was sitting there at the surface. This brings me to a concept that I developed about a decade ago which I want to tell you about called impact frustration of the origin of life. I think that very likely prebiotic synthesis, the sequence of chemical events leading to the formation of life was in fact frustrated by the trauma of frequent impact events. But of course we know from general considerations that the rate of impacts will decline with time. The Earth of course is still being hit. We suspect that the demise of the dinosaur 65 million years ago was the consequence of one of those hits. Fortunately for us those hits are rather infrequent but that very event that occurred 65 million years ago was very common on the early Earth, happening every 10,000 every 100,000 years or every million years depending on which epoch you look at. So the flux of impacting material declines with time. The time between the impact traumas increases as the solar system is cleared out of debris. Basically the idea is the Earth forms in about a few tens of millions of years that clears out most of the stuff in the Earth forming zone at about one astronomical unit from the sun. But there's a lot of stuff further out, stuff being scattered in and to Earth crossing orbits as indeed it still is and that material evidently for reasons that are imperfectly understood was arising at a greater rate during this period of time than in subsequent times. So perhaps the origin of life, whatever that was, could be placed at the epoch in which the time to develop complexity and that's how I'm defining life, self replicating complexity. The time to develop complexity was less than the time between impact erasures. Of course it follows automatically from this picture that life probably started many times over at many locations and got hit on the head many times over in those many locations until finally it caught root somewhere and survived the subsequent major impact events and that is our heritage. What about this impact flux business? What am I talking about here? Well, it's hard to do it on the Earth. It's hard to figure out how many things were hitting the Earth early in its history. We can look at the moon though and in fact, this was one of Gene Schumacher's crowning accomplishments as a scientist to look at the moon, figure out the impact flux on the moon and of course what he was doing was using a calibration for the clock for the time obtained through the dating of rocks returned by the Apollo astronauts. This is the crowning achievement of course of the Apollo program. You could perhaps argue that it might have been possible to do it without astronauts. I'm not sure about that but whichever way you argue it there's absolutely no doubt that the rocks returned from the moon provided us with a clock, a chronometer with which we could assess the age of various regions on the moon as a function of the number of impact craters for that region. So the way that works is you look at various parts of the moon, you say, oh yes, the places that are heavily cratered are very old and I can pick up rocks in those locations, bring them back to Earth and date them using radioactive isotopes. Places that are young, the Murray basalt regions for example, have fewer impact craters although the Murray basalt is itself filling an ancient impact structure you will notice. And in that way one can calibrate this business of the rate at which impacts are occurring and this was done for the moon and can be also extended to the Earth because the population of objects that hit the Earth is the same as the population of objects that hit the moon. And here it is shown as a logarithmic scale measure of the flux of impacts onto the moon and this is real data here. These boxes represent real data and there's a climb in the flux prior to about 3.8 billion years ago. There are disputes about whether this is a spike or a steady rise and there are lots of uncertainties associated with that. But the basic idea is that if you go back to very early times you get a very high impact flux and that's the essence of the idea of the impact frustration of the origin of life. What then can we say about the actual conditions in which life began? I've said that you have to wait until you're not being hit on the head too often but exactly where were you doing this waiting? Where was it happening? And there are various ideas about that of course on this early earth with lots of basic constituents available perhaps lightning, volcanic events. The moon is shown here as being rather close although it probably should not have Maori at this time. That's a detailed artistic license if you will. But what are we to say then about where exactly life began? One possibility of course is to appeal to experiments and there are some famous experiments by Miller and Yuri carried out in the 1950s showing that you can get amino acids from simple constituents through which an electric discharge is passed. They showed in fact that with a mixture of methane, ammonia, water they could get complex biological molecules and so they favored a very reducing environment on the earth's surface. In fact, for the most part, a non-aqueous environment. There is another way of looking at this issue though and that appeals to this influence of impacts that I've been describing because although the impacts occurring at four billion years ago are not enough to profoundly change the internal dynamics of the earth which is now being dominated by the heat released from the deep interior due to previous impacts. Nonetheless, at the surface of the earth the impacts are very important for discriminating among different environments and here I get to my point the deep oceans are a wonderful place to start life because it's a protected environment. That layer of water above you although it will get partially vaporized during an impact and in the biggest impacts completely vaporized. There are impacts that will completely vaporize the earth's oceans. Nonetheless, it's a protective blanket so other things being equal the deepest ocean on the earth or at least in the deep ocean not necessarily the deepest place is a great place to start life and of course in the volcanic environment of the early earth there would have been energy sources and you may recall back to Ed Stone's talk where he told you about the biological communities that have colonized they didn't start there they colonized the mid-ocean ridges which are the volcanic activity centers on the ocean floor right now. I would say that that's a great place to start life next to hydrofermal vents at the bottom of the ocean where you are protected things don't hit you on the head they hit the ocean above instead. And in fact you can quantify this and I've done it and other people have done it and I won't go through the details but the basic idea is that you try and plot as a function of time time between nasty things happening such as complete vaporization of the ocean mass extinction events and so forth and in this way try and get an idea of when life had to begin and a relative measure of merit of different environments for the beginning of that life. But again the main point is that this is a typical earth and this is where I think it began hot water environment and if you look at what the molecular phylogenous are saying where they look at DNA and try and construct the tree of life from DNA from the comparison among the species then by their measure the most primitive things on earth are the archaebacteria that like to live in temperatures above 100 degrees Celsius in hot water. So here's our timeline again the origin of the earth over to the left earliest evidence fossils and stromatolites at 3.5 and we had planet sterilizing impacts up to who knows exactly in this particular figure 3.8 I would actually put it more around 4.0 near the time of the oldest rocks. Obviously we don't know exactly but clearly the impacts played an important role. A fundamental question in all this of course is exactly what material are you using when you start life. And there are two points of view that you could make here. One is that the material is home grown that the molecules that you use as your building blocks amino acids whatever it is you want to use are synthesized on earth. Another point of view which is quite popular is that they are delivered from space. This is not an unreasonable idea given the fact that we see these complex molecules in interstellar space and in particular we see them in meteorites. There are amino acids in meteorites this has been known for a long time. There are particular chemical pathways that are possible in the interstellar medium and difficult on the earth. I would suggest nonetheless that delivery from space is not a particularly sensible idea because the way I see it this early earth environment was nasty enough that almost anything that arrives from space gets reprocessed. The analogy of the present earth where meteorites are gently delivered to the surface of earth slowed down to terminal velocity that analogy is a bad one. Most of the material arrives at high velocity gets destroyed upon impact and if not gets recycled through these hot hydrophilmal systems, hot volcanic systems, extremely energetic early environment of the earth. Reset, you don't keep the memory of these nice building blocks. It must be admitted of course that if there were some magic bullet, something special that you make out in space, it doesn't have to survive most of the time but if a magic bullet were needed and if you need it to get it from space then the earth could certainly use it. But homegrown would seem to me to prevail. It's much more voluminous, closer in time to the epoch in which we need to start life. We ought to look elsewhere of course on this question of life and the most obvious place to look is Mars. Parts of Mars are of course very heavily created as in this example but it has been known for more than 20 years that Mars has volcanic structures. Here is Olympus Mons, massive volcanic structure on Mars and volcanoes are important to my story because volcanoes are the means of expelling water from CO2 from the interior. Right now on earth, the main thing that comes out of volcanoes is water, CO2 as well. These things would also come out on Mars and associated with these volcanic structures there would be hydrothermal systems such as Yellowstone as Ed Stone mentioned in his talk yesterday. And of course we see evidence of flow on Mars as it discussed. There is however one difference on Mars, it does seem likely that because Mars is a smaller body it never had as much water as a fraction of its total mass as the earth. Material tends to get lost. Mars probably never had an ocean and so one part of my story, the protection provided by an ocean is not there. So in a relative sense Mars is probably not as good a place as the earth to have life but if you believe that life is rather easily started and I do then Mars is still a very good place to look for evidence of past life and maybe even evidence of present life because hydrothermal vents ought still exist. Volcanism has not ended I think on Mars so there are still places where water is created by the contact between molten rock and permafrost. So I come to my conclusions. First that no discussion of life's origin can omit careful consideration of the physical conditions on the early earth. You have to worry about impacts. You have to worry about being hit on the head. Very important part of the picture that we now have of the early earth. Shouldered environments are obviously favored. You can reduce your likelihood of being hit on the head by carrying out your synthesis in the deep ocean near hydrothermal vent environment. I say near because right in the hydrothermal vent system itself it's too hot again. You get pyralized, you get cooked. You don't want to do that so you stand off a bit. It's possible of course that there was some dry land and that some of the synthesis was done there and then the material migrated into the ocean and the protection afforded by the ocean then helped the survival of that prebiotic synthesis process in the deep ocean even though the starting recipe called for dry land. I think that homegrown building blocks are more likely than delivered from space. This is of course a speculation. We don't really know what the building blocks are. I think that Mars should have supported the same processes and suffered the same traumas. The absence of a deep ocean may reduce a little bit the likelihood of life, but Mars is still a great place to look. Finally I want to come back to a philosophical point about science. I must admit in my own work I sometimes feel like this poor guy. A pile of theories, not much in the way of facts or proofs. What are we doing in this science? I began by describing it as being like a detective story, but with elements of mythology in fact. Mythology however that is guided by good basic science. What were those elements? There was a little bit of earth science. I didn't tell you precisely how we do it. It's done with isotopes and I didn't get into the details of how it's done, but there are ways of constraining these events that I described by using the stuff on earth. Not a rock that you can hold in the hand, but by looking at the isotopic systematics of the mantle and by inference the differentiation processes in the earth. So that part of the detective story is very important. I talked about the astronomical connection. The astronomical connection is important. As Alan Boss described, we can see processes that we think lead to the formation of planets out there in space right now and we can describe those processes. And I also made a point about the planetary connection. To be sure we want to understand the earth a little bit better, but going to Mars is certainly an important part of the story. And indeed if this were a Sherlock Holmes story, I think at this point Sherlock Holmes would be saying to Dr. Watson, book us a ticket to Mars. Thank you. How do you do? How do you do? How do you do? I have both of them. You may, part of the, the mythological language is the hitting us in the head. Once we get involved we have myth, but I could feel it. When he kept saying hitting us in the head, I was feeling it. Please make out your questions and pass them to the aisles, to the people. And here's Professor Nita Rainer. Once again, we'll begin with comments from the panelists. Professor Boss. David, you didn't really have time to tell us too much about the formation of the moon. You just gave us a bit of a teaser about it, but perhaps you could take a moment or two to tell us about what you think are the leading dynamical questions left about how the impact that produced the moon occurred. Yes, the idea for the formation of the moon that is currently favored runs as follows. During one of these giant impacts where the Earth was hit by an object about the mass of Mars or somewhat more, some of the material from the impact finds its way into Earth orbit as a cloud of debris. The material in Earth orbit then aggregates quite quickly, perhaps on a time scale of just years to form the moon as we know it. In this picture, the moon would be composed of material similar to the outer regions of the Earth, which is good because the moon does not have a composition like the Earth as a whole, does not have much of an iron core, if any. And so this picture would indeed explain that. It also is a high energy event, which is nice because we see evidence in the high lands, the oldest rocks of the moon for extensive melting associated with lunar formation. And the picture is one in which the moon starts out in closer Earth orbit and then spirals away from the Earth over a subsequent geologic time. The dynamic questions that remain unanswered include exactly what set of conditions for the impact are needed to get the right angular momentum of the Earth moon system. It's not quite clear yet whether that will work out as you would like it. And a very interesting question that has puzzled me and others for a decade now is whether you get one moon. We have only one moon. And very recently, some nice numerical simulations by Robin Knopp and others, University of Colorado, has shown that indeed this debris disk around the Earth can aggregate to form one moon. There is some other stuff there which probably gets pushed back on the Earth. And so it does look as though the story is very promising indeed. The alternative theories for the origin of the moon, it would take too long to describe them in detail, but they all have major difficulties. Professor Roland. I have a question that relates to the biology of the life found near hydrothermal vents. It's certainly at the present time with the biology that concerns us. It's based on DNA which has an ultraviolet absorption spectrum, very similar to that of ozone. So it's widely said that life could not have crawled out of the ocean based on DNA until after the ozone layer was there. And that meant after there was an oxygen atmosphere. But that doesn't relate to the hydrothermal vents. They don't need that protection. So what is the nature of the biology at the vents? I'm not an expert on this, but my understanding is that the colonies of organisms that you find at the hydrothermal vents now did indeed come from elsewhere that this is not life that developed at that location. And for that reason one cannot use what you see there right now as a guide to the early Earth. That's unfortunate, but we are stuck with that circumstance. And so it might turn out that depending on your point of view, you could on the one hand argue that perhaps life did not actually begin at those locations that it did actually begin on dry land and then move to these sheltered locations. Or it could be that you can't make any connection between the ecosystem that you see now and the absorption characteristics of the DNA and so on. Of course, the way that the earliest life functioned was not to use DNA. There was probably an earlier evolutionary period that depended on RNA, or perhaps even something more primitive. So it might not, unfortunately, might not be possible to make these connections between the UV properties of the early atmosphere and the ecosystem we now see. Professor Zagreb. David, you spoke about very early experiments of Harold Yuri, who was able to reconstruct in Earth-like environment with electric discharges. The formation of early blocks for prebiotic material, these amino acids. Do you think that contemporary chemistry, biochemistry, in continuing this type of recreation experiment could contribute to the understanding of the origin of life? What has happened since those famous experiments is that people have looked at a variety of different alternatives, including catalysis on clay and other less reducing environments. The impression that you get from these experiments is that Miller and Yuri did indeed strike upon a particularly favorable set of circumstances for creating amino acids that it's quite hard to do if you make the conditions less reducing. And Stanley Miller has long made the point that there is a major difficulty, a conflict, if you will, between the picture that he would favor where you have a highly reducing environment and the picture that most people like myself would favor in which a lot of the carbon is in the form of CO2. It does, however, seem quite likely that the mixture of gases on the early earth is disequilibrium, that is a natural outcome of the volcanic process. There will be some methane. Methane is fertilized quickly in the atmosphere, so the methane mixing ratio is likely to be low, but there will be some reducing species. I think that this kind of experimentation is still very important, but I'm not at all sure that we're close to the final answer. I'm not at all sure that the particular circumstances that Miller and Urie studied are the ones that actually pertained on the earth simply because they were in such a strongly reducing set of conditions. Chuck, I'd like to make a comment there, because I was a graduate student at the University of Chicago when Stan Miller was, and you can have this apocryphal conversation between graduate students when Stan comes back and says, I just talked to Harold Urie, and I've got a great PhD thesis. It's the origin of life. The other graduate student says, do you think you'll finish it in four years? Ha ha ha ha ha ha ha. Philip, do you have a question? I think it was very wise to point out that the hydrothermal stuffs are pretty modern. They all live in oxygen, as you can tell from the red tube worms, and it's pretty hard to do that. It's not easy to break up those exudations of that condition. But what about the other possibility? That it's really deep in the rock, which can be inhomogeneous in space as well as in time. Your guys are mostly inhomogeneous in time because it's a common atmosphere and a more or less common ocean that affects them all. But you can have all kinds of places, all kinds of streams of methane, temperature gradients, other constituents of the earth, plenty of energy, and water coming up in different proportions of different circumstances in many, many spots in this kind of loose, spongy crust or formative crust, active crust. Isn't that another possibility? We don't have the evidence either of life originating there, but it looks to me like a pretty good place. It's sheltered as well. I did not hear everything you said, Phil, but were you asking about the possibility of starting life within rocks? Is that the basic point? Certainly, as Ed Stone mentioned yesterday, there is this fascinating discovery of life kilometers down below us in the rocks. And of course, when we say rocks, what we really mean is a permeable medium where water is flowing through, so there is the possibility of free energy sources and so forth. Yes, Phil, I think you're right that in principle, the ingredients, the free energy sources that you need could exist inside of rocks and the question then becomes whether that material survives in that form during the accumulation of the earth. My inclination is to think not, but that doesn't quite answer your question or at least only answers part of it. I guess what I'm saying there is I'm suspicious about the notion that the material you're using came from space because I believe that that nearly entirely gets obliterated. But if you're asking the more general question, can we have homegrown life, starts on earth, inside rocks? I think that's a fascinating possibility. I have not looked at it myself. I would not exclude it. Okay, we have time for a couple of questions from the audience. The first question, do large body impacts explain the retrograde rotation of Venus and the tilt for Uranus's rotation axis? It is certainly possible that the spin axis of some of the other planets has been greatly affected by impact. In the story that I described, the sense of rotation of the planets that are mostly solid is a matter of chance. That is to say the earth could just as likely have ended up spinning in the opposite sense to the way it currently spins. And indeed, Venus ended up in that manner. The story for Venus is, however, somewhat more complicated because the current rotation state has also been affected by tides. Venus, because it is closer to the sun, will have had its length of day changed through geologic time, through the action of two kinds of tides. So we are actually much less sure in the case of Venus than we are on the earth about the angular momentum budget, the amount of spin that the planet had at the end of its formation. Nonetheless, I think it is highly likely that part of the story for Venus is impact. In the case of Uranus, Uranus is tipped on its side. Yes, indeed, that is a possible outcome of a large impact. Uranus is mostly ice and rock. It has some gas that makes it different from Jupiter and Saturn. And maybe its unusual spin axis is due to a giant impact. However, I want to stress to you that in many of these issues that we're discussing, including this one, we are far from a complete understanding. It does turn out that this particular issue of an impact on Uranus, although you can make it work for the spin axis, it leads to problems for the orbit. And nobody has yet figured out how to get everything come out right, the eccentricity of the orbit, as well as the tilt of the spin axis. Time for one last one. Dr. Stevenson, it is quite an honor to have an asteroid named after you. The question is, who owns the asteroid and the mining rights? As far as I know, I do not own mining rights. The most important thing to me was that it was not an earth-crossing asteroid. I don't want to be responsible. Should it hit the earth? Although it is an honor, I do want to point out that there are a very, very large number of asteroids. And so a lot of people, and I think probably Ed Stone has one, lots of people have asteroids named after them. But it is nonetheless very gratifying, especially because this was one that was found by Gene and Carolyn Schumacher. I have a comment to this, too. Actually, I own One Acre on the Moon, which I bought from somebody at selling it at the entrance to the Berkeley campus about 20 years ago. He was selling, I bought it for $1, and he had for $2 you could get a view site where you could see the earth. And in either way, he wasn't furnishing transportation. By the way, as I understand it, there is an international treaty which says that no country can own, or people can own parts of other planets. Is that correct, is somebody? Maybe you can clarify. I don't know. Thank you, Zakir. You have probably heard that there are a lot of different auctions where former Soviet space memorabilia were resolved, like diaries of Gagarin and so on. And one of most interesting objects, which was bought by unnamed American collector, was a lunar rover, which was landed by Soviets in late 60s. Lunahot. And so the question was, what is an interest in buying it? It's symbolic purchase because you cannot get easily there. And besides this, Lunahot is dead, is not functioning for last maybe 30 years. And the answer is, it's American tradition. It's like making a homestead, you call it homestead. So declare your property rights on the land around. Okay, thank you. We will take a break now. There will be refreshments served on Ekman Mall in the center of campus once again.