Harold Urey (1970) - A Review of the Evidence in Regard to the Structure of the Moon

Thank you Professor Fuchs. And I would like to add, in regard to my experiences in life, that I spent a year with Niels Bohr in Copenhagen, it was one of the most important years of my life. I have been working with Gordon MacDonald on a review of the evidence in regard to the structure and history of the moon for the past two years. It has resulted in a manuscript of considerable size, as you see, and in the next 45 minutes I'm expected to give you a review of what we have put into about 150 pages of manuscript. It will be difficult to do but I will do my best. First of all, I think all of us know pretty well that the surface of the moon has been fashioned by great collisions mostly. These collisions have covered the surface of the moon on the far side almost completely with the results of the collisions. And on this side, including the maria to a certain extent, the whole surface again has been fashioned largely by collisions. Of course, if collisions fall on the moon, and it was anywhere near the earth or moving in an orbit about the sun, of about the same magnitude as the orbit of the earth, one would expect that collisions would also cover the earth in great density. This means of course that any geological features of the earth that had been formed before the time of the collision would be quite completely destroyed. Or would result in great fragmentation of these materials. It thus seems necessary to assume that the collisional history, this intense collisional history of the moon, must have anticipated the oldest craters, the oldest geological formations on the earth. These are known now to be in the neighbourhood of 3 to 3½ billion years, American billions that is, 10 to the 9th years. And hence this collisional process must have occurred early in the history of the moon. This is the conclusion to which I came some years ago and it seemed reasonable to assume that the collisional history had occurred very early in history and possibly at the terminal stage, the formation of the earth moon system, whatever that may have been. The first slide will show us what we see in regard to this at the present time. First slide, please. This is a composite picture of the moon which I made quite some years ago. In which two halves of the moon have been joined together at the centre in order to see more of the features. You will see that in this region we have an exhibition of the intense collisional character of this part of the moon. It also extends around in this way, around the moon, this near side of the moon. But it also extends to the far side. Now, the great maria that you see, illustrated here in Imbrium and Serenitatis, were also produced by great collisions and, as a result of this or after it or in some way associated with it, we believe that the great smooth areas of the moon were produced. Today we know these great smooth areas are covered with a fine dust. And not the surface of a lava flow, which they may have been one time but which they are not at the present time. A very fine powdery dust with rocks that must have been melted after the formation of the moon. I want to speak first of all about the ages of this surface, as they have been developed by the study of the materials that have been returned to the earth by Apollo 11 and Apollo 12. Rubidium-strontium dating has been used, as you know, rubidium 87 spontaneously is converted to strontium 87 with a half time of about 13 times 10 to the 9th years. And this has been used most effectively by one of my students, Dr. Wasserburg at Caltech, to secure excellent ages from the rocks that had been returned. The result of this was that if one assumes that the strontium in the dust material, the soil as it’s called, is assumed to have gotten its chemical composition at some time in the past. And that at that time it had a composition of approximately that of the strontium of the meteorites at the early age which he refers to as a composition called Bobby, for reasons that I don’t quite understand. We find that the soil must have gotten its chemical composition 4.6 billion years ago. Also there is confirming evidence of this in two special rocks. In the one rock extrapolating back from the usual method of determining ages by the rubidium strontium, we find that the composition of the strontium at the time that rock was last melted was so nearly the same as the early strontium of the early history of the solar system, that it must have been separated from material of meteoritic composition, not later than 11 million years after the time that the meteorites formed. In addition to that one other rock that arrived on the earth in Apollo 12 shows by the usual method of determination of rubidium-strontium dating that its age since it was last melted was 4.53 billion years. This shows that very early in the history of the solar system, and of the meteorites and presumably of the earth, the moon on its surface received its general chemical composition. The uranium-thorium-lead dating isn’t so good. But it does indicate very great ages. I think that we’re having trouble with the lead dates because a certain volatilisation of the lead has probably occurred and that some lead has been transferred from some rocks to other rocks. But at the same time the age of the chemical differentiation of the surface of the moon must have occurred about 4.6 billion years ago, and there are some indications that it might have been earlier than that. Now, in addition to that, rocks are found on the surface of the moon, which when dated by these methods indicate that they were last melted 3.65 billion years ago. So we must keep these dates in mind as we discuss the early history of the moon. I’m exceedingly gratified that these ages are so great. I have studied the moon for some 20 years with the hopes that it would be an interesting object. And it would be an interesting object if it should prove to be older than the oldest rocks on the earth. If it should cover history of the solar system not covered by the earth at all. Water erosion and volcanism have completely destroyed the first billion years of earth history. And it is gratifying that the moon will tell us something about events that were happening at that time. It was for this reason that I became interested in the moon and have spent considerable time studying it in the years since. Now, the chemical composition of the moon can be estimated from the density of the moon. The density of the moon is about 3.36 grams per cubic centimetre. And if we try to correct that for temperature and for pressure, the density is increased slightly, the mean density is increased slightly to about 3.4 grams per cubic centimetre. There is considerable evidence that the outer parts of the moon, to some slight depth of 10, 20 kilometres perhaps, has a rather low density. Also at the same time there is some evidence that the moment of inertia of the moon is slightly larger than .4 mass times radius squared. A homogeneous sphere should have a moment of inertia of 4/10th ma squared and there is a slight indication that the moment of inertia is at least that large and perhaps slightly larger. This would seem to indicate that there is some high density material lying in the outer parts of the moon and that it has remained there from some time very early in the history of the moon. This indicates that if we assume that the moon has otherwise the chemical composition of the meteorites that there is about 8 or 10% by weight of iron in the constitution of the moon. The rest being meteoritic in composition. Or we might assume 2% of water and secure this density of the moon from meteoritic material. I am not very fond of the idea of 2% of water in a moon which has been shown to be so exceedingly dry on its surface. But I think it’s not impossible that that might be the explanation of the low density of the moon. Until the last year or so I had thought that the composition of the moon was very similar to what one would expect from material of the sun if the gases were removed and this idea led me to suggest that perhaps the moon is a very fundamental object having a very early history in the solar system. This has been spoiled recently by the astronomers who now claim that the material of the earth or of the more ordinary meteorites is more nearly the same as the composition of the sun than the moon. I’m very sorry they changed their minds in this way but one must accept evidence when it is presented. The analysis of the surface material of the moon have gone very far indeed. There will be published three books extending about so large, shortly, covering the work that has been done largely on the chemical composition of the lunar samples. And of course we know something about them. The surveyor gave us some evidence in regard to this question and the next slide will show what the results were, surveyor 5, 6 and 7. I call your attention to the very high concentration of titanium and a number of other elements are very greatly increased in concentration, barium, zirconium, the rare earths and so forth. In fact the amount of titanium is so great that one must have extracted all of the titanium from about 75 times the volume of material that is presented here. And concentrated this all in our sample in order to have secured this and that is true of a number of others as well. The aluminium is fairly high, iron is rather low and the sodium would be very characteristic of meteorites but not of such concentrated material as this, such as we find on the earth. The sodium should be a percent or 2. Now, in surveyor 6 was very similar except that the titanium was 3.5% and so forth. Now, these were secured by surveyors. Not that when we go to surveyor 7 that the titanium is low, the aluminium is very high and the calcium oxide is very high. In fact this looks like what the geologists call an orthosite, it is a feldspar containing calcium. And it appears that near Tycho on the moon we have material of this very special composition. Now, in the Apollo landings the soil had chips in it that also had a very similar composition as you see here. It looks very much as though these chips of material in the soil, in the tranquillity base material were thrown from some location in the highlands of the moon that had about this composition. It is an assumption but one that I think is reasonable, that indicates that the mountainous areas have approximately this composition, whereas the maria areas, sinus medii and tranquillity base has this composition. The study of the Apollo material, let me just make a few general decisions in regard to it, shows that the surface materials of the maria are high in certain elements, very high. They are also low in certain other ones that look as though they could have been volatilised out of the material. This includes the alkalis and such things as mercury, zinc, cadmium, indium and some others of this kind. Then the siderophile elements, those that dissolve readily in metallic iron are very low in concentration, this includes nickel. Nickel apparently is lower in the surface of the moon than it is in the surface rocks of the earth. Gold, indium and so forth are also missing to a very high degree. And finally elements that go with iron sulphide, molten iron sulphide, the so called chalcophile elements such as silver, zinc, mercury are also very low. What this indicates is that an extensive differentiation of the surface of the moon has occurred. The next slide will show the analytical results. Now, these are from the A type rocks, the B type rocks and the soil which is labelled D. These are the numbers that are given to the particular rocks whose analysis are given here. Notice the titanium is high in the two rocks and is somewhat lower in the soil. As though the soil had been diluted by something, probably from the highlands where the titanium is low. If we look at aluminium, the aluminium is fairly high in the rocks and it’s higher in the soil. Just as though some material had been thrown by the highlands and mixed with this material. And so we go through this list, the difference in the calcium is not so marked, in these particular samples. The potassium is far lower in rocks of this kind than is usual in the surface of the earth. What we must conclude as a result of this is that an extensive differentiation process has occurred on the moon. We do not know how to produce these materials except by a melting and crystallisation process. This must have occurred 4.6 billion years ago. And if our assumption is right in regard to the land areas of the moon, it means that this must have occurred over the entire surface of the moon at the very beginning of lunar history. You might note that this is not the usual geological melting process that we see on the earth. The melting process on the earth is a result of the slow heating of materials deep inside the earth, several 100 kilometres down, with the transfer of the melted materials to the surface. And that has required a long period of time for this heat to develop. The heat, melting process we’re talking about occurred early in the history of the moon. I had expected that the age of the moon would be old, I had not expected such a general heating process. We must conclude that the entire surface was melted in some way, that it solidified and then it was intensely bombarded with objects to produce the many craters and indeed the circular maria on the surface of the moon. These craters could not have been made in liquid material or material near the melting point. This is an interesting bit of the early history of the moon. The next subject I wish to discuss are the mascons, so called, which is an abbreviation for mass concentration. First of all let me show you what they are, may I have the next slide, please. Oh I’m sorry, I missed this slide, let me mention a few things about it. I have calculated, made some calculation. Here is the meteoritic composition, I have assumed that the highlands of the moon consist of this, that the maria represent another kind of material. And I’ve asked what kind of material and how much must I mix with these two, what proportions, in order to produce the meteorites. And that is shown in this column here. And this is very close to estimates that are made in regard to the deep mantel of the earth. Of course, the figures are approximate and writing the analysis to a 100th of a percent is not justified at all. The Apollo 12 rocks had this composition and if we use this instead of the Apollo 11 rocks, we get this composition for the third constituent. The proportions have to be 9.79%, 1.42, 88.64, for the Apollo 11. For Apollo 12 you will see we use more of this constituent, a little less than this and get this for the third constituent. A differentiation process of this kind has occurred and I have already given you the conclusions I draw from this. Next slide, please. Now, this is a map of the moon showing gravitational anomalies. Sjogren and Muller at Pasadena were studying the gravitational field of the moon. And found that expanding the Legendre polynomials, it was very difficult to describe the field exactly. As a result of this they began tracing the orbiters as they travelled around the moon. And measuring their velocity directly with high precision. The precision that they were able to secure is within 1 millimetre per second. A rather good measurement at a distance of 384,000 kilometres I think. As a result of this, they found that an orbiter that was approaching Mare Imbrium, that you see here, was accelerated, and when it left it was decelerated. As though there was extra mass in this region and this is the map of these extra accelerations which they secured. These are the gravity anomalies. A milligal is about 1 millionth of the gravitational field of the earth. And this area here is 230 milligals positive, this one about 180, as I remember, this one about 10, 10, 10, that we see here. And more recent measurements have indicated that there’s an area in here that is really a positive anomaly of some magnitude. And way over in this area is Orientale and it shows a positive anomaly, a slight one, and Mare Smythii in this region also. Altogether some 10 of these gravitational anomalies were detected. In order to try to understand the magnitude of this effect, I have made some models of it. Most recently Muller and Sjogren have estimated that the excess mass that is located in these areas is 2 times 10 to the 21st grams. That’s a large number. Let me see, if this is buried mass beneath the surface and what they detect is the excess mass over what would be there if the moon were uniform. If we have material over here at the side that has a density of 3 grams per cubic centimetre and the density of the material in this cavity were 3.3 grams per cubic centimetre, it would require 20 times 10 to the 20th grams of material in this area to account for that. And I have worked out the volume and I ask how thick a layer, on the state of California would be produced by this. The state of California is just a little bit larger than combined East and West Germany. And the depth would be 15 kilometres deep. That is a very large object if the densities are of this kind. One can make other assumptions, if the difference in density were twice as great, of course the mass can be one half as great, it would still be 7.5 kilometres covering the entire state of California, 400,000 square kilometres. And about 350,000 square kilometres of Germany, East and West Germany combined, it’s a very large mass. The mass in Serenitatis is about the same and the others are less by small factors. And one wonders how this compares to similar things on the earth. The Oregon river Basalt covers an area of about 500,000 square kilometres and is a kilometre deep, it is therefore considerably smaller in volume than the figures that I have just given you. This flowed out upon the surface of the earth about 10 million years ago and was presumably, merely displaced the atmosphere of the earth and not some rocks. In the case of the moon we must displace rocks because this material is below the surface and does not lie high on the surface as do the Oregon river Basalt. Yet there is no gravity anomaly remaining from that lava flow. What has happened is that this layer of material has just sunk by about a kilometre and then the whole effect disappears. If the material in these maria on the moon have sunk a kilometre or a kilometre and a half, we would never have detected the effect that I am discussing. What is true is that this material has been supported on the outer parts of the moon since the time that they were formed, presumably in the very early history of the moon. This indicates a great strength on the part of the moon in a very rigid structure. Suggestions have been made that these mascons are due to lava flows with various modifications of the suggestion which I will not go into here. Now, lava flows occur on the earth because melting several 100 kilometres below the surface. And the pressure of the rocks supply pressure to drive the liquid to the surface. And the liquid comes to the surface because it has a lower density than the solid rocks of the outer part of the earth. And hence, in the Hawaiian islands for example, the liquid can be driven high above the ocean, to a very high level above the rocky surface of the oceans at that time, adjusted of course for some amount of mass due to the water on the surface. And there is a positive gravity anomaly because as the liquid flows out it solidifies and becomes more dense than the liquid. And as a result an excess mass builds up. In the course of some millions of years this disappears and it sinks into the earth and no gravity anomaly is observed. This is the sort of thing that has not occurred on the moon. And it is my contention that it is very difficult to believe that we can have a generally hot plastic interior of the moon where vast lava flows can be produced. And at the same time we produce mascons which are supported on the surface for 4½ billion years. I am having a considerable argument about this, I may lose the argument, I don’t know, but we will see how it turns out. I have another suggestion for this, namely that the objects that fell on the moon to produce the great circular mare came in at moderate velocities, flattened out on the surface of the moon, some 100 kilometres down say, and added an additional mass of high density material. The next slide will show the results of my calculations and those of Gordon MacDonald in regard to this. These are the names of the maria, this has a lot of data on it, you can’t possibly absorb it during the time that we have, let’s talk about Mare Imbrium. It is 680 kilometres in diameter. Now, Ralph Baldwin gives us a formula, from the diameter we can calculate the energy of the object that produced this mare. And from his energy, assuming a velocity of approach, we can calculate the mass. I’ve assumed that the velocity of approach is the escape velocity 2.38 kilometres per second and I get 20 times 10 to the 21st grams of material that is required to produce it. If I doubled the mass, I would of course get 5 instead of 20. If I double the velocity, pardon me, I would get 5 instead of 20. Nordyke gave a formula, slightly different, which makes it 52 instead of 20, and again the same remark holds. Van Doren has studied the waves in the surface of the moon and compared them to waves in water, he’s an oceanographer primarily and not a student of the moon. And he found that the waves surrounding the maria, the circular maria on the moon are very similar to the waves that are produced when a drop of water falls on a liquid surface. And from that he calculated the energies again and I have calculated the mass again in this way. And he gets considerably smaller masses. Collie used a different one, he used, oh I’m sorry, name escapes me, the great mathematician who showed us how to secure the mass from the gravitational anomaly. Then here I’ve used a spherical model, 50 kilometres below the surface to get masses in this way, excess masses. And here is the latest results of Sjogren and Muller, 2 times 10 to the 21st grams, this I regard as the most reliable set of figures. If I assume that the mass of the object that fell on the moon had the composition of the earth, namely it was part of the accumulation process that produced the earth and that its density was the density of the earth at low pressures in temperatures, namely about 4. And that the density of the moon is 3.36, so the difference in density is 0.64 grams per cc, then I get these masses. And you will notice that somewhere in the range of these three masses we get agreement as you’ve seen. I think it’s a striking agreement at least that the objects that were required to produce these circular maria arriving at modern velocities do give us approximately the correct mass for the mascons. It is a relationship that no one else has succeeded in getting so far. It does seem to me that it is difficult to believe that we could have enormously massive lava flows from the interior of the moon to supply this material and at the same time have an exceedingly rigid moon that would retain its structure for 4.5 billion years. Estimates have been made on the temperature of the interior of the moon, because magnetic fields flowing over the moon from the solar wind, to a first approximation, the solar wind with a magnetic field and it sweeps over the moon as though it were not there. This of course means that it is a non conductor. And it means that its temperature must be fairly low because igneous rocks at high temperatures become conducting. Ness in Washington estimated this some time ago as about a mean temperature of 800 degrees centigrade. Sonett and his colleagues at Palo Alto have been making estimates of this and conclude that the mean temperature is somewhere between 800 and 1000 degrees centigrade, that’s for 9/10 of the volume of the moon. They don’t know what is true about the deep centre of the moon. I had a surprising telephone call this week at breakfast from California and of course I thought what's happened to my wife, it wasn’t my wife at all, it was the people at Palo Alto calling me to tell me that they had made a mistake in what they had told me some time ago about the temperature of the moon. They had said that it was 1,300 degrees when they should have been saying in the neighbourhood of 900, I greatly appreciated the telephone call, I can tell you, and was glad that it was this and not some other things. From my first, earliest studies of the surface of the moon and this sort of thing, I concluded that the moon probably was cold and you’ve probably been told that I’m all wrong about it. As I have said about the space program one time that the object of the space program seemed to be to try to prove that I was wrong about everything about the moon. And that the space program was a complete flop and didn’t succeed in doing it at all. I think the moon is indeed exceedingly old and I think there is very considerable evidence that it is an object that formed at low temperatures and has not become exceedingly hot since then. Next slide will show us early evidence of this kind. Now, the alpha, beta and gamma that’s illustrated here, alpha is equal to the moment of inertia around the axis of the moon, rotational axis of the moon. Minus the moment of inertia around the east-west axis. Divided by the moment of inertia around the axis pointing to the earth. And beta is the axis of rotation, moment of inertia about the axis of rotation minus the moment of inertia around the east-west axis. No, minus the moment of inertia around the axis pointing to the earth divided by the moment of inertia about the east-west axis and gamma is a similar definition. The calculated values for the moon under the gravitational field of the earth, the sun and its own gravitational field are given in the last column. The observed values are given in the first column. These are the results of …, and all multiplied by 10 to the minus 4th, of course, and you will see that the agreement is not good at all. It is quite evident that the moon has an irregular shape for certain reasons. Levin in Moscow argues that it is due to temperature variations, Runcorn thinks that it is due to convection, I have been sort of on the fence about the convection idea. Recently it seems to me improbable that there is convection in the solid material of the moon because the seismic effects from the Apollo 11 and Apollo 12 experiments, observations, indicate that the interior of the moon is exceedingly quiet. It’s doubtful that they have observed a single event from deep inside the moon. And I would think that if convection were active on the moon that we might expect such events. I have written to Runcorn about that and have received no reply, I’ll have to try again to see if I can get him to agree or argue strongly against it. One other way of considering the rigidity of the moon is to treat it like a viscous body, formulae for the time that it takes for the Scandinavian shield to rise after the ice melted off of it, have been made by Vening-Meinesz and Heiskanen. And the formula is quite simple. If we calculate moments of inertia for the earth from this formula, we find about 10 to the 22nd poise. If we do exactly a similar thing for the moon, it is 10,000 times as great. Assuming that these irregularities have been on the moon for 4½ billion years. You can change it to 2 if you wish, and you can only change the viscosity by a factor of 2. Of course, neither the earth or the moon are really good viscose liquids, they’re solids and of course they have some physical strength and hence one must regard this as only an approximate calculation. Now, I wish to talk a little bit about the temperature problem that results from this. We had a melting process generally on the surface, we have mascons and an irregular shape of the moon that probably mean that the interior of the moon is too cold to melt. And this is a definite disagreement. We might ask something about the temperature history of the moon. I started this some 20 years ago when the earth was supposed to be 3 billion years old, modified it, many other people have worked on it and there isn’t much disagreement, I think some men have done a very refined job of calculating temperatures. It hardly does much good to make extensive calculations with bad data. I just warn all you young people that it’s a good idea to, if you're going to make extensive calculations, to look around to see if you can’t find some problem that gives you reliable data, so that you can draw a reliable conclusion. No amount of good mathematics will give you a good conclusion if the data is unreliable. First of all, if one assumes that the amount of potassium, uranium and thorium is that in the chondritic meteorites, the interior of the moon will get very hot, it will melt to a very considerable distance towards the surface, even if it started out a low temperature. However, Wasserburg and some others found evidence that the amount of potassium in the earth is not the same as that in the meteorites. And, as a matter of fact, there is one group of carbonaceous chondrites, the type 3, that have a rather consistently low concentration of potassium, 360 parts per million. And if one uses that or makes a calculation of the temperature of the moon, one can start it out at low temperature and it will not melt during geologic time. And I don’t know any reason why that particular choice of potassium isn’t just as good as any other choice of potassium. The meteorites have been through a very complicated chemical process and I believe it is not true that we can trust any of the abundances in the meteorites within a factor of 2 or 3. And if we do that we can keep a cold moon. Some people wish to assume that the moon was completely melted in its early history. Now, if it was melted in its early history then it had to cool off in some way and as a result would concentrate a great deal of radioactivity in the outer layers of the moon. Just from well-known geochemical observations we are sure that that is the case. And of course, if that is the case, they would generate a great deal of heat, 4.6 billion years ago, uranium 235 was 90 times as abundant, potassium 11 times as abundant, uranium 238, twice as abundant. The amount of heat produced per unit time would be much greater. So we had to have rather good conduction of heat to the surface in order to accomplish this. And finally, as a solid body, it has to conduct its heat out by thermal conduction or by convection in the liquid and solid materials. And finally then, after this has solidified, we must then produce the great collisions on its surface. And then 1 billion years later we must come along and melt some of these rocks. One wonders why it cooled off and solidified in the first place if it is going to turn around and melt itself a billion years later. I think we must assume that the surface of the moon changed its character somewhat after the cooling process. Probably collisions turned it into an immense powder. The soil on the surface of the moon has a conductivity of about 10 to the minus 5th. That’s its thermal diffusivity. Whereas rocks on the surface of the earth have a conductivity a 1000 times greater. I think probably the collisions on the early moon produced a fine powder over its surface which enabled material below to heat up and produce the melted material. But how do we support the mascons on the basis of this model? Our whole experience on the earth to, as nearly as I can understand it, indicates that the mascons would not stay in place at all, they would sink a kilometre or so and the whole effect would disappear. It looks to me as though the moon must have been melted on the surface by an external source of heat. One asks what it is and Dr. Charles Sonett and his colleagues have come up with this intense solar wind in the solar system. A strong magnetic field needed to slow down the rotation of the sun. And this magnetic field sweeping across a rather warm surface of the moon would melt the surface of the moon, even if the interior were not melted. And this melted surface could differentiate into layers of the type that I have described. I think before any of us wish to accept this model for the origin of these materials and physical processes on the moon, that we would be glad to have more data. I’m sure that I myself would. I have made quite a number of suggestions in regard to the moon and as I tell people I wasn’t there to observe this directly myself. And they are only suggestions which I will modify immediately if evidence is produced justifying this. This surface melting of the moon however fits in with certain things. The siderophile elements disappeared because there was some metallic iron that sank below the surface and carried them down. The chalcophile elements disappeared because iron sulphide melted and settled down. Elements were volatilised off the moon and it’s difficult to volatilise heavy metals from the moon. But a solar wind would do it quite effectively. If one should bring some element to the surface, let’s say mercury and volatilise it into a gas, light from the sun ionised it and the magnetic field of the solar wind would sweep it right off the moon immediately, since a moving magnetic field will carry charged particles with it and move them right off the moon. I think perhaps this accounts for the loss of the volatiles in this way. Now, the question comes up as to what the origin of the moon is. If you’ll bear with me just a few minutes. Three origins for the moon have been talked about, escape from the earth which was started by Sir George Darwin and has been argued about for 75 years or thereabouts. Mostly the most competent people in this field - and I do not regard myself as one of those – people that are acquainted with the mechanics of the earth, celestial mechanics in great detail, have concluded that it is very difficult to remove the earth from the moon - or the moon from the earth, one is as good as the other. Perhaps it’s not certain at all but I think, evidence is piling up that this is not the correct solution. The second one is that the moon accumulated in the neighbourhood of the sun, simultaneously and all models for this become exceedingly complicated. It is a model that has been put forward by Ringwood, and he talked about it last January down at Houston when the first results of Apollo 11 were discussed. And a great many people took it seriously, I did not. I do not believe that a complicated problem such as the origin of the moon from the earth by some sort of a condensation process, can possibly be accepted, except on the basis of very definite evidence of some kind. Natural phenomena are often so complicated that they cannot be described a priori by theoretical methods. And I believe that is true in this case. If the moon accumulated out of solid objects, which is the way one would think that it has to be done if it is to be kept cold and rigid, then one wonders why the earth acquired 30% of metallic iron, or about that, whereas the moon acquired only some 8%. It’s difficult to understand how a fractionation of that kind could occur. It is also difficult to understand how the moon might have accumulated in the neighbourhood of the earth out of solid particles, except rather rapidly. Epique suggests 80 years for this process. The moon was certainly formed at a high temperature in that case. And if it was at a high temperature, I do not understand how the mascons are supported. Finally there is the capture of the moon by the earth. And in this case the moon had an independent existence, was travelling around the sun for some time. It then of course is an independent planet from the earth. It is an object then probably antedated the earth. I wouldn’t believe that the moon could be captured by the earth unless there were a lot of moons around at one time. I’ve a pocket full of moons and I’m shooting them at the earth. What fraction of these moons that are tossed in the direction of the earth would be captured in any kind of an orbit at all? In fact I’ve said that I wouldn’t believe that the moon could be captured by the earth as a single body, made somewhere in space, with an odd composition of a low density, unless I saw it happen with my own eyes. And I don’t expect to do that. Well, the question is: If we have all of these troubles, how will we settle the problem? And we’ve been looking for evidence of one kind which we have not found. Iodine 129 is a radioactive element with a half life of 17 million years and it decays to xenon 129. And we have found it in the meteorites. Which means that the meteorites got together as solid bodies at a small enough time after the last synthesis of the elements, so that some iodine 129 was present, still in these solid objects. Now, it may be that the earth contains a little xenon 129 but it has been very heavily degassed and even an exceedingly small of iodine 129 would probably produce the xenon 129 that might be present on the earth. Estimated at perhaps 6% of the total amount of xenon is this. By a very complicated argument by these people who are working on this xenon composition problem of meteorites. The question is how about xenon from the moon, but there isn’t any xenon, excess xenon 129 in the samples of the moon that we have found. The evidence is negative as to the moon being older than the earth. But I’ve just described to you how we volatilised the heavier elements from the outer parts of the moon. And of course iodine and xenon 129 would be among these, so they would be swept off with everything else. And so our negative results tell us nothing at all in my opinion. We’re keeping an eye out for possible evidence of this kind. So far as I am aware, at least so far as I will accept at the present time, this is the only type of evidence I know of that would enable us to say definitely that the moon is older than the earth and might be a primitive planetary body. And part of an important history of the early solar system. I think it would be immensely interesting if that were the case. But just because I would find it interesting, doesn’t necessarily mean that old mother nature will agree. And we must accept in the end what Mother Nature hands down to us. For what she does is perfect. I thank you very much.

Harold Urey (1970)

A Review of the Evidence in Regard to the Structure of the Moon

Harold Urey (1970)

A Review of the Evidence in Regard to the Structure of the Moon

Comment

When Harold Urey in 1970 came to Lindau to give a lecture for the fourth and last time, the moon had been a hot topic for more than a year. The reason was, of course, the Apollo project, which brought the first men to the moon in 1969. Urey had already been interested in problems of the solar system, meteorites and the abundance of elements for many years. Already in 1952 he had put forward a theory of the origin of the solar system. So with the samples brought back by the astronauts, it was natural that he now focused his attention on the geological history of the moon. One way of investigating samples from the surface of the moon is to use radioactive dating methods. Since radioactive elements and their isotopes decay on many different time scales, one typically needs to find several “clocks” that “tick” with rates that fits the problem under study. This is where Urey’s old speciality isotope separation enters. In the beginning of the 1930’s, he developed a technique of fractional distillation to produce the isotope heavy hydrogen (deuterium). This isotope occurs at a fraction of 1 in 5000 in ordinary hydrogen and the same ratio appears in the heavy water formed with deuterium instead of ordinary hydrogen. Even though Alfred Nobel wrote in his will that he only wanted discoveries, physics inventions or chemical improvements done the year before to be awarded a prize, the rule is that you have to wait for many years before you are asked to come to Stockholm. Urey was asked already in 1934, but his answer was that he was sorry, but that he couldn’t come: His wife was due to bear a child in December. And, true enough, a daughter was born on the 10th of December, the Nobel Day! Thus, on photographs from the prize-giving ceremony of 1934, Urey is missing. On one of them, though, I recognize my grandfather Robert Bárány, who took part in the Nobel Day for the last time before passing away in 1936.

Anders Bárány

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