Priestley Medal Address
Chemistry and Physics of the Moon Harold C. Urey
I wish to thank the American Chemical Society and its committee for the award of the Priestley Medal. I was surprised when I heard of this award. I had thought that I had wandered so far over the fields of science, without developing any particular field of chemistry in detail, that I could hardly expect to win this medal in honor of the discoverer of oxygen. Then, I began to think a little about the things I had really done. I began by making some thermodynamic calculations at Berkeley, and then tried to do some work in molecular spectra, and you know, one field got named the Urey-Bradley field. I have sort of forgotten what it is, but it had to do with calculating the vibration frequencies of diatomic molecules. At about the same time, Arthur Ruark and I wrote our book on "Atoms, Molecules and Quanta." All young men should write books. This gives one a wonderful education. This was followed by the discovery of deuterium, and I must mention that Brickwedde and Murphy helped me very much in this. In fact, Murphy and I put our heads together to solve the theoretical problems involved. Then, this was followed by some very important developments, and today it looks as though deuterium might be the fuel of the future. I very much hope that it can be that, rather than these highly radioactive substances such as plutonium. This, of course, was followed by the fractionation of isotopes by chemical means, and this, I think, was an interesting chemical development. It led to methods of the concentration of isotopes first begun in the 30's by myself and a number of students and colleagues, particularly Ivan Taylor and David Rittenberg, who helped me with that, and others of course. This next led to the temperatures of the ancient oceans, which was furthered by Prof. Lowenstam of California Institute of Technology and Prof. Emiliani of the University of Miami at the present time, and very important extensions have been made by Prof. Clayton of the University of Chicago. It depends upon the abundance of oxygen-18 in carbonate rocks as the temperature varies. Prof.
Emiliani has been using this very effectively to show the variations in recent years, and he points out that temperatures of the oceans are a maximum at the present time. Normally we could expect to have glaciers next. Of course, it may be that our activity will reverse this in some way. At this time, I wrote "The Planets." It seems to be dreadfully out of date at the present time, but I cannot rewrite it because the subject has grown so enormously. Next, I turned to the study of atmospheres and published a long article on this in the Handbuch der Physik. This led to the so-called "Urey Equilibrium" as it was named by Lloyd V. Berkner. It is interesting that we live under very precarious conditions. The concentration of carbon dioxide in the air might decrease and the growth of plants be severely inhibited. The study of the atmospheres of the earth led to some ideas about the origin of life. After I had written a paper about this, I discovered that very much the same ideas had been put forward by Prof. Oparin in Moscow. Stanley Miller was my student, and he very quickly showed that electrical discharges in gases would lead to the production of amino acids, these very important compounds which are needed for the origin of life. I ran across a book by Ralph Baldwin on the moon and became interested in it. I have been studying it pretty much since during the last 20 years. I am going to talk about that particularly in this lecture, and I would like to pay my respects to the many younger men who have supplied the observational data and many ideas in regard to lunar history. It has not been done by myself. We wish, first of all, to review some of the older evidence in regard to the moon. First of all, we might mention that the density of the planets varies very considerably. Mercury has a high density, Venus and the earth intermediate
densities, and Mars has a density that for low pressures and low temperatures would be approximately that calculated from the meteorites, which varies from about 3.6 to 3.8, the latter figure being more probable. The moon has a density of about 3.34, and, if one tries to correct this for pressure and temperature, which I have tried to do several times, it comes out to be about 3.4 at low temperatures and pressures, distinctly different from the meteorites and the other planets. It is difficult to explain this, and one of the explanations that has been current for a long time is that the moon escaped from the earth, and, hence, has the composition of the mantle of the earth. The abundance of the elements has been a subject of considerable interest for the last 20 years, and I think the question is not completely settled, although most people are accepting the chemical composition of the nonvolatile elements as found in the carbonaceous chondrites Type I, at the present time. This leads to a density of the mineral fraction, the nonvolatile fraction of this material of about a density of 3.8, if the iron is completely oxidized. If water or the carbonaceous materials were present to some extent, of course, the density would be considerably less. Some years ago, working on this, I thought that the moon was a primitive object because it appeared that the iron abundance in the sun was considerably less than that indicated by the meteorites. Recently, more analyses have been made of the solar abundances, and it appears that they agree more nearly with the meteorites. However, some people have not agreed with this, and the matter is still somewhat open to discussion. I think it still may be that the moon has the composition of the nonvolatile fraction of the sun, perhaps with some volatile materials included in the deep interior of the moon. Also, one of the older bits of evidence in regard to the moon was that conApril 16, 1973 C&EN
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cerned with the moments of inertia of Thus the highland areas could be the moon. The three moments of inertia anorthositic material. This agreed with can be calculated for a plastic object the previous results of Turkevich. rotating about the earth at the distance These rocks from Apollo 11 changed of the moon, and we would find only all our minds in regard to the surface of very small differences in the moments of the moon. The surface of the moon is inertia if that were the case. However, highly differentiated, and the rocks were astronomers working on this problem, certainly separated from primitive solar particularly Koziel in Germany and material by a melting process such as Jeffreys and Kopal in England, find that we find on earth except that on earth we the differences in the moments of inertia have very little anorthositic rock. The differ by more than what they expected difficulty is this. Anorthositic rock is a by a rather large factor, an order of high-melting type of material, and at magnitude or more. This is supple- the same time has a low density. What mented by some work from the space flows out first on the earth is basalticprogram on the gravitational field of the like material, since this is the first mamoon by Michaels and others, and this terial to melt from the mixture of rocks indicates that the moment of inertia is of approximately meteoritic type. In very nearly that of a sphere of uniform fact, there are anorthositic rocks on the density throughout. These older data earth in various places, and I have have not proved to be wrong, so far as I asked some of my geologic friends how know, though we do have great diffi- they account for them. The first reaction culty explaining this in terms of the re- has been that we do not know where they cent data on the moon which indicate came from. The second one has been that the surface regions, for a consider- that they froze from a melted pool of able depth, have a low density. The silicate rocks with anorthositic rocks moment of inertia can hardly be that of floating on the surface as solids as ice a sphere of uniform density. I shall floats on water, because of their low discuss this later. density. Of course, if we had meteoritic These data, some years ago, led me to material and we produce this anorthothe suggestion that the moon was really sitic material, we should also expect to formed at a low temperature and had find some dunitic-type material, which not been melted in its history at all. The is high in the metasilicates and orthoreason for this was the evidence for a silicates of our common rocks. We moon of irregular shape, such as we see might just have a small percentage of from the data that I have just quoted. I basaltic-type material also. It looks as thought the moon accumulated at low though there was a liquid layer of some temperatures. This was partly because depth on the moon that solidified with Mars appeared to be a planet of uniform the anorthositic material floating mostly density and perhaps accumulated at to the top, dunitic material sinking low temperatures. It appeared, at the down below somewhere, and in between time, that the abundance of iron in the we may have had some basaltic-type sun was low, and that the material of the rocks. This was the conclusion we came moon was about the mean composition to at the time. of solar nongaseous material. Now it At the same time that these studies appears that the iron content of the sun were going on, we had the observations is higher, and the moon could not be an made from exact data on the orbits of accumulative object of the kind that I objects moving about the moon. From supposed. I shall discuss this again this, Sjogren and Muller succeeded in later. deducing that there were heavy masses Let us turn to what we know from the lying under the great circular maria space program and more recent data. of the moon. Of course, it is evident that When Surveyors VI and VII had gone to these great circular maria were made the moon and Prof. Turkevich's data many aeons ago, something dating tocame back, it indicated that the surface ward the origin of the moon, and, hence, of the moon was not composed of the rocks that supported them had to be meteoritic-type material, but was com- strong all through geologic time. They posed of highly differentiated material. reported positive gravity anomalies, That was the first shock to my ideas, and this means that certain areas of the and, of course, we had to accept them. moon had more mass per unit area than It appeared from this that the surface other areas. These areas had excess had been differentiated. When Apollo masses per unit area. Such masses occur 11 went to the Moon and returned sam- on the earth, and they are pushed up by ples, this became very much substan- the great convections in the mantle of tiated and very definite. The indication the earth. was that the dark material in the maria Depressions have been made by the was basaltic-type rocks having rather great ice sheets of some 10,000 years ago, high concentrations and intermediate and, on the earth, slow adjustment of concentrations of alumina, silica, and these negative anomalies is taking calcium. And imbedded in the dust of place. Assuming that both the moon the maria were some small chips of and the earth act as viscous liquids, the anorthositic-type rocks which, on analy- coefficient of the viscosity of the moon sis by John Wood of Harvard, showed has been found to be 10,000 times as that they had been highly differentiated. great as that of the earth, and this inIn fact, this material was of a kind of dicates a considerably colder and more rock that is very scarce on the earth. rigid moon. This must have been true 14
C&EN April 16, 1973
throughout geologic time. Also, we find that the center of gravity is displaced from the center of the figure toward the earth by somewhat over 2 kilometers, and this indicates an irregular shape of the moon, but we cannot argue as to the time that this occurred. We also sent seismic instruments to the moon, and seismologists have been dating these under the direction of Prof. Latham, mostly. A great many expert seismologists have been interested in this. Their conclusions from several stations located toward the middle of the lunar disk indicate that there is a layer of low seismic velocity material some 20 km. deep on the moon, followed
by material that has the velocity of anorthositic rock-type materials, and below that of dunitic-type materials of considerable thickness. They have also found recently that there are reflections at about 900 km. depth, and also that seismic S waves will not be transmitted through the moon, thus indicating a very soft structure at the middle of the moon. I think they are unable to say that this is an iron core, but it is a weak core of some kind. Thus, we have evidence for strong materials supporting the mascons, and, at the same time, we have evidence that a considerable melting process has taken place. The dates of melting of the rocks have been studied extensively by the uranium, thorium, and lead methods, by the rubidium-87/strontium-87 method, and by the potassium-40/argon-40 method. Many data have been piled up in this regard. I shall not try to review them in detail, but wish to point out that two dates generally seem to be indicated. First, many of the rocks seem to have gotten their chemical composition about 4.5 billion years ago, and then have been remelted without a chemical fractionation taking place sometime later. The later dates vary from about 3.2 to 4 billion years ago. Thus, it would appear from this that a general melting process occurred on the moon 4.5 or perhaps 4.6 billion years ago, at about the time the meteorites were made, and then later, a second, rather mild melting process did not result in a chemical fractionation in a
marked degree and produced basalticlike flows on the moon. This seems to be approximately the way the data stand at present. Of course, this limits, in a very definite way, the early history of the moon, as I shall outline later. The analyses of the rocks of the moon, with respect to the more volatile elements, indicate that the surface has a very low abundance of these elements. For example, the potassium/uranium ratio is nearly constant on the earth at about a ratio of 10,000 to 1. On the moon, it is about 2000 to 1, and in the meteorites something in the neighborhood of 60,000 to 1. This variation in this more volatile element potassium, and also other volatiles, indicates some volatilization process must have occurred, at least as far as the outer regions of the moon are concerned. Whether this extends to the interior of the moon, of course, we do not know, for we have no certain evidence so far in regard to the lunar interior. At this writing, it appears that little water has been found in the rocks of any Apollo site, including Apollo 17, though some volatiles in some of the Apollo 17 rocks are considerably more abundant than they are in other lunar rocks. This may indicate that higher abundances of the volatiles may exist in the deeper parts of the moon. It is also interesting that the siderophile elements are of low concentration in the lunar surface. By this we mean nickel, cobalt, the platinum metals, and the palladium metals, which dissolve readily in liquid iron. It would appear from this that someplace the materials of the lunar surface had liquid iron trickling through them and extracted these elements from the silicate materials. At the same time, we must look at the older evidence in regard to the moon. The whole surface of the moon has been covered by collisions which have kept their shape through geologic time. It is especially interesting that the big craters, Alphonsus and Ptolemaeus, have kept a negative gravitational anomaly. This means that whereas the surface was melted, it must have cooled down in some way before the objects which produced these craters fell on the moon and allowed them to scoop out material from the surface and leave the negative amount of mass per unit area in these regions. This requires considerable strength, and the material of the moon must have cooled down very considerably before these craters and others like them were produced by great collisions. This indicates that considerable time elapsed between the general melting process and the time of collisions on the lunar surface. In the light of what has been said, I should like to discuss briefly some current ideas in regard to the origin of the moon and the solar system. In fact, this has been the principal interest of the space program. We ask how the various data that can be secured from the space program can be applied to this interest-
ing problem of how the moon originated, how the earth originated, where the planets were formed, and how the solar system got its structure. Several ideas have been suggested in regard to the origin of the moon. Possibly one of the oldest is that it separated from the earth. This was one put forward by Sir George Darwin in the latter years of the 19th century. It is assumed that the earth had a high rate of rotation, and that the rotating earth was unstable and the moon separated from the earth. Recently, it has been put forward enthusiastically by a number of people. At the turn of the century, Jeffreys and Moulton discussed this and concluded that it was impossible. At the present time, most of us are very doubtful of this origin for the reason that the chemical composition of the moon is rather markedly different from that of the earth. For example, the volatiles and siderophiles are much rarer in the moon, and we do not understand how an object separating from the earth should have such a different chemical composition with respect to quite a number of elements. The iron is higher in the basalts of the moon while the nickel is lower than in terrestrial basalts. Things of this sort have made it very difficult to accept this origin for the moon. The second idea in regard to the origin is that the moon and the earth both condensed from a very-hightemperature sphere of gas. This is associated today with Prof. Ringwood's ideas, though Prof. Kuiper also had suggested such ideas. A gas sphere, with a high-temperature center ( ~ 10,000° K.) in which the earth and moon condensed, is a very complicated model chemically. It involves equations of state of the silicate gases. No reliable calculation can be made of this process, simply because of the lack of knowledge of the gas laws and the vapor pressures under these conditions. Also, it is a little difficult to explain how a moon of the composition that we have lacks volatiles as compared with the earth. I would have thought that the moon would have more volatiles than the earth, since it was farther from the center of the condensation process and, hence, at a lower temperature. If volatiles appear in the interior, as seems possible, then I think this is quite impossible as a method of forming the moon. Possibly I have overlooked something, and I will not exclude this origin completely. A third process is the accumulation of the moon in the neighborhood of the earth from solid objects; that is, the earth was accumulating out of solid objects, and it accumulated in its neighborhood in orbits about it a considerable number of small objects, meters in size, 10 meters in size, kilometers in size, or something of this sort; and that these then accumulated into the moon. Of course, if this is the general process, one wonders why Venus does not have a
moon also. Venus is a planet of about the same mass as the earth at about the same distance from the sun, forming under very similar conditions, one would think. Why does one planet manage to accumulate a large number of these objects, and, hence, the whole system have a high angular momentum, whereas Venus rotates in the wrong direction and has almost no angular momentum at all? Also, one wonders as to the mechanism of accumulating the earth from objects that must have had large amounts of metallic iron in them and the moon from objects that had very small amounts of metallic iron as indicated by the moon's density. It seems most unlikely that a separation of iron-rich objects and iron-poor objects could have occurred. This method of producing the earth seems unlikely to me. The fourth theory of origin is that the moon was captured by the earth, and this is a very improbable process. It is interesting to note that there is evidence for rather large objects present in the solar system during the accumulation of the planets. There are four groups of objects, I think, in the solar system. There are the planets themselves. If we added the solar proportion of gases that should be associated with the material of the earth to the earth, it would have about the mass of Jupiter. If all of the planets, the terrestrial planets as well as the major planets, were given their solar component of gases, they would be a very massive set of objects. Then there is a second group. There are seven moons in the solar system which, within a factor of 2, have the mass of our moon. It may be that Pluto has the mass of a moon, too. Apparently, there has been a great disagreement in regard to the mass of this object. Then there are the smaller objects in the solar system—the asteroidal belt objects and the smaller moons of the planets. These altogether have a mass of about one fourth of our moon, or something in that neighborhood. The masses are not well known. They may be fragments of larger objects. Finally, there are the comets. But the moon-sized objects are a striking, puzzling, and possibly a significant group. Also, the tilts of the axes of the planets are difficult to explain unless some rather big objects such as the moon collided with the growing planets. Again, it seems that the moons might have been an important constituent of the early solar system. Prof. Kurt Marti has shown that a moon rock contains xenon isotopes from the fission of plutonium-244, and such isotopes have not been observed in the earth's atmosphere, and, as yet, not in terrestrial rocks. At present, this indicates that the moon is older than the earth and that the moon was captured. If the moon was captured by the earth, it is reasonable to suppose that many moons were present in the early solar system. It hardly seems probable April 16, 1973 C&EN
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that we could have had one odd moon in the solar system, and this would be captured by one of the terrestrial planets. Capture of the moon by the earth by any proposed capture process is highly improbable, and, hence, we must propose methods for producing many moons. Some 15 years ago, I suggested t h e possibility that gas spheres existed, and I made a little calculation seeming to indicate that this might be the case. The temperatures required turned out to be rather low, and the mass of the solar nebula rather large. Of course, the theory is very approximate when applied to the solar nebula consisting of a mixture of solids and gases. Janet Bainbridge made some calculations on gas spheres some 10 years ago, and, in her conclusions, she mentions that volatiles should be missing from the surface of the moon. I think she was the first to predict this. I am kind of an old guy, and people do not believe old guys like me, but Janet Bainbridge was 29 years old (all women are 29 years old, you know), and she used a computer in her theory which, of course, makes it much more reliable, and I think that maybe the formation of gas spheres is not an unreasonable idea of the way to get many moons in the solar system in its early history. There are other methods undoubtedly. Hayashi has made a model of origin for solar systems in which a flattened sphere of gas spins off gases and solids at its equator as it contracts, and gives us a solar nebula. Prof. Anders and his colleagues have been calculating the condensation process in t h e solar nebula, and have done a very nice piece of work in regard to this. This is the process that occurs as material spins off the Hayashi model, as I have suggested previously. Of course, this condensation may have nothing to do with the formation of lunar-type objects which may have formed later. I have proposed a history for the moon. Perhaps I should not put this forward because I have made quite a number of mistakes in the past by so doing, and perhaps this subject is better solved by waiting for evidence to come in and then talking about the process. However, with the present evidence, as I see it, I should like to outline what I think may be an origin for the moon. The moon accumulated in a gas sphere which was produced by gravitational instability in the solar nebula. Solid particles, those calculated by Anders and his group, settled in the gas sphere, fell toward the center, and began to accumulate the moon at low temperatures. This material, at the center of the moon, had a low density. It grew to about half the radius of the moon at these low temperatures, but, at the same time, the gas sphere was shrinking and the temperature grew higher. Finally, some solid materials free of volatiles began to accumulate. Some iron began to be reduced. Later the temperatures
rose, and a melted surface of the moon of considerable depth was produced. I am not saying what the depth of this melted layer was. After this, the gases in which the settling occurred were blown off by a hightemperature sun, and the surface of the moon began to crystallize with anorthositic rocks floating at the surface, dunitic rocks settling out below, and metallic iron being reduced by the gases and settling through the liquid to the interior. Then it cooled off because the gases were blown away by a high-temperature sun, which Herbig again has observed. He has observed a star that increased in intensity by a factor of 100 in the course of a year or so. I expect that this blew the gases and the remaining solids out of the solar system and left us with moons about. These moons broke up in collisions with each other, scattered objects about, and the accumulation of the planets began. Seven of these moons were not captured into the body of one of the planets, and were probably captured in an orbit about the earth and other planets, because of collisions of other objects near the planets or by some other process. Gerstenkorn has proposed that the moon was captured in a retrograde orbit which later became direct. After the heating process and loss of gases, the surface solidified and anorthositic material floated on top, dunitic material settled below, and we had a moon with a cool interior and a hot exterior which began its history in orbit about the earth. Many collisions on the surface occurred, and the beginning of geologic time began. It is very difficult to say exactly when this occurred. I have thought, in the past, that they occurred very early in the history of the solar system, but now it seems that they may have occurred somewhat later, though there is not complete agreement about this. Of course, if the interior accumulated in the way that I say, it retained its radioactive elements within the cool, central body. In time, i.e., some 4.5 billion years, the deep interiors of the moon partially melted because of radioactive heating. Therefore transverse waves, i.e., seismic S waves, are not transmitted. Also, the magnetic fields disappeared when the temperature of the interior rose above the Curie point. I do not believe that this model has been proved to be true, nor do I believe that it has been proved false. If Prof. Marti's observations on the fission Xe isotopes are correct, something like this model must be real. If the abundance of iron should prove to be lower or if some volatiles should be proved to exist on the lunar interior, I would be much inclined to believe the model to be substantially correct. Again, I wish to thank the American Chemical Society for awarding the Priestley Medal to me on this occasion. Would it not be nice to have Priestley present at one meeting a year?
