The Origin of the Earth The emergence of the theory that the solar system coagulated from a vast cloud of dust has led to a new inquiry into the chemical history of our planet By Harold C. Urey It is probable that as soon as man acquired a large brain and the mind that goes with it he began to speculate on how far the earth extended, on what held it up, on the nature of the sun and moon and stars, and on the origin of all these things. He embodied his speculations in religious writings, of which the first chapter of Genesis is a poetic and beautiful example. For centuries these writings have been part of our culture, so that many of us do not realize that some of the ancient peoples had very definite ideas about the earth and the solar system which are quite acceptable today. Aristarchus of the Aegean island of Samos first suggested that the earth and the other planets moved about the sun—an idea that was rejected by astronomers until Copernicus proposed it again 2,000 years later. The Greeks knew the shape and the approximate size of the earth, and the cause of eclipses of the sun. After Copernicus the Danish astronomer Tycho Brahe watched the motions of the planet Mars from his observatory on the Baltic island of Hveen; as a result Johannes Kepler was able to show that Mars and the earth and the other planets move in ellipses about the sun. Then the great Isaac Newton proposed his universal law of gravitation and laws of motion, and from these it was possible to derive an exact description of the entire solar system. This occupied the minds of some of the greatest scientists and mathematicians in the centuries that followed. Unfortunately it is a far more difficult problem to describe the origin of the solar system than the motion of its parts. The materials that we find in the earth and the sun must originally have been in a rather different condition. An understanding of the process by which these materials were assembled requires the knowledge of many new concepts of science such as the molecular theory of gases, thermodynamics, radioactivity and quantum theory. It is not surprising that little progress was made along these lines until the 20th century. The Earlier Theories It is widely assumed by well-informed people that the moon came out of the earth, presumably from what is now the Pacific Ocean. This was proposed about 60 years ago by Sir George Darwin. The notion was considered in detail by F. R. Moulton, who concluded that it was not possible. In 1917 it was again considered by Harold Jeffreys, who thought that his analysis indicated the possibility that the moon had been removed from a completely molten earth by tides. In 1931, however, Jeffreys reviewed the subject and concluded that this could not have happened; since then most astronomers have agreed with him. But although Moulton and Jeffreys showed the improbability of the origin of the moon from the earth, they proposed theories for the origin of the solar system involving the removal of the earth and the other planets from the sun. Together with James Jeans and T. C. Chamberlin they proposed that another star passed near or collided with the sun, and that the loose material resulting from this cosmic encounter later coagulated into planets. This idea of the origin of the solar system has been widely held right up to the present. The evidence gathered by our great telescopes now tells us that most of the stars in the heavens are pairs or triplets or quadruplets. We have determined the masses of multiple stars by means of Newtons laws of motion and his universal law of gravitation; we have also studied the velocities of these stars by significant changes in their spectra and by actually measuring the motions of nearby examples. We find that the two stars of a pair seldom have exactly the same mass, and that the ratio of the mass of one star to that of the other varies considerably. Gerard P. Kuiper of the University of Chicago concludes that the number of pairs of stars is entirely independent of the ratios of their masses; that is, there is very little probability that one ratio of masses would occur more often than another. In fact, it would appear that there is about as much chance of finding a pair of stars in which one has one-thousandth the mass of the other as there is of finding a pair in which one is 999 thousandths as massive as the other. Of course it would be very difficult to see a double star in which the secondary was only a thousandth as large as the primary, particularly if the second emitted no light. The sun and Jupiter, the largest of the planets, might be viewed as such a double star: Jupiter weighs about a thousandth as much as the sun, and it shines only by reflected sunlight. Even from the nearest star Jupiter would be invisible. There is much evidence, however, that a double star such as the sun and Jupiter should occur as a regular event in our galaxy, and the same considerations would seem to indicate that there may be as many as a hundred million solar systems within it. Solar systems are almost certainly commonplace, and not the special things that one might expect from the collision of two stars. The Dust Cloud Hypothesis Many years ago E. E. Barnard of the Yerkes Observatory observed certain black spots in front of the great diffuse nebulae that occur throughout our galaxy. Bart J. Bok of Harvard University has investigated these opaque globules of dust and gas; they have about the mass of the sun and about the dimensions of the space between the sun and the nearest star. Lyman Spitzer, Jr., of Princeton University has shown that if large masses of dust and gas exist in space, they should be pushed together by the light of neighboring stars. Eventually, when the dust particles are sufficiently compressed, gravity should collapse the whole mass, and the pressure and temperature in its interior should be enough to start the thermonuclear reaction of a star. It would seem reasonable to believe that if a star such as the sun resulted from a process of this kind, there might be enough material left over to make a solar system. And if the process was more complex we might even end up with two stars instead of one. Or again we might have triple stars or quadruple stars. Theories along this line are more plausible to us today than the hypothesis that the planets were in some way removed from the sun after its formation had been completed. In my opinion the older hypotheses were unsatisfactory because they attempted to account for the origin of the planets without accounting for the origin of the sun. When we try to specify how the sun was formed, we immediately find ways in which the material that now comprises the planets may have remained outside of it. One piece of evidence that must be included in any theory about the origin of the solar system consists in our observation of the angular momentum that resides in the spinning sun and the planets that travel around it. The angular momentum of a planet is equal to its mass times its velocity times its distance from the sun. Jupiter possesses the largest fraction of the angular momentum in the solar system; only about two per cent resides in the sun. Another fact that must be encompassed by any theory is the so-called Titus-Bode law, which points out in a simple mathematical way how the distances of the planets from the sun vary: the inner planets are closer together and the outer ones are farther apart. This is only an approximate law which does not hold very well, and perhaps more emphasis has been put upon it than it deserves. In my own study of the problem I have looked for other evidence regarding the origin of the solar system. Some 15 years ago Henry Norris Russell of Princeton and Donald H. Menzel of Harvard pointed out that there was a very curious relationship between the proportions of the elements in the atmosphere of the earth and the atmospheres of the stars, including the sun. It is particularly noteworthy that neon, the gas that we use in electric signs, is very rare in the atmosphere of the earth but is comparatively abundant in the stars. Russell and Menzel concluded that neon, which forms no chemical compounds, escaped from the earth during a hot early period in its history, together with all of the water and other volatile materials that constituted its atmosphere at that time. The present atmosphere and oceans, they proposed, have been produced by the escape of nitrogen, carbon and water from the interior of the earth. The German physicist C. F. von Weizsäcker similarly suggested that the argon of the air has resulted mostly from the decay of radioactive potassium during geologic time, and has escaped from the interior of the earth. F. W. Aston of Cambridge University also pointed out that the other inert gases, krypton and xenon, were virtually missing from the earth. The Chemical Approach My own studies in the origin of the earth started with such thoughts about the loss of volatile chemical elements from the earths surface. Exactly how did these elements escape from the earth, and when? I came to the conclusion that it was impossible that they were evaporated from a completely formed earth; the evaporation must have occurred at some earlier time in the earths history. Once the earth was formed its gravitational field was much too strong for volatile gases to escape into space. But if these gases escaped from the earth at an earlier stage, what is the origin of those that we find on the earth today? Water, for example, would have tended to escape with neon, yet now it forms oceans. The answer seems to be that the chemical properties of water are such that it does not enter into volatile combinations at low temperatures. Thus if the earth had been even cooler than it is today, it might have retained some water in its interior that could have emerged later. But meteorites contain graphite and iron carbide, which require high temperatures for their formation. If the earth and the other planets were cool, how did these chemical combinations come about? Indeed, what was the process by which the earth and other planets were formed? None of us was there at the time, and any suggestions that I may make can hardly be considered as certainly true. The most that can be done is to outline a possible course of events which does not contradict physical laws and observed facts. For the present we cannot deduce by rigorous mathematical methods the exact history that began with a globule of dust. And if we cannot do this, we cannot rigorously include or exclude the various steps that have been proposed to account for the evolution of the planets. However, we may be able to show which steps are probable and which improbable. Kuiper believes that the original mass of dust and gas became differentiated into one portion that formed the sun and others that eventually became the planets. The precursors of the so-called terrestrial planets—Mercury, Venus, the earth and Mars—lost their gases. The giant planets Jupiter and Saturn retained the gases, even most of their exceedingly volatile hydrogen and helium. Uranus and Neptune lost much of their hydrogen, helium, methane and neon, but retained water and ammonia and less volatile materials. All this checks with the present densities of the planets. It seems reasonably certain that water and ammonia and hydrocarbons such as methane condensed in solid or liquid form in parts of these protoplanets. The dust must have coagulated in vast snowstorms that extended over regions as great as those between the planets of today. After a time substantial objects consisting of water, ammonia, hydrocarbons and iron or iron oxide were formed. Some of these planetesimals must have been as big as the moon; indeed, the moon may have originated in this way. The accumulation of a body as large as the moon would have generated enough heat to evaporate its volatile substances, but a smaller body would have held them. Most of the smaller bodies doubtless fell into the larger; Deimos and Phobos, the two tiny moons of Mars, may be the survivors of such small bodies. Massive chunks of iron must also have been formed. On the moon there is a huge plain called Mare Imbrium; it is encircled by mountains gashed by several long grooves. It would seem that the whole formation was created by the fall of a body perhaps 60 miles in diameter; this has been suggested by Robert S. Dietz of the U. S. Naval Electronics Laboratory, and by Ralph B. Baldwin, the author of a book entitled The Face of the Moon. The grooves must have been cut by fragments of some very strong material, presumably an alloy of iron and nickel, that were imbedded in this body. Of course large objects of iron still float through interplanetary space; occasionally one of them crashes into the earth as a meteorite. How were such metallic objects made from the fine material of the primordial dust cloud? In addition to dust the planetesimals contained large amounts of gas, mostly hydrogen. I suggest that the compression of the gases in a contracting planetesimal generated high temperatures that melted silicates, the compounds that today form much of the earths rocky crust. The same high temperatures, in the presence of hydrogen, reduced iron oxide to iron. The molten iron sank through the silicates and accumulated in large pools. It now seems that the meteorites were once part of a minor planet that traveled around the sun between the orbits of Mars and Jupiter. The pools of iron that formed in this body may have been a few yards thick. In the case of the object that was responsible for Mare Imbrium and its surrounding grooves, the depth of the pools must have been several miles. If the temperature of such a planetesimal had been high enough, its silicates would have evaporated, leaving it rich in metallic iron. The object must eventually have cooled off, for otherwise its nickel-iron fragments could scarcely have been hard enough to plow 50-mile grooves on the surface of the moon. It was at this stage that the planetesimals lost their gases; Kuiper believes that they were probably driven off by the pressure of light from the sun. This left the iron-rich bodies that are today the earth and the other planets. The whole process bequeathed a few meaningful fossils to the modern solar system: the meteorites and the surface of the moon, and perhaps the moons of Mars. The Moment of Inertia Recently we have redetermined the density of the various planets and the moon. The densities of some, calculated at low pressures, are as follows: Mercury, 5; Venus, 4.4; the earth, 4.4; Mars, 3.96, and the moon, 3.31. The variation is most plausibly explained by a difference in the iron content of these bodies. And this in turn is most plausibly explained by a difference in the amount of silicate that had evaporated from them. Obviously a planet that had lost much of its silicate would have proportionately more iron than one that had lost less. It is assumed by practically everyone that the earth was completely molten when it was formed, and that the iron sank to the center of the earth at that time. This idea, like the conception of an earth torn out of the sun, and a moon torn out of the earth, almost has the validity of folklore. Was the earth really liquid in the beginning? N. L. Bowen and other geologists at the Rancho Santa Fe Conference of the National Academy of Sciences in January, 1950, did not think so. They argued that if the earth had been liquid we should expect to find less iron and more silica in its outer parts. There is other evidence. Mars, which should resemble the earth in some respects, contains about 30 per cent of iron and nickel by weight, and yet we have learned by astronomical means that the chemical composition of Mars is nearly uniform throughout. If this is the case, Mars could never have been molten. The scars on the face of the moon indicate that at the terminal stages of its formation metallic nickel-iron was falling on its surface. The same nickel-iron must have fallen on the earth, but there it would have been vaporized by the energy of its fall into a much larger body. Even so, if the earth had not been molten at the time, some of the nickel-iron might still be found in its outer mantle. If there is iron in the mantle of the earth, it may be sifting toward the center of the earth; and if it is moving toward the center of the earth, it will change the moment of inertia of the earth. The moment of inertia may be defined as the sum of the mass at each point in the earth multiplied by the square of the distance to the axis of rotation, and added up for the whole earth. If iron were flowing toward the interior of the earth, this quantity should decrease. It is a requirement of mechanics that if the moment of inertia of a rotating body decreases, its speed of rotation must increase. Finally if the speed of the earths rotation is increasing, our days should slowly be getting shorter. Now we know that our unit of time is changing; but it is getting longer, not shorter. That is, the earth is not speeding up but is slowing down. Very precise astronomical measurements, some of them dating back to the observation of eclipses 2,500 years ago, indicate that the day is increasing in length by about one- or two-thousandths of a second per day per century. It has been thought that the lengthening of the day was due to the friction of the tides caused by the sun and the moon. But if we attempt to predict changes in the apparent position of the moon on the basis of this effect alone, we find that our calculations do not agree with the observations at all. If on the other hand we assume that iron is sinking to the core of the earth, the changing moment of inertia would also influence the length of the day. Indeed, calculations made on the basis of both the tides and the changing moment of inertia do agree with the observations. In order to make the calculations agree we must postulate a flow of 50,000 tons of iron from the mantle to the core of the earth every second. Staggering though this flow may seem, it would take 500 million years to form the metallic core of the earth. Some calculations indicate that it may have taken as long as two billion years. The important thing is that the order of magnitude approaches that of the age of the earth, which is generally given as two to three billion years. If this reasoning is correct, the earth was made initially with some iron in its exterior parts, and it could not have been completely molten. To complicate matters Walter H. Munk and Roger Revelle of the Scripps Institution of Oceanography have shown that the moment of inertia of the earth is probably decreasing because water is slowly being transferred from the oceans to the ice caps of Greenland and Antarctica, and that this process can account for the lengthening of the day without assuming that iron is moving to the center of the earth, at least not so rapidly as I have calculated. In view of the argument of Munk and Revelle we really have no evidence for the flow of iron to the center of the earth. However, we have little evidence to the contrary. New observations are needed. The Last Stages Let us briefly retell what the course of events may have been. A vast cloud of dust and gas in an empty region of our galaxy was compressed by starlight. Later gravitational forces accelerated the accumulation process. In some way which is not yet clear the sun was formed, and produced light and heat much as it does today. Around the sun wheeled a cloud of dust and gas which broke up into turbulent eddies and formed protoplanets, one for each of the planets and probably one for each of the larger asteroids between Mars and Jupiter. At this stage in the process the accumulation of large planetesimals took place through the condensation of water and ammonia. Among these was a rather large planetesimal which made up the main body of the moon; there was also a larger one that eventually formed the earth. The temperature of the planetesimals at first was low, but later rose high enough to melt iron. In the low-temperature stage water accumulated in these objects, and at the high-temperature stage carbon was captured as graphite and iron carbide. Now the gases escaped, and the planetesimals combined by collision. So, perhaps, the earth was formed! But what has happened since then? Many things, of course, among them the evolution of the earths atmosphere. At the time of its completion as a solid body, the earth very likely had an atmosphere of water vapor, nitrogen, methane, some hydrogen and small amounts of other gases. J. H. J. Poole of the University of Dublin has made the fundamental suggestion that the escape of hydrogen from the earth led to its oxidizing atmosphere. The hydrogen of methane (CH4) and ammonia (NH3) might slowly have escaped, leaving nitrogen, carbon dioxide, water and free oxygen. I believe this took place, but many other molecules containing hydrogen, carbon, nitrogen and oxygen must have appeared before free oxygen. Finally life evolved, and photosynthesis, that basic process by which plants convert carbon dioxide and water into foodstuffs and oxygen. Then began the development of the oxidizing atmosphere as we know it today. And the physical and chemical evolution of the earth and its atmosphere is continuing even now. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
Posted on: Tue, 06 Jan 2015 06:54:47 +0000
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