THE BEGINNING OF THE EARTH
THE origin of our planet is a problem which has appealed to the intellect of thoughtful men from the most remote times, and the earliest recorded speculations concerning it—those of the Mosaic cosmogony (INTRODUCTION TO THE CREATION OF THE UNIVERSE ACCORDING GENESIS)—possess a peculiar interest, since they embody the views of the ancient Chaldeans, who were not only systematic observers of the heavens, but made practical use of their results.
The Mosaic cosmogony is not unworthy of the great people among whom it took its rise; it recognizes the fact that the earth had a history antecedent to the advent of man, and its account of the order of events in this history is not only remarkable as a feat of a priori reasoning, but accords in some respects with the results achieved alter much labor by modern science.
It was not until the middle of the eighteenth century that the reign of evolution began, and attempts were made to trace the history of a planetary system from its source in a primeval nebula on purely mechanical grounds. Swedenborg (1735) was the pioneer in this direction, then came Thomas Wright (1750) of Durham, whose work furnished inspiration to Emanuel Kant (1755), and led him to construct a consistent scheme of the Universe. The last of this group of cosmic philosophers is Laplace (1796), whose admirable description of the evolution of the solar system was arrived at independently, and without knowledge of the previous work of Kant.
Laplace assumed as his starting-point the existence of a nebula formed of incandescent gas, and extending beyond the limits of the outermost planet of our system. It was in rotation about a central axis, and possessed in consequence a disc-like or lenticular form. Radiating its heat away in all directions through surrounding space, it grew continually colder, and in cooling diminished in bulk. As a consequence of this contraction its rate of rotation increased, till at length the centrifugal force of the outermost part became so great that this could no longer continue to follow the contracting mass within, and thus remained behind as a great rotating ring. The continued contraction of the internal mass, and the resulting increase in the velocity of rotation, again brought about the same condition of things, and a fresh ring was left behind.
This process was repeated time after time, till as many rings were formed as there are planets in the solar system; the central mass which survived within the innermost ring condensed to form the sun The rings were highly unstable—that is to say, a slight disturbing force was sufficient to destroy their continuity; they broke, across and rolled up into great nebulous globes, which revolved round the sun in the same direction as the original nebula, and rotated on their axes in the same direction as that in which they revolved. Most of them repeated the behavior of the original nebula, leaving behind rings as they contracted, and these rings either rolled up to form moons or satellites, or, in the solitary instance of Saturn’s rings, retained their annular form. The rings are now known to consist of a multitude of solid bodies, as proved by Clerk-Maxwell.
By this hypothesis, so beautiful in its simplicity, an explanation was afforded embracing all the more important facts of our system; the revolution of all the planets in nearly circular orbits and in the same direction as that in which the sun rotates, and the revolution of their satellites, also in circular orbits and in the same direction as their primaries; the comparatively high temperature and consequent low density of the larger planets and the sun, as well as a variety of other phenomena, all seem to follow naturally from it. The fundamental assumption seems to be in harmony with a number of known facts. Thus in the case of our own planet the volcanoes distributed around the margins of the oceans, and the hot springs scattered irregularly over the whole terrestrial surface, suggest that great stores of heat exist beneath our feet, a presumption which finds confirmation in the fact that whenever we descend towards the interior of the earth, as in deep mines or wells, the temperature continues steadily to rise after we have passed a depth below which seasonal and diurnal changes of temperature cease to be felt, the rise being in some cases as much as 3 deg. for 100 ft., in others only 1 deg. for the same distance, but on the average 1 deg. for 60 ft. or 70 ft. If this increase of temperature continues down to great depths, and there seems to be no reason why it should not, then a point will be reached, say, at thirty or forty miles down, where the interior will attain a white heat.
The Earth as a Star
Thus the earth might be regarded as a white hot body surrounded with a film of rock growing continually cooler towards the surface. But such a hot body suspended in space must be cooling, just as all bodies which are hotter than their surroundings. It is cooler today than it was yesterday, or—what is the same thing—it was hotter yesterday than it is today, and so of all previous yesterdays. And thus as we travel backwards in time we perceive that the earth will be growing hotter, the level of white heat will be mounting upwards towards the surface, and will at last reach it, so that the earth, instead of being, as it now is, a dark body shining only with the reflected light of the sun, will be self-luminous, a tiny star of a magnitude so diminutive as to have awakened resentment on the part of some terrestrial inhabitants, who have regarded it as disproportionate to their dignity. But we cannot arrest imagination at this stage; our thought still extends its retrospective glance into the abyss of past time, and we perceive the earth still growing hotter, till its temperature transcends those limits at which it can exist in the solid state. It becomes molten—nay, more, it becomes gaseous, and thus resumes the nebular state from which it sprang. Precisely the same argument applies to the sun ; our mighty luminary is also a cooling body, and if we could restore to it the heat which it has lost in the course of past aeons it would resume a completely gaseous state. Modified in one way or another, this chain of reasoning seemed irrefragable in those happy days which preceded the discovery of radium.
The question may be considered from another point of view. On searching the heavens we find that many of the stages which are assumed in Laplace’s hypothesis are still represented by actual existences. There are, to begin with, those immense diffused nebula; almost incapable of definition, which are proved, on spectroscopic examination, to emit that kind of light which is characteristic of glowing gas; from these we pass to others which are resolvable by the telescope into a central and more condensed nucleus, with two mighty nebulous arms whirled round in a spiral, and bearing more condensed masses in their midst; even ring nebulae are known to exist; and, finally, there are nebulous halos which surround some of the stars. Then we come to the stars themselves, which are suns of various degrees of magnitude, some immensely larger than our own luminary, and these are evidently in various stages of existence. Some are blue, and afford evidence of a higher temperature than that of our sun; others are yellow, and make a nearer approach to the solar temperature; while, again, others are red, and certainly colder.
These, in conjunction with other considerations, lead to the conviction that the universe is in a state of evolution, and that the solar system at one time existed in a nebular state. But whether Laplace’s description of the series of events through which the original nebula passed is the true one or not is a very different matter; it presents so many difficulties that scarcely any student now supports it.
A fundamental difficulty is the extreme tenuity of the gas which is assumed to have formed the planetary rings. A second difficulty, which has been emphasized by Professors Chamberlin and Moulton, is to be found m the comparatively small amount of rotational energy which the system at present possesses, for this is less than 1/200 of that which, on the most favorable assumption, must have been contained within the original nebula. Less fundamental, but equally fatal, is the fact that one of the satellites of Saturn revolves round its primary in a direction opposed to that of the rotation of the planet itself. [Recently Mr. Stratton, following out a suggestion of Professor W. H. Pickering, has shown that this is quite consistent, and, indeed, is a natural deduction from Laplace’s hypothesis]. Hence for these and other reasons we are reluctantly compelled to abandon an hypothesis which for over a century has exercised an influence on out conception of the cosmos not less profound, penetrating, and far-reaching than that of the famous Darwinian doctrine of natural selection, now on its trial.
At present, unanimity of opinion, even on questions of the most primary kind, is far to seek. Philosophers are not even agreed as to the constitution of the nebulae. It is questioned whether even those least resolvable and most diffused forms which give bright line spectra really consist of masses of incandescent gas. Many observers, among them Sir Norman Lockyer, now maintain that they are formed of swarms of meteorites, which, moving with prodigious velocity, meet in frequent collision, and by their impact evolve sufficient heat to become self-luminous. Others, again, like the distinguished investigator Arrhenius, while admitting the gaseous nature of these nebulae, deny that they are incandescent, and assert that their temperature is not much above that of surrounding space. Their exterior parts consist of the lighter gases in a highly rarefied state, and minute particles of negative electricity, which are always careering through space, on penetrating these gases produce a luminous discharge. A nebula composed of swarms of meteorites would, as Sir George Darwin has shown, behave very much in the same way as one composed of gas, and if in rotation would rotate as a solid mass. The meteorites would stand in the same relation to the nebula as molecules to a gas, and thus the question of the constitution of the nebula, although of great interest in itself, becomes of subsidiary importance in tracing its subsequent history.
One of the latest attempts to frame a nebular, hypothesis is that of Professor J. H. Jeans. His reasoning is of a highly mathematical character, and his conclusions are expressed in the most general terms. Starting with a spherical nebula of gas or meteorites endowed with a small amount of rotation, he shows that as it cools or loses energy the temperature of the interior will not fall continuously in precise correspondence with the cooling of the outer parts, and this "lag" of the interior temperature will bring about a tendency to instability. The contraction of the nebula due to cooling will increase the velocity of rotation, and this again will tend to instability. As a result of the instability so produced the nebula will change its form, and become more or less pear-shaped. The narrow end of the pear will then separate from the body and assume an independent existence as a primitive planet. This process will recur again and again till the nebula is resolved into a sun with its attendant planets. The planets, existing at first as gaseous masses or quasi gaseous masses, will be liable to the same kind of transformation, and may thus bud off moons or satellites.
If the nebula were not in rapid rotation, a slight disturbing cause, acting at the critical moment when a planet was being ejected, might determine the inclination of the planet’s orbit, which might thus be very oblique to the equatorial plane of the nebula. Thus the hypothesis is not open to one of the objections which have been urged against that of Laplace— namely, that the orbits of some of the planets in the solar system are inclined at a large angle with the plane of the sun’s equator.
Jeans mentions two disturbing causes in particular which might easily arise— one the penetration of the nebula by a wandering meteorite, which might precipitate an event already on the verge of happening, and simultaneously determine both the birth of a planet and the obliquity of its orbit; the second, the presence of some distant mass, such as a star, which, by raising a quasi-tide in the nebula, would give the final touch required to overturn its equilibrium. The influence of a distant body, such as a star, has been invoked by Moulton in another version of the nebular hypothesis. In conjunction with Chamberlin, he calls special attention to the spiral nebulas, which are by far the commonest kind, as presenting the closest approach to the conditions which obtain when planets are actually in course of formation. Chamberlain and Moulton enter on a detailed account of the manner in which they suppose the planets to have grown by the gradual accretion of meteoric masses as these encountered each other while moving in various elliptical orbits.
At present it would seem impossible to speak with certainty as to the precise history of the solar system. Meanwhile, we may console ourselves with the closing words of Professor Jeans’ paper, to the effect that “no difficulty need be experienced in referring existing planetary systems to a nebulous or meteoric origin on the ground that the configurations of these systems are not such as could have originated out of a rotating mass of liquid”.
An investigation by Sir George Darwin, which has furnished inspiration to such hypotheses as that of Jeans, bungs us nearer the immediate subject of this essay, since it treats of one of the last acts in the great drama of planetary existence, and attempts to derive the earth and moon from a common origin in a single rotating sphere.
It is well known that, owing to the gravitational effects produced by the tides, the earth is being gradually slowed down as it rotates upon its axis. Thus the day is constantly getting longer, so that in a few millions of years it will have increased in length from twenty-four to twenty-five hours. On the other hand, in past time it must have been shorter than at present: a few millions of years ago it was only twenty-three hours in length, and many millions of years earlier it was still less, only some five hours or so. At that time the earth was hotter than it is now, less rigid, more yielding, and, owing to its rapid rotation, less stable. The action on the moon of the tides produced in it by the earth is similar, and the rotation of the moon has been so far diminished by them that its day has become as long as the month—i.e., our satellite only turns once round on its axis in the time that it takes to revolve once round the earth; it is for this reason that our satellite keeps always the same face turned towards us.
The retardation of the earth in its rotation has, however, a very remarkable effect on the revolution of the moon; it involves—by the principle of the conservation of moment of momentum—a corresponding acceleration of the moon in its orbit, and, as a consequence of this, an enlargement of this orbit—that is, the moon is pushed away from us, as it were, and thus becomes more remote. But if so, the moon must have been nearer to us in times past. It is possible to trace the approach of the moon to the earth as we go backwards a time till the distance between them was only two and a halt terrestrial radii instead of the sixty radii which now separate them. Mathematics do not take us farther back than this. But it is difficult to resist the suggestion that in the immediately preceding stage of development the earth and moon formed together a single sphere.
If we may adopt this view, then we must regard the sphere as subject to the tidal influence of the sun. It was much hotter, and therefore more yielding, than the present earth; it was also rotating much faster, probably once in about four or five hours. It would be contracting as a consequence of cooling, and the contraction would lead to instability (gravitational instability); its rapid rotation would also tend toward instability (rotational instability). It is difficult to say which of these two, gravitational or rotational instability, would be the most effective; but the combined result would be to give a pear-shaped form to the rotating mass, and eventually to deepen the constriction between the narrow and the broad end, till the smaller protuberance became completely dissevered from the larger mass, and so entered on an independent existence as the moon. This final step in the process would probably depend on the tide-producing power of the sun; the larger mass remained behind as the earth, whose individual existence may be said to date from this event.
The young earth would be subject to very much the same conditions after as before the ejection of the moon, and might very possibly again pass into a pear-shaped form, but without proceeding further through those consequent changes, which would have led to the formation of another satellite; and while possessing some such form as this, she might very well have consolidated. With advancing years she would lose, as we have seen, the activity of her youth, the drag of the tides would cause her to spin ever more slowly on her axis, till the day would become prolonged to the twenty-four hours of the present. With this diminished rate of spin, the earth, if free to yield, would lose the pear-shaped form and become an oblate spheroid, and the oblateness of this spheroid would continually diminish, so that it continually approach towards a true sphere. Suppose, however, that the earth as it cooled lost its power of readily yielding and at present it is more rigid than a globe of steel then it would pass from form to form, not by a flowing movement, but by a series of ruptures, and its form at any moment might be a little in arrear of that which it would have possessed if it had been in the fluid state.
Thus it might indeed be possible still to discover some trace of an old-fashioned form in the existing planet; and a careful examination of the distribution of land and sea as represented on a terrestrial globe does, in fact, reveal a remarkable symmetry, in which we seem to recognize a surviving vestige of its early state. The great continent of Africa projects like the narrow end of a pear; around it are oceans—the Atlantic, the Indian Ocean, and the Mediterranean Sea, which was once of far greater extent; then comes a great dismembered ring of land, the two Americas, the Antarctic continent, Australia, Asia, and Europe. Within these, on the side opposite to Africa, is the great Pacific Ocean, which covers over the broad end of the pear.
A line drawn from somewhere in Central Africa to its antipodes in the Pacific, through the center of the earth, would correspond to the long axis of the pear; a second, at right angles to this, would correspond to its breadth; and a third, at right angles to both, would correspond to the axis on which it rotates. A diameter of the earth taken through the equator is almost 8,000 miles in length, the Polar diameter is about sixteen miles shorter, and this slight difference measures the oblateness of the spheroid, or the departure of the form of the earth from a true sphere. Further, it would appear that the diameter drawn through Africa is about half a mile longer than the equatorial diameter taken at right angles to it, and this insignificant quantity measures the departure of the form of the earth from that of an oblate spheroid to that of a pear, so nearly complete is the adjustment of its form to existing conditions. Before this nice adjustment was reached, the earth must have suffered many changes, passed through many times of stress and storm, and witnessed many geological revolutions. If, at the beginning of her career, the earth was molten, or at a very high temperature, she must have been surrounded by a very deep and dense atmosphere, for all the waters which now rest on her surface—oceans, lakes, and rivers—would have contributed to it in the state of steam; and not till the temperature of the ground had fallen to 380 deg. C. could liquid water have begun to accumulate. Then a steady downpour of almost red-hot rain would have set in, filling up the neck of the pear and extending lax and wide over its broad end
The temperature would now fall somewhat rapidly, and in a short space of time the surface of the earth would have become as cool as it is at the present day. Directly the waters of the firmament had collected into the oceans, leaving behind an atmosphere like that which now exists, geological agencies of the kind we are now familiar with would begin their sway. Air and rain would exert their insidious power upon the rocks, sapping their strength, converting the hardest granite into soft sand and clay, which would be washed away by the rain through brooks and rivulets into the channels of many rivers, all hastening with their burden of sediment, to deposit it finally in the sea. Here it would accumulate, layer after layer, building up those mighty masses of strata which now form the greater part of the visible land. While this general action was everywhere in progress, wearing down continents and islands towards the level of the sea, more specialized activities were assisting to the same end.
The waves which fall upon our coasts are now constantly undermining the cliffs and extending the margin of the sea at the expense of the land, and rivers not only serve to transport sediment, but cut down their channels deep into the rock, and so carve out the most varied landscapes of hill and valley from monotonous tableland.
When we enter into calculations we are astonished at the rapidity with which these agents perform their work even at the present day; but as we proceed farther back into the past, when the earth was full of youthful energy, their power must have been greatly enhanced. We might almost take the measure of the day as the measure of their work, for they probably accomplished as much during the eight hours’ day which once existed as they do now in twenty-four hours. A little consideration will make this clear. It is the winds which, blowing over the surface of the ocean, produce the sea waves, and it is these falling on our coasts that perform the work of marine denudation. But the winds are due in the first place to the heat of the sun, and the difference of temperature established at the equator and the poles; and, in the next place, to the rotation of the earth. Thus, with the increased rapidity of rotation which we know to have existed, and with increased and radiation from the sun, a very probable contingency, the winds would increase in strength and more powerfully erode our coasts. Again, with the moon in greater proximity, and with a more rapid rotation of the earth; the tides would be much higher and more frequent, and these, raising and lowering the cutting edge of the sea, greatly assist it in its work of destruction. The winds and the tides produce various marine currents, and these help to distribute the sediment which the rivers deliver into the sea, so that when stronger currents flowed as a result of more powerful tides and more violent winds, the sediments would be strewn over wider areas; hence, the more ancient strata of our planet far more widely distributed than are those of later time.
In the days when the earth’s crust had formed but was still unstable, the process of cooling not having gone far enough, there would not be the mountains which now characterize it. These came when the earth contracted and crumpled up along certain well defined lines, which are now represented by the three great mountain chains of the world.
Finally, a heavier rainfall would result from a more active atmospheric circulation, creating larger rivers, and thus, at the beginning, all those denuding agents which are engaged in wearing the land down into the sea would be working at a more rapid pace. Correspondingly, all the agents which are occupied in building up deposits of sediments would have extended their operations over a wider area, laying down a foundation broad and deep.
On the other hand, the contraction of the earth, due to the loss of its energy of rotation as well as of its internal heat, would also have proceeded more rapidly, new land would have emerged from the sea, old lands would have been submerged beneath it far less slowly than at the present day; ruptures of the crust, accompanied by earthquakes and volcanic action, would have been more frequent; and thus, by the more rapid loss of its intrinsic energy, the renovation of the earth would have kept pace with its accelerated destruction.
One effect of the contraction of the earth which has manifested itself in even late geological times is the crumpling up of the terrestrial crust into the sharp folds of mountain chains; but at the beginning this crumpling must have been far more universal and energetic. In this connection it is interesting to observe that the most ancient rocks known to us—the Archaean—never present themselves under any other form than as intensely plicated masses. They originally consisted of lava flows and volcanic ashes, of ancient sediments and limestones, into which subterranean masses of granite and other molten, deep-seated rocks have been injected: but under the intense pressures to which they were subjected after their formation they and the invading granite have entirely lost their original character, and have been metamorphosed into gneisses, schists, and marble, all sharply and closely molded together. In any given district the direction of their folding is maintained with wonderful constancy over great distances. There is no succeeding system of rocks that has been so completely transformed, so universally plicated, as this ancient Archaean complex.
In later times we can pass from stratum to stratum of the sedimentary series and read their history almost as we turn over the pages of a book; in the Archaean all are kneaded together into a state of such desperate entanglement as to defy the powers of human ingenuity to unravel them. Thus the line of demarcation between the Archaean and subsequent sedimentary systems is the sharpest and most absolute that is known to us in the history of the earth. It marks the close of our planet’s infancy, the several events of which have passed into oblivion as profound as that of our own forgetfulness of our earliest days: later events, on the other hand, are recorded in the stratified series with a faithfulness which increases as we approach existing times.
A history without dates must seem very unsatisfactory to a historian, and the question will naturally arise whether we can assign any definite time to the various critical events recorded in the evolution of the Earth. At present we can only make more or less plausible estimates. Thus, from a consideration of the thickness of the sedimentary crust, and the rate at which sediments are now being deposited, it has been asserted that the interval which separates us from the close of the Archaean era may amount to about twenty-six millions of years. Professor Joly, basing his argument on the undoubted fact that the ocean derives the greater part of its salt from the dissolved material contributed to it by rivers, comes to the conclusion that the ocean first came into existence about one hundred millions of years ago. As regards the birth of the moon, Sir George Darwin has given a minimum limit of fifty-four millions of years, but he adds that it may have taken place many hundreds of millions of years before this. Lord Kelvin has attempted to determine the time which has elapsed since the earth first acquired a solid crust. If we only knew the rate at which the earth is cooling we might calculate back to this time with some assurance of certainty, always, however, on the assumption that the earth is simply a hot body cooling like any other hot body—such, say, as a red-hot cannon-ball. But a, few years ago it began to be seriously suspected that this assumption was a very doubtful one, for a new element—radium—was discovered in 1898, which possesses the remarkable property of spontaneously liberating heat, and this not in small quantities, but at an astonishing rate. One gramme of radium, for example, gives out enough heat in one hour to raise the temperature of one gramme of water to boiling point; hour after hour, year in, year out, this wonderful substance is setting free the energy it contains, and will continue to do so until, some thousands of years hence, it has exhausted its store. If this element should happen to exist in sufficient quantity within the earth, then the earth could not be said to be cooling just like a piece of hot iron, and the increase of temperature we experience as we descend towards the interior of the earth might possibly be due to the heat set free from radium. Indeed, the argument is not confined to earth; it may apply also to the sun, and much of the heat we derive from that luminary may be provided by bursting atoms of radium. This was pointed out by Sir George Darwin and Professor Joly in 1903.
It became obviously a question of the first importance to discover what proportion of the earth’s crust consists of radium, and an investigation was undertaken for this purpose by the Hon. R. J. Strutt, who finds that the rocks composing the earth’s crust contain a superabundance of radium—sufficient, if this element is uniformly distributed through the whole earth in the same proportion as it occurs at the surface, not only to make good the heat which is radiated away into space, but actually to raise the temperature of our planet, which, on this evidence, should, therefore, be growing not colder, but hotter.
This is a result as disconcerting at first sight as it is astonishing, and its effects are very wide-reaching. Of course, it completely destroys the validity of Lord Kelvin’s argument, but it also deprives the nebular hypothesis of one of its cherished lines of evidence—a loss which the force of the general argument enables us to bear with equanimity.
In any case, the vast body of facts bearing on the history of the earth suffices to show that its temperature cannot be rising. Mr. Strutt has, therefore, imagined that the radium is not uniformly distributed throughout the mass of the planet, and supposes that it is restricted to an external zone forty-five miles in thickness; this would suffice to maintain the earth at its existing temperature. If, however, we admit a restriction of this kind, we are in no way bound to fix the limit at forty-five miles. All we can say is that we do not know how far downwards the radium reaches—for aught we know five miles, or even less, is as likely a limit as forty-five miles. Professor Joly, indeed, maintains that the radium we meet with is not proper to the earth at all, but comes from the sun.
Radium is a short lived element, its existence being limited to a few thousand years; but as fast as it decays it is reproduced at the expense of another element—uranium—the lifetime of which is measured by hundreds of millions of years.
The last quarter of a century has proved fertile in great discoveries—more so than any corresponding period in the past. As a result, the whole world of scientific, thought has been thrown into commotion; old-established theories, and even the most fundamental notions, seem to be in a state of flux. Under the stimulus of new ideas great questions, such as the constitution of matter, the origin of species, and the birth of worlds are being re-investigated with renewed energy, and we seem to be on the eve of great events.
William Johnson Sollas