Public domain
[p. 415]
The bedded rocks of the earth’s shell reveal its history far back into the past with great fidelity; but the record of the still earlier ages is indistinct, and if an attempt be made to follow the history back to its beginning, the indistinctness merges into obscurity. The rocks below the well-bedded strata are so broken and altered, and so cut up by intrusions, that their history is read with the greatest difficulty. Still lower lies the inaccessible interior of the earth whose nature is more a matter of inference than of knowledge.
Some suggestions as to the origin of the earth may be found in its relations to the other bodies of the solar system, and certain features of the solar system give pointed hints as to its early history. The interpretation of these outside relations and of the secrets of the hidden interior is yet far from clear, and our only recourse at present is to hypotheses. It is nevertheless important that we should study, with due reserve, these hypotheses, and note with care the ways in which they enter into the interpretations of the earth’s phenomena, for not a few of the leading doctrines of geology hang on some hypothesis of the earth’s beginning, and have no greater strength than the hypothesis on which they depend.
It is the nearly unanimous conviction of astronomers that the solar system was evolved in some way from a nebula of some form. Until recently, astronomers so generally accepted the view of Laplace [p. 416] that it came to be known as “The Nebular Hypothesis”; but the advance of inquiry makes it necessary now to consider other hypotheses which maintain that the solar system arose from a nebula, but a nebula whose constitution and mode of evolution differed from that postulated by Laplace. The leading hypotheses of the earth’s origin may be grouped in three classes:
I. The gaseous hypotheses, in which the parent nebula is assumed to have been formed of gas collected into a spheroid by gravity in accordance with the laws of gases, and to have been evolved into the present solar system by loss of heat, and the separation of the outer parts into planets. The type of the class is the Laplacian hypothesis.
II. The meteoritic hypotheses, in which the parent nebula is assumed to have been a swarm of meteorites, the members of which moved in diverse directions, and suffered frequent collisions giving rise to heat, light, and vaporization. The swarm of meteorites is thought to have behaved essentially as a coarse gas, and the evolution of the system to have been dynamically like that of the gaseous system.
III. The planetesimal hypothesis, in which the original constituents are assumed to have been small bodies, molecules, or aggregates, moving in orbits about a common center and forming a disklike system. The bodies are supposed to have been controlled by revolution about the center of the nebula, and not by impact on one another. The evolution consisted in the gathering of these small bodies, planetesimals, into planets and satellites. Dynamically, this hypothesis differs more from the other two than they do from one another.
I. The Laplacian or “Nebular” Hypothesis
During the last century the Laplacian hypothesis was very generally accepted, and geological theories as to the early states of the earth, and as to many later events in its history, were built upon it, and these views are still prevalent. The hypothesis is so well known that a few sentences will recall its essential features. It holds that the sun, the planets, and the satellites were once parts of a glowing, rotating, spheroidal, gaseous nebula, which was ex
[p. 417]
panded enough to embrace the whole space of the present system. The nebula was assumed to have cooled by radiation of heat, and in cooling to have shrunk. This shrinkage accelerated the rate of rotation, and this in turn increased the equatorial bulge which rotation developed. The progressive increase of cooling, rotation, and bulging finally led to the separation of an equatorial ring. As this ring also cooled and contracted, it was disrupted and its substance gathered into a planet whose orbit lay in the plane the ring had occupied. A series of rings, separated in this way, gave rise to the several planets in turn, while the central mass formed the sun. The orbit of any planet bounds approximately the space assigned to the nebula at the birth of that planet. At the time of their formation, the several planets were thought to be hot, gaseous, and rotating. Cooling and shrinkage increased the rate of their rotation, and this caused a bulging of their equatorial zones, till some of them, following the example of their parent body, shed rings which became satellites.
In support of this ingenious theory many harmonies in the motions of the members of the solar system were cited, and in the early days of the hypothesis, the phenomena of existing nebulae were thought to give it much support, for among them, as then known, there seemed to be nebulous aggregations in various stages of development, from diffuse nebulous patches on the one hand, to forms almost as concentrated as suns on the other. Critical students of celestial mechanics have always recognized difficulties in the hypothesis, but to those who did not follow the closer reasoning on which its truth or falsity must rest, the hypothesis seemed to afford a plausible starting point for the history of the solar system, and was accepted as if its foundations were firm. Without a knowledge of celestial mechanics and of the molecular activities of gases, it is scarcely possible to appreciate the full force of the arguments which bear against it, but some of them may be stated briefly.
In the evolution of a gaseous nebula, it is highly improbable that rings would be formed, for wthe molecular activities would cause the molecules to separate from the parent nebula one by one.
There are grave difficulties in the contraction of a ring [p. 418] into a spheroid as simply and promptly as postulated by this theory.
In the highly heated condition assigned the earth-moon ring, its gravity could hardly have held the common gases together, because of their intense molecular activities. The earth, even now, does not appear to be able to hold permanently very light gases, though it does hold the heavier ones which have slower molecular velocities.
It is probable that the attenuated substance of a ring, such as the supposed earth-moon ring, would have cooled to solid particles long before it could collect into a spheroid, and hence no secondary ring to form a moon would be developed.
The inner satellite of Mars (Phobos) revolves about that planet three times while the planet rotates once. According to the Laplacian theory they must have rotated together at the time of separation, and the planet should have kept on increasing its rotation by cooling after the separation. Its period of rotation should therefore have been shorter than the period of the satellite’s revolution. It has been suggested that the planet’s rotation was lengthened by its tides, but so much lengthening is very unlikely. If accepted in this case, it is hard to apply it consistently to the similar anomaly of the small bodies that make up the inner edge of the inner ring of Saturn, and they revolve in about half the time of that planet’s rotation.
If the solar system were converted into a gaseous spheroid, and so expanded as to reach out to Neptune’s orbit, and had its matter distributed according to the laws of gases, and if this nebula were endowed with the total value of the momentum (technically the moment of momentum) now possessed by the solar system, it would not have had a rate of rotation rapid enough to detach matter from its equator at that stage, or at any later stage until it had contracted well within the orbit of the innermost planet.
If the expanded spheroid were given the rates of rotation necessary to shed rings at the proper stages (if rings could be formed), the moment of momentum at each stage must have equalled that of the matter it then contained; for the total moment of momentum of any such system must remain constant so far as its [p. 419] own evolution is concerned. Now computations show that at the stage which marked the birth of Neptune, according to this hypothesis, the moment of momentum of the nebula must have been more than 200 times as great as the present moment of momentum of the solar system; that at the stage of Jupiter’s formation, the moment of momentum must have been 140 times too great; that at the Earth stage, it must have been 1,800 times too great; and so on. Here is not only an enormous discrepancy, but one that varies greatly and irregularly from stage to stage.
If the masses of the planets are compared with the moments of momenta they carried off from the parent nebula, strange discrepancies are disclosed. The matter in the ring supposed to have formed Jupiter and his moons had a mass less than onethousandth of that of the nebula at the time of its separation; but Jupiter and his moons now have about 95 per cent of the total moment of momentum which the nebula then had. The Laplacian hypothesis thus calls on us to believe that an equatorial ring having a mass less than a thousandth of the mass of the parent body, carried off in its separation 95 per cent of the total moment of momentum. The supposed separation of other rings involves similar incredible ratios.
Under the Laplacian hypothesis, the satellites should all revolve about their planets in the direction in which the planets rotate on their axes; but the recently discovered ninth satellite of Saturn revolves m the opposite direction.
Though our knowledge of nebula? has been extended greatly in recent years, nebulae with such rings as the Laplacian theory postulates have not been found. The larger number of those at first supposed to support the theory have been found by improved methods of research to be spiral nebulae or anomalous forms.
II. The Meteoritic Hypotheses
It was long ago noted that shooting stars entered the upper atmosphere nightly in great numbers, and that occasionally fragments of stony and metallic matter fall from the heavens. Out of this grew the notion that the earth may have been built up in this way, save that the process was more rapid in the early ages of the
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Earth’s building. This notion, however simple and natural, may be dismissed without serious consideration, for the diverse directions of motion and the various velocities of meteorites are such as to forbid the belief that a system of such symmetrical discoid al form and of such harmonious motions as the solar system presents, could have been formed in this manner.
The hypothesis of Lockyer and Darwin. A meteoritic hypothesis of more logical character has been based on the conception that meteorites may be aggregated into swarms and constitute nebulae. This hypothesis is, therefore, nebulo-meteoritic. Inquiry into the mechanical possibilities of the case led Sir George Darwin to the conclusion that such a swarm of meteorites would act very much like a gas, and that the laws of gases may be applied in determining its mechanical properties. So far as it applies to the origin of the earth, this form of the meteoritic hypothesis is thus found to be practically identical with the gaseous hypothesis, and so far as it applies to the dynamics of the solar system, it is subject to the criticisms already urged against that hypothesis. It is to be noted that these criticisms apply only to such meteoroidal nebulae as may be formed of swarms of meteorites the members of which move in diverse directions, and frequently collide with one another. If the meteorites were assembled so as to pursue concentric orbits and form a disk-like system, they would be controlled by planetoidal or planetesimal dynamics, and fall into the following class. The term meteoritic hypothesis is here used essentially as employed 1 >y Lockyer and Darwin, and should be carefully distinguished from the planetesimal hypothesis, whose mode of evolution is radically different. The meteoritic hypothesis as thus defined is applied by its authors chiefly to the earlier and more scattered states of the nebulae, and has not been specifically applied to the formation of a planet, perhaps because a meteoritic nebula tends to pass into a gaseous nebula as it condenses.
III. The Planetesimal Hypothesis
When the shortcomings of the Laplacian hypothesis came to be so clear and cogent that there was no apparent way of escape from them, an alternative better in accord with the facts was sought. [p. 421] It had been held that the matter of a nebula, if formed of particles revolving independently about their common center of gravity, could not be gathered into planets without giving these a backward rotation, at least normally; but the six inner planets have forward rotations. All hypotheses of the planetesimal type appeared, therefore, to be shut out; but closer analysis showed that the earlier conclusion was an error and thus a field, previously supposed to be closed, was opened. It has also been shown by astronomic photography that there are a multitude of nebulae, — at least ten times as many as were known a few years ago, — and that in this multitude, there is one dominant form, the spiral nebula (Fig. 334). This form has a central nucleus, from which two arms or sets of arms start forth on opposite sides and curve spirally outward. The arms often branch, and are much interrupted and knotted, and between them there is much scattered hazy matter; but even in the more diffuse forms, the presence of two arms is discernible. The prevalence of this form of nebula implies that it is due to some process which is pervasive. The numerous nebulous knots on the arms, and sometimes more or less outside them, are significant features. Clearly the matter has a very unequal dispersal and does not conform to the symmetric laws of gaseous distribution.
[p. 422]
Recent advances in spectroscopy throw much light on the constitution of nebulae. As just inferred from their forms, the spiral nebulas seem to be composed, not of gaseous molecules, but of solid or liquid particles. These tiny bodies are believed to revolve about the center of the nebula, like little planets, but this has not yet been proved. If it were positively known that the particles of the spiral nebula? revolve about the centers of the nebula in elliptical orbits like planets, and were thus planetesimals, these nebulae might well be called planetesimal nebulae. The planetesimal hypothesis is based on a spiral nebula of this supposed organization.[1]
[p. 423]
For the building up of the solar system, the planetesimal hypothesis starts with a spiral nebula consisting of the following elements: (1) the main knots to serve as nuclei for the planets, (2) small scattered knots as the nuclei of the asteroids, (3). other small knots near to the large knots and controlled by them, as the nuclei of the satellites, and (4) scattered matter or nebulous haze to be gathered into these nuclei to give them their mature sizes, and (5) the great central mass of the nebula, forming the nucleus of the sun. The gathering of the scattered planetesimals into the knots to form the planets, planetoids, and satellites, is assigned to the coming together of these bodies as they pursued their slightly different orbits, not as the result of falling directly together under the control of gravity. It is assumed that the planetesimals had rather highly elliptical orbits arranged in disk-like form, and such orbits would be favorable for the conjunction of the bodies following them. It can be shown mathematically that under such conditions the addition of planetesimals to the nuclei would give them more and more circular orbits as the accessions took place, and it is significant that the planetoids (asteroids), which presumably have grown little, usually have the most eccentric orbits, that Mercury and Mars, the smallest of the planets, have the next most eccentric orbits, while the orbits of the larger planets approach circularity most closely. The photographs of spiral nebulae show large knots with small ones near them, which appear quite susceptible of evolution into planets attended by satellites. They also show small scattered knots susceptible of forming planetoids. The earth-moon system is assumed to have been derived from companion nuclei of very unequal sizes.
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The knots might have had a rotary motion at the outset, arising from inequalities of projection at the time of their formation; but, in the main, the rotations of the planets are assigned to the impacts of the planetesimals as they joined the nuclei to form the planets, and this would naturally give rise to notable inequalities. There would be no fixed relation between the rotation of a planet and the revolution of its satellites; the period of the latter might be either longer or shorter than that of the former. Even if the revolutionperiod of a satellite-nucleus was originally the same as the rotationperiod of the planetary-nucleus, the growth of the planet might draw the satellite nearer to itself and shorten the time of its revolution, and thus the difficulty of Phobos and of the innermost particles of the ring of Saturn be obviated. The mode of accretion thus assigned might give rise to forward rotation, or to retrograde rotation of the planets and satellites; the forward rotation should be the rule, and retrograde rotation the exception, as is actually the case. In a spiral nebula, formed in the way assigned, the outer parts of the arms should be composed of lighter materials [p. 425] than the inner parts, and since the planets were formed from these arms, the inner ones should have higher specific gravities than the outer ones, as is the fact.
Other peculiarities of the solar system seem to find a fitting explanation in the planetesimal hypothesis, but most of these must be passed without mention here.
The meetings and unions of planetesimals and nuclei at the crossings of their orbits imply a relatively slow evolution of the nebula into a solar system, however inevitable the evolution must be with the requisite time. The planetesimal hypothesis therefore implies a relatively slow growth of the earth. With such a mode of growth, the stages of the earth’s early history necessarily depart widely from those postulated by the Laplacian and the meteoritic hypotheses.
The manner in which it may have arisen is discussed in the authors’ larger work on Geology, Vol. II. ↩︎