Popular Science Monthly/Volume 31/May 1887/Origin of Comets and Meteors
|ORIGIN OF COMETS AND METEORS.|
By RICHARD A. PROCTOR.
THERE appeared in these pages not long since a valuable essay, by M. Daubrée, on the structure of meteorites, and a little later a very interesting paper by Professor Newton, of Yale College, in which the general question of the origin of meteors, meteorites, and comets was discussed, without any definite conclusion being indicated, except that there are objections against all the various opinions which have been expressed by Schiaparelli, Tschermak, Meunier, Daubrée, and others, respecting this very difficult subject. I should be glad if permission could be accorded to me to bring before the readers of a magazine, so high in scientific standing as "The Popular Science Monthly," the theory to which my own researches have led me—the more so, that I find my ideas quite commonly misapprehended, insomuch that objections have been urged (as by Professor Newton and by my friend Professor Young) which, though, full of force in themselves, have no bearing whatsoever on my theory as it really is.
I may say, indeed, at the outset, that I am in thorough agreement with all or very nearly all which Professor Newton has urged in the way of objection against the views of those who have theorized on this subject on special lines, including the view (which he attributes to myself) that comets and meteors have been expelled from the sun, or from the giant planets. But, at the same time, I find in all those theories, including even the one mistakenly attributed to myself, the germs of truth. If Schiaparelli is quite mistaken in regarding comets as captured meteor-flights, we yet owe to Schiaparelli the now established theory, admitted by all (unless Mr. Denning can be counted as an exception), that meteors are closely connected with comets, and the probable theory that comets are in reality flights of meteors, though their origin in our system may not be that which Schiaparelli has assigned to them. Again, Tschermak is undoubtedly mistaken in supposing that meteorites were originally expelled from the earth's interior, yet the evidence which he has adduced to show that a certain class of meteoric bodies most probably had such an origin can not be lightly overlooked. In like manner when Daubrée speaks of meteorites as having had their origin in the stars, and regards all orders of them as telling us of stellar interiors, he unquestionably lays himself open to the objection that certain orders of meteoric bodies can not possibly have had that origin, their orbits being entirely inconsistent with any such supposition. Yet, in the work Daubrée has done to indicate the conditions under which meteorites were first formed, he has as unquestionably supplied material, of which the true theory, whatsoever it may be, must take account. So with the theory which I have been supposed to entertain. It is manifest that bodies shot forth from the sun would either return to him, or, if their velocities of ejection were great enough, would pass away not only from him, but from the solar system, forever. It is manifest, also, that the giant planets can not now possess power to expel bodies from their interior with such force as would be required for absolute rejection as distinguished from mere temporary ejection; and certainly nothing in the present condition of our earth, or in the evidence given by her crust as to past volcanic action, suggests the likelihood, if even the possibility, that during her career as a planet she has had the power of rejecting matter from her interior.
But I have mostly found that, in endeavoring to form a true general theory on a subject of this kind, it is important to gather together the good features of the several special theories, not merely to note such weak features as become associated with them through a mistaken endeavor to make them parts of a general theory on the same scale in regard to details. A specialist, striving to generalize, nearly always goes more or less astray; but, instead of following him, even though with the idea of setting him right, it is best to take the special results he has obtained at the cost of much labor and research. If we do this with the work of all who have dealt specially with a subject, the chances are that we shall find we have gathered nearly enough to indicate a general theory, which shall include all these specially ascertained details. This the true theory, whatsoever it may be, should unquestionably do. My theory, at any rate, has been obtained in this way, and is intended in its wide generality to cover all the known facts.
So much premised, I note that my reasoning on the subject of comets and meteors starts from the idea which Professor Newton seems very properly to regard as almost necessarily true, that shooting-stars, fireballs, aerolites, and in fine all orders of meteoric bodies, belong to the same general class, differing only inter se in size, distribution, orbital motion, and the proportions in which the materials constituting them are distributed. It appears to me, as it does to him, that a theory which will account for such streams as supply the August and November displays, but not for the meteoric masses which fall sporadically, can not be the true general theory of meteors; nor can any theory be accepted as at once true and general which accounts for the holosiderites while it leaves unexplained the asiderites (so called, though in reality no meteors are free from iron). Again, noting that meteors have been associated with comets, we require for any theory which shall be accepted as generally true, that while it shall explain this connection between streams of bodies producing falling stars and certain special comets, it shall also be able to account for all comets as possibly associated with meteor-streams, and for all meteor-streams as possibly associated with comets. How much resides in this last condition, those only can guess who have put the matter to the test by striving to find a general theory of comets and meteors which shall not be found to be in conflict with some fact known about particular comets or some other fact known about particular meteor systems. Yet no general theory of comets and meteors can possibly be accepted which fails when thus tested, trying though the test may be. These trying tests are, indeed, particularly valuable for the seeker after truth, seeing that they serve to diminish his field of research by fencing out portions which can not usefully be dealt with. My own experience has convinced me that negative indications serve often to lead more directly to the truth than the most seemingly decisive evidence of the positive sort, though in reality it is by combining the two kinds of evidence, rejecting because of decisive negative evidence theory after theory from among those to which we are directed by decisive positive evidence, that we can alone hope to arrive at the true theory.
With a wide choice as to a starting-point, I take first the results of M. Daubrée's analysis of meteorites in regard to chemical composition and physical structure; and I combine the positive evidence he has obtained with Professor Newton's argument—very just and of great negative weight—that no theory can reasonably be accepted with regard to meteorites which may not be extended in its general sense to all orders of meteoric bodies.
M. Daubrée tells us, then (nay, he shows by demonstrative experimental evidence), that meteorites resemble so closely in composition and structure volcanic products such as are only found deep below the earth's crust, that we may be assured they were formed under similar conditions of temperature and pressure. He constructs masses of matter under such conditions which the most experienced student of meteorites could not distinguish from true meteoric masses; and he points out how the earth in her interior laboratories has constructed and presently ejected bodies which in like manner deceived the most experienced, taking their place for a long time in museums as "the Ovifak meteorites."
M. Daubrée very naturally draws the inference that meteorites were actually formed under such conditions. But a mass formed as such volcanic products are being now formed, deep beneath the crust of the earth, could not possibly escape from such a birthplace except by such energetic extrusion as a body like our earth, now or during the ages recorded in the geologic strata, could not possibly have effected. Hence, M. Daubrée infers (again, quite naturally) that meteorites were ejected from the interiors of stars.
Applying to this result the principle indicated by Professor Newton, we see that it requires to be at once generalized and modified, for there are classes of meteoric bodies which can not possibly be regarded as coming from any of those orbs which we call stars. Among these may be specially mentioned, first, those orders of meteoric bodies which Stanislas Meunier and Tschermak have been led to regard as ejected from the earth. Without for the moment attaching any specific importance to this idea as involving a positive theory of the origin of these meteors, it is certain that the evidence adduced by Tschermak and Meunier, confirmed also by the mathematical inquiries of Sir Robert Ball, definitely negatives the idea of an origin outside the sun's special domain. In like manner we must exclude those meteor-streams which, like the Leonides, the Perseids, and the Bielids, travel in closed paths, indicating an origin within the solar system. I have myself adduced evidence which is really demonstrative, and admitted (even by those who think there may be some escape from it) to be for the present unanswerable, to show that these meteor-streams can not have been captured as meteor-flights by the giant planets, as Schiaparelli suggested. But, apart from this, I believe that no one who considers the nature of these streams, or the character of the orbits in which their components move, will for a moment adopt the belief that they have been ejected from the stars, even though he may accept the colorless theory (which explains nothing) that they were captured by the giant planets from out the star-depths.
Nor are we at all helped by remembering that the sun himself is a star, and that certain among the meteors which reach the earth may be supposed to have come from him. For assuredly the meteors regarded by Meunier and Tschermak as of terrestrial origin can not be attributed to the sun as their source, while the orbits of all the recognized meteor-streams are entirely inconsistent with such an origin. Mr. Matthew Williams, in his "Fuel of the Sun," has pictured bodies ejected from the sun which somehow come to be traveling afterward in orbits nowhere approaching within millions of miles of his surface; but no such processes are within the range of dynamical possibilities.
How, then, are we to retain at the same time what we regard as proved by Daubrée, and also those facts, inconsistent with Daubrée's theory as actually presented, which have been shown with equal certainty, either in their positive aspect (as in the case of the November and August meteor-streams) or negatively, to be certainly inconsistent with the supposed origin of meteorites from the stars? Clearly we must widen our range of survey so as to recognize an origin for meteors and meteorites which, while including Daubrée's facts, shall not exclude the others; and I think there can be very little doubt how such widening of the range of survey should be effected. Widening our survey of space will be of no service, for we only bring in more distant regions, and the meteors we have to explain require a nearer origin; but if we widen our survey of time, as assuredly we are justified in doing (for many meteorites must be millions, nay, tens, hundreds of millions of years old), we shall find other stars than those considered in Daubrée's theory, and some of these may meet our difficulty.
If there is one fact about the past of our earth and the other members of the solar system which may be regarded as certain (amid all our uncertainties in regard to the possible nebular origin of the system, or its possible origin by aggregation, or by a combination of both processes), it is that each planet began its career in a state of intense heat. I suppose no one doubts now that the giant planets retain much more of their primeval heat than the earth or Venus or Mars; nor, on the other hand, can it be reasonably doubted that the moon has parted with much more of her original heat than our earth, insomuch that, whereas, once she was the scene of such activities as we recognize in our world, she is now a cold and lifeless orb. It is in their aspect as records of the past of the planets that I note these facts. They indicate a progressive loss of heat which we only have to trace back to recognize each one of the planets, in the earliest stages of its career, as a sun-like body.
Extending, then, thus our survey in time, we find another set of stars, or rather of suns (we must now use the more general word), to be added to those regarded by Daubrée as the bodies from which meteorites (and meteors of all classes, according to Professor Newton's just generalization) have proceeded. We may in one sense, indeed, be said to have multiplied Daubrée's sun-sources of meteors manifold, since for every sun now existing in space our views as extended show a whole family of sun-like orbs. But in reality we have only strengthened our theory by the addition of the suns which once belonged to our present sun's domain, for these alone could in any way explain the meteors and meteor-streams which had prevented us from accepting stellar (or rather extra-planetary) origin for meteoric bodies. It is to be observed, however, that these suns which we now introduce into the theory were not all active at the same time. We must regard them as distributed in time much as the stars are distributed in space—some very far off, others far off, indeed, but yet comparatively near; and in determining the distance of time at which they were active as suns, we can not range them in any definite order according to their mass. For instance, our own earth, though much more advanced in planetary life—that is, far cooler—than a giant planet like Jupiter, was probably in an actively sun-like state at a much more recent time: since the interval of time during which Jupiter has been cooling from the sun-like stage to his present fiery condition enormously exceeded, in all probability (owing to the vastness of his mass), the time occupied by the earth in passing from the sun-like stage through the fiery stage to her present cool and habitable condition; and, on the other hand, though Mars is much more advanced in planetary life than the earth, yet it is quite possible (though we can form no definite opinion in this case as in the former) that Mars might have been in the sun-like stage later than the earth.
We may observe here that we not only remove from Daubrée's theory, by this extension of it, the difficulties which had before prevented us from accepting it as a general theory of the origin of meteors, but we place the theories suggested by Tschermak, Meunier, Schiaparelli, and others, in a much more satisfactory light than before. It is properly objected to Tschermak's theory, by Professor Newton and others, that our earth can not be supposed to have ever had while a world explosive energy such as that theory imagines; but when the earth was in the sun-like state she could do all that might befit a sun. We know that our sun can eject matter from his interior with velocities sufficing to carry such matter forever away from him, for he has been caught, first by Professor Young in 1872, and several times since, in the act. What our sun with his much vaster energies can do ejectively to overcome the withdrawing power of his own much mightier mass, we may well believe that our earth in her sun-like days could do to overcome the attraction of her smaller mass. The only difference would be that while the sun on such occasions expels matter so as to pass forever away not only from himself but from the solar system, the earth, even in the full energy of her sun-like condition, could not probably have expelled bodies from her interior with velocity sufficing to carry them beyond the control of the sun. The bodies ejected with velocities freeing them from the earth would thenceforward travel around the sun on orbits of different dimensions within certain limits. Their orbits would at first intersect the earth's path, and, even under the perturbing actions of the planets, would always pass very near it, oscillating, in this respect, on either side of the earth's track so as at intervals to cross it for a while as at first. Hence there would always remain a chance of recapture, and indeed there would be a certainty that, in the fullness of time, every body ejected from the earth would be recaptured, though the fullness of time might in some cases mean many millions of years. Thus the capture of sporadic meteors of terrestrial origin would be fully accounted for.
In like manner with those streams of meteors whose orbits lie within the solar system so as to preclude the supposition that these bodies could have reached the system from without. I say definitely that the supposition is precluded, because the argument from the consideration of the laws of motion, by which I have shown that the giant planets could not possibly have captured these meteor-streams in the manner imagined by Schiaparelli, is admitted to be sound even by those who have not weighed my own theory in explanation of the origin of these systems. It is suggested by some, as by Professor Young, that there may be some way of explaining away the difficulty I have indicated; but I am not prepared to regard a vague suggestion of this kind as having any present weight. It seems, to say the truth, much as though one should say, for example, It has indeed been demonstrated mathematically that the circumference of a circle is not arithmetically commensurable with the diameter, but there may be some way of representing the ratio of the circumference of a circle to the diameter as a fraction of some sort. The fact remains that this has been proved to be impossible; and it has equally been shown to be impossible that any flight of meteors could be captured (that is, brought into the solar domain for good) as such, by any planet, even by the giant Jupiter. Either the meteors must lie so very close together that their mutual attractions would make them practically one body, and keep them such after capture by the planet, which is not the case with any of the meteor-streams, or else the differences of perturbing action on the meteors would be so great that the flight must be entirely dispersed in the act of capture—not merely dispersed so as to form a stream or a larger flight, but so dispersed as no longer to form a meteor system at all.
But, extending our generalized theory to the case of the giant planets—as, be it observed, we are not only entitled but bound to do—we see that as there would be flights of meteors passing always near the earth's orbit because of their original derivation from the earth in her sun-like stage, so would there be flights of meteors passing always near the orbit of each one of the giant planets; unless, indeed, in the fullness of the vast periods of time which must have elapsed since their formation, processes such as seem to affect Encke's comet should have altered their orbits considerably. Even then they would exhibit an approach to the orbits of their parent planets such as to suggest the idea of some sort of physical association. And accordingly, we find this peculiarity so far manifested, that years before the idea ever occurred to me that any comets or meteor systems could have been expelled from the giant planets, I wrote an essay on what I called the "Comet Families of the Giant Planets," describing just such comets, though I was unable to find any interpretation at that time of the peculiarity in question.
It is worthy of notice that quite a number of difficulties, some of them very serious ones, disappear, even as these last two have done, so soon as we adopt this generalization of the special theory to which we found ourselves forced first by direct evidence.
For example, if the capture theory advanced by Schiaparelli were accepted, not only would it leave all our perplexities unexplained, not only would it involve our running directly against mathematical certainties, but it would introduce this tremendous difficulty: the number of meteor-flights traveling about in interstellar space must exceed many millions of times those which visit our solar system, and the number of such flights visiting our solar system must exceed millions of millions of times those which chanced, by a strange combination of accidents, to come within capturing distance of a planet. Again, if many of the meteor systems which cross our earth's orbit are not to be attributed to a terrestrial origin, then the number of meteor systems within the solar domain must exceed millions of millions of millions of times those which have thus been recognized. In the former case the capturing-places in the solar system are very limited in extent compared with the dimensions of the system; but in the latter case it is the whole domain of the sun which we have to compare with that mere thread of space traversed by our earth within the solar system.
I might have arrived at the same result, however, in entirely different ways—a consideration which is at once the most marked characteristic and the surest test of truth in a general theory of this sort.
Suppose, for instance, I had begun with the discovery by Professor Young that the sun has tremendous ejective might. I have shown, from the circumstances attending the formation of the eruptive prominences, that they do not indicate the ejection of glowing hydrogen and helium, but of small masses of denser matter through those gases. (I note, in passing, that Tacchini's observations during the last eclipse have practically demonstrated the justice of this view.) I have further proved that such masses of ejected matter have in some cases had velocities exceeding the three hundred and eighty-two miles per second which the sun can master, and therefore must have passed forever away from him. From this demonstrated fact, as surely as from M. Daubrée's demonstrated facts about meteorites, we can work out the whole theory of cometic and meteoric ejection. For our sun, being one of the stars, we may infer that what he does each star also does. Again, what he does now he must have done (perhaps once with even greater energy) during all the millions of years that he has been a sun and doing sun-work. So also must all the suns which people space, during the past millions of years of their sun-work, have expelled from time to time flights of small bodies (whose nature we have yet, so far as this discussion of our theory is concerned, to determine). We may conclude that from the total matter ejected at any outburst many millions of small bodies would be formed as the originally vaporous matter vomited forth condensed into the liquid form and then into the solid—perhaps quite close to the parent orb. But the total mass ejected would bear to the ejecting body some such relation as the total mass of the dust ejected at Krakatoa bore to the six hundred millions of millions of millions of tons of the earth's mass; a hundred millions of years of such ejective work from an orb like our sun might well be unable to eject a total mass from out of which such a globe as even the least of the asteroids could be formed. Moreover, what we have thus inferred about each sun during the whole of its career up to the present time, we must infer also of each one of the bodies attending on each of those suns, during the sun-like portion of the career of each such attendant orb.
Hence, taking an average meteor-flight to represent the number of bodies at each ejection, ten effective ejections per annum for each sun-like orb, an average of a million years only for the sun-like duration of each orb in space, a thousand millions of suns in our galaxy (a three-inch telescope shows a million stars), and ten orbs of various size depending (on the average) on each, then we have a grand total of 10 × 1,000,000 × 1,000,000,000 × 10 = 100,000 millions of millions of meteor flights, as representing the total number of bodies ejected from the various orbs peopling space, including those now sun-like, and also those which, though now in the fiery stage, or further advanced still in planetary life, were once as surely suns as the stars are now.
When we remember that with so many millions of millions of flights of bodies, each flight to be counted probably by millions of millions, our earth must from time to time be saluted by some of these, while we know that during all the years over which observation has continued, absolutely nothing has reached our earth from outside except the various orders of meteors, while no flights of bodies can be recognized as ever visiting us from interplanetary space except the various orders of comets, we are justified in concluding that these represent products of ejection. We infer this on the safe ground of the argument that if these bodies do not, no other bodies exist which can represent the product of the ejective processes we have certainly recognized. It would have been a rather bold thought, yet not wanting in reasonableness, and certainly ingenious, to have said that therefore comets and meteors are but different appearances of the same objects. This, though it might have been shown to be probable, could not have been shown to be certain; for the simple reason that the ejected bodies might have been only discernible when any of them entered our atmosphere, in which case only meteors would have been required by the facts or accounted for by the theory of ejection. But now that we know comets to be but flights of meteors, and meteors to be but attendants on comets, we see that one of the prettiest discoveries of modern astronomy, Schiaparelli's recognition of the connection between comets and meteors, is implicitly associated with the results of inquiry applied to the sun's power of volcanic ejection. We might further have inferred the discoveries of Tschermak, Daubrée, Sorby, Graham, and others, as to the structure of meteorites, even though none of these bodies had ever reached our earth from interplanetary space—seeing that our earth's interior, beneath the regions now relieved by volcanic outbursts, would afford good information as to the nature of bodies ejected from such deep-seated regions of her interior, or of the interior of other celestial orbs.
A theory which could not be true except in its most generalized form, but which in that form (1) agrees with every one of the known facts, (2) accounts for many of them, (3) alone accounts for some of them, (4) shows that certain of them are its necessary consequences, would probably be right, even though it had not been yet shown that among all other possible theories there is not one which is not directly contradicted by some known facts. As, however, the general theory of the ejection of all cometic and meteoric bodies from orbs—suns of all orders, giant planets, terrestrial planets, planetoids, and moons—is as strongly supported by such negative evidence as it is by direct positive evidence, I venture to say that a case not easily shaken has been made out in its favor. No one, so far as I know, has yet indicated any objection against the theory in the generalized form in which alone I have ever advanced it. Objections have been urged against it in the form in which it has been supposed that I have maintained it. It has been very clearly shown that meteors can not come to the earth from the sun unless they strike the earth on their first course out from the central orb; it has been proved that a considerable proportion of the meteoric and cometic systems known can not have had their origin either in our sun or in any of his fellow-suns, the stars; it has been urged as effectively that the giant planets can not eject comets or meteors; and it has been shown clearly that our earth can not, in any stage of which geology has traced the records, have ejected bodies which could thenceforth travel in interplanetary space as meteors or meteor-flights. But, in these objections against specific theories of the possible origin of comets and meteors, we may find some of the strongest, if not the very strongest, arguments for that general theory to which each specific theory points, so soon as we notice that the arguments supporting each specific theory are such as decline to be limited to that theory alone.
In fine, as I suggested at the outset, if we apply to the several specific theories of comets and meteors the general principles laid down by Professor Newton, we find ourselves led irresistibly to that general theory which I have sketched above, and presented with more elaboration of detail elsewhere.
- It would not be absolutely impossible that some of the matter ejected from the earth in this way would pass away even from the solar system. It appears from the very existence of earth-ejected meteors, which we regard as demonstrated by Tschermak and Meunier, that she had power of ejecting matter with velocities up to seven miles per second. A velocity of little more than eight miles per second would suffice to carry matter away from the solar system if the matter chanced to be ejected from the middle of the advancing face of the earth, for then there would result relatively to the sun a velocity of more than twenty-six miles per second, which at the earth's distance is a velocity corresponding to parabolic motion around the sun; but this would very rarely happen.
- If we suppose our earth's eruptive power to be unable to give greater velocity of ejection than eight miles per second, then the velocities of bodies expelled from the earth would, at the distance from the sun where they began their independent careers, range between twenty-six miles and ten miles per second, the earth's orbital velocity being eighteen miles per second. Hence the orbits of the expelled bodies around the sun would range between parabolic orbits with perihelia at the earth's distance, and ellipitical orbits with aphelia at the earth's distance and perihelia at a distance of one third the earth's, with an eccentricity of ·866. None of the expelled bodies could come nearer to the sun than this last-named perihelion distance.
- I take some pleasure in noting that I was the original proposer, and an intimate friend of mine the seconder, of the proposition that the Council of the Astronomical Society should bestow their gold medal on Signor Schiaparelli for this discovery. If we must have the prize-pig system of rewarding scientific research, let us at least, according to the good old English saying, "catch the right pig by the tail."