Scientia - Vol. VII/The origin and nature of comets
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THE ORIGIN AND NATURE OF COMETS
I do not think that the time has yet arrived when we can give a complete solution of the problems of the nature and origin of comets. Considerable progress has, however, been made in this direction during the last half-century, and it seems worth while to collect and put in order some considerations, which at all events limit the field within which the solutions must lie; this limitation is really a help to us in framing hypotheses, since it enables us to eliminate those that have little chance of proving fruitful.
It is now about fifteen years since M. Fabry published an essay concerning the true significance of the parabolic form in cometary orbits. The essay does not seem to be very widely known, as is shown by the fact that Prof. C. L. Poor, in «The Solar System» p. 281 argues as if the prevalence of parabolic orbits shows that the majority of comets come to the Solar System from outside. In reality, as M. Fabry shows, if comets really came to our system from outside, quite a large number would have strongly marked hyperbolic orbits. It is easy to see that this is so, if we reflect that the velocity at any point of a parabolic orbit is the same as that due to a fall from an infinite distance, (or, practically, from any distance that is very great compared with the actual distance of the body from the sun) under the influence of the sun’s attraction; for a hyperbolic orbit the speed is greater than this, for an elliptical orbit less. Hence to assert that a comet moving in an apparently parabolic orbit has come to our system from outside is equivalent to saying that it entered the sun’s sphere of influence, (by which we mean the region where the sun’s influence is paramount, and to which we may assign an arbitrary radius, such as 1000 times the radius of Neptune’s orbit) with zero relative velocity; in other words, it was previously moving through space with exactly the same motion as the translational drift of the solar system. The chances are slight of this being the case in a single instance, and it is out of the question that any considerable number of the comets whose orbits are sensibly parabolic should have reached our system from outside. The relative velocities of the stars are of the order of many kilometres per second, which would suffice to produce an orbit of most markedly hyperbolic character in a comet reaching us from another system.
It is then clear that it is to our own system that we must look for the origin of the comets with apparently parabolic paths, and still more those of elliptical character. Within this system there are three different modes of origin that have been suggested: 1) that they are the products of eruptions from the sun; 2) that they are the products of eruptions from the larger planets in a sunlike state; 3) that they are stray fragments of the nebula, which is supposed to have been the parent of our system, and that they remained unattached to any of the large masses that were formed from that nebula.
There are two points that are in favour of the solar origin; first, we can see, in the solar prominences, eruptions of gas at sufficient velocities to carry some of the projected matter away from the sun. Secondly comets by their spectrum, and meteors by actual analysis, reveal the presence of large amounts of hydrogen and its compounds, suggesting their origin in an atmosphere like that of the sun. The obvious drawback to this theory is that all matter ejected by the sun would travel in orbits intersecting his globe, and so, if their speed of ejection was less than 383 miles per second (the parabolic velocity) they would on their return fall back on the sun. Planetary perturbations might suffice to avert this, and produce an orbit just clearing his surface. We have instances of such orbits in the remarkable group of comets of 1680, 1843, 1880, 1882, 1887, and a solar origin does not seem impossible for these; but there are many other comets whose perihelion distance equals or even greatly exceeds the earth’s distance from the sun, and we can hardly suppose, without straining probabilities, that these changes were entirely produced by perturbations. There is a further difficulty in the way of the solar origin; the ejected matter would leave the sun in the form of vapour, and would only liquify and solidify when it reached outer space. Probably it would solidify into particles of extreme minuteness, very much smaller than the meteoric masses that enter our atmosphere, many of which are known to have a connection with comets.
We turn next to the giant planets as possible comet-producers. The late Mr R. A. Proctor warmly advocated the view that the giant planets were the actual parents of the comet-families which are attached to them. Jupiter has a large and ever-growing family of comets, which obviously owe him allegiance, since in the case of Lexell’s and Brooks’ comets he has been caught in the act of profoundly modifying their orbits; Saturn has two, of which Tuttle’s has been observed at several returns; Uranus two, one being the comet of the November meteors; and there are six whose orbits are associated with Neptune’s in such a way as to suggest a connection. Three of these have been observed in two or more revolutions, one being the famous comet of Halley, which has been traced with tolerable certainty back to B. C. 240, and with some probability to B. C. 467 and 625. We have to go back to very remote antiquity to find a time when Neptune could have exerted any considerable influence on Halley’s comet; at present there is no near approach of their orbits, and Neptune’s influence is trifling.
Proctor argued (1) that very close approaches to the giant planets would be required, for their orbits to be transformed from an approximately parabolic form to an ellipse with a period half that of the planet; hence (2) that the number of captured comets would be a very small fraction of the total number that approach the sun. It would require an immense period to produce such a number of comets attached to the various groups. The suggestion has been made that the number might be greatly increased by the consideration that comets that have their orbits changed into ellipses, even of long period, would sooner or later make other approaches to the planet, and might undergo further shortening. But I doubt whether there is much weight in this consideration, for the action of Jupiter is just as likely to increase as to diminish the period, and in only a few cases would the successive perturbations be all in the same direction.
On the other hand, if the planets be regarded as themselves the parents of these comets, their orbits would necessarily pass through the point of ejection, and consequently the difficulty (2) disappears. The only difficulty is the question whether the giant planets are now in a physical condition capable of expelling matter with a velocity of several miles per second. I do not think that we can put back the epoch of ejection to a date several million years ago, when the giant planets may have been in a quasi-sunlike state; for the life of a short-period comet seems to be measured by centuries, not by millions of years. In the last 130 years Lexell’s and Brooks’ comets have had their orbits greatly modified, two others, Biela’s and Brorsen’s, have definitely perished, while Encke’s comet was extremely faint in 1908, which may indicate its approaching dissolution; to these we may perhaps add the numerous comets for which short periods have been found, but which have never been seen again.
The conclusion is plain that if the giant planets are the parents of their comet families, they must be, at the present time capable of ejecting them. It does not appear to me that the possibility of such ejection can be summarily rejected. We have evidence of disturbances of intense violence in the Jovian atmosphere; the great red spot denotes a mighty cataclysm. Also the long rows of white spots that occur on Jupiter seem to indicate a series of eruptions far below; There are occasional outbursts of white spots on Saturn. Moreover, there is no reason to suppose that the ejection of comets is a common phenomenon; one ejection per century would probably suffice to balance the loss from dissipation and perturbation. Even on the earth we have occasional volcanic outbursts of extraordinary violence, as Skaptar Jokull in 1783, Krakatoa in 1883, etc.; paroxysmal outbursts on a far grander scale may be expected on the giant planets. In 1883 it was computed that over a cubic mile of solid matter was blown to a height of many miles. This is probably comparable with the total mass of the smaller comets. A difficulty has been raised that the ejection of the matter would take some hours, in which case the rotation of the planet would greatly alter the direction of projection. I do not think there is much in this objection, as if the whole mass of a small comet were concentrated into a compact form, it would not occupy many cubic miles, and its ejection might be practically instantaneous. In fact there is less difficulty in this than in supposing that if the comet came from regions outside the planetary system, and was merely captured by the planet, it should be sufficiently small and compact for all parts of it to suffer the same perturbations. For a very close appulse is required for capture, and if the comet were a few thousand miles in diameter, the perturbations of different parts would be sensibly different.
In discussing the solar origin of comets, we pointed out the difficulty of conceiving how the ejected matter could form such large masses of iron as we find in meteors. The difficulty would be less in the case of the planets, the temperature being probably low enough for iron to assume a solid form quite close to their surfaces.
I conclude from all this that the hypothesis of the planetary origin of short-period comets deserves consideration, and should not be dismissed so summarily as it is by many astronomers.
Bredichin has given a different hypothesis for the origin of the short-period comets in his pamphlet «Sur l’origine des comètes périodiques» Moscow, 1889. He supposes that they have simply arisen from the splitting up of larger comets, on the analogy of Biela’s comet, that of Liais in 1860, of the great comet of 1882, and Brooks’s comet in 1889. But in none of these cases is there evidence of notable alteration in the period, nor does the theory explain in any way the relation of the orbits to those of the four giant planets.
The third hypothesis of the origin of comets is that they are unattached outlying fragments of the nebula, which is conjectured to have been the parent of the solar system. I consider that we are practically driven to this theory by exclusion in the case of those comets whose perihelion distance is large, and which do not belong to the planetary families. Seeing that comets have a large amount of meteoric matter associated with them, we must assume a meteoric, rather than a purely gaseous nebula.
Meteors are such complex bodies both in structure and composition that it is difficult to conceive of them as the primitive form of matter; they are more like the debris of earlier worlds. This idea naturally leads us on to the planetesimal hypothesis, which has been lately put forward by Professors Moulton and Chamberlin. According to this view, the sun existed in past ages in solitary state, without any attendant worlds; at some very remote epoch, another sun is supposed to have passed extremely near it, without actually colliding, but causing immense tidal disturbance, by which an appreciable fraction of its total mass was torn off; 1/700 of its mass went into the known planets, a much larger amount returned to the sun, but some remained unattached, and formed our comets and meteor swarms. It was the perturbing action of the other sun that gave the ejected mass moment of momentum, and thus prevented it from falling back on the sun. If we suppose that before the cataclysm our sun had already cooled sufficiently for a solid crust to form, which would absorb a portion of the hydrogen and other gases in the atmosphere, we seem to get an explanation of the large solid meteoric masses that frequently fall on the earth and which contain a large quantity of occluded gases. The theory of course implies that the sun’s temperature was again raised as a result of the appulse, either by actual collision, or the impact of the return of part of the ejected matter.
The chief drawback to the adoption of the planetesimal hypothesis seems to me to be the extreme improbability of such a near approach of two suns to each other. The interstellar distances are so immense compared with the size of each sun that such encounters must be excessively rare. The frequent appearance of Novae is sometimes quoted as evidence in favour of such appulses; these outbursts occur, almost without exception, within the Galaxy, where we have good reasons for supposing the star-density to be much greater than elsewhere, moreover the very rapid decline in the light of Novae suggests that they are not stars in an advanced stage of condensation, but are in a much more diffused and tenueus state. On account of these difficulties I think that we should only regard the planetesimal hypothesis as a plausible conjecture, not as an establised conclusion.
Passing on now to the question of the nature of comets, I assume as established that they all have as their nucleus a more or less dense swarm of meteors. This conclusion rests partly on the clearly proved connection of the Leonid, Perseid, and Andromedid systems with the comets of 1866, 1862 and Biela’s comet, partly on the impossibility of conceiving that such comets as Halley’s could persist for so many returns if they were mere bunches of vapour. According to the generally accepted views of Dr Johnstone Stoney, even the moon and the smaller planets are incapable of permanently retaining atmospheres, in consequence of the rapid motion of the gaseous molecules. Since we are certain that the mass of Halley’s comet is much less than that of the moon, it is evident that its gravitational power would be too weak to hold it together if wholly gaseous. It is probable that the meteors are continually giving off small quantities of gas (at least while in the neighbourhood of the sun) since otherwise we should expect the vapourous envelope to be dissipated with fair rapidity. The fact that Halley’s comet has been emitting such large tails at every return for at least 2000 years makes it probable that in its case the meteoric masses are of considerable size, perhaps larger than the large masses in our museums, since these must have suffered loss in their passage through the air. For we should expect small lumps, under a foot in diameter to give up their whole supply of gas at a single apparition.
I put forward here the conjecture that, since it is only near perihelion that the loss of gas occurs, a large periodic time is favourable to a long life of the comet; hence the prevalence of nearly parabolic orbits may be a case of «Survival of the fittest», the comets with shorter periods having already exhausted their supply of gas, and therefore ceased to exist as visible comets. The disappearance of Biela’s comet presumably means only the loss of the gas contained in its meteors; these are still moving in the same orbit, as is shown by the many showers of Andromedids that have been observed since the comet disappeared.
I now pass to an independent proof of the meteoric constitution of a comet’s nucleus. This rests on the extremely close agreement between theory and observation in the date of the return of Halley’s comet to perihelion. The calculation was based on the assumption that no forces were acting except the gravitation of known matter. The discordance amounts to three days at most, showing that any non-gravitational forces acting on the head are of the order of 1/10,000 of gravity.
But in the case of the tail the non-gravitational forces are predominant; further, the gases in the head are frequently almost indistinguishable in the spectroscope from those in the tail. If then the head contained nothing except these gases, it would move in the same manner as the tail does. The inference is plain that the head contains much denser matter, on which the influence of the tail-forming forces is inappreciable, and that this matter emits the gases which form the coma and tail. It seems certain that Halley’s comet will transit the sun’s disc about May 18.6, and it will be of interest to examine whether any trace of the comet on the sun can be detected. If so, it will enable us to form an estimate of the density of the head. The earth will probably pass through the tail at the same time, as happened with the great comet of 1861; it is rather curious to reflect that even passing through the tail does not make it any easier to settle the question of its composition; for in all probability it is far too rarified to have the slightest discernible effect on our atmosphere. A kind of auroral glow over the whole sky was suspected in 1861, and this should be looked for next May.
It only remains now to consider the question of the forces that produce the tail. It does not seem necessary to invoke any other agency than the solar heat to explain the emission of gas from the meteors when the comet approaches perihelion. There are at least three theories to explain the repulsion of the tail from the sun: 1) Light-pressure; 2) electrical repulsion; 3) Mechanical bombardment by electrons, or other tiny particles violently ejected from the sun. It is quite possible that all three act conjointly, as no one of them seems capable of explaining all the facts. The first and second, being central forces, could not alter the rate of description of areas of any particle of the comet about the sun. It is easy to deduce that tails produced by them would always start from the nucleus in the direction of the radius-vector produced. Now the photographs taken during the last few years give many instances of streamers leaving the head in directions making a considerable angle with this line. It seems clear that the forces producing these are not wholly solar; some expulsive force, for whose nature I can only suggest electrical repulsion, shoots particles violently from the head in various directions; perhaps there is a slight favouring of the direction towards the sun, as we seem thus to get an explanation of the paraboloid envelopes so often seen on the sunward side of the nucleus, resembling the jet of a fountain. Morehouse’s comet of 1908 showed these very distinctly, and they were discussed by Mr Eddington at the Royal Institution on 1909 March 26. He deduced a velocity of projection from the nucleus of 70,000 miles per hour, and a solar repulsive force 800 times gravity, a startling result, being far in excess of the values previously found by Bredichin or Jaegermann. Another point that shows that the tail-producing forces reside partly in the nucleus is the cycle of changes in the aspect of the tail that several comets, and notably Morehouse’s, have shown. In the latter comet the Greenwich photographs revealed a fairly regular cycle that was many times repeated, so that it was even found possible, at the beginning of a cycle to predict the subsequent behaviour. Since we cannot plausibly assign these short-period variations to any change in the solar action, we must suppose the source is in the nucleus.
Allied to the last are the numerous instances of apparent rotation of the tail, it appearing alternately broad and narrow, like a sword seen broadside and edgewise. Halley’s comet showed features of this kind in 1835, which were attributed by Bessel to rotation of the tail, in a period of about 5 days. There is a difficulty in supposing the tail to rotate, since this implies either a rigid body or a powerful central force to control the rotating particles, neither of which is present in the tail. A rotating head would however produce a semblance of rotation in the tail emitted by it. The conception of a rotating head involves the assumption that its separate meteoric masses are concentrated pretty densely, since they would not otherwise have sufficient mutual gravitation to control the rotation. The other cyclical variations of the tail, referred to above, also seem to involve concentration in the head, for if it was scattered over a wide area it is difficult to see how the separate particles could all act in unison, as they appear to do.
Allusion should be made to the theory recently put forward both by Prof. Newall and Mr G. Burns (possibly by others also) that the tail of a comet consists of matter already in situ, but in some way excited to luminosity by the passage of the comet. They point out that this would explain the similarity of spectrum shown by most comets’ tails, and Prof. Newall has found evidence of the presence of cyanogen in the interplanetary spaces. The idea of scattered gases is supported by the occasional shattering of comets’ tails; instances are Brooks’ comet photographed by Barnard in 1893, and Morehouse’s comet on Oct. 15, 1908; there can scarcely be any doubt that the tail matter met some obstruction in space, which suddenly checked its onward movement.
But I find it difficult to believe that, at least, the beginning of the tail is not an emission from the head, since spectrograms with an objective prism frequently show head and tail absolutely continuous, so that it is not easy to say where the one passes into the other.
I pass now to the idea of the tail being due to the ejection of material particles from the sun. I think that Prof. Schaeberle was the first to put forward this idea in 1893. He thought that both the corona and comets’ tails are formed in this manner, his words in «Astronomical Journal», 306, being «The tail of a comet is produced by the visible particles of matter originally forming the comet’s atmosphere, and by the previously invisible particles of a coronal stream which moving with great velocity, finally produce by repeated impact of the successive particles almost the same motions in the visible atmosphere of the comet as would be communicated by a continuously accelerating force directed away from the sun.... The coronal matter, owing to its retardation, grows so dense that it also becomes visible, and with the comet’s atmosphere is finally driven into the tail by the repeated bombardment of unretarded following portions of the stream». Since this was written, the radio-active elements have been discovered, and the theory of the dissociation of the atom into electrons has been adopted. Hence the particles suggested by Prof. Newall and Mr G. Burns are much smaller than those suggested by Prof. Schaeberle. Mr Burns («Journal Brit. Astron. Assoc.», XIX, 5) says. «The radiant matter emitted by the sun is identical with the Beta rays given off by radium.... The impact of this radiant matter on the meteorites composing the comet’s head generates light, and the spectrum of this light will be that of the atmosphere surrounding the meteorites.... We can account for the formation of the tail by the known property possessed by radiant matter of acting as nuclei for the condensation of molecular aggregates. I suppose that as the particles of radiant matter from the sun pass through the head of a comet they collect matter round them, and become of sufficient size to reflect light. And Prof. Newall wrote in the «Monthly Notices» for Feb. 1909 «Is anyone who is familiar with the phenomena and theory of comet’s tails prepared to say that the repulsion of these tails is not simply a phenomenon indicating the existence of this constant radial outstreaming of dust, rendered manifest by the glowing of the vapours set free by the nucleus of the comet, possibly under the influence of the incessant bombardment by the dust?» It is rather remarkable that these two passages were written simultaneously, Mr Burns knowing nothing of Prof. Newall’s view. The idea of the bombardment by ultramicroscopical particles ejected from the sun is coming more and more to the fore to explain various phenomena in the solar system, in particular the corona, the aurora, and magnetic storms. In connection with the last, I recall Mr Maunder’s papers in the Monthly Notices, in which he showed that the matter supposed to produce the storms was not projected equally from the sun in all directions, but from definite areas in the sun (frequently marked by some notable spot or other disturbance) and outwards along definite streamlines, like the jet of water from a fireman’s hose. For this reason I am not inclined to attribute the whole of the tail-phenomena of comets to this action, though I think it would be decidedly rash at the present time to deny its connection with special outbursts, such as those exhibited by Morehouse’s comet, of which Mr Eddington said «I am not sure that the exceptional activity of this comet is not due to the physical state of the sun at the time, rather than to the constitution of the object itself».
We also recall Prof. Backlund’s discovery that the epochs of change in the rate of acceleration of Encke’s comet coincided with times of maximum solar activity, and some reasons have lately been adduced for believing that the brightness of the same comet at different apparitions also varies with the state of the sun’s surface.
The electrical theory of the repulsion of tails explains the fact that the acceleration of some of the knots in the tail has been found to cease at a distance from the head which we may ascribe to a leakage of the charge. Also electrical excitement gives a more reasonable explanation of the glow of the gases in the tail than to suppose that they shine by actual incandescence, which can hardly occur except in the case of comets with very small perihelion distances.
To conclude, each of the three explanations of tail-repulsion seems to me to be a vera causa which we have good reason to believe is actually in operation; the only difficulty is to discriminate between the separate effects of each. This is work for the future; the rapid advance of cosmical physics in recent years gives ground for hope that the full solution is only a question of time.
Andrew C. D. Crommelin