Popular Science Monthly/Volume 14/February 1879/Planetary Rings and New Stars
|PLANETARY RINGS AND NEW STARS.|
By Professor DANIEL VAUGHAN.
THE study of celestial phenomena which represent changes of the greatest magnitude is an important source of intelligence respecting the course of creation and the diversified condition of the universe. In all well-directed efforts to explore the more profound secrets of space and of time, it is necessary to be guided by certain conspicuous marks which even unseen planets may leave behind them, on assuming new forms or in closing an inconceivably long term of existence. Within the solar domain there is, perhaps, no object which claims more interest or value for original inquiries in astronomy than Saturn's rings—whether they be regarded as presenting a picture of the first or the last stages of planetary life. With a wide deviation from the ordinary figure of worlds, they exhibit changes which are interesting on account of the vast scale on which they occur, and the light which they throw on the past and the future history of the solar system. In the absence of those restraints which secure much repose on the surface of our globe, the Saturnian girdle is abandoned to rage of the most violent commotions and becomes occasionally the seat of disturbances which, though transpiring at the distance of about 900,000,000 miles, are yet revealed by the telescope. The temporary divisions which have been so often observed in the rings are evidences of the terrific scenes of turmoil in this remote theatre of chaotic activity; as the opening and closing of visible chasms must be attended with physical convulsions immeasurably greater than any ever witnessed on our terrestrial abode.
From well-established principles of physical astronomy, it is easy to prove the impossibility of tranquil movements in a region so close to the great planet. Whether tenanted by innumerable solid masses or even by a vast expanse of fluid, the zones in which the Saturnian sway is so powerful must present a long-continued struggle for opposite ends. The matter spread over the wide annular fields is ever urged by its own attraction to collect together and form satellites, which are soon destroyed by the attractive disturbance of the primary, and have their parts scattered once more over a wide space. From the gravity due to their preponderating masses, the mountains or inequalities observed on the ring can not be prevented from growing at the expense of the matter along the zone in which they circulate; but with the increasing size the vast structures become at last incapable of sustaining the crushing strain in certain directions; so that a dilapidation and a dispersion of their materials become inevitable. It is in consequence of the ephemeral character of these mountains or embryonic satellites, that observations on them have sometimes led astronomers to absurd and discordant conclusions in regard to the general movements and the stability of the rings.
With the resources which spectrum analysis has conferred on science, more advantages are to be expected from inquiries in regard to the exact nature of the matter of distant space; and the basis for opinions or conclusions in regard to the composition of celestial objects is more then usually favorable in the case of Saturn's bright rings. That they are not entirely liquid or gaseous is evident from their serrated edges shown in the observations of Trouvelot, and from the peculiar character of the inequalities observed during their disappearance by Lassell and other astronomers. And yet there are few solid substances which could endure the long course of turmoil and ruin without being reduced to powder, and thus rendered incapable of raising mountainous structures high enough to be visible from our earth. But if the great annular appendage were largely or wholly composed of water with a temperature near 32° Fahr., the readiness of the fluid to assume a frozen condition would be a remedy to the ceaseless work of destruction, and would give solidity enough to enable incipient satellites to rise to the height of more than one hundred miles before tumbling to pieces.
The range of temperature necessary for the continuance of such operations must be maintained chiefly by the thermal effects with which they are attended. Any large stock of primitive heat which (according to the more generally received opinions) the rings might have possessed at their origin, must have been long since wasted by radiation from their extensive surfaces. In that remote and frigid zone of our planetary domain, the rays of our sun are too feeble to mitigate the rigors of extreme cold; and the outer ring at least can obtain but little calorific relief from the great planet which it environs. But, from the incessant changes and convulsions in the restless fields of matter, heat is abundantly produced by the violent mechanical action which, in a ring of aqueous composition, would proceed in a manner calculated to give a uniformity of temperature. If such a half-frozen ocean were abnormally heated throughout much of its vast expanse, so that a large portion of its ice were liquefied, the water, on obtaining a preponderance, would perform its movements and fluctuations with less violence and loss of living force. The heat produced mechanically would be then less than the amount lost by radiation, and a return of cold would again give ice the ascendancy. Yet, as the temperature declined and the freezing extended, mechanical violence would again become more energetic; and heat would be more copiously developed by the collisions of icy blocks, and by the rise and fall of gigantic mountains.
Reasoning from the most reliable principles of physics, and guided by the light of recent discoveries, many eminent scientists have come to the very just conclusion that the movement in Saturn's rings must be attended with a loss of energy and a reduction in the size of the orbits described by the innumerable disconnected masses of which the curious appendage is now generally believed to be composed. The opening and closing of its chasms, and other observed signs of its restless character, are calculated to give the impression that the extent to which it is altered during many years must be considerable. But, considering the conditions necessary for the phenomena it exhibits, it would seem that the rapidity with which its changes proceed might be approximately determined from the amount of heat which it radiates into space, and which must have been, for the most part, produced at the expense of its motion. Now, though the rings may be principally composed of water, and have a superficial temperature near the freezing-point, yet, their surfaces being over a hundred times as extensive as that of the earth, they may be reasonably supposed to lose by radiation about thirty times as much heat as our planet receives from the sun and allows to escape into space. Taking Bessel's estimate for the mass of the double girdle, it will be found that such an amount of heat might be generated by the conflicting movements of its parts without reducing their orbits more than one per cent, in ten thousand years. There is, indeed, reason to believe that, in this case, Bessel's results are unreliable, in consequence of the uncertain and defective character of the data with which they were obtained. If we assign to each ring the probable amount of matter in the neighboring moons of the gigantic planet, it would seem that their permanent change of size may be so considerable that it might be detected by the observations of a few centuries.
It is likely that, in the inner ring, especially at the zone nearest to the primary, the temperature is much higher than that which I have supposed, and that alterations in its condition might proceed at a rate sufficiently rapid to be discoverable by the telescope. More than twenty years ago Otto Struve, having carefully compared observations since the time of Huygens, announced as the result of his labors that the inner ring is changing its dimensions so rapidly that before two hundred years it will be united to the planet. Other astronomers have expressed a belief of the recent origin and of the mutable nature of the obscure or vapor ring which lies closer to Saturn. The conclusions of Struve, however, have been disputed; and indeed it is probable that they give an exaggerated picture of the transitory state of things in the Saturnian dominions; nor can the conflict of opinions on this point be settled by observation alone. But, though taking place too slowly to be at once detected in this way, the changes in question are still inevitable; and they give safe ground for tracing the history of past events in this part of the celestial regions. It is evident that the matter composing Saturn's wonderful appendage must have once moved in a wider zone, where it could exist only in the form of two secondary planets. I have shown in a previous article that a dismemberment and a conversion into a ring must be the general fate of every planetary body which, by a slow contraction of its orbit, revolves at last too close to its primary. From these and other considerations it may be legitimately concluded that the rings of Saturn are the remains of two former satellites; and their origin and their present condition must be regarded as the ultimate consequence of a rare medium disseminated through space.
Though the dismemberment which I have shown to be inevitable in small orbits differs, in some features, from that supposed to take place in the nebular hypothesis, yet in tracing the effects of both it is necessary to be guided by certain mathematically demonstrated principles in regard to stability. Two homogeneous fluid planets varying widely in size, but having the same density and occupying the same time in their diurnal movement, would be similar spheroids, or have the same relative deviation from a true sphere. Stability would cease to be possible in both if they were as dense as the earth and turned once in two hours and twenty-five minutes. If they were as rare as hydrogen gas at the level of our seas, they could not endure a rotation which took place in a less time than twenty-five days. In the investigations which I have given in the "Philosophical Magazine" in regard to secondary planets close to their primaries, the results have a like generality. Twelve hours would be very nearly the shortest time of revolution for an homogeneous fluid satellite as dense as water, whether its diameter were a hundred or a thousand miles, or whether it revolved around the earth or around Jupiter. Such a body, however, would require to be about thirteen times as dense as its primary in order to circulate in safety a little beyond the surface of the latter orb. A small satellite composed of fluid quicksilver would be capable of maintaining a planetary form if revolving just outside the atmosphere of Saturn or of Neptune; but it would be doomed to dismemberment if moving in a similar proximity to the surface of the earth or even of Jupiter.
The results are not very different even in cases of the greatest possible deviation from the homogeneous character I have ascribed to the bodies. If both planet and satellite were composed of rare gas enveloping a central nucleus, the smaller body would require to have an average density nearly eight times that of the greater, in order to preserve its integrity in such a dangerous proximity. A modern advocate of the nebular hypothesis supposes that each planet, when formed from the rarefied matter of a previous solar ring, was fifteen times less dense than the sun would be if it were an homogeneous sphere inclosed by the planetary orbit. In his own words, "After all their contraction during their condition as rings, and during their aggregation into globes, we may assume at a moderate estimate that when their rotation began they were fifteen times less dense than the average density of the sun expanded to their orbits." To change from a ring to a planet, however, the nebulous matter should have about one hundred times the density which the writer ascribes to it. Before it could become dense enough for the transformation, the nebulous expanse would, like Saturn's rings, be a prey to great commotions, sustaining a constant loss of energy and contracting the circles which its parts described around the sun. In endeavoring to account for the direct motion in secondary systems, Laplace contends that, in consequence of friction, the supposed primitive solar rings would have a greater velocity in their outer than in their inner zones. Now, if friction were so potent as to counteract to such an extent the normal effects of gravitation, it must be an eternal bar against the origin of worlds by nebulous dismemberment; and if a ring of attenuated matter were placed under the circumstances suggested by the eminent astronomer, it would be ultimately doomed, not to form a planet, but to coalesce with the immense spheroid of fiery vapor which it is supposed to have environed.
For further progress in the task of tracing the course of celestial events and thus obtaining materials for an astronomical history of worlds, it is necessary to consider, not only the theory of motion and stability in comparatively small orbits, but also the effects which a resisting medium of space may produce during long periods of time. Though the evidence which cometary motion gives of this rare fluid is far from being satisfactory, more reliable information on the subject may be gleaned from other sources, as I pointed out in an article in the "Philosophical Magazine" for June, 1861. In the September number of "The Popular Science Monthly" I alluded to the form of Mars as bearing marks of a former rapid rotation which appears to have been considerably reduced by the friction of a space-pervading fluid. But by far the most acceptable evidence on this question has been lately derived from the peculiarities of the nearest moon of Mars, as the small size of its orbit and its brief period of revolution have been ascribed to a resisting medium, even by advocates of the nebular hypothesis. The bearing of the new discoveries on certain astronomical doctrines has been already pointed out, in the July number of this journal, by my long-esteemed friend whose useful life has been since lost to the cause of science. The effects of the space-pervading fluid will appear more decided if we regard the diminutive satellites as former asteroids which became so far a prey to the Martian attraction as to be reduced to their present subordinate condition. Such views respecting their origin were first suggested by Prof. Kirkwood, and were subsequently advocated by Prof. Alexander in a paper read before the National Academy of Science.
The possibility that some straggling fragments from the ruins of one world might become the satellites of another, depends on a principle which has been long applied in tracing the origin of the cometary members of the solar family. It has been maintained by Laplace that comets were at first strangers in our system, and that many of them, coming from remote interstellar regions and entering the sphere of the sun's attraction, obtained a permanent domicile in his extensive domain; their orbits being changed from hyperboles to ellipses chiefly by some planetary disturbances. There would, however, be less probability that a small asteroid would become a satellite to Mars on passing through the region over which his attraction is effective; as the change would depend mainly on the intervention of solar influence, and would require rare conditions which the movements of many hundred asteroids could furnish only during long periods of time. But it may be considered certain that a moon obtained in this way must have had its primitive path so very extensive, that its revolution occupied a term of several months. Accordingly, if the asteroidal origin of the Martian moons be adopted, their present condition must be indicative of greater deviations from their primitive arrangement; and it is evident that their early large orbits could be reduced to the present small size only by a resisting medium. This theory is, perhaps, not wholly proof against all objections, but another which accounts better for the movements of both small secondaries in the plane of the equator of the planet, and which I intend to set forth in a future article, involves also the necessity of supposing the imperfect vacuity of celestial space.
As, in traversing a rare medium, such small masses must lose more velocity in one year than our globe would in many centuries under the same circumstances, they afford the most available means for indicating the slow alterations in the state of the universe. In the absence of the diversified cases which are, no doubt, concealed from our knowledge in distant solar systems, the small moons will serve as the means of illustration of the ultimate effects of the slight but constant resistance to celestial motion. It is evident that not many million years can elapse before Phobos will have its orbit so far reduced that it will sweep through the atmosphere of Mars, and then its career as a small secondary world will close with a meteoric exhibition. A term of existence several hundred times longer must be ascribed to our moon, or to the first satellite of Jupiter; but their end would be signalized by a far greater display of meteoric effulgence. A secondary planet two or three thousand miles in diameter, even if solid, would become unstable before coming in contact with its primary, and would undergo a sudden dilapidation; so that a numberless host of its fragments, scattering into smaller orbits and plunging as meteors into the atmosphere of the great central orb, would send forth a flood of brilliancy sufficient to rival solar light, and to proclaim the great work of destruction to the most distant parts of the universe.
The paroxysmal manifestation of light which a planet could scarcely fail to call forth in thus passing through its final stages of existence, corresponds in every feature to the mysterious effulgence of temporary stars. The accordance of the theory with facts appears more satisfactory, in proportion as the problems involved in the inquiry are more accurately solved with the aids of mathematics, and as new means of observation reveal the true nature of these rare and transitory apparitions of stellar light in the skies. The theatre of one of the great meteoric exhibitions in question may be the aerial envelope, not only of a large planet or a vast dark central body presiding over unseen planetary members, but even the corona and the photosphere of a sun. Whenever any of the large members of a solar or of a secondary system become unstable in too small an orbit, a vast portion of its dilapidated mass would be quickly sweeping as innumerable meteors through the atmosphere of the immense primary sphere. This accounts for the incipient brilliancy of the temporary stars, a fact hitherto unexplained, though it is generally admitted, and though it has been recognized by a high authority as a valuable guide in the study of these mysterious phenomena. "The circumstance," says Humboldt, "that nearly all the new stars burst forth at once with extreme brilliancy as stars of the first magnitude, and even with still stronger scintillations, and that they do not appear, at least to the naked eye, to increase gradually in brightness, is in my opinion a singular peculiarity, and one well deserving of consideration." Recent discoveries, though calling for some modification in this statement, detract little from its value; for the three new stars of the present century, though all below the first magnitude, yet showed their greatest effulgence at an early period of their visibility, and afterward exhibited a constant decline. According to the present theory, a rapid weakening of brilliancy in these objects would be an inevitable result: as a large portion of the meteors must have been successively precipitated to the surface of the great central sphere; while the balance assumed a closer array, changing their orbits into circles and forming a solar or a planetary ring.
The most favorable circumstances for such sudden outbursts of light are presented in cases where, in mass and size, the subordinate world is little more than one per cent, of the solar or the primary orb with which it is doomed to incorporate. If, for instance, our moon were caused to revolve so near us that it would be rendered unstable by terrestrial attraction, its dismemberment, though occurring on a large scale, would be confined to the region nearest to the earth. A vast portion of the lunar matter torn from this locality would be hurled to our globe or would fly as innumerable meteors through our atmosphere. But the remainder of our satellite would retire to a greater distance from the earth; and millions of centuries would elapse before it became again close enough to our world to suffer another great dilapidation and to give occasion for another gigantic display of meteoric light. It would thus appear that many great luminous exhibitions would attend the awful paroxysms with which a large planet passes away from the stage of existence in a solar or in a secondary system. Though a small satellite, if fluid, may meet its final doom in an obscure manner, yet, if solid, it would be likely to maintain a planetary form until it came very close to the primary; so that on its dilapidation a large portion of the resulting fragments would sweep through the atmosphere of the latter and call forth a sudden effulgence which in very remote worlds would appear as the transitory glare of a temporary star in the firmament.
In carefully tracing the conspicuous scenes which must mark the end of a planetary career, and thus obtaining a more correct interpretation of the rare and mysterious characters occasionally inscribed in our skies, very interesting information may be obtained of the diversified contents of space, and the long term of existence assigned to each of the numerous worlds of creation. If the dominions of other suns be equally rich as our solar region in mundane objects, it may not be extravagant to suppose that, in our universe, the large primary and secondary planets enlivened by the genial influence of more than twenty millions of stellar bodies might equal in number half the population of our globe. Now. the average mortality in the human family is about one death every second, while astronomical records show that only twenty-three temporary stars appeared within the past two thousand years. Taking their appearance as records of planetary fate, it would follow that a century is as small a part of the career of a planet as two seconds is of human life; and that the few thousand years in which the history of our race is comprised is scarcely two minutes in the immeasurable age of our world.
Yet these considerations will perhaps give an inadequate idea of the long endurance of the great works in creation's wide domain. According to the opinions of Laplace, besides the systems over which visible stars preside, there are others, equally numerous in which the central bodies, though of sunlike magnitude, are not self-luminous. Madler and Bessel embraced similar views. Those who believe, with Helmholtz, that a sun's heat and light are produced by the contraction of its mass, and that solar activity has a limited duration, might be naturally led to consider dark systems a hundred or even a thousand times as numerous as those which are illuminated. Yet I think it more reasonable to take the moderate estimate of Laplace for the comparative numbers of the dark and the bright occupants of space. But it is, moreover, necessary to consider that great planets and satellites meet their ultimate doom by a number of dismemberments and great meteoric scenes, each separated by intervals of many millions of centuries. Taking all these circumstances into account, the age of a world, as inferred from the observed indications of catastrophes in the heavens, may reach as high as 500,000,000,000 years.
If the feelings of some readers will revolt from the idea of having a primary or even a secondary mundane orb occasionally sacrificed in some part of the wide celestial domain, they must be powerfully shocked by the views of Dr. Croll, who gives destruction a far more oppressive sway over the great works of creation, when he regards the collisions of suns as the normal means of perpetuating the economy of nature. If the great centers of unfailing light were thus hurled into ruin, their attendant worlds, if saved from a worse fate, would be sent adrift in hyperbolic orbits and doomed to a long pilgrimage in the cold interstellar regions. Without denying the possibility of such rare and terrific convulsions, I must pronounce it as certain that they can not have any part in giving birth to the new stars which astonished Tycho Brahe and Kepler, or to those which have blazed forth in the heavens in our own times. Though the greater cosmical bodies might, as Dr. Croll supposes, be heated by collisions so intensely as to be capable of diffusing heat and light for many millions of years, they could not undergo the rapid decline of brilliancy which temporary stars exhibit. Lockyer takes similar grounds in speaking of Nova-Cygni. "We are driven," says he, "from the idea that these phenomena are produced by the incandescence of large masses of matter, for, if so produced, the running down of brilliancy would be exceedingly slow," A planetary wreck, incorporating with the sun in the manner I have described, would sweep through his external matter at the rate of about two hundred and eighty miles a second. The heat produced mechanically at the expense of this high velocity would not be so great in quantity as that which might be expected from Dr. Croll's solar encounters; but, being confined to a very limited zone, it would attain much greater intensity, be more effective for dissociation, and prove a more efficient means for giving nebulæ their existence and the peculiar character which they exhibit.
Since spectrum analysis has been brought to bear on the new stars, the doctrine of their meteoric origin has obtained more currency in astronomical circles. Though the incorporation of a remote world with a greater sphere around which it previously revolved has been suggested as the cause of such meteoric action, the idea has been somewhat unproductive, in consequence of the loose manner in which inquiries on the subject have been conducted, and the little care which has been taken for obtaining correct solutions for the problems of motion and stability involved in the questions at issue. The consequences of instability and dismemberment in small orbits have been generally overlooked. Recent developments, however, show some steps for correcting the early errors and oversights in this new field of investigation. In his recent work on "The Struggle for Existence in the Heavens," Du Prel (alluding to the ultimate doom of the earth near the center of our system) states that our world will end its career, not as one gigantic meteor, but as numberless meteoric fragments; and then the great shower of stones in the solar atmosphere will show the inhabitants of some very remote orb such a spectacle as was to be seen by terrestrial astronomers in the constellation of Corona on the 12th of May, 1866. A similar conclusion has been expressed frequently in my writings during the past twenty-five years, especially in my papers published in the reports of the British Association for 1857 and 1861, and in my communications in the "Philosophical Magazine" for 1858, 1861, and 1872.
The meteoric phenomena of distant space may be profitably studied in connection with those to be seen on a diminutive scale in our atmosphere. The visits of shooting-stars to the earth have been lately brought within the province of astronomy, and numbers of these bright objects, which take part in extraordinary showers, have been found to come from the tracks of certain comets. It would, accordingly, seem that cometary bodies have portions of their matter separated from them, and occasionally sent as meteors into the atmospheres of the planets. The dismemberment which, in such cases, is occasioned by the heat, and more rarely by the attraction of the sun, is analogous to that which planets would suffer in very narrow orbits; but it occurs on a scale infinitely smaller, and can never be productive of any very conspicuous results. The greatest exhibition of shooting-stars in our atmosphere could never be observed from any of the neighboring planets; and, if armies of meteors were sent from many systems to invade a single one, and had their orbits and positions, best arranged for a simultaneous charge on the atmosphere of one of its larger orbs, the light which they could produce would fail to exhibit the remarkable features observed in incipient brightness and the gradual decline of temporary stars.