Popular Science Monthly/Volume 1/July 1872/A Giant Planet
|A GIANT PLANET.|
By RICHARD A. PROCTOR, F. R. A. S.
WE propose to give a brief sketch of what is known respecting the noble planet Jupiter. He is the giant of the solar system, himself the primary of a scheme of orbs whose movements resemble in regularity the motions of the planets round the sun. Much has been discovered during the last few years—nay, even during the last few months—to render such a sketch interesting.
We must, in the first place, dispossess ourselves of the notion, not uncommonly entertained, that Jupiter is one of a family of orbs, nearly equal in dignity and importance, and comprising the Earth and Venus, Mars and Mercury, among its members. This idea still prevails, cause in our books on astronomy we commonly see a set of concentric circles at regularly increasing distances, assigned as the paths of the several planets of the solar system. And besides, there yet remains, in the modern teaching of astronomy, a perceptible trace of the ancient astronomical systems, in which Saturn and Jupiter, Mars, Venus, and Mercury, played parts of equal importance.
Let it be carefully remembered, then, that the four planets which circle nearest the sun—the family of which our earth is a member—differ in all their characteristics from the outer family (also consisting of four planets) to which Jupiter belongs. The whole of the inner family—the whole of the space within which its members travel—could be placed between the paths of Jupiter and his next neighbor Saturn, with a clear space many millions of miles wide on either side. The actual area between the paths of Jupiter and Saturn exceeds nearly thirty times the whole area within which the four lesser planets pursue their paths. And, when we consider the dimensions of the four inner planets, we find a like disproportion. Four circles representing these orbs can be enclosed within a circle representing Uranus, the smallest of the four outer planets; yet even this circumstance does not adequately represent the enormous disparity between the two families of planets; for, in fact, the volume of Uranus exceeds the combined volume of all the inner planets upward of thirty times. We might adduce many other illustrations of the complete dissimilarity between the inner and outer families of planets; but what has been already stated will suffice for our present purpose. It will be evident that, in considering the members of one or other family, we must be prepared to meet with relations which differ not merely in degree, but in kind. We may thus, at the outset, dismiss from our thoughts the idea that the planet Jupiter is necessarily to be regarded as an inhabited world merely because the only planet we are actually acquainted with is inhabited. The latter circumstance may be an excellent reason for regarding Mars or Venus as the abode of life; but the analogy can no more be extended to Jupiter than to the fixed stars, which certainly are not inhabited worlds. We must, in fact, consider the physical habitudes of Jupiter independently of all conceptions based upon terrestrial analogies. Studied thus, he will be found, as we conceive, to hold a position in the scheme of creation differing considerably from that which has been assigned to him, until of late, in treatises on astronomy.
It is necessary briefly to state the dimensions, mass, and general characteristics of the planet, before proceeding to discuss its probable physical condition.
Jupiter has a diameter exceeding the earth's rather more than ten times, and a volume exceeding hers 1,230 times. It is not far from the truth to say that Jupiter's dimensions exceed the earth's in very nearly the same degree that those of the sun exceed Jupiter's. But his mass, though gigantic compared with the earth's, does not altogether correspond to his bulk, for it exceeds the mass of the earth only three hundred times. So that, if the disk our astronomers see and measure actually represents the true globe of the planet, his substance must be, on the average, much less dense than that of the earth. In fact, while the earth's density is nearly six times as great as that of water, the density of Jupiter (thus judged) would exceed that of water by barely one-third. This vast globe rotates, in less than ten hours, on an axis nearly upright or square to the level in which the planet travels. This rapidity of rotation—so great that points on the planet's equator travel twenty seven times as fast as points on the terrestrial equator—results in a considerable flattening of the planet's globe; insomuch that the polar diameter is less than the equatorial by about a twelfth part, or by fully 7,000 miles. And it may be remarked in passing, that this circumstance—the fact, namely, that the poles of the planet are drawn in, as it were, 3,500 miles as compared with the equatorial regions, or 1,750 miles as compared with the mid-latitudes in either hemisphere—affords a striking illustration of the enormous amount of energy really represented by the rotation of Jupiter. It may also be added that the velocity with which points on Jupiter's equatorial zone are carried round, exceeds the corresponding velocity in the case of all the planets in the solar system, and is nearly six times greater than the equatorial velocity of the sun himself. It amounts, in fact, to about 7½ miles per second.
We do not propose to consider here at any length the system of satellites over which Jupiter bears sway; but this preliminary sketch would be incomplete without a few words on the subject. It is worthy of notice that, although our earth in some sort resembles the outer planets in being accompanied by a satellite, yet the relation which our moon bears to the earth is altogether different from that which the satellites of the outer planets bear to their respective primaries. Our moon is by no means a minute body by comparison with the earth, and compared with Mars or Mercury she may be regarded as having very respectable dimensions. We may, indeed, look upon the moon as a fifth member of the inner family of planets—a member inferior to the rest, doubtless, but still not so far inferior to Mercury as Mercury is inferior to the earth. In the case of the outer planets, however, and especially in Jupiter's case, moons hold an utterly subordinate position. Taking the accepted measurements, we find the largest of Jupiter's moons less than the 16,000th part of its primary as respects bulk, while its mass or weight is less than the 11,000th part of Jupiter's. So that these orbs may fairly be regarded as bearing the same relation to their primary that Jupiter himself bears to his primary—the sun. It will be seen presently that this consideration is an important one.
But the great interest of the study of Jupiter resides in the fact that, being the nearest of the outer family of planets, the aspect of his globe supplies the best available means of determining the condition of the giant orbs constituting that family.
The first feature which strikes us in the telescopic aspect of the planet is the presence of a series of belts, lying parallel to the planet's equator. Usually the equatorial regions are occupied by a broad, bright belt, of a creamy-white color, and bordered on the north and south by copper-colored belts. Beyond these, again, lie alternate bright and dark belts, the dark belts growing more and more bluish in hue as the pole is approached—while the poles themselves are usually of a somewhat decided blue color in telescopes adapted to display such features to advantage. There are commonly two or three dark belts on each hemisphere.
Now, before inquiring into the peculiarities presented by these belts, and into the remarkable changes which have been noted lately in their general aspect, it may be well for us to consider briefly what such belts seem to imply. That they are due to peculiarities in the planet's atmosphere is admitted on all hands. And it has been usual to compare them with the trade-wind zones and the great equatorial calm zone on our earth. The bright belts, according to this view, are regarded as zones where for the time clouds are prevalent, the dark belts being regions where the comparatively dark hues of the planet's surface are brought into view. And then it has been deemed sufficient to point out that the parallelism of the zones is due to the extreme rapidity of the planet's rotation.
But, setting aside the fact that the trade-wind zones and the great equatorial calm zone on our earth are, in reality, little better than meteorological myths, it must be regarded as a remarkable fact that, in the case of a planet so far away from the sun as Jupiter is, there should be a supply of clouds so abundant as to form belts discernible from the earth. Jupiter is rather more than five times farther from the sun than the earth is, and receives from him about one twenty seventh part of the light and heat which falls upon the earth (equal surface for equal surface). Making every allowance for the possibility pointed out by Prof. Tyndall, that some quality in Jupiter's atmosphere may prevent the solar heat from escaping, and so cause the climate of the planet to be not very different from the earth's, yet the direct heat falling on the planet's oceans cannot be increased in this way nay, it must be rather diminished. It chances, indeed, that the very quality by which the earth's atmosphere retains the solar heat is unquestionably possessed by Jupiter's atmosphere. When our air is full of aqueous vapor (invisible to the eye), the escape of heat is prevented, as Tyndall has shown, and thus the nights are warmer than where the air is dry. Now, in Jupiter's atmosphere there is much water, for observers, armed with that wonderful instrument, the spectroscope, have recognized the very same dark bands upon the spectrum of the planet which appear in the solar spectrum when the sun is low down, and therefore shining through the lower and denser atmospheric strata. The spectroscopist knows that these bands are due to the aqueous vapor in the air, because Janssen saw the very same bands when he examined the spectrum of a powerful light shining through tubes filled with steam. So that there is the vapor of water—and that, too, in enormous quantities in the atmosphere of Jupiter. But though we thus recognize the very quality necessary for an atmosphere which is to retain the solar heat, our difficulty is not a whit lessened; for it is as difficult to understand how the invisible aqueous vapor finds its way thus into the planet's atmosphere, as to understand how the great cloud-masses are formed.
Aqueous vapor in the atmosphere, whether its presence is rendered sensible to the sight or not, implies the action of heat. Other things being equal, the greater the heat the greater the quantity of watery vapor in the air. In the summer, for instance—though many imagine the contrary—there is much more of such vapor in the air than there is in winter, the greater heat of the air enabling it to keep a greater quantity of the vapor in the invisible form. In winter, clouds are more common, and the air seems moister; yet, in reality, the quantity of aqueous, vapor is reduced. Now, it cannot but be regarded as a remarkable circumstance that, though the sun supplies Jupiter with only one twenty seventh part of the heat which we receive, there should yet be raised from the oceans of Jupiter such masses of clouds as to form veritable zones; and that, moreover, above these clouds there should be so large a quantity of invisible aqueous vapor that the spectroscopist can recognize the bands of this vapor in the planet's spectrum.
Even more perplexing is the circumstance that the cloud-masses should form themselves into zones. We cannot get rid of this difficulty by a mere reference to the planet's rapid rotation, unless we are prepared to show how this rotation is to act in forcing the cloud-masses to become true belts. The whole substance of Jupiter and his whole atmosphere must take part in his rotation, and to suppose that aqueous vapor raised from his oceans would be left behind in the upper air, like the steam from a railway-engine, is to make a mistake resembling that which caused Tycho Brahe to deny the rotation of the earth, because bodies projected into the air are not left behind by the rotating earth. Nor is it conceivable that belts which vary remarkably, from time to time, in position and extent, should be formed by sun-raised clouds in the Jovian atmosphere, if the planet's surface is divided into permanent lands and seas.
But we are thus led to consider a circumstance which, as it appears to us, disposes finally of the idea that in the cloud-rings of Jupiter we have to deal with phenomena resembling those presented by our own earth.
We are too apt, in studying the celestial objects, to forget that where all seems at nearly perfect rest, there may be processes of the utmost activity—nay, rather, of the utmost violence—taking place as it were under our very eyes, and yet not perceptible save to the eye of reason. Looking at Jupiter, under his ordinary aspect, even in the finest telescope, one would feel certain that a general calm prevailed over his mighty globe. The steadfast equatorial ring, and the straight and sharply-defined bands over either hemisphere, suggest certainly no idea of violent action. And when some feature in a belt is seen to change slowly in figure—or, rather, when at the end of a certain time it is found to have so changed, for no eye can follow such changes as they proceed—we are not prepared to recognize in the process the evidence of disturbances compared with which the fiercest hurricanes that have ever raged on earth are as mere summer zephyrs.
Indeed, the planet Jupiter has been selected even by astronomers of repute as an abode of pleasantness, a sort of paradise among the planet-worlds. There exists, we are told, in that distant world, a perennial spring—"a striking display of the beneficence of the Creator," says Admiral Smyth; "for the Jovian year contains twelve mundane years; and, if there were a proportionate length of winter, that cold season would be three of the earthly years in length and tend to the destruction of vegetable life."
Even those who have denied that Jupiter can be the abode of life, and have formed altogether unfavorable ideas of his condition, have pictured him nevertheless as the scene of continual calm, though the calm is, according to their view, the calm of gloom and desolation. They recognize in Jupiter an eternal winter rather than a perpetual spring. Whewell, for example, in that once famous work the "Plurality of Worlds," maintained that, if living creatures exist at all in Jupiter, they must be wretched gelatinous monsters, languidly floating about in icy seas. According to him, Jupiter is but a great globe of ice and water, with perhaps a cindery nucleus—a glacial planet, with no more vitality in it than an iceberg.
But when we begin to examine the records of observers, and to consider them with due reference to the vast proportions of the planet, we recognize the fact that, whatever may be Jupiter's unfitness to be the abode of life, it is not of an excess of stillness that his inhabitants (if he have any) can justly make complaint. Setting aside the enormous activity of which the mere existence of the belts affords evidence, and even regarding such phenomena as the formation or a disappearance of a new belt in two or three hours as merely indicative of heavy rainfalls or of the condensation of large masses of invisible aqueous vapor into clouds—there have been signs, on more occasions than one, of Jovian hurricanes blowing persistently for several weeks together at a rate compared with which the velocity of our fiercest tornadoes seems utterly insignificant. During the year 1860, a rift in one of the Jovian cloud-belts behaved in such a way as to demonstrate the startling fact that a hurricane was raging over an extent of Jovian territory equalling the whole surface of our earth, at a rate of fully 150 miles per hour. It is not too much to say that a hurricane of like velocity on our earth would destroy every building in the territory over which it raged, would uproot the mightiest forest-trees, and would cause in fact universal desolation. At sea no ship that man ever made could withstand the fury of such a storm for a single minute. And yet this tremendous Jovian hurricane continued to rage with unabated fury for at least six weeks, or for fully one hundred Jovian days.
But during the last two or three years a change of so remarkable a nature has passed over Jupiter as to imply the existence of forces even more energetic than those at work in producing atmospheric changes.
In the autumn of 1870, Mr. Browning (the eminent optician and observer) called the attention of astronomers to the fact that the great equatorial zone, usually, as we have said, of a creamy-white color, had assumed a decidedly orange-tint. At the same time it had become much less uniform in outline, and sundry peculiarities in its appearance could be recognized, which have been severally compared to port-holes, pipe-bowls, and stems, oval mouldings, and other objects of an uncelestial nature. Without entering into descriptions which could only be rendered intelligible by means of a series of elaborate illustrations, let it suffice to say that the bright edges of the belts bordering on this ruddy equatorial zone seemed to be frayed and torn like the edges of storm-clouds, and that the knots and projections thus formed often extended so far upon the great orange zone from both sides as almost to break it up into separate parts.
Now, without inquiring into the particular form of action to which these remarkable changes were due, we can see at once that they implied processes of extreme energy. For, every one of the projections and knots, the seeming frayed edges of narrow cloud-streaks, had, in reality, an extent exceeding the largest of our terrestrial countries. Yet their aspect, and indeed the whole aspect of the ruddy belt, whose extent far exceeded the whole surface of our earth, changed obviously from night to night.
Strangely enough, these interesting observations, though they were presently confirmed by several well-known students of the heavens, did not attract that full attention, from the senior astronomers of the day, which they appeared to merit. Several, indeed, of our leading astronomers were disposed to deny that any thing unusual was in progress, though none asserted definitely that they based this opinion on a careful reexamination of the planet's face. But quite recently one of the most eminent of our modern observers—Mr. Lassell, lately president of the Royal Astronomical Society—(having been led to observe the planet by the fact that certain phenomena of interest in connection with the satellite-system are now in progress), found his attention attracted by the marvellous beauty of the colors presented by Jupiter's belts. After describing the appearances he had intended to observe in the first instance, he proceeds: "But this was not the phenomenon which struck me most in this rare and exquisite view of Jupiter. I must acknowledge that I have hitherto been inclined to think that there might be some exaggeration in the colored views I have lately seen of the planet; but this property of the disk, in the view I am describing, was so unmistakable that my skepticism is at last beginning to yield." Nor will this statement be thought to express more than the truth, when we add that, in the picture accompanying his paper, Mr. Lassell presented the equatorial zone as brown-orange, and three neighboring dark zones as purple; one of the intermediate light belts being pictured as of a light olive-green.
Let us compare these observations made in our brumous latitudes with those effected by Father Secchi with the fine equatorial of the Roman Observatory. "During the fine evenings of this month," he wrote last February, "Jupiter has presented a wonderful aspect. The equatorial band, of a very pronounced rose-color, was strewn with a large number of yellowish clouds. Above and below this band, there were many very fine zones, with others strongly marked and narrow, which resembled stretched threads. The blue and yellow colors formed a remarkable contrast with the red zone, a contrast doubtless increased by a little illusion. The surface of the planet is actually so different from that which I have formerly seen, that there is room for the study of the planet's meteorology."
It appears to us that, when these remarkable changes are considered in combination with the circumstance that on a priori grounds we should expect the sun to have very little influence on the condition of the planet's atmosphere, the idea cannot but be suggested that the chief source of all this energy resides in the planet itself. The idea may seem startling at a first view, but, when once entertained, many arguments will be found to present themselves in its favor.
For instance, it does not seem to have been noticed, heretofore, as a very remarkable circumstance, if the Jovian belts are sun-raised, that they pass round to the nocturnal half of Jupiter and reappear again, with the same general features as before, and this often for weeks at a stretch. Even that remarkable feature whose changes led to the conclusion that mighty hurricanes were in progress, yet changed continuously and regularly during the Jovian nights as well as during the Jovian days, for one hundred such days in succession. This is perfectly intelligible if the seat of disturbance is in the planet itself, but it is perfectly inexplicable (as it seems to us) if the sun occasions all these meteorological changes in Jupiter, as he occasions all the changes which take place in our earth's atmosphere. The alternation of day and night, which is one of the most potent of all the circumstances affecting the earth's meteorological condition, appears to have no effect whatever on the condition of Jupiter's atmosphere!
Now, as respects the alternation of summer and winter, we can form no satisfactory opinion in Jupiter's case, because he has no seasons worth mentioning. For instance, in latitudes on Jupiter corresponding to our own, the difference between extreme winter and extreme summer corresponds to the difference between the warmth on March 12th and March 28th, or between the warmth on September 15th and on September 30th. Yet we are not without evidence as to seasonal meteorological effects in the case of the sun's outer family of planets.. Saturn, a belted planet like Jupiter, and in all other respects resembling him, so far as a telescopic study can be trusted, has seasons even more markedly contrasted than those on our own earth. We see now one pole now another bowed toward us, and his equatorial zone is curved now downward now upward, so as to form two half ovals (at these opposite seasons), which, taken together, would make an ellipse about half as broad as it is long. As no less than fourteen years and a half separate the Saturnian summer and winter, we might fairly expect that the sun's action would have time to exert itself. In particular, we might fairly expect the great equatorial zone to be displaced; for our terrestrial zone of calms or "doldrums" travels north and south of the equator as the sun shifts northward and southward of the celestial equator, accomplishing in this way a range of no less than three thousand miles. But the Saturnian equatorial zone is not displaced at all during the long Saturnian year. It remains always persistently equatorial! Nothing could be more easy than the detection of its change of place if it followed the sun; yet no observer has ever suspected the slightest degree of systematic change corresponding with the changes of the Saturnian seasons. Or, rather, it is absolutely certain that no such change takes place.
It appears, then, that night and day, and summer and winter, are alike without influence on the Jovian and Saturnian cloud-zones. Can it reasonably be questioned that, this being the case, we must look for the origin of the cloud-zones in these planets themselves, and not in the solar orb, whose action must needs be largely influenced by the alternation of night and day and of the seasons?
But further, we find that a circumstance which had seemed perplexing, when we compared the Jovian belts with terrestrial trade-wind zones, finds an explanation at once when we regard the belts as due to some form of action exerted by the planet itself. For, let us suppose that streams of vapor are poured upward to vast heights and with great velocity from the true surface of the planet. Then such streams, starting from the surface with the rational movement there prevailing, would be carried to regions where (owing to increase of distance from the centre) the movement due to the planet's rotation would be greater. They would thus be caught by the more swiftly-moving upper air and carried forward, the modus operandi being the reverse of that observed when an engine leaves a trail of condensed steam behind it; or, rather, it may be compared to what would take place if a steam-engine were moving in the same direction as the wind, but less swiftly, so that steam-clouds would be carried in front instead of behind.
Now, heat is the only form of force which could account for the formation of the enormous masses of cloud suspended in the atmosphere of Jupiter. And it seems difficult to conceive that the clouds could be maintained at a great height above the real surface of the planet, unless that surface were intensely hot—as hot perhaps as red-hot iron. If we supposed this to be the case, we should find at once an explanation of the ruddy aspect of the dark belts. Nor would the change of the great equatorial belt from white to red imply more than that, owing to some unknown cause, clouds had not formed during the last two years over the planet's equatorial zone, or, having formed, had been dispersed in some way. We need not even imagine a complete dispersion, since the best telescopes, and notably Mr. Buckingham's fine twenty-one-inch refractor, have shown always a multitude of minute cloud-like objects over the ruddy equatorial zone.
But the idea of a red-hot planet, or of a planet partially red-hot, will appear at first view too bizarre to be entertained even for a moment. We have been so accustomed to regard Jupiter and Saturn as other worlds, that the mind is disposed to reject the conception that they can be so intensely heated as to be utterly unfit to be the abode of living creatures.
This unwillingness to accept startling ideas is not to be altogether reprehended, since it prevents the mind from forming rash and baseless speculations. Yet we must not suffer this mental habitude, excellent though it may be in its proper place, to interfere with the admission of conclusions which seem based on trustworthy evidence. Let us, then, inquire whether the startling hypothesis to which we have been led by the study or observed facts may not be found to be in agreement with other facts not yet considered.
It will be obvious that, if the real globe of Jupiter is thus intensely heated, a portion of the planet's light must be inherent. Therefore we might expect that the planet would shine somewhat more brightly than a globe of equal size and similarly placed, shining merely by reflecting the sun's light. Now, two series of good observations have been made upon the luminosity of Jupiter. One was made by the late Prof. Bond, of America, the other by Dr. Zollner, of Germany. According to the former, Jupiter shines more brightly than he would if he reflected the whole of the light falling upon him! According to the latter, and more trustworthy series, Jupiter does not indeed shine quite so brightly as Prof. Bond supposed, but the planet yet shines three times as brightly as a globe of equal size would shine, if similarly placed, but constituted like Mars, and four times as brightly as such a globe would shine if constituted like our moon. Jupiter shines, in fact, very nearly as brightly as though he were constituted like one of our terrestrial clouds!
This result is highly significant. If Jupiter showed no belts and shone with a pure white color, we could explain it at once by simply regarding Jupiter as wholly cloud-covered or snow-covered (for snow and cloud shine with nearly equal lustre when similarly illuminated). But the great dark belts which occupy so large a proportion of the planet's disk altogether negative this supposition. We seem compelled to believe that some considerable portion of the planet's lustre is inherent.
Let us, however, proceed carefully here. We have to inquire first how far Zöllner's results can be trusted; and, secondly, whether they are corroborated by any independent evidence. Now, Zöllner carefully estimated the weight of his observations—we may say he jealously estimated their weight, for it must be remembered that he was in no way interested in securing a greater or less result, while he was greatly interested in so stating the value of his results that those who might succeed him in the inquiry should not detect any serious error in his estimate. But his opinion of the probable degree of error in his observations was such as scarcely to affect to an appreciable extent the statements we have made above. Taking Zöllner's lowest estimate of Jupiter's brightness, that statement remains appreciably correct.
And next as to corroborative evidence.
It happens that we have a very delicate means of measuring the degree of Jupiter's luminosity, as compared with that of other orbs similarly placed. For his satellites pass across his face, and nothing can be easier than to observe whether they appear darker or brighter than his surface.
It was an observation such as this which Mr. Lassell had made on the night when he noticed the ruddiness of Jupiter's great medial belt. By a singular chance Father Secchi made a similar observation during his researches, and the reader will see, when we have quoted the narratives of both these observers, that the comparative darkness of all four satellites will have been established. "The fourth satellite," says Lassell, "has begun again for a season to cross the planet's disk, and I have looked out for opportunities of observing its passages, and was favored on the night of the 30th December last by witnessing a part of its passage under circumstances more than usually propitious. On its first entrance it was scarcely to be distinguished from the edge, not appearing at all as the others do, as a round bright spot. As it advanced, it grew gradually manifestly darker than the surface of the planet, and, by the time it had advanced a fourth of the way across, it had become a very dark if not a black spot—so dark, indeed, that, if I had looked at Jupiter without knowing any thing of the positions of his satellites, I should have said that a shadow (of a satellite) was passing. I remember having seen the like phenomenon many years ago; but my impression is that I had never seen the disk of the satellite so near to absolute blackness before. Of course, it is only by contrast that it can possibly so appear; and we have in this fact a striking proof of the exceeding brilliancy of the surface of the planet. In the same way, the solar spots, if not surrounded by the marvellous splendor of the sun's surface, would doubtless appear as brilliant objects."
Next let us hear Secchi's account. "On the evening of February 3d," he says, "I observed the transit of the third satellite and that of its shadow. The satellite seemed almost black when it was upon the middle of the planet's disk, and notably smaller than its shadow, which was visible at the same time; one would have estimated it at only one-half. In approaching the edge the satellite disappeared, and reappeared soon after, close by the edge, but as a bright point. This fact is not a new one for the other satellites, but for the third it is unique. This result shows also the great difference of luminosity at the centre and near the edge of the planet, a difference already confirmed by photography."
It is hardly necessary to point out how strikingly these facts illustrate and confirm Dr. Zöllner's observations. But they also supply fresh evidence of a very interesting nature.
Although a part of the difference dwelt on in Secchi's closing words may be ascribed to the oblique incidence of the light near the planet's edge, yet it does not appear to us that the whole difference can be thus explained. A difference so great that a satellite appears as a bright point close by the planet's edge, and almost black near the middle of the disk, suggests that the light near the edge is not reënforced by the inherent luminosity of our theory, that luminosity adding only to the brightness of the central parts of the disk. We would not insist too strongly on this inference, because the darkening due to oblique incidence is, under certain circumstances, very obvious to direct observation. But it seems to us that a portion of the difference should be referred to the inherent luminosity of the central parts of the disk. This being admitted, it would follow that the real solid globe of the planet is much smaller than the globe measured by astronomers; and that, therefore, instead of that amazingly small density which is so perplexing a feature of the planet's physical condition, Jupiter's globe may have a density equalling or exceeding that of the earth.
And, after all, let us remember that the theory that Jupiter is an intensely heated globe—a theory to which we have been led by the consideration of many observed facts, and which in its turn suggests very satisfactory explanations of other observed facts—would merely show that, as Jupiter and Saturn hold an intermediate position between the sun and the minor planets in respect to size, so those giant orbs hold a corresponding position in respect of inherent heat. Roughly speaking, the earth is 8,000 miles, the sun 840,000 miles, in diameter, and Jupiter, with his diameter of 82,000 miles, comes midway between these orbs. Now, the sun is a white heat, and the earth gives out only what is called obscure heat; and, if Jupiter's globe is at a red heat, he again comes midway between the sun and the earth.
We should be led by the theory here maintained to regard the major planets which travel outside the zone of asteroids as in a sense secondary suns. So viewed, they could not be regarded as orbs fit for the support of living creatures. Yet, as each of them is the centre of a scheme of dependent worlds, of dimensions large enough to supply room for many millions of living creatures, we should not merely find a raison d'être for the outer planet's, but we should be far better able to explain their purpose in the scheme of creation than on any theory hitherto put forward respecting them. Jupiter as an abode of life is a source of wonder and perplexity, and his satellites seem scarcely to serve any useful purpose. He appears as a bleak and desolate dwelling-place, and they together supply him with scarcely a twentieth part of the light which we receive from our moon at full. But, regarding Jupiter as a miniature sun, not indeed possessing any large degree of inherent lustre, but emitting a considerable quantity of heat, we recognize in him the fitting ruler of a scheme of subordinate orbs, whose inhabitants would require the heat which he affords to eke out the small supply which they receive directly from the sun. The Saturnian system, again, is no longer mysterious when thus viewed. The strange problem presented by the rings, which naturally conceal the sun from immense regions of the planet for years together in the very heart of the winter of those regions, is satisfactorily solved when the Saturnian satellites are regarded as the abodes of life, and Saturn himself as the source of a considerable proportion of their heat-supply. We do not say that, in thus exhibiting the Jovian and Saturnian systems in a manner which accords with our ideas respecting the laws of life in the universe, we have given irrefragable testimony in favor of our theory. The theory must stand or fall according to the evidence in its favor or against it. But, so long as men believe that there is design in the scheme of the universe, they will be readier to accept conclusions which exhibit at once the major planets and their satellites as occupying an intelligible position in that scheme, than views which leave the satellites unaccounted for, and present the giant planets themselves as very questionable abodes for any known orders of living creatures.—Cornhill Magazine
- It is not uncommonly stated in our text-books of astronomy, that the density of Jupiter's moons is far less than Jupiter's density; and Lardner goes so far as to say that "the density of the matter composing these satellites is much smaller than that of any other body of the system whose density is known." But this is a mistake. All the satellites, save one, are of greater density than Jupiter, and that one—the innermost—is denser than Saturn, Uranus, or Neptune.