It is hardly worth while to speculate about any solid nucleus within, because, even if such a thing were possible, or probable, it must lie forever hidden from our eyes. But if we accept the theory that Saturn is in an early formative stage, and that, millions of years hence, it may become an incrusted and habitable globe, we shall, at least, follow the analogy of what we believe to have been the history of the earth, except that Saturn's immense distance from the sun will always prevent it from receiving an amount of solar radiation consistent with our ideas of what is required by a living world. Of course, since one can imagine what he chooses, it is possible to suppose inhabitants suited to existence in a world composed only of whirling clouds, and a poet with the imagination of a Milton might give us very imposing and stirring images of such creatures and their chaotic surroundings, but fancies like these can have no basis in human experience, and consequently can make no claim upon scientific recognition.
Or, as an alternative, it might be assumed that Saturn is composed of lighter elements and materials than those which constitute the earth and the other solid planets in the more immediate neighborhood of the sun.
But such an assumption would put us entirely at sea as regards the forms of organic life that could exist upon a planet of that description, and, like Sir Humphry Davy in the Vision, that occupies the first chapter of his quaintly charming Consolations in Travel, or, the Last Days of a Philosopher, we should be thrown entirely upon the resources of the imagination in representing to ourselves the nature and appearance of its inhabitants. Yet minds of unquestioned power and sincerity have in all ages found pleasure and even profit in such exercises, and with every fresh discovery arises a new flight of fancies like butterflies from a roadside pool. As affording a glimpse into the mind of a remarkable man, as well as a proof of the fascination of such subjects, it will be interesting to quote from the book just mentioned Davy's description of his imaginary inhabitants of Saturn:
"I saw below me a surface infinitely diversified, something like that of an immense glacier covered with large columnar masses, which appeared as if formed of glass, and from which were suspended rounded forms of various sizes which, if they had not been transparent, I might have supposed to be fruit. From what appeared to me to be analogous to bright-blue ice, streams of the richest tint of rose color or purple burst forth and flowed into basins, forming lakes or seas of the same color. Looking through the atmosphere toward the heavens, I saw brilliant opaque clouds, of an azure color, that reflected the light of the sun, which had to my eyes an entirely new aspect and appeared smaller, as if seen through a dense blue mist.
"I saw moving on the surface below me immense masses, the forms of which I find it impossible to describe. They had systems for locomotion similar to those of the morse, or sea-horse, but I saw, with great surprise, that they moved from place to place by six extremely thin membranes, which they used as wings. Their colors were varied and beautiful, but principally azure and rose color. I saw numerous convolutions of tubes, more analogous to the trunk of the elephant than to anything else I can imagine, occupying what I supposed to be the upper parts of the body. It was with a species of terror that I saw one of them mounting upward, apparently flying toward those opaque clouds which I have before mentioned.
"'I know what your feelings are,' said the Genius; 'you want analogies, and all the elements of knowledge to comprehend the scene before you.
You are in the same state in which a fly would be whose microscopic eye was changed for one similar to that of man, and you are wholly unable to associate what you now see with your former knowledge. But those beings who are before you, and who appear to you almost as imperfect in their functions as the zoophytes of the polar sea, to which they are not unlike in their apparent organization to your eyes, have a sphere of sensibility and intellectual enjoyment far superior to that of the inhabitants of your earth. Each of those tubes, which appears like the trunk of an elephant, is an organ of peculiar motion or sensation. They have many modes of perception of which you are wholly ignorant, at the same time that their sphere of vision is infinitely more extended than yours, and their organs of touch far more perfect and exquisite.'"
After descanting upon the advantages of Saturn's position for surveying some of the phenomena of the solar system and of outer space, and the consequent immense advances that the Saturnians have made in astronomical knowledge, the Genius continues:
"'If I were to show you the different parts of the surface of this planet you would see the marvelous results of the powers possessed by these highly intellectual beings, and of the wonderful manner in which they have applied and modified matter. Those columnar masses, which seem to you as if rising out of a mass of ice below, are results of art, and processes are going on within them connected with the formation and perfection of their food. The brilliant-colored fluids are the results of such operations as on the earth would be performed in your laboratories, or more properly in your refined culinary apparatus, for they are connected with their system of nourishment. Those opaque azure clouds, to which you saw a few minutes ago one of those beings directing his course, are works of art, and places in which they move through different regions of their atmosphere, and command the temperature and the quantity of light most fitted for their philosophical researches, or most convenient for the purposes of life.'"[11]
[Footnote 11: Davy, of course, was aware that, owing to increase of distance, the sun would appear to an inhabitant of Saturn with a disk only one ninetieth as great in area as that which it presents to our eyes.]
But, while Saturn does not appear, with our present knowledge, to hold out any encouragement to those who would regard it as the abode of living creatures capable of being described in any terms except those of pure imagination, yet it is so unique a curiosity among the heavenly bodies that one returns again and again to the contemplation of its strange details. Saturn has nine moons, but some of them are relatively small bodies--the ninth, discovered photographically by Professor Pickering in 1899, being especially minute--and others are situated at great distances from the planet, and for these reasons, together with the fact that the sunlight is so feeble upon them that, surface for surface, they have only one ninetieth as much illumination as our moon receives, they can not make a very brilliant display in the Saturnian sky. To astronomers on Saturn they would, of course, be intensely interesting because of their perturbations and particularly the effect of their attraction on the rings.
This brings us again to the consideration of those marvelous appendages, and to the statement of facts about them which we have not yet recalled.
If the reader will take a ball three inches in diameter to represent the globe of Saturn, and, out of the center of a circular piece of writing-paper seven inches in diameter, will cut a round hole three and three quarter inches across, and will then place the ball in the middle of the hole in the paper, he will have a very fair representation of the relative proportions of Saturn and its rings. To represent the main gap or division in the rings he might draw, a little more than three eighths of an inch from the outer edge of the paper disk, a pencil line about a sixteenth of an inch broad.
Perhaps the most striking fact that becomes conspicuous in making such a model of the Saturnian system is the exceeding thinness of the rings as compared with their enormous extent. They are about 170,000 miles across from outer edge to outer edge, and about 38,000 miles broad from outer edge to inner edge--including the gauze ring presently to be mentioned--yet their thickness probably does not surpass one hundred miles! In fact, the sheet of paper in our imaginary model is several times too thick to represent the true relative thickness of Saturn's rings.
Several narrow gaps in the rings have been detected from time to time, but there is only one such gap that is always clearly to be seen, the one already mentioned, situated about 10,000 miles from the outer edge and about 1,600 miles in width. Inside of this gap the broadest and brightest ring appears, having a width of about 16,500 miles. For some reason this great ring is most brilliant near the gap, and its brightness gradually falls off toward its inner side. At a distance of something less than 20,000 miles from the planet--or perhaps it would be more correct to say above the planet, for the rings hang directly over Saturn's equator--the broad, bright ring merges into a mysterious gauzelike object, also in the form of a ring, which extends to within 9,000 or 10,000 miles of the planet's surface, and therefore itself has a width of say 10,000 miles.
In consequence of the thinness of the rings they completely disappear from the range of vision of small telescopes when, as occurs once in every fifteen years, they are seen exactly edgewise from the earth. In a telescope powerful enough to reveal them when in that situation they resemble a thin, glowing needle run through the ball of the planet. The rings will be in this position in 1907, and again in 1922.
The opacity of the rings is proved by the shadow which they cast upon the ball of the planet. This is particularly manifest at the time when they are edgewise to the earth, for the sun being situated slightly above or below the plane of the rings then throws their shadow across Saturn close to its equator. When they are canted at a considerable angle to our line of sight their shadow is seen on the planet, bordering their outer edge where they cross the ball.
The gauze ring, the detection of which as a faintly luminous phenomenon requires a powerful telescope, can be seen with slighter telescopic power in the form of a light shade projected against the planet at the inner edge of the broad bright ring. The explanation of the existence of this peculiar object depends upon the nature of the entire system, which, instead of being, as the earliest observers thought it, a solid ring or series of concentric rings, is composed of innumerable small bodies, like meteorites, perhaps, in size, circulating independently but in comparatively close juxtaposition to one another about Saturn, and presenting to our eyes, because of their great number and of our enormous distance, the appearance of solid, uniform rings. So a flock of ducks may look from afar like a continuous black line or band, although if we were near them we should perceive that a considerable space separates each individual from his neighbors.
The fact that this is the constitution of Saturn's rings can be confidently stated because it has been mathematically proved that they could not exist if they were either solid or liquid bodies in a continuous form, and because the late Prof. James E. Keeler demonstrated with the spectroscope, by means of the Doppler principle, already explained in the chapter on Venus, that the rings circulate about the planet with varying velocities according to their distance from Saturn's center, exactly as independent satellites would do.
It might be said, then, that Saturn, instead of having nine satellites only, has untold millions of them, traveling in orbits so closely contiguous that they form the appearance of a vast ring.
As to their origin, it may be supposed that they are a relic of a ring of matter left in suspension during the contraction of the globe of Saturn from a nebulous mass, just as the rings from which the various planets are supposed to have been formed were left off during the contraction of the main body of the original solar nebula. Other similar rings originally surrounding Saturn may have become satellites, but the matter composing the existing rings is so close to the planet that it falls within the critical distance known as "Roche's limit," within which, owing to the tidal effect of the planet's attraction, no body so large as a true satellite could exist, and accordingly in the process of formation of the Saturnian system this matter, instead of being aggregated into a single satellite, has remained spread out in the form of a ring, although its substance long ago passed from the vaporous and liquid to the solid form. We have spoken of the rings as being composed of meteorites, but perhaps their component particles may be so small as to answer more closely to the definition of dust. In these rings of dust, or meteorites, disturbances are produced by the attraction of the planet and that of the outer satellites, and it is yet a question whether they are a stable and permanent feature of Saturn, or will, in the course of time, be destroyed.[12]
[Footnote 12: For further details about Saturn's rings, see The Tides, by G.H. Darwin, chap. xx.]
It has been thought that the gauze ring is variable in brightness. This would tend to show that it is composed of bodies which have been drawn in toward the planet from the principal mass of the rings, and these bodies may end their career by falling upon the planet. This process, indefinitely continued, would result in the total disappearance of the rings--Saturn would finally swallow them, as the old god from whom the planet gets its name is fabled to have swallowed his children.
Near the beginning of this chapter reference was made to the fact that Saturn's rings have been regarded as habitable bodies. That, of course, was before the discovery that they were not solid. Knowing what we now know about them, even Dr. Thomas Dick, the great Scotch popularizer of astronomy in the first half of the nineteenth century, would have been compelled to abandon his theory that Saturn's rings were crowded with inhabitants. At the rate of 280 to the square mile he reckoned that they could easily contain 8,078,102,266,080 people.
He even seems to have regarded their edges--in his time their actual thinness was already well known--as useful ground for the support of living creatures, for he carefully calculated the aggregate area of these edges and found that it considerably exceeded the area of the entire surface of the earth. Indeed, Dr. Dick found room for more inhabitants on Saturn's rings than on Saturn itself, for, excluding the gauze ring, undiscovered in his day, the two surfaces of the rings are greater in area than the surface of the globe of the planet. He did not attack the problem of the weight of bodies on worlds in the form of broad, flat, thin, surfaces like Saturn's rings, or indulge in any reflections on the interrelations of the inhabitants of the opposite sides, although he described the wonderful appearance of Saturn and other celestial objects as viewed from the rings.
But all these speculations fall to the ground in face of the simple fact that if we could reach Saturn's rings we should find nothing to stand upon, except a cloud of swiftly flying dust or a swarm of meteors, swayed by contending attractions. And, indeed, it is likely that upon arriving in the immediate neighborhood of the rings they would virtually disappear! Seen close at hand their component particles might be so widely separated that all appearance of connection between them would vanish, and it has been estimated that from Saturn's surface the rings, instead of presenting a gorgeous arch spanning the heavens, may be visible only as a faintly gleaming band, like the Milky Way or the zodiacal light. In this respect the mystic Swedenborg appears to have had a clearer conception of the true nature of Saturn's rings than did Dr. Dick, for in his book on The Earths in the Universe he says--using the word "belt" to describe the phenomenon of the rings:
"Being questioned concerning that great belt which appears from our earth to rise above the horizon of that planet, and to vary its situations, they [the inhabitants of Saturn] said that it does not appear to them as a belt, but only as somewhat whitish, like snow in the heaven, in various directions."
In view of such observations as that of Prof. E.E. Barnard, in 1892, showing that a satellite passing through the shadow of Saturn's rings does not entirely disappear--a fact which proves that the rings are partially transparent to the sunlight--one might be tempted to ask whether Saturn itself, considering its astonishing lack of density, is not composed, at least in its outer parts, of separate particles of matter revolving independently about their center of attraction, and presenting the appearance of a smooth, uniform shell reflecting the light of the sun. In other words, may not Saturn be, exteriorly, a globe of dust instead of a globe of vapor? Certainly the rings, incoherent and translucent though they be, reflect the sunlight to our eyes, at least from the brighter part of their surface, with a brilliance comparable with that of the globe of the planet itself.
As bearing on the question of the interior condition of Saturn and Jupiter, it should, perhaps, be said that mathematical considerations, based on the figures of equilibrium of rotating liquid masses, lead to the conclusion that those planets are comparatively very dense within.
Professor Darwin puts the statement very strongly, as follows: "In this way it is known with certainty that the central portions of the planets Jupiter and Saturn are much denser, compared to their superficial portions, than is the case with the earth."[13]
[Footnote 13: The Tides, by G.H. Darwin, p. 333.]
The globe and rings of Saturn witness an imposing spectacle of gigantic moving shadows. The great ball stretches its vast shade across the full width of the rings at times, and the rings, as we have seen, throw their shadow in a belt, whose position slowly changes, across the ball, sweeping from the equator, now toward one pole and now toward the other. The sun shines alternately on each side of the rings for a space of nearly fifteen years--a day fifteen years long! And then, when that face of the ring is turned away from the sun, there ensues a night of fifteen years' duration also.
Whatever appearance the rings may present from the equator and the middle latitudes on Saturn, from the polar regions they would be totally invisible. As one passed toward the north, or the south, pole he would see the upper part of the arch of the rings gradually sink toward the horizon until at length, somewhere in the neighborhood of the polar circle, it would finally disappear, hidden by the round shoulder of the great globe.
URANUS, NEPTUNE, AND THE SUSPECTED ULTRANEPTUNIAN PLANET
What has been said of Jupiter and Saturn applies also to the remaining members of the Jovian group of planets, Uranus and Neptune, viz., that their density is so small that it seems probable that they can not, at the present time, be in a habitable planetary condition. All four of these outer, larger planets have, in comparatively recent times, been solar orbs, small companions of the sun. The density of Uranus is about one fifth greater than that of water, and slightly greater than that of Neptune. Uranus is 32,000 miles in diameter, and Neptune 35,000 miles.
Curiously enough, the force of gravity upon each of these two large planets is a little less than upon the earth. This arises from the fact that in reckoning gravity on the surface of a planet not only the mass of the planet, but its diameter or radius, must be considered. Gravity varies directly as the mass, but inversely as the square of the radius, and for this reason a large planet of small density may exercise a less force of gravity at its surface than does a small planet of great density.
The mean distance of Uranus from the sun is about 1,780,000,000 miles, and its period of revolution is eighty-four years; Neptune's mean distance is about 2,800,000,000 miles, and its period of revolution is about 164 years.
Uranus has four satellites, and Neptune one. The remarkable thing about these satellites is that they revolve _backward_, or contrary to the direction in which all the other satellites belonging to the solar system revolve, and in which all the other planets rotate on their axis.
In the case of Uranus, the plane in which the satellites revolve is not far from a position at right angles to the plane of the ecliptic; but in the case of Neptune, the plane of revolution of the satellites is tipped much farther backward. Since in every other case the satellites of a planet are situated nearly in the plane of the planet's equator, it may be assumed that the same rule holds with Uranus and Neptune; and, that being so, we must conclude that those planets rotate backward on their axes. This has an important bearing on the nebular hypothesis of the origin of the solar system, and at one time was thought to furnish a convincing argument against that hypothesis; but it has been shown that by a modification of Laplace's theory the peculiar behavior of Uranus and Neptune can be reconciled with it.
Very little is known of the surfaces of Uranus and Neptune. Indications of the existence of belts resembling those of Jupiter have been found in the case of both planets. There are similar belts on Saturn, and as they seem to be characteristic of large, rapidly rotating bodies of small density, it was to be expected that they would be found on Uranus and Neptune.
The very interesting opinion is entertained by some astronomers that there is at least one other great planet beyond Neptune. The orbits of certain comets are relied upon as furnishing evidence of the existence of such a body. Prof. George Forbes has estimated that this, as yet undiscovered, planet may be even greater than Jupiter in mass, and may be situated at a distance from the sun one hundred times as great as the earth's, where it revolves in an orbit a single circuit of which requires a thousand years.
Whether this planet, with a year a thousand of our years in length, will ever be seen with a telescope, or whether its existence will ever, in some other manner, be fully demonstrated, can not yet be told. It will be remembered that Neptune was discovered by means of computations based upon its disturbing attraction on Uranus before it had ever been recognized with the telescope. But when the astronomers in the observatories were told by their mathematical brethren where to look they found the planet within half an hour after the search began. So it is possible the suspected great planet beyond Neptune may be within the range of telescopic vision, but may not be detected until elaborate calculations have deduced its place in the heavens. As a populous city is said to furnish the best hiding-place for a man who would escape the attention of his fellow beings, so the star-sprinkled sky is able to conceal among its multitudes worlds both great and small until the most painstaking detective methods bring them to recognition.
CHAPTER VIII
THE MOON, CHILD OF THE EARTH AND THE SUN
Very naturally the moon has always been a great favorite with those who, either in a scientific or in a literary spirit, have speculated about the plurality of inhabited worlds. The reasons for the preference accorded to the moon in this regard are evident. Unless a comet should brush us--as a comet is suspected of having done already--no celestial body, of any pretensions to size, can ever approach as near to the earth as the moon is, at least while the solar system continues to obey the organic laws that now control it. It is only a step from the earth to the moon. What are 240,000 miles in comparison with the distances of the stars, or even with the distances of the planets? Jupiter, driving between the earth and the moon, would occupy more than one third of the intervening space with the chariot of his mighty globe; Saturn, with broad wings outspread, would span more than two thirds of the distance; and the sun, so far from being able to get through at all, would overlap the way more than 300,000 miles on each side.
In consequence, of course, of its nearness, the moon is the only member of the planetary system whose principal features are visible to the naked eye. In truth, the naked eye perceives the larger configurations of the lunar surface more clearly than the most powerful telescope shows the details on the disk of Mars. Long before the time of Galileo and the invention of the telescope, men had noticed that the face of the moon bears a resemblance to the appearance that the earth would present if viewed from afar off. In remote antiquity there were philosophers who thought that the moon was an inhabited world, and very early the romancers took up the theme. Lucian, the Voltaire of the second century of our era, mercilessly scourged the pretenders of the earth from an imaginary point of vantage on the moon, which enabled him to peer down into their secrets. Lucian's description of the appearance of the earth from the moon shows how clearly defined in his day had become the conception of our globe as only an atom in space.
"Especially did it occur to me to laugh at the men who were quarreling about the boundaries of their land, and at those who were proud because they cultivated the Sikyonian plain, or owned that part of Marathon around Oenoe, or held possession of a thousand acres at Acharnae. Of the whole of Greece, as it then appeared to me from above, being about the size of four fingers, I think Attica was in proportion a mere speck.
So that I wondered on what condition it was left to these rich men to be proud."[14]
[Footnote 14: Ikaromenippus; or, Above the Clouds. Prof. D.C. Brown's translation.]
Such scenes as Lucian beheld, in imagination, upon the earth while looking from the moon, many would fain behold, with telescopic aid, upon the moon while looking from the earth. Galileo believed that the details of the lunar surface revealed by his telescope closely resembled in their nature the features of the earth's surface, and for a long time, as the telescope continued to be improved, observers were impressed with the belief that the moon possessed not only mountains and plains, but seas and oceans also.
It was the discovery that the moon has no perceptible atmosphere that first seriously undermined the theory of its habitability. Yet, as was remarked in the introductory chapter, there has of late been some change of view concerning a lunar atmosphere; but the change has been not so much in the ascertained facts as in the way of looking at those facts.