Other Worlds<br />Their Nature, Possibilities and Habitability in the Light of the Latest Discoveries, Garrett Putman Serviss [reading diary .txt] 📗

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[Pg 201] 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[Pg 202] and permanent feature of Saturn, or will, in the course of time, be destroyed.[12]

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[Pg 203] 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.[Pg 204]

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[Pg 205] 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 sur[Pg 206]face, 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]

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[Pg 207] 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[Pg 208] 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; Nep[Pg 209]tune'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 ar[Pg 210]gument 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.[Pg 211]

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.[Pg 212]

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