Popular Science Monthly/Volume 51/July 1897/The Planet Saturn
|THE PLANET SATURN.|
DOUBTLESS many observers of the sky are familiar with the planet Saturn as he slowly moves through the constellations from year to year, but how many of them stop to think of the wonders and mysteries connected with this far-off member of the solar system? Very few, probably; and yet this planet is well worth a closer acquaintance, for, as Prof. Langley says, "In all the heavens there is no more wonderful object than the planet Saturn, for it preserves to us an apparent type of the plan on which all the worlds were originally made."
Saturn was the remotest planet known to the ancients, and it was probably on account of his sluggish motion along the sky that a malignant influence over human affairs was attributed to him by the astrologers. This slow movement is only apparent, however, for he is really bowling along through space more than twenty thousand miles every hour; but such is his distance from us that we can scarcely detect any change of position from night to night, and must wait thirty years for him to make his circuit of the heavens.
In point of size Saturn stands next to Jupiter, the "giant of the solar system," and upon his diameter nine earths could be strung like beads on a wire, while from his vast bulk seven hundred planets like ours could be formed. But just here comes a factor which has an important bearing upon the present condition of Saturn. In spite of his enormous bulk, he "weighs" only ninety times as much as the earth, which at once shows us that the materials of which he is formed are much lighter than those composing our world. In fact they are but three quarters the weight of an equal amount of water, so that theoretically, if placed in an ocean large enough to hold it, this huge planet would float on the surface like a wooden ball.
There is but one conclusion from this and also from some other facts connected with the planet. Saturn is not, like the earth, a solid sphere covered with oceans and continents capable of supporting animal and vegetable life, but is midway between this state and that of the sun. In other words he might be called a semi-sun, perhaps giving forth but little light, yet so intensely heated still that its vast bulk is probably but a distended mass of liquid fire—a world where "the solid land as yet is not, and the foot could find no resting place." It is too bad to destroy the pleasant theories we often see about the inhabitants of this far-off world and the conditions of life upon its surface, but we can not evade the facts as they open up to us.
When viewed through a good telescope the planet presents a most beautiful sight—a huge golden ball, crossed by parallel belts of a brownish tinge, and capped at the poles with a bluish or greenish gray; and, most wonderful of all, surrounded by a thin, broad, flat ring, likewise of a golden hue. As if this were not enough, it is accompanied by a retinue of at least eight satellites or moons, some of which will be in the field of view.
Under very favorable conditions faint markings can be discerned on the belts, which seem in every way similar to those of Jupiter, and like his may safely be assumed to be masses of rolling clouds ranged in belts parallel to the equator by currents analogous to our trade winds. It seems very probable that these clouds may be mostly aqueous, and we may thus regard them as the future oceans of these planets, suspended in the air at present because the surface is not yet sufficiently cool to allow them to settle and remain as bodies of water upon it.
That this must be the case is shown by a moment's thought. We know that on the earth clouds are formed by the condensation, in the upper and cooler portions of the air, of the water vapor raised from the surface waters by the sun's heat. But at Saturn, nearly ten times farther away, this heat is reduced to one one-hundredth of its intensity here. On the earth too, as a rule, the clouds are somewhat sparsely distributed, so that a large part of the globe has usually fairly clear weather. On Saturn, however, we never yet have caught a glimpse, so far as known, of the real surface, whatever that surface may be. The rolling cloud masses completely envelop the planet and shut it out entirely from the sun's light.
We can scarcely suppose, then, that these clouds are raised upon this distant world by the solar heat, especially when we see how feeble that heat is compared with what the earth receives. And this is but another argument to prove the theory of Saturn's present condition as already given, for it is most probable that the planet holds in its own vast bulk the immense amount of heat whose presence is so certainly revealed in these phenomena.
Of course the rings are the unique and most wonderful feature of the whole system. When Galileo first turned his rude telescope upon Saturn, in 1610, he announced that the planet was triple, the projection of the ring on either side making it appear to him as if two smaller planets were joined to the larger one. Gradually, however, these smaller companions decreased in size and finally vanished altogether, much to Galileo's amazement. Later on they reappeared and still further increased his perplexity.
Saturn thus remained an enigma to astronomers until an increase in the power of telescopes brought out the fact that it was surrounded by a thin, flat ring, which, by its varying positions as seen from the earth, caused the peculiar appearances that so puzzled Galileo.
This so-called ring, when seen through large telescopes, appears as a very thin, flat disk with a circular opening in the center in which the planet itself is situated. It lies exactly in the plane of Saturn's equator, and extends considerably more than half the planet's diameter on either side of it. The breadth of the ring is just half the planet's diameter, so that there is quite a space left between its inner edge and the surface of the planet.
We speak of it as a ring, but in reality there are many of them. When favorably situated, a dark division can easily be detected which separates it into an "outer" and an "inner" bright ring; while within the last fifty years a third one, inside of the others, was discovered at Cambridge. This innermost of all, known as the "dark" or "crêpe" ring, is a most peculiar object. In appearance it is more like a shadow than anything else, for it seems to be semi-transparent, inasmuch as the outline of the planet can be seen through it where it crosses the planet's disk. It shades away gradually from the inner edge of the inner bright ring, and becomes fainter until it disappears at some nine thousand miles from the planet's surface.
What the nature of these rings may be is still in some degree a mystery. They are not gaseous, and it has been shown that they are not liquid, for no liquid could be suspended in such a manner without being precipitated eventually upon the surface of the planet. Nor are they solid; for it has been demonstrated that no solid could hold together under such strains, such tremendous forces, as the attraction of the monster planet would subject it to; it would soon be broken up entirely.
The only supposition remaining is that it is composed of myriads of solid particles—a ring of dust and fragments of rock and stone. In this case we may imagine it as being an immense swarm of tiny moons or satellites, each revolving in its own particular path around the planet, and the aggregation presenting to us at this distance the appearance of a solid mass. Of course, the word "tiny" must be taken in an astronomical sense, which would not preclude one of these "dust" particles or fragments from being as large as a house, or even a mountain.
That the ring is composed of solid matter of some kind is proved by the fact that it reflects the sunlight which it receives, apparently unchanged in quality, and deprives of sunlight those portions of the planet on which its shadow falls. But here comes the question, If we know the ring is composed of solid matter, how do we know that it is in the form of dust and fragments? This question was long a stumbling-block, but, as Prof. George Darwin points out, the investigations of M. Roche, a French mathematician, seem to have solved the difficulty.
Briefly, the reasoning is as follows: We know that our moon always keeps the same face toward the earth, but perhaps it is not so generally known that the cause of this is in the moon's own shape, which is that of an egg with the longer diameter pointing toward the earth. Not that this egg shape is so very pronounced, but it is sufficient to keep the moon from rotating as the earth does, and to keep its longer diameter pointed toward the seat of that force which holds our satellite in its path.
The cause of this egg shape is simply in what is termed the "tide-generating force." The moon's effect upon the earth due to this force is rendered noticeable and well known in our tides. The earth also exerts the same force upon the moon, only, as the former is eighty times more massive, the effect is correspondingly greater, and the moon's globe has suffered under the strain has—been pulled out of shape, so to speak.
Now this force of course increases as its source is approached, and were the moon brought nearer and nearer the earth, a point might finally be reached where the solid materials of which she is composed could no longer hold together, and her globe would be torn to pieces by the tremendous forces to which she would be subjected. To determine this point was the problem which M. Roche solved, and his conclusions led him to place it at a distance just under a diameter and a quarter from the planet's center. Within this distance, then, no satellite of any considerable size can circulate for the reasons above stated.
Now, the most remarkable fact remaining is that the outer edge of Saturn's ring system lies just within this limit, so that the conclusion as to its nature seems to point to the "meteoric theory," as it is called, as the only possible one. Either a satellite has been drawn within the fatal circle and disrupted, or the materials now present as a ring have been prevented from uniting to form a single satellite, as they might otherwise have done.
So much, then, for theory. The next point is. What proof can we get to substantiate it? This might seem at first a hopeless task, but that wonderful instrument, the spectroscope, has recently given us direct testimony on the subject.
One of the peculiarities of the spectroscope is its ability to detect the motion of a luminous body in the line of sight, by the shifting of the dark (Fraunhofer) lines of its spectrum from their normal position as seen in the spectrum of direct sunlight. Advantage was taken of this fact by Mr. J. E. Keeler, who obtained photographs of the spectrum of Saturn and its rings which plainly showed that the shifting of the lines due to the motion of the rings was greater in each case for the inner edge than for the outer, proving conclusively that the portions of the ring nearer the planet move faster than those farther away.
Let us see what this means. In the first place, if we suppose the rings to be solid, it is evident that they must rotate as a whole, the angular velocity of all parts being the same, but the linear or actual velocity being much greater at the outer edge of the ring than the inner, because of the greater circumference of the circle traveled over in rotation.
If, on the other hand, the ring is composed of separate particles, each in effect a little moon, it is apparent that the nearer these tiny satellites are to the planet the faster they must revolve to overcome the increasing pull of the planet and save themselves from being drawn to destruction upon its surface. In this case, therefore, the inner edge of the ring will have a much greater velocity than the outer.
Thus we see that the two theories require opposite conditions to obtain, and that the proof given by the spectroscope confirms directly the approximate correctness of the "meteoric theory."
This latter theory offers a ready explanation for the curious "crêpe" ring. Shading off gradually as this ring does from the inner edge of the bright one, it is natural to suppose that it is a portion of the former ring in which the fragments or "meteorites" are more sparsely distributed, their numbers growing gradually less as the distance from the main ring increases, until the eye can no longer detect their mass and the ring apparently ends.
This explains why the outline of the planet can be seen through the dark ring; but if this fact is not enough, an observation made on November 1, 1889, at the Lick Observatory will further confirm the theory. This observation was of the outer satellite, which was in such a position behind the planet as to pass through the shadow of the rings and be eclipsed by it. Watching the satellite, then, as it left the planet's shadow and slowly passed on into the shadow of the rings, its light was seen to grow gradually fainter as it passed through the shadow of the dark ring, but did not wholly disappear until the moon had entered the shadow of the inner bright ring. This shows clearly that the dark ring is partially transparent, but becomes more opaque as the bright ring is approached.
With regard to the satellites there is little to be said. There are eight known at present, and there may be more, for they are mostly quite small, as heavenly bodies go. Still, they form the most numerous as well as the most extended family within the sun's domain, for the outer one of all swings around Saturn at a distance of two and a quarter millions of miles—ten times as far away as our own moon. This one, which is named Japetus, is just about the size of the moon, and apparently shares the latter's peculiar trait of always keeping one side toward its ruling planet. This supposition is due to the fact that when on the western side of Saturn Japetus is always very much brighter than when to the eastward; in fact, though easily seen with a telescope of moderate power when brightest, it will almost entirely disappear when faintest. It is difficult to explain the cause of such a marked change, for one half of the satellite must be extremely bright and the other half very much darker to produce it, but the fact remains.
Titan, as its name implies, is the largest of the group, and in size is midway between Mars and Mercury—in fact, it would make a very respectable planet itself, for it is nearly half the diameter of the earth. The other six are all considerably smaller than our moon, and have been discovered in the order of their brightness, their discovery keeping pace with the increase in the power of telescopes, so it is quite possible that there may be others in this already numerous family to be introduced later on.
We spoke in the beginning of this article of destroying the theories often put forth concerning the inhabitants and conditions of life upon this far-off world. There are certain facts and deductions, however, from which we can gain an idea of some of the conditions which may prevail when Saturn has finally reached a stage where life will be possible upon its surface, and it may not be uninteresting to consider some of their peculiarities.
In considering the climatic conditions of a planet we find they depend principally upon three factors: the distance of the planet from the sun, the inclination of its axis, and the length of its year, with incidentally the length of its day. What the results of this combination may lead us to expect in the case of Saturn we will point out by using the earth, naturally, for analogy or contrast.
In the first place, as affecting animal and vegetable life, the greater distance of the sun, and the corresponding decrease in its lighting and heating power compared with the same effects on the earth, would materially change in itself the character of such life on Saturn. As already noted, the heat and light are reduced to nearly one one-hundredth of their intensity here, but no one can tell what compensating features may ultimately be provided for retaining the internal heat of the globe or storing up the sun's heat. As an instance of such adaptation we have only to turn to the planet Mars, where we have visual proof, in the melting of its polar "snows," of a much milder climate than the earth possesses, although the intensity of the sun's heat there is reduced by half.
In connection with the foregoing is the question of the composition of the atmosphere, and whether it could support such organisms as we are familiar with in terrestrial life. The spectroscope has told us but little about Saturn's atmosphere, but it is known that the planet is provided with one of considerable extent, and apparently of a similar constitution to our own. The presence of water vapor has been detected, according to some observers, but not positively; yet it is fair to suppose from other considerations that this most necessary adjunct of all life is plentifully supplied.
The change in the seasons will, of course, depend upon the inclination of the axis, which in Saturn's case is twenty-six and a half degrees from the perpendicular to its orbit. When we remember that the corresponding inclination of the earth's axis is twenty-three and a half degrees, it will be apparent that the change of seasons would be quite similar to ours, the sun merely rising three degrees higher in the heavens at the summer solstice and three degrees lower at the winter solstice. But the length of the seasons, determined by Saturn's long journey around the sun, will be, on the average, nearly seven and a half years, a fact which would render unlikely much similarity in organic life to the forms found on the earth. If we add to this the rapid succession of day and night, each being at the equator of but five and a quarter hours' duration, we may look for still further dissimilarity; but the greatest difficulty comes when we consider the effect of the rings.
At first thought it might seem that the rings would have little to do with the climate of the planet, and in fact such is the case during the summer of either hemisphere; but winter tells a different tale, as we shall see. Since the rings lie exactly in the plane of the planet's equator, they will be presented edgewise to the sun at the equinoxes, when the sun is "vertical over the equator." At this time their shadow, part of which must fall on the planet, will lie directly on the equator, and presumably be about as wide as the general thickness of the ring system, which is estimated to be not more than one hundred miles.
As the sun travels northward from the equinox, it is apparent that the shadow will fall farther and farther south of the equator until it has covered the whole southern hemisphere, save a portion of the torrid zone where the light comes through the space between the rings and the planet. After the summer solstice the effects are reversed: the shadow retreats toward the equator, and after the succeeding equinox the southern hemisphere will have its summer undisturbed, and the northern hemisphere in turn will have its long winter made still more dreary by this remarkable daily eclipse of the sun. It thus appears that only in a relatively narrow belt lying on either side of the equator would be likely to occur climatic conditions approaching those with which we are familiar.
One often sees in articles on astronomy some reference to the grandeur of the Saturnian heavens at night, where, in addition to the starry host familiar to us all, would be the wonderful ring spanning the sky as an arch of golden light, and eight moons in their various phases. In a measure this is true, but it depends upon circumstances. During the summer half of the year in either hemisphere the illuminated side of the rings is, of course, visible perhaps—even faintly so in the daytime, as is the case with our moon; but when the twilight falls and the golden arch shines forth in all its beauty against the darkness of the sky, it must certainly be a sight which for grandeur surpasses any celestial phenomenon known to us, save possibly a total eclipse of the sun.
As soon as the sun has set, however, the shadow of the planet, where it falls upon the rings, rises in the east and mars the beauty of the arch as it travels across it during the short night and disappears in the west at sunrise. At the summer solstice, though, the sun rises high enough in the heavens, or, more correctly, the planet's axis is inclined far enough toward the sun to bring the outer ring clear of the shadow, which then appears somewhat conical in shape and reaches across the inner bright ring nearly to the outer one.
But after the autumnal equinox and during the winter season all this is changed. Not only do the rings cause daily eclipses of the sun, but they give no illumination at night, for their dark side is then toward the observer, and they can be only "negatively visible," so to speak—that is, their position in the sky is shown merely by the absence of stars in that portion.
As to their appearance from various positions on the planet, it might be said that the whole system is visible above the horizon as far as latitude 41°—that of New York and Constantinople in our northern hemisphere, and Tasmania and New Zealand in the southern. At this latitude the inner edge of the dark ring will be upon the south point of the horizon, and the arch will extend about a third of the way toward the zenith. When latitude 51° is reached, that of Dresden and Winnipeg, Manitoba, the dark ring will have sunk below the horizon, but the whole width of the bright rings will be above it; and, finally, at latitude 66° 30′, that of our Arctic and Antarctic Circles, the entire system will have disappeared.
Of the illumination given by the moons in the absence of the rings we must say a little, since one often sees some statement to the effect that so many moons must compensate in some measure for the diminution of sunlight. But as the moons are illuminated by this very sunlight, their brilliancy is reduced in the same ratio, and in Saturn's case their total light in no wise makes up for this loss.
Reckoning from the best estimates of their sizes, we find that the total area on the sky covered by the moons when full is about two and a half times the area of our own moon, but their illumination, could they all be full at once, would be only the fortieth part of what we are accustomed to at the full. Then, again, as all of them except Japetus, the outer one, lie in the plane of the equator, it is evident that at the equinoxes, when this plane passes through the sun, they will all suffer total eclipse at the full, and will continue thus until the increasing inclination of the axis toward the sun brings their orbits one by one outside the shadow at this point. Thus we see that this numerous retinue does not amount to so much, after all, in the matter of illumination.
One other feature, and one which would doubtless be noticed first of all were any of us suddenly transferred to another planet, would be our change in weight due to the change in surface gravity. If we take the dimensions of Saturn as revealed by the telescope to represent its true size, we should find much less difference than one would expect, considering the tremendous size of the planet. The combination of three factors—the much greater distance of the surface from the center of the planet, which is the center of the attraction we call gravity; the much greater "lightness" of the materials composing the planet; and the great centrifugal or "throwing-off" force at the equator, due to the rapid rotation, and which would, of course, counteract to some extent the downward pull of gravity—results in making but a slight increase, so that a man weighing one hundred and fifty pounds on the earth would weigh only about six pounds more at Saturn's equator. At the poles, however, the change is more marked, since there is no centrifugal force, and the polar flattening, due to the rapid rotation and consequent bulging at the equator, brings one nearer the center of the planet. In this case the increase would be about thirty-six pounds, and would probably be found somewhat uncomfortable to us.
However, it is by no means certain that the dimensions seen through the telescope are the right ones to consider in this manner. If all we have ever seen of the planet is the outer side of its cloud envelope, it may be that the true surface, provided there is one at all, is far beneath the tops of these rolling cloud masses; and if there is no real surface yet—if the terrible struggle of fire and water for the mastery is still in full sway—no one can tell just what the size of the globe may be when the crust finally forms and the real planetary life begins. This "distended mass of liquid fire" may have shrunk perceptibly by that time.
This also brings up one other interesting query. The spectroscope has proved that the sun and stars are composed of materials with which we are familiar in our laboratories, and Saturn as well as the other planets must be composed of the same chemical elements, though probably with wide variations in combination and distribution. If, then, Saturn were to approach the earth in the density of its composition when it reaches a corresponding stage in its planetary growth, it must shrink to one eighth its present volume, or one half its present diameter. On the other hand, if its size remains anywhere near the present dimensions, we shall almost be forced to the conclusion that this great globe may eventually become one vast ocean—a dreary expanse of water with perhaps only a relatively small solid center, thousands of miles below the surface.
But whatever its future, it will always remain a most interesting object of study, and no one can consider it thoroughly without being inclined to agree with Richard Proctor, that here certainly must be a world "altogether more important in the scheme of creation than the globe on which we live."