The projections of light on Mars can be explained much more simply and reasonably. Various suggestions have been made about them; among others, that they are masses of cloud reflecting the sunshine; that they are areas of snow; and that they are the summits of mountains crowned with ice and encircled with clouds. In fact, a huge mountain mass lying on the terminator, or the line between day and night, would produce the effect of a tongue of light projecting into the darkness without assuming that it was snow-covered or capped with clouds, as any one may convince himself by studying the moon with a telescope when the terminator lies across some of its most mountainous regions. To be sure, there is reason to think that the surface of Mars is remarkably flat; yet even so the planet may have some mountains, and on a globe the greater part of whose shell is smooth any projections would be conspicuous, particularly where the sunlight fell at a low angle across them.
Another form in which the suggestion of interplanetary communication has been urged is plainly an outgrowth of the invention and surprising developments of wireless telegraphy. The human mind is so constituted that whenever it obtains any new glimpse into the arcana of nature it immediately imagines an indefinite and all but unlimited extension of its view in that direction. So to many it has not appeared unreasonable to assume that, since it is possible to transmit electric impulses for considerable distances over the earth's surface by the simple propagation of a series of waves, or undulations, without connecting wires, it may also be possible to send such impulses through the ether from planet to planet.
The fact that the electric undulations employed in wireless telegraphy pass between stations connected by the crust of the earth itself, and immersed in a common atmospheric envelope, is not deemed by the supporters of the theory in question as a very serious objection, for, they contend, electric waves are a phenomenon of the ether, which extends throughout space, and, given sufficient energy, such waves could cross the gap between world and world.
But nobody has shown how much energy would be needed for such a purpose, and much less has anybody indicated a way in which the required energy could be artificially developed, or cunningly filched from the stores of nature. It is, then, purely an assumption, an interesting figment of the mind, that certain curious disturbances in the electrical state of the air and the earth, affecting delicate electric instruments, possessing a marked periodicity in brief intervals of time, and not yet otherwise accounted for, are due to the throbbing, in the all-enveloping ether, of impulses transmitted from instruments controlled by the savants of Mars, whose insatiable thirst for knowledge, and presumably burning desire to learn whether there is not within reach some more fortunate world than their half-dried-up globe, has led them into a desperate attempt to "call up" the earth on their interplanetary telephone, with the hope that we are wise and skilful enough to understand and answer them.
In what language they intend to converse no one has yet undertaken to tell, but the suggestion has sapiently been made that, mathematical facts being invariable, the eternal equality of two plus two with four might serve as a basis of understanding, and that a statement of that truth sent by electric taps across the ocean of ether would be a convincing assurance that the inhabitants of the planet from which the message came at least enjoyed the advantages of a common-school education.
But, while speculation upon this subject rests on unverified, and at present unverifiable, assumptions, of course everybody would rejoice if such a thing were possible, for consider what zest and charm would be added to human life if messages, even of the simplest description, could be sent to and received from intelligent beings inhabiting other planets! It is because of this hold that it possesses upon the imagination, and the pleasing pictures that it conjures up, that the idea of interplanetary communication, once broached, has become so popular a topic, even though everybody sees that it should not be taken too seriously.
The subject of the atmosphere of Mars can not be dismissed without further consideration than we have yet given it, because those who think the planet uninhabitable base their opinion largely upon the assumed absence of sufficient air to support life. It was long ago recognized that, other things being equal, a planet of small mass must possess a less dense atmosphere than one of large mass. Assuming that each planet originally drew from a common stock, and that the amount and density of its atmosphere is measured by its force of gravity, it can be shown that Mars should have an atmosphere less than one fifth as dense as the earth's.
Dr. Johnstone Stoney has attacked the problem of planetary atmospheres in another way. Knowing the force of gravity on a planet, it is easy to calculate the velocity with which a body, or a particle, would have to start radially from the planet in order to escape from its gravitational control. For the earth this critical velocity is about seven miles per second; for Mars about three miles per second. Estimating the velocity of the molecules of the various atmospheric gases, according to the kinetic theory, Dr. Stoney finds that some of the smaller planets, and the moon, are gravitationally incapable of retaining all of these gases in the form of an atmosphere. Among the atmospheric constituents that, according to this view, Mars would be unable permanently to retain is water vapor. Indeed, he supposes that even the earth is slowly losing its water by evaporation into space, and on Mars, owing to the slight force of gravity there, this process would go on much more rapidly, so that, in this way, we have a means of accounting for the apparent drying up of that planet, while we may be led to anticipate that at some time in the remote future the earth also will begin to suffer from lack of water, and that eventually the chasms of the sea will yawn empty and desolate under a cloudless sky.
But it is not certain that the original supply of atmospheric elements was in every case proportional to the respective force of gravity of a planet. The fact that Venus appears to have an atmosphere more extensive and denser than the earth's, although its force of gravity is a little less than that of our globe, indicates at once a variation as between these two planets in the amount of atmospheric material at their disposal. This may be a detail depending upon differences in the mode, or in the stage, of their evolution. Thus, after all, Dr. Stoney's theory may be substantially correct and yet Mars may retain sufficient water to form clouds, to be precipitated in snow, and to fill its canals after each annual melting of the polar caps, because the original supply was abundant, and its escape is a gradual process, only to be completed by age-long steps.
Even though the evidence of the spectroscope, as far as it goes, seems to lend support to the theory that there is no water vapor in the atmosphere of Mars, we can not disregard the visual evidence that, nevertheless, water vapor exists there.
What are the polar caps if they are not snow? Frozen carbon dioxide, it has been suggested; but this is hardly satisfactory, for it offers no explanation of the fact that when the polar caps diminish, and in proportion as they diminish, the "seas" and the canals darken and expand, whereas a reasonable explanation of the correlation of these phenomena is offered if we accept the view that the polar caps consist of snow.
Then there are many observations on record indicating the existence of clouds in Mars's atmosphere. Sometimes a considerable area of its surface has been observed to be temporarily obscured, not by dense masses of cloud such as accompany the progress of great cyclonic storms across the continents and oceans of the earth, but by comparatively thin veils of vapor such as would be expected to form in an atmosphere so comparatively rare as that of Mars. And these clouds, in some instances at least, appear, like the cirrus streaks and dapples in our own air, to float at a great elevation. Mr. Douglass, one of Mr. Lowell's associates in the observations of 1894 at Flagstaff, Arizona, observed what he believed to be a cloud over the unilluminated part of Mars's disk, which, by micrometric measurement and estimate, was drifting at an elevation of about fifteen miles above the surface of the planet. This was seen on two successive days, November 25th and November 26th, and it underwent curious fluctuations in visibility, besides moving in a northerly direction at the rate of some thirteen miles an hour. But, upon the whole, as Mr. Lowell remarks, the atmosphere of Mars is remarkably free of clouds.
The reader will remember that Mars gets a little less than half as much heat from the sun as the earth gets. This fact also has been used as an argument against the habitability of the planet. In truth, those who think that life in the solar system is confined to the earth alone insist upon an almost exact reproduction of terrestrial conditions as a sine qua non to the habitability of any other planet. Venus, they think, is too hot, and Mars too cold, as if life were rather a happy accident than the result of the operation of general laws applicable under a wide variety of conditions. All that we are really justified in asserting is that Venus may be too hot and Mars too cold for us. Of course, if we adopt the opinion held by some that the temperature on Mars is constantly so low that water would remain perpetually frozen, it does throw the question of the kind of life that could be maintained there into the realm of pure conjecture.
The argument in favor of an extremely low temperature on Mars is based on the law of the diminution of radiant energy inversely as the square of the distance, together with the assumption that no qualifying circumstances, or no modification of that law, can enter into the problem. According to this view, it could be shown that the temperature on Mars never rises above -200° F. But it is a view that seems to be directly opposed to the evidence of the telescope, for all who have studied Mars under favorable conditions of observation have been impressed by the rapid and extensive changes that the appearance of its surface undergoes coincidently with the variation of the planet's seasons. It has its winter aspect and its summer aspect, perfectly distinct and recognizable, in each hemisphere by turns, and whether the polar caps be snow or carbon dioxide, at any rate they melt and disappear under a high sun, thus proving that an accumulation of heat takes place.
Professor Young says: "As to the temperature of Mars we have no certain knowledge. On the one hand, we know that on account of the planet's distance from the sun the intensity of solar radiation upon its surface must be less than here in the ratio of 1 to (1.524)^2—i.e., only about 43 per cent as great as with us; its 'solar constant' must be less than 13 calories against our 30. Then, too, the low density of its atmosphere, probably less at the planet's surface than on the tops of our highest mountains, would naturally assist to keep down the temperature to a point far below the freezing-point of water. But, on the other hand, things certainly look as if the polar caps were really masses of snow and ice deposited from vapor in the planet's atmosphere, and as if these actually melted during the Martian summer, sending floods of water through the channels provided for them, and causing the growth of vegetation along their banks. We are driven, therefore, to suppose either that the planet has sources of heat internal or external which are not yet explained, or else, as long ago suggested, that the polar 'snow' may possibly be composed of something else than frozen water."[4]
Even while granting the worst that can be said for the low temperature of Mars, the persistent believer in its habitability could take refuge in the results of recent experiments which have proved that bacterial life is able to resist the utmost degree of cold that can be applied, microscopic organisms perfectly retaining their vitality—or at least their power to resume it—when subjected to the fearfully low temperature of liquid air. But then he would be open to the reply that the organisms thus treated are in a torpid condition and deprived of all activity until revived by the application of heat; and the picture of a world in a state of perpetual sleep is not particularly attractive, unless the fortunate prince who is destined to awake the slumbering beauty can also be introduced into the romance.[5]
To an extent which most of us, perhaps, do not fully appreciate, we are indebted for many of the pleasures and conveniences and some of the necessities of life on our planet to its faithful attendant, the moon. Neither Mercury nor Venus has a moon, but Mars has two moons. This statement, standing alone, might lead to the conclusion that, as far as the advantages a satellite can afford to the inhabitants of its master planet are concerned, the people of Mars are doubly fortunate. So they would be, perhaps, if Mars's moons were bodies comparable in size with our moon, but in fact they are hardly more than a pair of very entertaining astronomical toys. The larger of the two, Phobos, is believed to be about seven miles in diameter; the smaller, Deimos, only five or six miles. Their dimensions thus resemble those of the more minute of the asteroids, and the suggestion has even been made that they may be captured asteroids which have fallen under the gravitational control of Mars.
The diameters just mentioned are Professor Pickering's estimates, based on the amount of light the little satellites reflect, for they are much too small to present measurable disks. Deimos is 14,600 miles from the center of Mars and 12,500 miles from its surface. Phobos is 5,800 miles from the center of the planet and only 3,700 from the surface. Deimos completes a revolution about the planet in thirty hours and eighteen minutes, and Phobos in the astonishingly short period—although, of course, it is in strict accord with the law of gravitation and in that sense not astonishing—of seven hours and thirty-nine minutes.
Since Mars takes twenty-four hours and thirty-seven minutes for one rotation on its axis, it is evident that Phobos goes round the planet three times in the course of a single Martian day and night, rising, contrary to the general motion of the heavens, in the west, running in a few hours through all the phases that our moon exhibits in the course of a month, and setting, where the sun and all the stars rise, in the east. Deimos, on the other hand, has a period of revolution five or six hours longer than that of the planet's axial rotation, so that it rises, like the other heavenly bodies, in the east; but, because its motion is so nearly equal, in angular velocity, to that of Mars's rotation, it shifts very slowly through the sky toward the west, and for two or three successive days and nights it remains above the horizon, the sun overtaking and passing it again and again, while, in the meantime, its protean face swiftly changes from full circle to half-moon, from half-moon to crescent, from crescent back to half, and from half to full, and so on without ceasing.
And during this time Phobos is rushing through the sky in the opposite direction, as if in defiance of the fundamental law of celestial revolution, making a complete circuit three times every twenty-four hours, and changing the shape of its disk four times as rapidly as Deimos does! Truly, if we were suddenly transported to Mars, we might well believe that we had arrived in the mother world of lunatics, and that its two moons were bewitched. Yet it must not be supposed that all the peculiarities just mentioned would be clearly seen from the surface of Mars by eyes like ours. The phases of Phobos would probably be discernible to the naked eye, but those of Deimos would require a telescope in order to be seen, for, notwithstanding their nearness to the planet, Mars's moons are inconspicuous phenomena even to the Martians themselves. Professor Young's estimate is that Phobos may shed upon Mars one-sixtieth and Deimos one-twelve-hundredth as much reflected moonlight as our moon sends to the earth. Accordingly, a "moonlit night" on Mars can have no such charm as we associate with the phrase. But it is surely a tribute to the power and perfection of our telescopes that we have been able to discover the existence of objects so minute and inconspicuous, situated at a distance of many millions of miles, and half concealed by the glaring light of the planet close around which they revolve.
If Mars's moons were as massive as our moon is they would raise tremendous tides upon Mars, and would affect the circulation of water in the canals, but, in fact, their tidal effects are even more insignificant than their light-giving powers. But for astronomers on Mars they would be objects of absorbing interest.
Upon quitting Mars we pass to the second distinctive planetary group of the solar system, that of the asteroids.
Beyond Mars, in the broad gap separating the terrestrial from the Jovian planets, are the asteroids, of which nearly five hundred have been discovered and designated by individual names or numbers. But any statement concerning the known number of asteroids can remain valid for but a short time, because new ones are continually found, especially by the aid of photography. Very few of the asteroids are of measurable size. Among these are the four that were the first to be discovered—Ceres, Pallas, Juno, and Vesta. Their diameters, according to the measurements of Prof. E.E. Barnard, of the Yerkes Observatory, are as follows: Ceres, 477 miles; Pallas, 304 miles; Juno, 120 miles; Vesta, 239 miles.
It is only necessary to mention these diameters in order to indicate how wide is the difference between the asteroids and such planets as the earth, Venus, or Mars. The entire surface of the largest asteroid, Ceres, does not equal the republic of Mexico in area. But Ceres itself is gigantic in comparison with the vast majority of the asteroids, many of which, it is believed, do not exceed twenty miles in diameter, while there may be hundreds or thousands of others still smaller—ten miles, five miles, or perhaps only a few rods, in diameter!
Curiously enough, the asteroid which appears brightest, and which it would naturally be inferred is the largest, really stands third in the order of measured size. This is Vesta, whose diameter, according to Barnard, is only 239 miles. It is estimated that the surface of Vesta possesses about four times greater light-reflecting power than the surface of Ceres. Some observations have also shown a variation in the intensity of the light from Vesta, a most interesting fact, which becomes still more significant when considered in connection with the great variability of another most extraordinary member of the asteroidal family, Eros, which is to be described presently.
The orbits of the asteroids are scattered over a zone about 200,000,000 miles broad. The mean distance from the sun of the nearest asteroid, Eros, is 135,000,000 miles, and that of the most distant, Thule, 400,000,000 miles. Wide gaps exist in the asteroidal zone where few or no members of the group are to be found, and Prof. Daniel Kirkwood long ago demonstrated the influence of Jupiter in producing these gaps. Almost no asteroids, as he showed, revolve at such a distance from the sun that their periods of revolution are exactly commensurable with that of Jupiter. Originally there may have been many thus situated, but the attraction of the great planet has, in the course of time, swept those zones clean.
Many of the asteroids have very eccentric orbits, and their orbits are curiously intermixed, varying widely among themselves, both in ellipticity and in inclination to the common plane of the solar system.
Considered with reference to the shape and position of its orbit, the most unique of these little worlds is Eros, which was discovered in 1898 by De Witt, at Berlin, and which, on account of its occasional near approach to the earth, has lately been utilized in a fresh attempt to obtain a closer approximation to the true distance of the sun from the earth. The mean distance of Eros from the sun is 135,000,000 miles, its greatest distance is 166,000,000 miles, and its least distance 105,000,000 miles. It will thus be seen that, although all the other asteroids are situated beyond Mars, Eros, at its mean distance, is nearer to the sun than Mars is. When in aphelion, or at its greatest distance, Eros is outside of the orbit of Mars, but when in perihelion it is so much inside of Mars's orbit that it comes surprisingly near the earth.
Indeed, there are times when Eros is nearer to the earth than any other celestial body ever gets except the moon—and, it might be added, except meteors and, by chance, a comet, or a comet's tail. Its least possible distance from the earth is less than 14,000,000 miles, and it was nearly as close as that, without anybody knowing or suspecting the fact, in 1894, four years in advance of its discovery. Yet the fact, strange as the statement may seem, had been recorded without being recognized. After De Witt's discovery of Eros in 1898, at a time when it was by no means as near the earth as it had been some years before, Prof. E.C. Pickering ascertained that it had several times imprinted its image on the photographic plates of the Harvard Observatory, with which pictures of the sky are systematically taken, but had remained unnoticed, or had been taken for an ordinary star among the thousands of star images surrounding it. From these telltale plates it was ascertained that in 1894 it had been in perihelion very near the earth, and had shone with the brilliance of a seventh-magnitude star.
It will, unfortunately, be a long time before Eros comes quite as near us as it did on that occasion, when we failed to see it, for its close approaches to the earth are not frequent. Prof. Solon I. Bailey selects the oppositions of Eros in 1931 and 1938 as probably the most favorable that will occur during the first half of the twentieth century.
We turn to the extraordinary fluctuations in the light of Eros, and the equally extraordinary conclusions drawn from them. While the little asteroid, whose diameter is estimated to be in the neighborhood of twenty or twenty-five miles, was being assiduously watched and photographed during its opposition in the winter of 1900-1901, several observers discovered that its light was variable to the extent of more than a whole magnitude; some said as much as two magnitudes. When it is remembered that an increase of one stellar magnitude means an accession of light in the ratio of 2.5 to 1, and an increase of two magnitudes an accession of 6.25 to 1, the significance of such variations as Eros exhibited becomes immediately apparent. The shortness of the period within which the cycle of changes occurred, about two hours and a half, made the variation more noticeable, and at the same time suggested a ready explanation, viz., that the asteroid was rapidly turning on its axis, a thing, in itself, quite in accordance with the behavior of other celestial bodies and naturally to be expected.
But careful observation showed that there were marked irregularities in the light fluctuations, indicating that Eros either had a very strange distribution of light and dark areas covering its surface, or that instead of being a globular body it was of some extremely irregular shape, so that as it rotated it presented successively larger and smaller reflecting surfaces toward the sun and the earth. One interesting suggestion was that the little planet is in reality double, the two components revolving around their common center of gravity, like a close binary star, and mutually eclipsing one another. But this theory seems hardly competent to explain the very great fluctuation in light, and a better one, probably, is that suggested by Prof. E.C. Pickering, that Eros is shaped something like a dumb-bell.
We can picture such a mass, in imagination, tumbling end over end in its orbit so as to present at one moment the broad sides of both bells, together with their connecting neck, toward the sun, and, at the same time, toward the observer on the earth, and, at another moment, only the end of one of the bells, the other bell and the neck being concealed in shadow. In this way the successive gain and loss of sixfold in the amount of light might be accounted for. Owing to the great distance the real form of the asteroid is imperceptible even with powerful telescopes, but the effect of a change in the amount of reflecting surface presented produces, necessarily, an alternate waxing and waning of the light. As far as the fluctuations are concerned, they might also be explained by supposing that the shape of the asteroid is that of a flat disk, rotating about one of its larger diameters so as to present, alternately, its edge and its broadside to the sun. And, perhaps, in order completely to account for all the observed eccentricities of the light of Eros, the irregularity of form may have to be supplemented by certain assumptions as to the varying reflective capacity of different parts of the misshapen mass.
The invaluable Harvard photographs show that long before Eros was recognized as an asteroid its light variations had been automatically registered on the plates. Some of the plates, Prof. E.C. Pickering says, had had an exposure of an hour or more, and, owing to its motion, Eros had formed a trail on each of these plates, which in some cases showed distinct variations in brightness. Differences in the amount of variation at different times will largely depend upon the position of the earth with respect to the axis of rotation.
Another interesting deduction may be made from the changes that the light of Eros undergoes. We have already remarked that one of the larger asteroids, and the one which appears to the eye as the most brilliant of all, Vesta, has been suspected of variability, but not so extensive as that of Eros. Olbers, at the beginning of the last century, was of the opinion that Vesta's variations were due to its being not a globe but an angular mass. So he was led by a similar phenomenon to precisely the same opinion about Vesta that has lately been put forth concerning Eros. The importance of this coincidence is that it tends to revive a remarkable theory of the origin of the asteroids which has long been in abeyance, and, in the minds of many, perhaps discredited.
This theory, which is due to Olbers, begins with the startling assumption that a planet, perhaps as large as Mars, formerly revolving in an orbit situated between the orbits of Mars and Jupiter, was destroyed by an explosion! Although, at first glance, such a catastrophe may appear too wildly improbable for belief, yet it was not the improbability of a world's blowing up that led to a temporary abandonment of Olbers's bold theory. The great French mathematician Lagrange investigated the explosive force "which would be necessary to detach a fragment of matter from a planet revolving at a given distance from the sun," and published the results in the Connaissance des Temps for 1814.
"Applying his results to the earth, Lagrange found that if the velocity of the detached fragment exceeded that of a cannon ball in the proportion of 121 to 1 the fragment would become a comet with a direct motion; but if the velocity rose in the proportion of 156 to 1 the motion of the comet would be retrograde. If the velocity was less than in either of these cases the fragment would revolve as a planet in an elliptic orbit. For any other planet besides the earth the velocity of explosion corresponding to the different cases would vary in the inverse ratio of the square root of the mean distance. It would therefore manifestly be less as the planet was more distant from the sun. In the case of each of the four smaller planets (only the four asteroids, Ceres, Pallas, Juno, and Vesta, were known at that time), the velocity of explosion indicated by their observed motion would be less than twenty times the velocity of a cannon ball."[6]
Instead, then, of being discredited by its assumption of so strange a catastrophe, Olbers's theory fell into desuetude because of its apparent failure to account for the position of the orbits of many of the asteroids after a large number of those bodies had been discovered. He calculated that the orbits of all the fragments of his exploded planet would have nearly equal mean distances, and a common point of intersection in the heavens, through which every fragment of the original mass would necessarily pass in each revolution. At first the orbits of the asteroids discovered seemed to answer to these conditions, and Olbers was even able to use his theory as a means of predicting the position of yet undetected asteroids. Only Ceres and Pallas had been discovered when he put forth his theory, but when Juno and Vesta were found they fell in with his predictions so well that the theory was generally regarded as being virtually established; while the fluctuations in the light of Vesta, as we have before remarked, led Olbers to assert that that body was of a fragmental shape, thus strongly supporting his explosion hypothesis.
Afterward, when the orbits of many asteroids had been investigated, the soundness of Olbers's theory began to be questioned. The fact that the orbits did not all intersect at a common point could easily be disposed of, as Professor Newcomb has pointed out, by simply placing the date of the explosion sufficiently far back, say millions of years ago, for the secular changes produced by the attraction of the larger planets would effectively mix up the orbits. But when the actual effects of these secular changes were calculated for particular asteroids the result seemed to show that "the orbits could never have intersected unless some of them have in the meantime been altered by the attraction of the small planets on each other. Such an action is not impossible, but it is impossible to determine it, owing to the great number of these bodies and our ignorance of their masses."[7]
Yet the theory has never been entirely thrown out, and now that the discovery of the light fluctuations of Eros lends support to Olbers's assertion of the irregular shape of some of the asteroids, it is very interesting to recall what so high an authority as Professor Young said on the subject before the discovery of Eros:
"It is true, as has often been urged, that this theory in its original form, as presented by Olbers, can not be correct. No single explosion of a planet could give rise to the present assemblage of orbits, nor is it possible that even the perturbations of Jupiter could have converted a set of orbits originally all crossing at one point (the point of explosion) into the present tangle. The smaller orbits are so small that, however turned about, they lie wholly inside the larger and can not be made to intersect them. If, however, we admit a series of explosions, this difficulty is removed; and if we grant an explosion at all, there seems to be nothing improbable in the hypothesis that the fragments formed by the bursting of the parent mass would carry away within themselves the same forces and reactions which caused the original bursting, so that they themselves would be likely enough to explode at some time in their later history."[8]
The rival theory of the origin of the asteroids is that which assumes that the planetary ring originally left off from the contracting solar nebula between the orbits of Mars and Jupiter was so violently perturbed by the attraction of the latter planet that, instead of being shaped into a single globe, it was broken up into many fragments. Either hypothesis presents an attractive picture; but that which presupposes the bursting asunder of a large planet, which might at least have borne the germs of life, and the subsequent shattering of its parts into smaller fragments, like the secondary explosions of the pieces of a pyrotechnic bomb, certainly is by far the more impressive in its appeal to the imagination, and would seem to offer excellent material for some of the extra-terrestrial romances now so popular. It is a startling thought that a world can possibly carry within itself, like a dynamite cartridge, the means of its own disruption; but the idea does not appear so extremely improbable when we recall the evidence of collisions or explosions, happening on a tremendous scale, in the case of new or temporary stars.[9]
Coming to the question of life upon the asteroids, it seems clear that they must be excluded from the list of habitable worlds, whatever we may choose to think of the possible habitability of the original planet through whose destruction they may have come into existence. The largest of them possesses a force of gravity far too slight to enable it to retain any of the gases or vapors that are recognized as constituting an atmosphere. But they afford a captivating field for speculation, which need not be altogether avoided, for it offers some graphic illustrations of the law of gravitation. A few years ago I wrote, for the entertainment of an audience which preferred to meet science attired in a garb woven largely from the strands of fancy, an account of some of the peculiarities of such minute globes as the asteroids, which I reproduce here because it gives, perhaps, a livelier picture of those little bodies, from the point of view of ordinary human interest, than could be presented in any other way.
One night as I was waiting, watch in hand, for an occultation, and striving hard to keep awake, for it had been a hot and exhausting summer's day, while my wife—we were then in our honeymoon—sat sympathetically by my side, I suddenly found myself withdrawn from the telescope, and standing in a place that appeared entirely strange. It was a very smooth bit of ground, and, to my surprise, there was no horizon in sight; that is to say, the surface of the ground disappeared on all sides at a short distance off, and beyond nothing but sky was visible. I thought I must be on the top of a stupendous mountain, and yet I was puzzled to understand how the face of the earth could be so far withdrawn. Presently I became aware that there was some one by me whom I could not see.
"You are not on a mountain," my companion said, and as he spoke a cold shiver ran along my back-bone; "you are on an asteroid, one of those miniature planets, as you astronomers call them, and of which you have discovered several hundred revolving between the orbits of Mars and Jupiter. This is the little globe that you have glimpsed occasionally with your telescope, and that you, or some of your fellows, have been kind enough to name Menippe."
Then I perceived that my companion, whose address had hardly been reassuring, was a gigantic inhabitant of the little planet, towering up to a height of three quarters of a mile. For a moment I was highly amused, standing by his foot, which swelled up like a hill, and straining my neck backward to get a look up along the precipice of his leg, which, curiously enough, I observed was clothed in rough homespun, the woolly knots of the cloth appearing of tremendous size, while it bagged at the knee like any terrestrial trousers' leg. His great head and face I could see far above me, as it were, in the clouds. Yet I was not at all astonished.
"This is all right," I said to myself. "Of course on Menippe the people must be as large as this, for the little planet is only a dozen miles in diameter, and the force of gravity is consequently so small that a man without loss of activity, or inconvenience, can grow three quarters of a mile tall."
Suddenly an idea occurred to me. "Just to think what a jump I can make! Why, only the other day I was figuring it out that a man could easily jump a thousand feet high from the surface of Menippe, and now here I actually am on Menippe. I'll jump."
The sensation of that glorious rise skyward was delightful beyond expression. My legs seemed to have become as powerful as the engines of a transatlantic liner, and with one spring I rose smoothly and swiftly, and as straight as an arrow, surmounting the giant's foot, passing his knee and attaining nearly to the level of his hip. Then I felt that the momentum of my leap was exhausted, and despite my efforts I slowly turned head downward, glancing in affright at the ground a quarter of a mile below me, on which I expected to be dashed to pieces. But a moment's thought convinced me that I should get no hurt, for with so slight a force of gravity it would be more like floating than falling. Just then the Menippean caught me with his monstrous hand and lifted me to the level of his face.
"I should like to know," I said, "how you manage to live up here; you are so large and your planet is so little."
"Now, you are altogether too inquisitive," replied the giant. "You go!"
He stooped down, placed me on the toe of his boot, and drew back his foot to kick me off.
It flashed into my mind that my situation had now become very serious. I knew well what the effects of the small attractive force of these diminutive planets must be, for I had often amused myself with calculations about them. In this moment of peril I did not forget my mathematics. It was clear that if the giant propelled me with sufficient velocity I should be shot into space, never to return. How great would that velocity have to be? My mind worked like lightning on this problem. The diameter of Menippe I knew did not exceed twelve miles. Its mean density, as near as I could judge, was about the same as that of the earth. Its attraction must therefore be as its radius, or nearly 660 times less than that of the earth. A well-known formula enables us to compute the velocity a body would acquire in falling from an infinite distance to the earth or any other planet whose size and force of gravity are known. The same formula, taken in the opposite sense, of course, shows how fast a body must start from a planet in order that it may be freed from its control. The formula is V = √2gr., in which "g" is the acceleration of gravity, equal for the earth to 32 feet in a second, and "r" is the radius of the attracting body. On Menippe I knew "g" must equal about one twentieth of a foot, and "r" 31,680 feet. Like a flash I applied the formula while the giant's muscles were yet tightening for the kick: 31,680 × 1/20 × 2 = 3,168, the square root of which is a fraction more than 56. Fifty-six feet in a second, then, was the critical velocity with which I must be kicked off in order that I might never return. I perceived at once that the giant would be able to accomplish it. I turned and shouted up at him:
"Hold on, I have something to say to you!"
I dimly saw his mountainous face puckered into mighty wrinkles, out of which his eyes glared fiercely, and the next moment I was sailing into space. I could no more have kept a balance than the earth can stand still upon its axis. I had become a small planet myself, and, like all planets, I rotated. Yet the motion did not dizzy me, and soon I became intensely interested in the panorama of creation that was spread around me. For some time, whenever my face was turned toward the little globe of Menippe, I saw the giant, partly in profile against the sky, with his back bent and his hands upon his knees, watching me with an occasional approving nod of his big head. He looked so funny standing there on his little seven-by-nine world, like a clown on a performing ball, that, despite my terrible situation, I shook my sides with laughter. There was no echo in the profundity of empty space.
Soon Menippe dwindled to a point, and I saw her inhospitable inhabitant no more. Then I watched the sun and the blazing firmament around, for there was at the same time broad day and midnight for me. The sunlight, being no longer diffused by an atmosphere, did not conceal the face of the sky, and I could see the stars shining close to the orb of day. I recognized the various planets much more easily than I had been accustomed to do, and, with a twinge at my heart, saw the earth traveling along in its distant orbit, splendid in the sunshine. I thought of my wife sitting alone by the telescope in the darkness and silence, wondering what had become of me. I asked myself, "How in the world can I ever get back there again?" Then I smiled to think of the ridiculous figure I cut, out here in space, exposed to the eyes of the universe, a rotating, gyrating, circumambulating astronomer, an animated teetotum lost in the sky. I saw no reason to hope that I should not go on thus forever, revolving around the sun until my bones, whitening among the stars, might be revealed to the superlative powers of some future telescope, and become a subject of absorbing interest, the topic of many a learned paper for the astronomers of a future age. Afterward I was comforted by the reflection that in airless space, although I might die and my body become desiccated, yet there could be no real decay; even my garments would probably last forever. The savants, after all, should never speculate on my bones.
I saw the ruddy disk of Mars, and the glinting of his icy poles, as the beautiful planet rolled far below me. "If I could only get there," I thought, "I should know what those canals of Schiaparelli are, and even if I could never return to the earth, I should doubtless meet with a warm welcome among the Martians. What a lion I should be!" I looked longingly at the distant planet, the outlines of whose continents and seas appeared most enticing, but when I tried to propel myself in that direction I only kicked against nothingness. I groaned in desperation.
Suddenly something darted by me flying sunward; then another and another. In a minute I was surrounded by strange projectiles. Every instant I expected to be dashed in pieces by them. They sped with the velocity of lightning. Hundreds, thousands of them were all about me. My chance of not being hit was not one in a million, and yet I escaped. The sweat of terror was upon me, but I did not lose my head. "A comet has met me," I said. "These missiles are the meteoric stones of which it is composed." And now I noticed that as they rushed along collisions took place, and flashes of electricity darted from one to another. A pale luminosity dimmed the stars. I did not doubt that, as seen from the earth, the comet was already flinging the splendors of its train upon the bosom of the night.
While I was wondering at my immunity amid such a rain of death-threatening bolts, I became aware that their velocity was sensibly diminishing. This fact I explained by supposing that I was drawn along with them. Notwithstanding the absence of any collision with my body, the overpowering attraction of the whole mass of meteors was overcoming my tangential force and bearing me in their direction. At first I rejoiced at this circumstance, for at any rate the comet would save me from the dreadful fate of becoming an asteroid. A little further reflection, however, showed me that I had gone from the frying-pan into the fire. The direction of my expulsion from Menippe had been such that I had fallen into an orbit that would have carried me around the sun without passing very close to the solar body. Now, being swept along by the comet, whose perihelion probably lay in the immediate neighborhood of the sun, I saw no way of escape from the frightful fate of being broiled alive. Even where I was, the untempered rays of the sun scorched me, and I knew that within two or three hundred thousand miles of the solar surface the heat must be sufficient to melt the hardest rocks. I was aware that experiments with burning-glasses had sufficiently demonstrated that fact.
But perforce I resigned myself to my fate. At any rate it would the sooner be all over. In fact, I almost forgot my awful situation in the interest awakened by the phenomena of the comet. I was in the midst of its very head. I was one of its component particles. I was a meteor among a million millions of others. If I could only get back to the earth, what news could I not carry to Signor Schiaparelli and Mr. Lockyer and Dr. Bredichin about the composition of comets! But, alas! the world could never know what I now saw. Nobody on yonder gleaming earth, watching the magnificent advance of this "specter of the skies," would ever dream that there was a lost astronomer in its blazing head. I should be burned and rent to pieces amid the terrors of its perihelion passage, and my fragments would be strewn along the comet's orbit, to become, in course of time, particles in a swarm of aerolites. Perchance, through the effects of some unforeseen perturbation, the earth might encounter that swarm. Thus only could I ever return to the bosom of my mother planet. I took a positive pleasure in imagining that one of my calcined bones might eventually flash for a moment, a falling star, in the atmosphere of the earth, leaving its atoms to slowly settle through the air, until finally they rested in the soil from which they had sprung.
From such reflections I was aroused by the approach of the crisis. The head of the comet had become an exceedingly uncomfortable place. The collisions among the meteors were constantly increasing in number and violence. How I escaped destruction I could not comprehend, but in fact I was unconscious of danger from that source. I had become in spirit an actual component of the clashing, roaring mass. Tremendous sparks of electricity, veritable lightning strokes, darted about me in every direction, but I bore a charmed life. As the comet drew in nearer to the sun, under the terrible stress of the solar attraction, the meteors seemed to crowd closer, crashing and grinding together, while the whole mass swayed and shrieked with the uproar of a million tormented devils. The heat had become terrific. I saw stone and iron melted like snow and dissipated in steam. Stupendous jets of white-hot vapor shot upward, and, driven off by the electrical repulsion of the sun, streamed backward into the tail.
Suddenly I myself became sensible of the awful heat. It seemed without warning to have penetrated my vitals. With a yell I jerked my feet from a boiling rock and flung my arms despairingly over my head.
"You had better be careful," said my wife, "or you'll knock over the telescope."
I rubbed my eyes, shook myself, and rose.
"I must have been dreaming," I said.
"I should think it was a very lively dream," she replied.
I responded after the manner of a young man newly wed.
At this moment the occultation began.
When we are thinking of worlds, and trying to exalt the imagination with them, it is well to turn to Jupiter, for there is a planet worth pondering upon! A world thirteen hundred times as voluminous as the earth is a phenomenon calculated to make us feel somewhat as the inhabitant of a rural village does when his amazed vision ranges across the million roofs of a metropolis. Jupiter is the first of the outer and greater planets, the major, or Jovian, group. His mean diameter is 86,500 miles, and his average girth more than 270,000 miles. An inhabitant of Jupiter, in making a trip around his planet, along any great circle of the sphere, would have to travel more than 30,000 miles farther than the distance between the earth and the moon. The polar compression of Jupiter, owing to his rapid rotation, amounts in the aggregate to more than 5,000 miles, the equatorial diameter being 88,200 miles and the polar diameter 83,000 miles.
Jupiter's mean distance from the sun is 483,000,000 miles, and the eccentricity of his orbit is sufficient to make this distance variable to the extent of 21,000,000 miles; but, in view of his great average distance, the consequent variation in the amount of solar light and heat received by the planet is not of serious importance.
When he is in opposition to the sun as seen from the earth Jupiter's mean distance from us is about 390,000,000 miles. His year, or period of revolution about the sun, is somewhat less than twelve of our years (11.86 years). His axis is very nearly upright to the plane of his orbit, so that, as upon Venus, there is practically no variation of seasons. Gigantic though he is in dimensions, Jupiter is the swiftest of all the planets in axial rotation. While the earth requires twenty-four hours to make a complete turn, Jupiter takes less than ten hours (nine hours fifty-five minutes), and a point on his equator moves, in consequence of axial rotation, between 27,000 and 28,000 miles in an hour.
The density of the mighty planet is slight, only about one quarter of the mean density of the earth and virtually the same as that of the sun. This fact at once calls attention to a contrast between Jupiter and our globe that is even more significant than their immense difference in size. The force of gravity upon Jupiter's surface is more than two and a half times greater than upon the earth's surface (more accurately 2.65 times), so that a hundred-pound weight removed from the planet on which we live to Jupiter would there weigh 265 pounds, and an average man, similarly transported, would be oppressed with a weight of at least 400 pounds. But, as a result of the rapid rotation of the great planet, and the ellipticity of its figure, the unfortunate visitor could find a perceptible relief from his troublesome weight by seeking the planet's equator, where the centrifugal tendency would remove about twenty pounds from every one hundred as compared with his weight at the poles.
If we could go to the moon, or to Mercury, Venus, or Mars, we may be certain that upon reaching any of those globes we should find ourselves upon a solid surface, probably composed of rock not unlike the rocky crust of the earth; but with Jupiter the case would evidently be very different. As already remarked, the mean density of that planet is only one quarter of the earth's density, or only one third greater than the density of water. Consequently the visitor, in attempting to set foot upon Jupiter, might find no solid supporting surface, but would be in a situation as embarrassing as that of Milton's Satan when he undertook to cross the domain of Chaos: