As to the gemination or duplication of many of the lines which, at the beginning of the season, appear single, it may be suggested that, in the course of the development of the vast irrigation system of the planet parallel bands of cultivation have been established, one receiving its water supply from the canals of the other, and consequently lagging a little behind in visibility as the water slowly percolates through the soil and awakens the vegetation. Or else, the character of the vegetation itself may differ as between two such parallel bands, one being supplied with plants that spring up and mature quickly when the soil about their roots is moistened, while the plants in the twin band respond more slowly to stimulation.
Objection has been made to the theory of the artificial origin of the canals of Mars on the ground, already mentioned, that the work required to construct them would be beyond the capacity of any race of creatures resembling man. The reply that has been made to this is twofold. In the first place, it should be remembered that the theory, as Mr. Lowell presents it, does not assert that the visible lines are the actual canals, but only that they are strips of territory intersected, like Holland or the center of the plain of Lombardy, by innumerable irrigation canals and ditches. To construct such works is clearly not an impossible undertaking, although it does imply great industry and concentration of effort.
In the second place, since the force of gravity on Mars is in the ratio of only 38 to 100 compared with the earth's, it is evident that the diminished weight of all bodies to be handled would give the inhabitants of Mars an advantage over those of the earth in the performance of manual labor, provided that they possess physical strength and activity as great as ours. But, in consequence of this very fact of the slighter force of gravity, a man upon Mars could attain a much greater size, and consequently much greater muscular strength, than his fellows upon the earth possess without being oppressed by his own weight. In other words, as far as the force of gravity may be considered as the decisive factor, Mars could be inhabited by giants fifteen feet tall, who would be relatively just as active, and just as little impeded in their movements by the weight of their bodies, as a six-footer is upon the earth. But they would possess far more physical strength than we do, while, in doing work, they would have much lighter materials to deal with.
Whether the theory that the canals of Mars really are canals is true or not, at any rate there can now be no doubt as to the existence of the strange lines which bear that designation. The suggestion has been offered that their builders may no longer be in existence, Mars having already passed the point in its history where life must cease upon its surface. This brings us to consider again the statement, made near the beginning of this chapter, that Mars is, perhaps, at a more advanced stage of development than the earth. If we accept this view, then, provided there was originally some resemblance between Mars's life forms and those of the earth, the inhabitants of that planet would, at every step, probably be in front of their terrestrial rivals, so that at the present time they should stand well in advance. Mr. Lowell has, perhaps, put this view of the relative advancement in evolution of Mars and its inhabitants as picturesquely as anybody.
"In Mars," he says, "we have before us the spectacle of a world relatively well on in years, a world much older than the earth. To so much about his age Mars bears witness on his face. He shows unmistakable signs of being old. Advancing planetary years have left their mark legible there. His continents are all smoothed down; his oceans have all dried up.... Mars being thus old himself, we know that evolution on his surface must be similarly advanced. This only informs us of its condition relative to the planet's capabilities. Of its actual state our data are not definite enough to furnish much deduction. But from the fact that our own development has been comparatively a recent thing, and that a long time would be needed to bring even Mars to his present geological condition, we may judge any life he may support to be not only relatively, but really older than our own. From the little we can see such appears to be the case. The evidence of handicraft, if such it be, points to a highly intelligent mind behind it. Irrigation, unscientifically conducted, would not give us such truly wonderful mathematical fitness in the several parts to the whole as we there behold.... Quite possibly such Martian folk are possessed of inventions of which we have not dreamed, and with them electrophones and kinetoscopes are things of a bygone past, preserved with veneration in museums as relics of the clumsy contrivances of the simple childhood of the race. Certainly what we see hints at the existence of beings who are in advance of, not behind us, in the journey of life."[3]
[Footnote 3: Mars, by Percival Lowell, p. 207 _et seq._]
Granted the existence of such a race as is thus described, and to them it might not seem a too appalling enterprise, when their planet had become decrepit, with its atmosphere thinned out and its supply of water depleted, to grapple with the destroying hand of nature and to prolong the career of their world by feats of chemistry and engineering as yet beyond the compass of human knowledge.
It is confidence, bred from considerations like these, in the superhuman powers of the supposed inhabitants of Mars that has led to the popular idea that they are trying to communicate by signals with the earth.
Certain enigmatical spots of light, seen at the edge of the illuminated disk of Mars, and projecting into the unilluminated part--for Mars, although an outer planet, shows at particular times a gibbous phase resembling that of the moon just before or just after the period of full moon--have been interpreted by some, but without any scientific evidence, as of artificial origin.
Upon the assumption that these bright points, and others occasionally seen elsewhere on the planet's disk, are intended by the Martians for signals to the earth, entertaining calculations have been made as to the quantity of light that would be required in the form of a "flash signal"
to be visible across the distance separating the two planets. The results of the calculations have hardly been encouraging to possible investors in interplanetary telegraphy, since it appears that heliographic mirrors with reflecting surfaces measured by square miles, instead of square inches, would be required to send a visible beam from the earth to Mars or _vice versa_.
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]
[Footnote 4: General Astronomy, by Charles A. Young. Revised edition, 1898, p. 363.]
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]
[Footnote 5: Many of the present difficulties about temperatures on the various planets would be beautifully disposed of if we could accept the theory urged by Mr. Cope Whitehouse, to the effect that the sun is not really a hot body at all, and that what we call solar light and heat are only local manifestations produced in our atmosphere by the transformation of some other form of energy transmitted from the sun; very much as the electric impulses carried by a wire from the transmitting to the receiving station on a telephone line are translated by the receiver into waves of sound. According to this theory, which is here mentioned only as an ingenuity and because something of the kind so frequently turns up in one form or another in popular semi-scientific literature, the amount of heat and light on a planet would depend mainly upon local causes.]
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.
CHAPTER V
THE ASTEROIDS, A FAMILY OF DWARF WORLDS
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.