If the markings were evidently of a permanent nature and attached to the solid shell of the planet, and if they were of sufficient distinctness to be seen in substantially the same form by all observers armed with competent instruments, then whatever conclusion was drawn from their apparent motion as to the period of the planet's rotation would have to be accepted. In the case of Mercury the markings, while not easily seen, appear to be sufficiently distinct to afford confidence in the result of observations based upon them, but Venus's markings have been represented in so many different ways that it seems advisable to await more light before accepting any extraordinary, and in itself improbable, conclusion based upon them.
It should also be added that in 1900 spectroscopic observations by Belopolski at Pulkova gave evidence that Venus really rotates rapidly on her axis, in a period probably approximating to the twenty-four hours of the earth's rotation, thus corroborating the older conclusions.
Belopolski's observation, it may be remarked, was based upon what is known as the Doppler principle, which is employed in measuring the motion of stars in the line of sight, and in other cases of rapidly moving sources of light. According to this principle, when a source of light, either original or reflected, is approaching the observer, the characteristic lines in its spectrum are shifted toward the blue end, and when it is retreating from the observer the lines are shifted toward the red end. Now, in the case of a planet rotating rapidly on its axis, it is clear that if the observer is situated in, or nearly in, the plane of the planet's equator, one edge of its disk will be approaching his eye while the opposite edge is retreating, and the lines in the spectrum of a beam of light from the advancing edge will be shifted toward the blue, while those in the spectrum of the light coming from the retreating edge will be shifted toward the red. And, by carefully noting the amount of the shifting, the velocity of the planet's rotation can be computed. This is what was done by Belopolski in the case of Venus, with the result above noted.
Secondly, the theory that Venus rotates but once in the course of a revolution finds but slight support from the doctrine of tidal friction, as compared with that which it receives when applied to Mercury. The effectiveness of the sun's attraction in slowing down the rotation of a planet through the braking action of the tides raised in the body of the planet while it is yet molten or plastic, varies inversely as the sixth power of the planet's distance. For Mercury this effectiveness is nearly three hundred times as great as it is for the earth, while for Venus it is only seven times as great. While we may admit, then, that Mercury, being relatively close to the sun and subject to an enormous braking action, lost rotation until—as occurred for a similar reason to the moon under the tidal attraction of the earth—it ended by keeping one face always toward its master, we are not prepared to make the same admission in the case of Venus, where the effective force concerned is comparatively so slight.
It should be added, however, that no certain evidence of polar compression in the outline of Venus's disk has ever been obtained, and this fact would favor the theory of a very slow rotation because a plastic globe in swift rotation has its equatorial diameter increased and its polar diameter diminished. If Venus were as much flattened at the poles as the earth is, it would seem that the fact could not escape detection, yet the necessary observations are very difficult, and Venus is so brilliant that her light increases the difficulty, while her transits across the sun, when she can be seen as a round black disk, are very rare phenomena, the latest having occurred in 1874 and 1882, and the next not being due until 2004.
Upon the whole, probably the best method of settling the question of Venus's rotation is the spectroscopic method, and that, as we saw, has already given evidence for the short period.
Even if it were established that Venus keeps always the same face to the sun, it might not be necessary to abandon altogether the belief that she is habitable, although, of course, the obstacles to that belief would be increased. Venus's orbit being so nearly circular, and her orbital motion so nearly invariable, she has but a very slight libration with reference to the sun, and the east and west lunes on her surface, where day and night would alternate once in her year of 225 days, would be so narrow as to be practically negligible.
But, owing to her extensive atmosphere, there would be a very broad band of twilight on Venus, running entirely around the planet at the inner edge of the light hemisphere. What the meteorological conditions within this zone would be is purely a matter of conjecture. As in the case of Mercury, we should expect an interchange of atmospheric currents between the light and dark sides of the planet, the heated air rising under the influence of the unsetting sun in one hemisphere, and being replaced by an indraught of cold air from the other. The twilight band would probably be the scene of atmospheric conflicts and storms, and of immense precipitation, if there were oceans on the light hemisphere to charge the air with moisture.
It has been suggested that ice and snow might be piled in a vast circle of glaciers, belting the planet along the line between perpetual day and night, and that where the sunbeams touched these icy deposits near the edge of the light hemisphere a marvelous spectacle of prismatic hills of crystal would be presented!
It may be remarked that it would be the inhabitants of the dark hemisphere who would enjoy the beautiful scene of the earth and the moon in opposition.
MARS, A WORLD MORE ADVANCED THAN OURS
Mars is the fourth planet in the order of distance from the sun, and the outermost member of the terrestrial group. Its mean distance is 141,500,000 miles, variable, through the eccentricity of its orbit, to the extent of about 13,000,000 miles. It will be observed that this is only a million miles less than the variation in Mercury's distance from the sun, from which, in a previous chapter, were deduced most momentous consequences; but, in the case of Mars, the ratio of the variation to the mean distance is far smaller than with Mercury, so that the effect upon the temperature of the planet is relatively insignificant.
Mars gets a little less than half as much solar light and heat as the earth receives, its situation in this respect being just the opposite to that of Venus. Its period of orbital revolution, or the length of its year, is 687 of our days. The diameter of Mars is 4,200 miles, and its density is 73 per cent of the earth's density. Gravity on its surface is only 38 per cent of terrestrial gravity—i.e., a one hundred-pound weight removed from the earth to Mars would there weigh but thirty-eight pounds. Mars evidently has an atmosphere, the details of which we shall discuss later.
The poles of the planet are inclined from a perpendicular to the plane of its orbit at very nearly the same angle as that of the earth's poles, viz., 24 deg. 50 min. Its rotation on its axis is also effected in almost the same period as the earth's, viz., 24 hours, 37 minutes.
When in opposition to the sun, Mars may be only about 35,000,000 miles from the earth, but its average distance when in that position is more than 48,000,000 miles, and may be more than 60,000,000. These differences arise from the eccentricities of the orbits of the two planets. When on the farther side of the sun—i.e., in conjunction with the sun as seen from the earth—Mars's average distance from us is about 235,000,000 miles. In consequence of these great changes in its distance, Mars is sometimes a very conspicuous object in the sky, and at other times inconspicuous.
The similarity in the inclination of the axis of the two planets results in a close resemblance between the seasons on Mars and on the earth, although, owing to the greater length of its year, Mars's seasons are much longer than ours. Winter and summer visit in succession its northern and southern hemispheres just as occurs on the planet that we inhabit, and the torrid, temperate, and frigid zones on its surface have nearly the same angular width as on the earth. In this respect Mars is the first of the foreign planets we have studied to resemble the earth.
Around each of its poles appears a circular white patch, which visibly expands when winter prevails upon it, and rapidly contracts, sometimes almost completely disappearing, under a summer sun. From the time of Sir William Herschel the almost universal belief among astronomers has been that these gleaming polar patches on Mars are composed of snow and ice, like the similar glacial caps of the earth, and no one can look at them with a telescope and not feel the liveliest interest in the planet to which they belong, for they impart to it an appearance of likeness to our globe which at first glance is all but irresistible.
To watch one of them apparently melting, becoming perceptibly smaller week after week, while the general surface of the corresponding hemisphere of the planet deepens in color, and displays a constantly increasing wealth of details as summer advances across it, is an experience of the most memorable kind, whose effect upon the mind of the observer is indescribable.
Early in the history of the telescope it became known that, in addition to the polar caps, Mars presented a number of distinct surface features, and gradually, as instruments increased in power and observers in skill, charts of the planet were produced showing a surface diversified somewhat in the manner that characterizes the face of the earth, although the permanent forms do not closely resemble those of our planet.
Two principal colors exist on the disk of Mars—dark, bluish gray or greenish gray, characterizing areas which have generally been regarded as seas, and light yellowish red, overspreading broad regions looked upon as continents. It was early observed that if the dark regions really are seas, the proportion of water to land upon Mars is much smaller than upon the earth.
For two especial reasons Mars has generally been regarded as an older or more advanced planet than the earth. The first reason is that, accepting Laplace's theory of the origin of the planetary system from a series of rings left off at the periphery of the contracting solar nebula, Mars must have come into existence earlier than the earth, because, being more distant from the center of the system, the ring from which it was formed would have been separated sooner than the terrestrial ring. The second reason is that Mars being smaller and less massive than the earth has run through its developments a cooling globe more rapidly. The bearing of these things upon the problems of life on Mars will be considered hereafter.
And now, once more, Schiaparelli appears as the discoverer of surprising facts about one of the most interesting worlds of the solar system. During the exceptionally favorable opposition of Mars in 1877, when an American astronomer, Asaph Hall, discovered the planet's two minute satellites, and again during the opposition of 1879, the Italian observer caught sight of an astonishing network of narrow dark lines intersecting the so-called continental regions of the planet and crossing one another in every direction. Schiaparelli did not see the little moons that Hall discovered, and Hall did not perceive the enigmatical lines that Schiaparelli detected. Hall had by far the larger and more powerful telescope; Schiaparelli had much the more steady and favorable atmosphere for astronomical observation. Yet these differences in equipment and circumstances do not clearly explain why each observer should have seen what the other did not.
There may be a partial explanation in the fact that an observer having made a remarkable discovery is naturally inclined to confine his attention to it, to the neglect of other things. But it was soon found that Schiaparelli's lines—to which he gave the name "canals," merely on account of their shape and appearance, and without any intention to define their real nature—were excessively difficult telescopic objects. Eight or nine years elapsed before any other observer corroborated Schiaparelli's observations, and notwithstanding the "sensation" which the discovery of the canals produced they were for many years regarded by the majority of astronomers as an illusion.
But they were no illusion, and in 1881 Schiaparelli added to the astonishment created by his original discovery, and furnished additional grounds for skepticism, by announcing that, at certain times, many of the canals geminated, or became double! He continued his observations at each subsequent opposition, adding to the number of the canals observed, and charting them with classical names upon a detailed map of the planet's surface.
At length in 1886 Perrotin, at Nice, detected many of Schiaparelli's canals, and later they were seen by others. In 1888 Schiaparelli greatly extended his observations, and in 1892 and 1894 some of the canals were studied with the 36-inch telescope of the Lick Observatory, and in the last-named year a very elaborate series of observations upon them was made by Percival Lowell and his associates, Prof. William C. Pickering and Mr. A.E. Douglass, at Flagstaff, Arizona. Mr. Lowell's charts of the planet are the most complete yet produced, containing 184 canals to which separate names have been given, besides more than a hundred other markings also designated by individual appellations.
It should not be inferred from the fact that Schiaparelli's discovery in 1877 excited so much surprise and incredulity that no glimpse of the peculiar canal-like markings on Mars had been obtained earlier than that. At least as long ago as 1864 Mr. Dawes, in England, had seen and sketched half a dozen of the larger canals, or at least the broader parts of them, especially where they connect with the dark regions known as seas, but Dawes did not see them in their full extent, did not recognize their peculiar character, and entirely failed to catch sight of the narrower and more numerous ones which constitute the wonderful network discovered by the Italian astronomer. Schiaparelli found no less than sixty canals during his first series of observations in 1877.
Let us note some of the more striking facts about the canals which Schiaparelli has described. We can not do better than quote his own words:
"There are on this planet, traversing the continents, long dark lines which may be designated as canals, although we do not yet know what they are. These lines run from one to another of the somber spots that are regarded as seas, and form, over the lighter, or continental, regions a well-defined network. Their arrangement appears to be invariable and permanent; at least, as far as I can judge from four and a half years of observation. Nevertheless, their aspect and their degree of visibility are not always the same, and depend upon circumstances which the present state of our knowledge does not yet permit us to explain with certainty. In 1879 a great number were seen which were not visible in 1877, and in 1882 all those which had been seen at former oppositions were found again, together with new ones. Sometimes these canals present themselves in the form of shadowy and vague lines, while on other occasions they are clear and precise, like a trace drawn with a pen. In general they are traced upon the sphere like the lines of great circles; a few show a sensible lateral curvature. They cross one another obliquely, or at right angles. They have a breadth of two degrees, or 120 kilometres [74 miles], and several extend over a length of eighty degrees, or 4,800 kilometres [nearly 3,000 miles]. Their tint is very nearly the same as that of the seas, usually a little lighter. Every canal terminates at both its extremities in a sea, or in another canal; there is not a single example of one coming to an end in the midst of dry land.
"This is not all. In certain seasons these canals become double. This phenomenon seems to appear at a determinate epoch, and to be produced simultaneously over the entire surface of the planet's continents. There was no indication of it in 1877, during the weeks that preceded and followed the summer solstice of that world. A single isolated case presented itself in 1879. On the 26th of December, this year—a little before the spring equinox, which occurred on Mars on the 21st of January, 1880—I noticed the doubling of the Nile [a canal thus named] between the Lakes of the Moon and the Ceraunic Gulf. These two regular, equal, and parallel lines caused me, I confess, a profound surprise, the more so because a few days earlier, on the 23d and the 24th of December, I had carefully observed that very region without discovering anything of the kind.
"I awaited with curiosity the return of the planet in 1881, to see if an analogous phenomenon would present itself in the same place, and I saw the same thing reappear on the 11th of January, 1882, one month after the spring equinox—which occurred on the 8th of December, 1881. The duplication was still more evident at the end of February. On this same date, the 11th of January, another duplication had already taken place, that of the middle portion of the canal of the Cyclops, adjoining Elysium. [Elysium is a part of one of the continental areas.]
"Yet greater was my astonishment when, on the 19th of January, I saw the canal Jamuna, which was then in the center of the disk, formed very rigidly of two parallel straight lines, crossing the space which separates the Niliac Lake from the Gulf of Aurora. At first sight I believed it was an illusion, caused by fatigue of the eye and some new kind of strabismus, but I had to yield to the evidence. After the 19th of January I simply passed from wonder to wonder; successively the Orontes, the Euphrates, the Phison, the Ganges, and the larger part of the other canals, displayed themselves very clearly and indisputably duplicated. There were not less than twenty examples of duplication, of which seventeen were observed in the space of a month, from the 19th of January to the 19th of February.
"In certain cases it was possible to observe precursory symptoms which are not lacking in interest. Thus, on the 13th of January, a light, ill-defined shade extended alongside the Ganges; on the 18th and the 19th one could only distinguish a series of white spots; on the 20th the shadow was still indecisive, but on the 21st the duplication was perfectly clear, such as I observed it until the 23d of February. The duplication of the Euphrates, of the canal of the Titans, and of the Pyriphlegethon also began in an uncertain and nebulous form.
"These duplications are not an optical effect depending on increase of visual power, as happens in the observation of double stars, and it is not the canal itself splitting in two longitudinally. Here is what is seen: To the right or left of a pre-existing line, without any change in the course and position of that line, one sees another line produce itself, equal and parallel to the first, at a distance generally varying from six to twelve degrees—i.e., from 350 to 700 kilometres (217 to 434 miles); even closer ones seem to be produced, but the telescope is not powerful enough to distinguish them with certainty. Their tint appears to be a quite deep reddish brown. The parallelism is sometimes rigorously exact. There is nothing analogous in terrestrial geography. Everything indicates that here there is an organization special to the planet Mars, probably connected with the course of its seasons."
[Footnote 1: L'Astronomie, vol. i, 1882, pp. 217 et seq.]
Schiaparelli adds that he took every precaution to avoid the least suspicion of illusion. "I am absolutely sure," he says, "of what I have observed."
I have quoted his statement, especially about the duplication of the canals, at so much length, both on account of its intrinsic interest and because it has many times been argued that this particular phenomenon must be illusory even though the canals are real.
One of the most significant facts that came out in the early observations was the evident connection between the appearance of the canals and the seasonal changes on Mars. It was about the time of the spring equinox, when the white polar caps had begun to melt, that Schiaparelli first noticed the phenomenon of duplication. As the season advanced the doubling of the canals increased in frequency and the lines became more distinct. In the meantime the polar caps were becoming smaller. Broadly speaking, Schiaparelli's observation showed that the doubling of the canals occurred principally a little after the spring equinox and a little before the autumn equinox; that the phenomenon disappeared in large part at the epoch of the winter solstice, and disappeared altogether at the epoch of the summer solstice. Moreover, he observed that many of the canals, without regard to duplication, were invisible at times, and reappeared gradually; faint, scarcely visible lines and shadows, deepened and became more distinct until they were clearly and sharply defined, and these changes, likewise, were evidently seasonal.
The invariable connection of the canals at their terminations with the regions called seas, the fact that as the polar caps disappeared the sealike expanses surrounding the polar regions deepened in color, and other similar considerations soon led to the suggestion that there existed on Mars a wonderful system of water circulation, whereby the melting of the polar snows, as summer passed alternately from one hemisphere to the other, served to reenforce the supply of water in the seas, and, through the seas, in the canals traversing the broad expanses of dry land that occupy the equatorial regions of the planet. The thought naturally occurred that the canals might be of artificial origin, and might indicate the existence of a gigantic system of irrigation serving to maintain life upon the globe of Mars. The geometrical perfection of the lines, their straightness, their absolute parallelism when doubled, their remarkable tendency to radiate from definite centers, lent strength to the hypothesis of an artificial origin. But their enormous size, length, and number tended to stagger belief in the ability of the inhabitants of any world to achieve a work so stupendous.
After a time a change of view occurred concerning the nature of the expanses called seas, and Mr. Lowell, following his observations of 1894, developed the theory of the water circulation and irrigation of Mars in a new form. He and others observed that occasionally canals were visible cutting straight across some of the greenish, or bluish-gray, areas that had been regarded as seas. This fact suggested that, instead of seas, these dark expanses may rather be areas of marshy ground covered with vegetation which flourishes and dies away according as the supply of water alternately increases and diminishes, while the reddish areas known as continents are barren deserts, intersected by canals; and as the water released by the melting of the polar snows begins to fill the canals, vegetation springs up along their sides and becomes visible in the form of long narrow bands.
According to this theory, the phenomena called canals are simply lines of vegetation, the real canals being individually too small to be detected. It may be supposed that from a central supply canal irrigation ditches are extended for a distance of twenty or thirty miles on each side, thus producing a strip of fertile soil from forty to sixty miles wide, and hundreds, or in some cases two or three thousands, of miles in length.
The water supply being limited, the inhabitants can not undertake to irrigate the entire surface of the thirsty land, and convenience of circulation induces them to extend the irrigated areas in the form of long lines. The surface of Mars, according to Lowell's observation, is remarkably flat and level, so that no serious obstacle exists to the extension of the canal system in straight bands as undeviating as arcs of great circles.
Wherever two or more canals meet, or cross, a rounded dark spot from a hundred miles, or less, to three hundred miles in diameter, is seen. An astonishing number of these appear on Mr. Lowell's charts. Occasionally, as occurs at the singular spot named Lacus Solis, several canals converging from all points of the compass meet at a central point like the spokes of a wheel; in other cases, as, for instance, that of the long canal named Eumenides, with its continuation Orcus, a single conspicuous line is seen threading a large number of round dark spots, which present the appearance of a row of beads upon a string. These circular spots, which some have regarded as lakes, Mr. Lowell believes are rather oases in the great deserts, and granting the correctness of his theory of the canals the aptness of this designation is apparent.
[Footnote 2: The reader can find many of these "canals" and "oases," as well as some of the other regions on Mars that have received names, in the frontispiece.]
Wherever several canals, that is to say, several bands of vegetation or bands of life, meet, it is reasonable to assume that an irrigated and habitable area of considerable extent will be developed, and in such places the imagination may picture the location of the chief centers of population, perhaps in the form of large cities, or perhaps in groups of smaller towns and villages. The so-called Lacus Solis is one of these localities.
So, likewise, it seems but natural that along the course of a broad, well-irrigated band a number of expansions should occur, driving back the bounds of the desert, forming rounded areas of vegetation, and thus affording a footing for population. Wherever two bands cross such areas would be sure to exist, and in almost every instance of crossing the telescope actually shows them.
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."
[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 deg. 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."
[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.
[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.
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.
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."
[Footnote 6: Grant's History of Physical Astronomy, p. 241.]
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."
[Footnote 7: Popular Astronomy, by Simon Newcomb, p. 335.]
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."
[Footnote 8: General Astronomy, by Charles A. Young. Revised edition, 1898, p. 372.]
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.
[Footnote 9: "Since the discovery of Eros, the extraordinary position of its orbit has led to the suggestion that possibly Mars itself, instead of being regarded as primarily a major planet, belonging to the terrestrial group, ought rather to be considered as the greatest of the asteroids, and a part of the original body from which the asteroidal system was formed."—J. Bauschinger, Astronomische Nachrichten, No. 3542.]
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.
A WAIF OF SPACE
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 = square root of (2 gr.), 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 x 1/20 x 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.
JUPITER, THE GREATEST OF KNOWN WORLDS
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:
"Fluttering his pinions vain, plumb down he drops, Ten thousand fathom deep, and to this hour Down had been falling had not, by ill chance, The strong rebuff of some tumultuous cloud. Instinct with fire and niter, hurried him As many miles aloft; that fury stayed, Quenched in a boggy Syrtis, neither sea Nor good dry land, nigh foundered, as he fares, Treading the crude consistence, half on foot, Half flying."
The probability that nothing resembling a solid crust, nor, perhaps, even a liquid shell, would be found at the visible surface of Jupiter, is increased by considering that the surface density must be much less than the mean density of the planet taken as a whole, and since the latter but little exceeds the density of water, it is likely that at the surface everything is in a state resembling that of cloud or smoke. Our imaginary visitor upon reaching Jupiter would, under the influence of the planet's strong force of gravity, drop out of sight, with the speed of a shot, swallowed up in the vast atmosphere of probably hot, and perhaps partially incandescent, gases. When he had sunk—supposing his identity could be preserved—to a depth of thousands of miles he might not yet have found any solid part of the planet; and, perchance, there is no solid nucleus even at the very center.
The cloudy aspect of Jupiter immediately strikes the telescopic observer. The huge planet is filled with color, and with the animation of constant movement, but there is no appearance of markings, like those on Mars, recalling the look of the earth. There are no white polar caps, and no shadings that suggest the outlines of continents and oceans. What every observer, even with the smallest telescope, perceives at once is a pair of strongly defined dark belts, one on either side of, and both parallel to, the planet's equator. These belts are dark compared with the equatorial band between them and with the general surface of the planet toward the north and the south, but they are not of a gray or neutral shade. On the contrary, they show decided, and, at times, brilliant colors, usually of a reddish tone. More delicate tints, sometimes a fine pink, salmon, or even light green, are occasionally to be seen about the equatorial zone, and the borders of the belts, while near the poles the surface is shadowed with bluish gray, imperceptibly deepening from the lighter hues of the equator.