Curiosities of the Sky Part 5

The great velocity ascribed to the supposed meteorite at the moment of striking could be accounted for by the fact that it probably plunged nearly vertically downward, for it formed a circular crater in the rocky crust of the earth. In that case it would have been less r.e.t.a.r.ded by the resistance of the atmosphere than are meteorites which enter the air at a lower angle and shoot ahead hundreds of miles until friction has nearly destroyed their original motion when they drop upon the earth. Some meteoric ma.s.ses of great size, such as Peary's iron meteorite found at Cape York, Greenland, and the almost equally large ma.s.s discovered at Bacubirito, Mexico, appear to have penetrated but slightly on striking the earth. This may be explained by supposing that they pursued a long, horizontal course through the air before falling. The result would be that, their original velocity having been practically destroyed, they would drop to the ground with a velocity nearly corresponding to that which gravity would impart within the perpendicular distance of their final fall. A six-hundred-and-sixty-pound meteorite, which fell at Knyahinya, Hungary, striking at an angle of 27 from the vertical, penetrated the ground to a depth of eleven feet.

It has been remarked that the c.o.o.n b.u.t.te meteorite may have fallen not longer ago than a few thousand years. This is based upon the fact that the geological indications favor the supposition that the event did not occur more than five thousand years ago, while on the other hand the rings of growth in the cedar-trees growing on the slopes of the crater show that they have existed there about seven hundred years.

Prof. William H. Pickering has recently correlated this with an ancient chronicle which states that at Cairo, Egypt, in the year 1029, ''many stars pa.s.sed with a great noise.'' He remarks that Cairo is about 100, by great circle, from c.o.o.n b.u.t.te, so that if the meteorite that made the crater was a member of a flock of similar bodies which encountered the earth moving in parallel lines, some of them might have traversed the sky tangent to the earth's surface at Cairo. That the spectacle spoken of in the chronicle was caused by meteorites he deems exceedingly probable because of what is said about ''a great noise;'' meteorites are the only celestial phenomena attended with perceptible sounds. Professor Pickering conjectures that this supposed flock of great meteorites may have formed the nucleus of a comet which struck the earth, and he finds confirmation of the idea in the fact that out of the ten largest meteorites known, no less than seven were found within nine hundred miles of c.o.o.n b.u.t.te. It would be interesting if we could trace back the history of that comet, and find out what malicious planet caught it up in its innocent wanderings and hurled it with so true an aim at the earth! This remarkable crater is one of the most interesting places in the world, for there is absolutely no record of such a ma.s.s, possibly an iron-headed comet, from outer s.p.a.ce having come into collision with our earth. The results of the future exploration of the depths of the crater will be awaited with much interest.

The Wrecking of the Moon

There are sympathetic moods under whose influence one gazes with a certain poignant tenderness at the worn face of the moon; that little ''fossil world'' (the child of our mother earth, too) bears such terrible scars of its brief convulsive life that a sense of pity is awakened by the sight. The moon is the wonder-land of the telescope.

Those towering mountains, whose ''proud aspiring peaks'' cast silhouettes of shadow that seem drawn with india-ink; those vast plains, enchained with gentle winding hills and bordered with giant ranges; those oval ''oceans,'' where one looks expectant for the flash of wind-whipped waves; those enchanting ''bays'' and recesses at the seaward feet of the Alps; those broad straits pa.s.sing between guardian heights incomparably mightier than Gibraltar; those locket-like valleys as secluded among their mountains as the Vale of Cashmere; those colossal craters that make us smile at the pretensions of Vesuvius, Etna, and Cotopaxi; those strange white ways which pa.s.s with the unconcern of Roman roads across mountain, gorge, and valley -- all these give the beholder an irresistible impression that it is truly a world into which he is looking, a world akin to ours, and yet no more like our world than Pompeii is like Naples. Its air, its waters, its clouds, its life are gone, and only a skeleton remains -- a mute but eloquent witness to a cosmical tragedy without parallel in the range of human knowledge.

One cannot but regret that the moon, if it ever was the seat of intelligent life, has not remained so until our time. Think what the consequences would have been if this other world at our very door had been found to be both habitable and inhabited! We talk rather airily of communicating with Mars by signals; but Mars never approaches nearer than 35,000,000 miles, while the moon when nearest is only a little more than 220,000 miles away. Given an effective magnifying power of five thousand diameters, which will perhaps be possible at the mountain observatories as telescopes improve, and we should be able to bring the moon within an apparent distance of about forty miles, while the corresponding distance for Mars would be more than seven thousand miles. But even with existing telescopic powers we can see details on the moon no larger than some artificial constructions on the earth. St Peter's at Rome, with the Vatican palace and the great piazza, if existing on the moon, would unquestionably be recognizable as something else than a freak of nature. Large cities, with their radiating lines of communication, would at once betray their real character. Cultivated tracts, and the changes produced by the interference of intelligent beings, would be clearly recognizable.

The electric illumination of a large town at night would probably be markedly visible. Gleams of reflected sunlight would come to us from the surfaces of the lakes and oceans, and a huge ''liner'' traversing a lunar sea could probably be followed by its trail of smoke. As to communications by ''wireless'' signals, which certain enthusiasts have thought of in connection with Mars, in the case of the moon they should be a relatively simple matter, and the feat might actually be accomplished. Think what a literature would grow up about the moon if it were a living world! Its very differences from the earth would only accentuate its interest for us. Night and day on the moon are each two weeks in length; how interesting it would be to watch the manner in which the lunarians dealt with such a situation as that. Lunar and terrestrial history would keep step with each other, and we should record them both. Truly one might well wish to have a neighbor world to study; one would feel so much the less alone in s.p.a.ce.

It is not impossible that the moon did at one time have inhabitants of some kind. But, if so, they vanished with the disappearance of its atmosphere and seas, or with the advent of its cataclysmic age. At the best, its career as a living world must have been brief. If the water and air were gradually absorbed, as some have conjectured, by its cooling interior rocks, its surface might, nevertheless, have retained them for long ages; but if, as others think, their disappearance was due to the escape of their gaseous molecules in consequence of the inability of the relatively small lunar gravitation to retain them, then the final catastrophe must have been as swift as it was inevitable. Accepting Darwin's hypothesis, that the moon was separated from the earth by tidal action while both were yet plastic or nebulous, we may reasonably conclude that it began its career with a good supply of both water and air, but did not possess sufficient ma.s.s to hold them permanently. Yet it may have retained them long enough for life to develop in many forms upon its surface; in fact, there are so many indications that air and water have not always been lacking to the lunar world that we are driven to invent theories to explain both their former presence and their present absence.

But whatever the former condition of the moon may have been, its existing appearance gives it a resistless fascination, and it bears so clearly the story of a vast catastrophe sculptured on its rocky face that the thoughtful observer cannot look upon it without a feeling of awe. The gigantic character of the lunar features impresses the beholder not less than the universality of the play of destructive forces which they attest. Let us make a few comparisons. Take the lunar crater called ''Tycho'', which is a typical example of its kind.

In the telescope Tycho appears as a perfect ring surrounding a circular depression, in the center of which rises a group of mountains. Its superficial resemblance to some terrestrial volcanic craters is very striking. Vesuvius, seen from a point vertically above, would no doubt look something like that (the resemblance would have been greater when the Monte del Cavallo formed a more complete circuit about the crater cone). But compare the dimensions. The remains of the outer crater ring of Vesuvius are perhaps half a mile in diameter, while the active crater itself is only two or three hundred feet across at the most; Tycho has a diameter of fifty-four miles! The group of relatively insignificant peaks in the center of the crater floor of Tycho is far more ma.s.sive than the entire mountain that we call Vesuvius. The largest known volcanic crater on the earth, Aso San, in j.a.pan, has a diameter of seven miles; it would take sixty craters like Aso San to equal Tycho in area! And Tycho, though one of the most perfect, is by no means the largest crater on the moon.

Another, called ''Theophilus,'' has a diameter of sixty-four miles, and is eighteen thousand feet deep. There are hundreds from ten to forty miles in diameter, and thousands from one to ten miles. They are so numerous in many places that they break into one another, like the cells of a crushed honeycomb.

The lunar craters differ from those of the earth more fundamentally than in the matter of mere size; they are not situated on the tops of mountains. If they were, and if all the proportions were the same, a crater like Tycho might crown a conical peak fifty or one hundred miles high! Instead of being cavities in the summits of mountains, the lunar craters are rather gigantic sink-holes whose bottoms in many cases lie two or three miles below the general surface of the lunar world. Around their rims the rocks are piled up to a height of from a few hundred to two or three thousand feet, with a comparatively gentle inclination, but on the inner side they fall away in gigantic broken precipices which make the dizzy cliffs of the Matterhorn seem but ''lover's leaps.'' Down they drop, ridge below ridge, crag under crag, tottering wall beneath wall, until, in a crater named ''Newton,'' near the south lunar pole, they attain a depth where the rays of the sun never reach. Nothing more frightful than the spectacle which many of these terrible chasms present can be pictured by the imagination. As the lazy lunar day slowly advances, the suns.h.i.+ne, unmitigated by clouds or atmospheric veil of any kind, creeps across their rims and begins to descend the opposite walls. Presently it strikes the ragged crest of a ridge which had lain hidden in such darkness as we never know on the earth, and runs along it like a line of kindling fire.

Rocky pinnacles and needles shoot up into the sunlight out of the black depths. Down sinks the line of light, mile after mile, and continually new precipices and cliffs are brought into view, until at last the vast floor is attained and begins to be illuminated. In the meanwhile the sun's rays, darting across the gulf, have touched the summits of the central peaks, twenty or thirty miles from the crater's inmost edge, and they immediately kindle and blaze like huge stars amid the darkness. So profound are some of these awful craters that days pa.s.s before the sun has risen high enough above them to chase the last shadows from their depths.

Although several long ranges of mountains resembling those of the earth exist on the moon, the great majority of its elevations a.s.sume the crateriform aspect. Sometimes, instead of a crater, we find an immense mountain ring whose form and aspect hardly suggest volcanic action. But everywhere the true craters are in evidence, even on the sea-beds, although they attain their greatest number and size on those parts of the moon -- covering sixty per cent of its visible surface -- which are distinctly mountainous in character and which const.i.tute its most brilliant portions. Broadly speaking, the southwestern half of the moon is the most mountainous and broken, and the northeastern half the least so. Right down through the center, from pole to pole, runs a wonderful line of craters and crateriform valleys of a magnitude stupendous even for the moon. Another similar line follows the western edge. Three or four ''seas'' are thrust between these mountainous belts. By the effects of ''libration'' parts of the opposite hemisphere of the moon which is turned away from the earth are from time to time brought into view, and their aspect indicates that that hemisphere resembles in its surface features the one which faces the earth. There are many things about the craters which seem to give some warrant for the hypothesis which has been particularly urged by Mr G.

K. Gilbert, that they were formed by the impact of meteors; but there are also many things which militate against that idea, and, upon the whole, the volcanic theory of their origin is to be preferred.

The enormous size of the lunar volcanoes is not so difficult to account for when we remember how slight is the force of lunar gravity as compared with that of the earth. With equal size and density, bodies on the moon weigh only one-sixth as much as on the earth.

Impelled by the same force, a projectile that would go ten miles on the earth would go sixty miles on the moon. A lunar giant thirty-five feet tall would weigh no more than an ordinary son of Adam weighs on his greater planet. To shoot a body from the earth so that it would not drop back again, we should have to start it with a velocity of seven miles per second; a mile and a half per second would serve on the moon. It is by no means difficult to believe, then, that a lunar volcano might form a crater ring eight or ten times broader than the greatest to be found on the earth, especially when we reflect that in addition to the relatively slight force of gravity, the materials of the lunar crust are probably lighter than those of our terrestrial rocks.

For similar reasons it seems not impossible that the theory mentioned in a former chapter -- that some of the meteorites that have fallen upon the earth originated from the lunar volcanoes -- is well founded.

This would apply especially to the stony meteorites, for it is hardly to be supposed that the moon, at least in its superficial parts, contains much iron. It is surely a scene most strange that is thus presented to the mind's eye -- that little attendant of the earth's (the moon has only one-fiftieth of the volume, and only one-eightieth of the ma.s.s of the earth) firing great stones back at its parent planet! And what can have been the cause of this furious outbreak of volcanic forces on the moon? Evidently it was but a pa.s.sing stage in its history; it had enjoyed more quiet times before. As it cooled down from the plastic state in which it parted from the earth, it became incrusted after the normal manner of a planet, and then oceans were formed, its atmosphere being sufficiently dense to prevent the water from evaporating and the would-be oceans from disappearing continually in mist. This, if any, must have been the period of life in the lunar world. As we look upon the vestiges of that ancient world buried in the wreck that now covers so much of its surface, it is difficult to restrain the imagination from picturing the scenes which were once presented there; and, in such a case, should the imagination be fettered? We give it free rein in terrestrial life, and it rewards us with some of our greatest intellectual pleasures. The wonderful landscapes of the moon offer it an ideal field with just enough half-hidden suggestions of facts to stimulate its powers.

The great plains of the Mare Imbrium and the Mare Serenitatis (the ''Sea of Showers'' and the ''Sea of Serenity''), bordered in part by lofty mountain ranges precisely like terrestrial mountains, scalloped along their sh.o.r.es with beautiful bays curving back into the adjoining highlands, and united by a great strait pa.s.sing between the nearly ab.u.t.ting ends of the ''Lunar Apennines'' and the ''Lunar Caucasus,''

offer the elements of a scene of world beauty such as it would be difficult to match upon our planet. Look at the finely modulated bottom of the ancient sea in Mr Ritchey's exquisite photograph of the western part of the Mare Serenitatis, where one seems to see the play of the watery currents heaping the ocean sands in waving lines, making shallows, bars, and deeps for the mariner to avoid or seek, and affording a playground for the creatures of the main. What geologist would not wish to try his hammer on those rocks with their stony pages of fossilized history? There is in us an instinct which forbids us to think that there was never any life there. If we could visit the moon, there is not among us a person so prosaic and unimaginative that he would not, the very first thing, begin to search for traces of its inhabitants. We would look for them in the deposits on the sea bottoms; we would examine the sh.o.r.es wherever the configuration seemed favorable for harbors and the sites of maritime cities -- forgetting that it may be a little ridiculous to ascribe to the ancient lunarians the same ideas that have governed the development of our race; we would search through the valleys and along the seeming courses of vanished streams; we would explore the mountains, not the terrible craters, but the pinnacled chains that recall our own Alps and Rockies; seeking everywhere some vestige of the transforming presence of intelligent life. Perhaps we should find such traces, and perhaps, with all our searching, we should find nothing to suggest that life had ever existed amid that universal ruin.

Look again at the border of the ''Sea of Serenity'' -- what a name for such a scene! -- and observe how it has been rent with almost inconceivable violence, the wall of the colossal crater Posidonius dropping vertically upon the ancient sh.o.r.e and obliterating it, while its giant neighbor, Le Monnier, opens a yawning mouth as if to swallow the sea itself. A scene like this makes one question whether, after all, those may not be right who have imagined that the so-called sea bottoms are really vast plains of frozen lava which gushed up in floods so extensive that even the mighty volcanoes were half drowned in the fiery sea. This suggestion becomes even stronger when we turn to another of the photographs of Mr Ritchey's wonderful series, showing a part of the Mare Tranquilitatis (''Sea of Tranquility''!).

Notice how near the center of the picture the outline of a huge ring with radiating ridges shows through the sea bottom; a fossil volcano submerged in a petrified ocean! This is by no means the only instance in which a buried world shows itself under the great lunar plains.

Yet, as the newer craters in the sea itself prove, the volcanic activity survived this other catastrophe, or broke out again subsequently, bringing more ruin to pile upon ruin.

Yet notwithstanding the evidence which we have just been considering in support of the hypothesis that the ''seas'' are lava floods, Messrs. Loewy and Puiseux, the selenographers of the Paris Observatory, are convinced that these great plains bear characteristic marks of the former presence of immense bodies of water. In that case we should be forced to conclude that the later oceans of the moon lay upon vast sheets of solidified lava; and thus the catastrophe of the lunar world a.s.sumes a double aspect, the earliest oceans being swallowed up in molten floods issuing from the interior, while the lands were reduced to chaos by a universal eruption of tremendous volcanoes; and then a period of comparative quiet followed, during which new seas were formed, and new life perhaps began to flourish in the lunar world, only to end in another cataclysm, which finally put a term to the existence of the moon as a life-supporting world.

Suppose we examine two more of Mr Ritchey's illuminating photographs, and, first, the one showing the crater Theophilus and its surroundings. We have spoken of Theophilus before, citing the facts that it is sixty-four miles in diameter and eighteen thousand feet deep. It will be noticed that it has two brother giants -- Cyrillus the nearer, and Catharina the more distant; but Theophilus is plainly the youngest of the trio. Centuries, and perhaps thousands of years, must have elapsed between the periods of their upheaval, for the two older craters are partly filled with debris, while it is manifest at a glance that when the south eastern wall of Theophilus was formed, it broke away and destroyed a part of the more ancient ring of Cyrillus.

There is no more tremendous scene on the moon than this; viewed with a powerful telescope, it is absolutely appalling.

The next photograph shows, if possible, a still wilder region. It is the part of the moon lying between Tycho and the south pole. Tycho is seen in the lower left-hand part of the picture. To the right, at the edge of the illuminated portion of the moon, are the crater-rings, Longomonta.n.u.s and Wilhelm I, the former being the larger. Between them are to be seen the ruins of two or three more ancient craters which, together with portions of the walls of Wilhelm I and Longomonta.n.u.s, have been honeycombed with smaller craters. The vast crateriform depression above the center of the picture is Clavius, an unrivaled wonder of lunar scenery, a hundred and forty-two miles in its greatest length, while its whole immense floor has sunk two miles below the general surface of the moon outside the ring. The monstrous shadow-filled cavity above Clavius toward the right is Blanca.n.u.s, whose aspect here gives a good idea of the appearance of these chasms when only their rims are in the sunlight. But observe the indescribable savagery of the entire scene. It looks as though the spirit of destruction had gone mad in this spot. The mighty craters have broken forth one after another, each rending its predecessor; and when their work was finished, a minor but yet tremendous outbreak occurred, and the face of the moon was gored and punctured with thousands of smaller craters. These relatively small craters (small, however, only in a lunar sense, for many of them would appear gigantic on the earth) recall once more the theory of meteoric impact. It does not seem impossible that some of them may have been formed by such an agency.

One would not wish for our planet such a fate as that which has overtaken the moon, but we cannot be absolutely sure that something of the kind may not be in store for it. We really know nothing of the ultimate causes of volcanic activity, and some have suggested that the internal energies of the earth may be acc.u.mulating instead of dying out, and may never yet have exhibited their utmost destructive power.

Perhaps the best a.s.surance that we can find that the earth will escape the catastrophe that has overtaken its satellite is to be found in the relatively great force of its gravitation. The moon has been the victim of its weakness; given equal forces, and the earth would be the better able to withstand them. It is significant, in connection with these considerations, that the little planet Mercury, which seems also to have parted with its air and water, shows to the telescope some indications that it is pitted with craters resembling those that have torn to pieces the face of the moon.

Upon the whole, after studying the dreadful lunar landscapes, one cannot feel a very enthusiastic sympathy with those who are seeking indications of the continued existence of some kind of life on the moon; such a world is better without inhabitants. It has met its fate; let it go! Fortunately, it is not so near that it cannot hide its scars and appear beautiful -- except when curiosity impels us to look with the penetrating eyes of the astronomer.

The Great Mars Problem

Let any thoughtful person who is acquainted with the general facts of astronomy look up at the heavens some night when they appear in their greatest splendor, and ask himself what is the strongest impression that they make upon his mind. He may not find it easy to frame an answer, but when he has succeeded it will probably be to the effect that the stars give him an impression of the universality of intelligence; they make him feel, as the sun and the moon cannot do, that his world is not alone; that all this was not made simply to form a gorgeous canopy over the tents of men. If he is of a devout turn of mind, he thinks, as he gazes into those fathomless deeps and among those bewildering hosts, of the infinite mult.i.tude of created beings that the Almighty has taken under his care. The narrow ideas of the old geocentric theology, which made the earth G.o.d's especial footstool, and man his only rational creature, fall away from him like a veil that had obscured his vision; they are impossible in the presence of what he sees above. Thus the natural tendency, in the light of modern progress, is to regard the universe as everywhere filled with life.

But science, which is responsible for this broadening of men's thoughts concerning the universality of life, itself proceeds to set limits. Of spiritual existences it pretends to know nothing, but as to physical beings, it declares that it can only entertain the supposition of their existence where it finds evidence of an environment suited to their needs, and such environment may not everywhere exist. Science, though repelled by the antiquated theological conception of the supreme isolation of man among created beings, regards with complacency the probability that there are regions in the universe where no organic life exists, stars which s.h.i.+ne upon no inhabited worlds, and planets which nourish no animate creatures. The astronomical view of the universe is that it consists of matter in every stage of evolution: some nebulous and chaotic; some just condensing into stars (suns) of every magnitude and order; some shaped into finished solar bodies surrounded by dependent planets; some forming stars that perhaps have no planets, and will have none; some const.i.tuting suns that are already aging, and will soon lose their radiant energy and disappear; and some aggregated into ma.s.ses that long ago became inert, cold, and rayless, and that can only be revivified by means about which we can form conjectures, but of which we actually know nothing.

As with the stars, so with the planets, which are the satellites of stars. All investigations unite to tell us that the planets are not all in the same state of development. As some are large and some small, so some are, in an evolutionary sense, young, and some old. As they depend upon the suns around which they revolve for their light, heat, and other forms of radiant energy, so their condition varies with their distance from those suns. Many may never arrive at a state suitable for the maintenance of life upon their surfaces; some which are not at present in such a state may attain it later; and the forms of life themselves may vary with the peculiar environment that different planets afford. Thus we see that we are not scientifically justified in affirming that life is ubiquitous, although we are thus justified in saying that it must be, in a general sense, universal. We might liken the universe to a garden known to contain every variety of plant. If on entering it we see no flowers, we examine the species before us and find that they are not of those which bloom at this particular season, or perhaps they are such as never bear flowers. Yet we feel no doubt that we shall find flowers somewhere in the garden, because there are species which bloom at this season, and the garden contains all varieties.

While it is tacitly a.s.sumed that there are planets revolving around other stars than the sun, it would be impossible for us to see them with any telescope yet invented, and no instrument now in the possession of astronomers could a.s.sure us of their existence; so the only planetary system of which we have visual knowledge is our own.

Excluding the asteroids, which could not from any point of view be considered as habitable, we have in the solar system eight planets of various sizes and situated at various distances from the sun. Of these eight we know that one, the earth, is inhabited. The question, then, arises: Are there any of the others which are inhabited or habitable?

Since it is our intention to discuss the habitability of only one of the seven to which the question applies, the rest may be dismissed in a few words. The smallest of them, and the nearest to the sun, is Mercury, which is regarded as uninhabitable because it has no perceptible supply of water and air, and because, owing to the extraordinary eccentricity of its...o...b..t, it is subjected to excessive and very rapid alterations in the amount of solar heat and light poured upon its surface, such alterations being inconsistent with the supposition that it can support living beings. Even its average temperature is more than six and a half times that prevailing on the earth! Another circ.u.mstance which militates against its habitability is that, according to the results of the best telescopic studies, it always keeps the same face toward the sun, so that one half of the planet is perpetually exposed to the fierce solar rays, and the other half faces the unmitigated cold of open s.p.a.ce. Venus, the next in distance from the sun, is almost the exact twin of the earth in size, and many arguments may be urged in favor of its habitability, although it is suspected of possessing the same peculiarity as Mercury, in always keeping the same side sunward. Unfortunately its atmosphere appears to be so dense that no permanent markings on its surface are certainly visible, and the question of its actual condition must, for the present, be left in abeyance. Mars, the first planet more distant from the sun than the earth, is the special subject of this chapter, and will be described and discussed a few lines further on. Jupiter, Saturn, Ura.n.u.s, and Neptune, the four giant planets, all more distant than Mars, and each more distant than the other in the order named, are all regarded as uninhabitable because none of them appears to possess any degree of solidity. They may have solid or liquid nuclei, but exteriorly they seem to be mere b.a.l.l.s of cloud. Of course, one can imagine what he pleases about the existence of creatures suited to the physical const.i.tution of such planets as these, but they must be excluded from the category of habitable worlds in the ordinary sense of the term. We go back, then, to Mars.

It will be best to begin with a description of the planet. Mars is 4230 miles in diameter; its surface is not much more than one-quarter as extensive as that of the earth (.285). Its mean distance from the sun is 141,500,000 miles, 48,500,000 miles greater than that of the earth. Since radiant energy varies inversely as the square of distance, Mars receives less than half as much solar light and heat as the earth gets. Mars' year (period of revolution round the sun) is 687 days. Its mean density is 71 per cent of the earth's, and the force of gravity on its surface is 38 per cent of that on the surface of the earth; i.e., a body weighing one hundred pounds on the earth would, if transported to Mars, weigh but thirty-eight pounds. The inclination of its equator to the plane of its...o...b..t differs very little from that of the earth's equator, and its axial rotation occupies 24 hours 37 minutes. so that the length of day and night, and the extent of the seasonal changes on Mars, are almost precisely the same as on the earth. But owing to the greater length of its year, the seasons of Mars, while occurring in the same order, are almost twice as long as ours. The surface of the planet is manifestly solid, like that of our globe, and the telescope reveals many permanent markings on it, recalling the appearance of a globe on which geographical features have been represented in reddish and dusky tints. Around the poles are plainly to be seen rounded white areas, which vary in extent with the Martian seasons, nearly vanis.h.i.+ng in summer and extending widely in winter. The most recent spectroscopic determinations indicate that Mars has an atmosphere perhaps as dense as that to be found on our loftiest mountain peaks, and there is a perceptible amount of watery vapor in this atmosphere. The surface of the planet appears to be remarkably level, and it has no mountain ranges. No evidences of volcanic action have been discovered on Mars. The dusky and reddish areas were regarded by the early observers as respectively seas and lands, but at present it is not believed that there are any bodies of water on the planet. There has never been much doubt expressed that the white areas about the poles represent snow.

It will be seen from this brief description that many remarkable resemblances exist between Mars and the earth, and there is nothing wonderful in the fact that the question of the habitability of the former has become one of extreme and wide-spread interest, giving rise to the most diverse views, to many extraordinary speculations, and sometimes to regrettably heated controversy. The first champion of the habitability of Mars was Sir William Herschel, although even before his time the idea had been suggested. He was convinced by the revelations of his telescopes, continually increasing in power, that Mars was more like the earth than any other planet. He could not resist the testimony of the polar snows, whose suggestive conduct was in such striking accord with what occurs upon the earth. Gradually, as telescopes improved and observers increased in number, the princ.i.p.al features of the planet were disclosed and charted, and ''areography,''

as the geography of Mars was called, took its place among the recognized branches of astronomical study. But it was not before 1877 that a fundamentally new discovery in areography gave a truly sensational turn to speculation about life on ''the red planet.'' In that year Mars made one of its nearest approaches to the earth, and was so situated in its...o...b..t that it could be observed to great advantage from the northern hemisphere of the earth. The celebrated Italian astronomer, Schiaparelli, took advantage of this opportunity to make a trigonometrical survey of the surface of Mars -- as coolly and confidently as if he were not taking his sights across a thirty-five-million-mile gulf of empty s.p.a.ce -- and in the course of this survey he was astonished to perceive that the reddish areas, then called continents, were crossed in many directions by narrow, dusky lines, to which he gave the suggestive name of ''ca.n.a.ls.'' Thus a kind of firebrand was cast into the field of astronomical speculation, which has ever since produced disputes that have sometimes approached the violence of political faction. At first the accuracy of Schiaparelli's observations was contested; it required a powerful telescope, and the most excellent ''seeing,'' to render the enigmatical lines visible at all, and many searchers were unable to detect them. But Schiaparelli continued his studies in the serene sky of Italy, and produced charts of the gridironed face of Mars containing so much astonis.h.i.+ng detail that one had either to reject them in toto or to confess that Schiaparelli was right. As subsequent favorable oppositions of Mars occurred, other observers began to see the ''ca.n.a.ls'' and to confirm the substantial accuracy of the Italian astronomer's work, and finally few were found who would venture to affirm that the ''ca.n.a.ls'' did not exist, whatever their meaning might be.

When Schiaparelli began his observations it was generally believed, as we have said, that the dusky areas on Mars were seas, and since Schiaparelli thought that the ''ca.n.a.ls'' invariably began and ended at the sh.o.r.es of the ''seas,'' the appropriateness of the t.i.tle given to the lines seemed apparent. Their artificial character was immediately a.s.sumed by many, because they were too straight and too suggestively geometrical in their arrangement to permit the conclusion that they were natural watercourses. A most surprising circ.u.mstance noted by Schiaparelli was that the ''ca.n.a.ls'' made their appearance after the melting of the polar snow in the corresponding hemisphere had begun, and that they grew darker, longer, and more numerous in proportion as the polar liquidation proceeded; another very puzzling observation was that many of them became double as the season advanced; close beside an already existing ''ca.n.a.l,'' and in perfect parallelism with it, another would gradually make its appearance. That these phenomena actually existed and were not illusions was proved by later observations, and today they are seen whenever Mars is favorably situated for observation.

In the closing decade of the nineteenth century, Mr Percival Lowell took up the work where Schiaparelli had virtually dropped it, and soon added a great number of ''ca.n.a.ls'' to those previously known, so that in his charts the surface of the wonderful little planet appears covered as with a spider's web, the dusky lines criss-crossing in every direction, with conspicuous knots wherever a number of them come together. Mr Lowell has demonstrated that the areas originally called seas, and thus named on the earlier charts, are not bodies of water, whatever else they may be. He has also found that the mysterious lines do not, as Schiaparelli supposed, begin and end at the edges of the dusky regions, but often continue on across them, reaching in some cases far up into the polar regions. But Schiaparelli was right in his observation that the appearance of the ''ca.n.a.ls'' is synchronous with the gradual disappearance of the polar snows, and this fact has become the basis of the most extraordinary theory that the subject of life in other worlds has ever given birth to.

Now, the effect of such discoveries, as we have related, depends upon the type of mind to whose attention they are called. Many are content to accept them as strange and inexplicable at present, and to wait for further light upon them; others insist upon an immediate inquiry concerning their probable nature and meaning. Such an inquiry can only be based upon inference proceeding from a.n.a.logy. Mars, say Mr Lowell and those who are of his opinion, is manifestly a solidly incrusted planet like the earth; it has an atmosphere, though one of great rarity; it has water vapor, as the snows in themselves prove; it has the alternation of day and night, and a succession of seasons closely resembling those of the earth; its surface is suggestively divided into regions of contrasting colors and appearance, and upon that surface we see an immense number of lines geometrically arranged, with a system of symmetrical intersections where the lines expand into circular and oval areas -- and all connected with the annual melting of the polar snows in a way which irresistibly suggests the interference of intelligence directed to a definite end. Why, with so many concurrent circ.u.mstances to support the hypothesis, should we not regard Mars as an inhabited globe?

But the differences between Mars and the earth are in many ways as striking as their resemblances. Mars is relatively small; it gets less than half as much light and heat as we receive; its atmosphere is so rare that it would be distressing to us, even if we could survive in it at all; it has no lakes, rivers, or seas; its surface is an endless prairie. and its ''ca.n.a.ls'' are phenomena utterly unlike anything on the earth. Yet it is precisely upon these divergences between the earth and Mars, this repudiation of terrestrial standards, that the theory of ''life on Mars,'' for which Mr Lowell is mainly responsible, is based. Because Mars is smaller than the earth, we are told it must necessarily be more advanced in planetary evolution, the underlying cause of which is the gradual cooling and contraction of the planet's ma.s.s. Mars has parted with its internal heat more rapidly than the earth; consequently its waters and its atmosphere have been mostly withdrawn by chemical combinations, but enough of both yet remain to render life still possible on its surface. As the globe of Mars is evolutionally older than that of the earth, so its forms of organic life may be proportionally further advanced, and its inhabitants may have attained a degree of cultivated intelligence much superior to what at present exists upon the earth. Understanding the nature and the causes of the desiccation of their planet, and possessing engineering science and capabilities far in advance of ours, they may be conceived to have grappled with the stupendous problem of keeping their world in a habitable condition as long as possible. Supposing them to have become accustomed to live in their rarefied atmosphere (a thing not inconceivable, since men can live for a time at least in air hardly less rare), the most pressing problem for them is that of a water-supply, without which plant life cannot exist, while animal life in turn depends for its existence upon vegetation. The only direction in which they can seek water is that of the polar regions, where it is alternately condensed into snow and released in the liquid form by the effect of the seasonal changes. It is, then, to the annual melting of the polar snow-fields that the Martian engineers are supposed to have recourse in supplying the needs of their planet, and thus providing the means of prolonging their own existence. It is imagined that they have for this purpose constructed a stupendous system of irrigation extending over the temperate and equatorial regions of the planet. The ''ca.n.a.ls'' represent the lines of irrigation, but the narrow streaks that we see are not the ca.n.a.ls themselves, but the irrigated bands covered by them. Their dark hue, and their gradual appearance after the polar melting has begun, are due to the growth of vegetation stimulated by the water. The rounded areas visible where several ''ca.n.a.ls'' meet and cross are called by Mr Lowell ''oases.'' These are supposed to be the princ.i.p.al centers of population and industry. It must be confessed that some of them, with their complicated systems of radiating lines, appear to answer very well to such a theory. No attempt to explain them by a.n.a.logy with natural phenomena on the earth has proved successful.

But a great difficulty yet remains: How to explain the seemingly miraculous powers of the supposed engineers? Here recourse is had once more to the relative smallness of the planet. We have remarked that the force of gravity on Mars is only thirty-eight per cent of that on the earth. A steam-shovel driven by a certain horse-power would be nearly three times as effective there as here. A man of our stature on Mars would find his effective strength increased in the same proportion. But just because of the slight force of gravity there, a Martian might attain to the traditional stature of Goliath without finding his own weight an enc.u.mbrance to his activity, while at the same time his huge muscles would come into unimpeded play, enabling him single-handed to perform labors that would be impossible to a whole gang of terrestrial workmen. The effective powers of huge machines would be increased in the same way; and to all this must be added the fact that the mean density of the materials of which Mars is composed is much less than that of the const.i.tuents of the earth.

Combining all these considerations, it becomes much less difficult to conceive that public works might be successfully undertaken on Mars which would be hopelessly beyond the limits of human accomplishment.

Certain other difficulties have also to be met; as, for instance, the relative coldness of the climate of Mars. At its distance it gets considerably less than half as much light and heat as we receive. In addition to this, the rarity of its atmosphere would naturally be expected to decrease the effective temperature at the planet's surface, since an atmosphere acts somewhat like the gla.s.s cover of a hot-house in retaining the solar heat which has penetrated it. It has been calculated that, unless there are mitigating circ.u.mstances of which we know nothing, the average temperature at the surface of Mars must be far below the freezing-point of water. To this it is replied that the possible mitigating circ.u.mstances spoken of evidently exist in fact, because we can see that the watery vapor condenses into snow around the poles in winter, but melts again when summer comes. The mitigating agent may be supposed to exist in the atmosphere where the presence of certain gases would completely alter the temperature gradients.

It might also be objected that it is inconceivable that the Martian engineers, however great may be their physical powers, and however gigantic the mechanical energies under their control, could force water in large quant.i.ties from the poles to the equator. This is an achievement that measures up to the cosmical standard. It is admitted by the champions of the theory that the difficulty is a formidable one; but they call attention to the singular fact that on Mars there can be found no chains of mountains, and it is even doubtful if ranges of hills exist there. The entire surface of the planet appears to be almost ''as smooth as a billiard ball,'' and even the broad regions which were once supposed to be seas apparently lie at practically the same level as the other parts, since the ''ca.n.a.ls'' in many cases run uninterruptedly across them. Lowell's idea is that these sombre areas may be expanses of vegetation covering ground of a more or less marshy character, for while the largest of them appear to be permanent, there are some which vary coincidently with the variations of the ca.n.a.ls.

As to the kind of machinery employed to force the water from the poles, it has been conjectured that it may have taken the form of a gigantic system of pumps and conduits; and since the Martians are a.s.sumed to be so far in advance of us in their mastery of scientific principles, the hypothesis will at least not be harmed by supposing that they have learned to harness forces of nature whose very existence in a manageable form is yet unrecognized on the earth. If we wish to let the imagination loose, we may conjecture that they have conquered the secret of those intra-atomic forces whose resistless energy is beginning to become evident to us, but the possibility of whose utilization remains a dream, the fulfillment of which n.o.body dares to predict.

Such, in very brief form, is the celebrated theory of Mars as an inhabited world. It certainly captivates the imagination, and if we believe it to represent the facts, we cannot but watch with the deepest sympathy this gallant struggle of an intellectual race to preserve its planet from the effects of advancing age and death. We may, indeed, wonder whether our own humanity, confronted by such a calamity, could be counted on to meet the emergency with equal stoutness of heart and inexhaustibleness of resource. Up to the present time we certainly have shown no capacity to confront Nature toe to toe, and to seize her by the shoulders and turn her round when she refuses to go our way. If we could get into wireless telephonic communication with the Martians we might learn from their own lips the secret of their more than ''Roman recovery.''

The Riddle of the Asteroids

Between the orbits of Mars and Jupiter revolves the most remarkable system of little bodies with which we are acquainted -- the Asteroids, or Minor Planets. Some six hundred are now known, and they may actually number thousands. They form virtually a ring about the sun.

The most striking general fact about them is that they occupy the place in the sky which should be occupied, according to Bode's Law, by a single large planet. This fact, as we shall see, has led to the invention of one of the most extraordinary theories in astronomy -- viz., that of the explosion of a world!

Bode's Law, so-called, is only an empiric formula, but until the discovery of Neptune it accorded so well with the distances of the planets that astronomers were disposed to look upon it as really representing some underlying principle of planetary distribution. They were puzzled by the absence of a planet in the s.p.a.ce between Mars and Jupiter, where the ''law'' demanded that there should be one, and an a.s.sociation of astronomers was formed to search for it. There was a decided sensation when, in 1801, Piazzi, of Palermo, announced that he had found a little planet which apparently occupied the place in the system which belonged to the missing body. He named it Ceres, and it was the first of the Asteroids. The next year Olbers, of Bremen, while looking for Ceres with his telescope, stumbled upon another small planet which he named Pallas. Immediately he was inspired with the idea that these two planets were fragments of a larger one which had formerly occupied the vacant place in the planetary ranks, and he predicted that others would be found by searching in the neighborhood of the intersection of the orbits of the two already discovered. This bold prediction was brilliantly fulfilled by the finding of two more -- Juno in 1804, and Vesta in 1807. Olbers would seem to have been led to the invention of his hypothesis of a planetary explosion by the faith which astronomers at that time had in Bode's Law. They appear to have thought that several planets revolving in the gap where the ''law'' called for but one could only be accounted for upon the theory that the original one had been broken up to form the several.

Gravitation demanded that the remnants of a planet blown to pieces, no matter how their orbits might otherwise differ, should all return at stated periods to the point where the explosion had occurred; hence Olbers' prediction that any asteroids that might subsequently be discovered would be found to have a common point of orbital intersection. And curiously enough all of the first asteroids found practically answered to this requirement. Olbers' theory seemed to be established.

After the first four, no more asteroids were found until 1845, when one was discovered; then, in 1847, three more were added to the list; and after that searchers began to pick them up with such rapidity that by the close of the century hundreds were known, and it had become almost impossible to keep track of them. The first four are by far the largest members of the group, but their actual sizes remained unknown until less than twenty years ago. It was long supposed that Vesta was the largest, because it s.h.i.+nes more brightly than any of the others; but finally, in 1895, Barnard, with the Lick telescope, definitely measured their diameters, and proved to everybody's surprise that Ceres is really the chief, and Vesta only the third in rank. His measures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, 239 miles; and Juno, 120 miles. They differ greatly in the reflective power of their surfaces, a fact of much significance in connection with the question of their origin. Vesta is, surface for surface, rather more than three times as brilliant as Ceres, whence the original mistake about its magnitude.

Nowadays new asteroids are found frequently by photography, but physically they are most insignificant bodies, their average diameter probably not exceeding twenty miles, and some are believed not to exceed ten. On a planet only ten miles in diameter, a.s.suming the same mean density as the earth's, which is undoubtedly too much, the force of gravity would be so slight that an average man would not weigh more than three ounces, and could jump off into s.p.a.ce whenever he liked.

Although the asteroids all revolve around the sun in the same direction as that pursued by the major planets, their orbits are inclined at a great variety of angles to the general plane of the planetary system, and some of them are very eccentric -- almost as much so as the orbits of many of the periodic comets. It has even been conjectured that the two tiny moons of Mars and the four smaller satellites of Jupiter may be asteroids gone astray and captured by those planets. Two of the asteroids are exceedingly remarkable for the shapes and positions of their orbits; these are Eros, discovered in 1898, and T. G., 1906, found eight years later. The latter has a mean distance from the sun slightly greater than that of Jupiter, while the mean distance of Eros is less than that of Mars. The orbit of Eros is so eccentric that at times it approaches within 15,000,000 miles of the earth, nearer than any other regular member of the solar system except the moon, thus affording an unrivaled means of measuring the solar parallax. But for our present purpose the chief interest of Eros lies in its extraordinary changes of light.