The Story of the Heavens

observations by Schiaparelli and several other observers seem to indicate that this phenomenon of the duplication of the canals is of a periodic character. It is produced about the times when Mars passes through its equinoxes. One of the two parallel lines is often superposed as exactly as possible upon the track of the old canal. It does, however, sometimes happen that both the lines occupy opposite sides of the former canal and are situated on entirely new ground. The distance between the two lines varies from about 360 miles as a maximum down to the smallest limit distinguishable in our large telescopes, which is something less than thirty miles. The breadth of each of these remarkable channels may range from the limits of visibility, say, up to more than sixty miles. The duplication of the canals is perhaps the most difficult problem which Mars offers to us for solution. Even if we admit that the canals themselves represent inlets or channels through which the melted polar snow makes its way across the equatorial continents, it is not easy to see how the duplicate canals can arise. This is especially true in those cases where the original channel seems to vanish and to be replaced by two quite new canals, each about the breadth of the English Channel, and lying one on each side of the course of the old one. The very obvious explanation that the whole duplication is an optical illusion has been brought forward more than once, but never in a conclusive manner. We must, perhaps, be content to let the solution of this matter rest for the present, in the hope that the extraordinary attention which this planet is now receiving will in due time explain the present enigma. The markings on the surface of this planet are, generally speaking, of a permanent character, so that when we compare drawings made one or two hundred years ago with drawings[Pg 222] made more recently we can recognise in each the same features. This permanence is, however, not nearly so absolute as it is in the case of the moon. In addition to the canals which we have already considered, many other parts of the surface of Mars alter their outlines from time to time. This is particularly the case with those dark spots which we call oceans, the contours of which sometimes undergo modifications in matters of detail which are quite unmistakable. Changes of colour are often observed on parts of the planet, and though some of these observations may perhaps be attributed to the influence of our own atmosphere on the planet's appearance, they cannot be all thus accounted for. Some of the phenomena must certainly be due to actual changes which have taken place on the surface of Mars. As an example of such changes, we may refer to the north-western part of the notable feature, to which Schiaparelli has given the name of Syrtis major.[18] This has at various times been recorded as grey, green, blue, brown, and even violet. When this region (about the time of the autumnal equinox of the northern hemisphere) is situated in the middle of the visible disc, the eastern part is distinctly greener than the western. As the season progresses this characteristic colour gets feebler, until the green tint is to be perceived only on the shores of the Syrtis. The atmosphere of Mars is usually very transparent, and fortunately allows us to scrutinise the surface of the planet without putting obstacles in the way m the shape of Martian clouds. Such clouds, however, are not invariably absent. Our view of the surface is occasionally obstructed in such a manner as to make it certain that clouds or mist in the atmosphere of Mars must be the cause of the trouble. Would we form an idea of the physical constitution of the surface of Mars, then the question as to the character of the atmosphere of the planet is among the first to be considered. Spectroscopic observations do not in this case render[Pg 223] us much assistance. Of course, we know that the planet has no intrinsic light. It merely shines by reflected sunlight. The hemisphere which is turned towards the sun is bright, and the hemisphere which is turned away from the sun is dark. The spectrum ought, therefore, like that of the moon, to be an exact though faint copy of the solar spectrum, unless the sun's rays, by passing twice through the atmosphere of Mars, suffered some absorption which could give rise to additional dark lines. Some of the earlier observers thought that they could distinctly make out some such lines due, as was supposed, to water vapour. The presence of such lines is, however, denied by Mr. Campbell, of the Lick Observatory, and Professor Keeler, at the Allegheny Observatory,[19] who, with their unrivalled opportunities, both instrumental and climatic, could find no difference between the spectra of Mars and the moon. If Mars had an atmosphere of appreciable extent, its absorptive effect should be noticeable, especially at the limb of the planet; but Mr. Campbell's observations do not show any increased absorption at the limb. It would therefore seem that Mars cannot have an extensive atmosphere, and this conclusion is confirmed in several other ways. The distinctness with which we see the surface of this planet tends to show that the atmosphere must be very thin as compared with our own. There can hardly be any doubt that an observer on Mars with a good telescope would be unable to distinguish much of the features of the earth's surface. This would be the case not only by reason of the strong absorption of the light during the double passage through our atmosphere, but also on account of the great diffusion of the light caused by this same atmosphere. Also, it is needless to say, the great amount of cloud generally floating over the earth would totally obscure many parts of our planet from a Martian observer. But though, as already mentioned, we occasionally find parts of Mars rendered indistinct, it must be acknowledged that the clouds on Mars are very slight. We should expect that the polar caps, if composed of snow, would, when melting, produce clouds which[Pg 224] would more or less hide the polar regions from our inspection; yet nothing of the kind has ever been seen. We have seen that there are very grave doubts as to the existence of water on Mars. No doubt we have frequently spoken of the dark markings as "oceans" and of the bright parts as "continents." That this language was just has been the opinion of astronomers for a very long time. A few years ago Mr. Schaeberle, of the Lick Observatory, came to the very opposite conclusion. He contended that the dark parts were the continents and the bright ones were the oceans of water, or some other fluid. He pointed to the irregular shading of the dark parts, which does not suggest the idea of light reflected from a spherical surface of water, especially as the contrasts between light and shade are strongest about the middle of the disc. It is also to be noticed that the dark regions are not infrequently traversed by still darker streaks, which can be traced for hundreds of miles almost in straight lines, while the so-called canals in the bright parts often seem to be continuations of these same lines. Mr. Schaeberle therefore suggests that the canals may be chains of mountains stretching over sea and land! The late Professor Phillips and Mr. H.D. Taylor have pointed out that if there were lakes or seas in the tropical regions of Mars we should frequently see the sun directly reflected from them, thus producing a bright, star-like point which could not escape observation. Even moderately disturbed water would make its presence known in this manner, and yet nothing of the kind has ever been recorded. On the question as to the possibility of life on Mars a few words may be added. If we could be certain of the existence of water on Mars, then one of the fundamental conditions would be fulfilled; and even though the atmosphere on Mars had but few points of resemblance either in composition or in density to the atmosphere of the earth, life might still be possible. Even if we could suppose that a man would find suitable nutriment for his body and suitable air for his respiration, it seems very doubtful whether he would be able to live. Owing to the small size of Mars and[Pg 225] the smallness of its mass in comparison with the earth, the intensity of the gravitation on the neighbouring planet would be different from the attraction on the surface of the earth. We have already alluded to the small gravitation on the moon, and in a lesser degree the same remarks will apply to Mars. A body which weighs on the earth two pounds would on the surface of Mars weigh rather less than one pound. Nearly the same exertion which will raise a 56-lb. weight on the earth would lift two similar weights on Mars. The earth is attended by one moon. Jupiter is attended by four conspicuous moons. Mars is a planet revolving between the orbits of the earth and of Jupiter. It is a body of the same general type as the earth and Jupiter. It is ruled by the same sun, and all three planets form part of the same system; but as the earth has one moon and Jupiter four moons, why should not Mars also have a moon? No doubt Mars is a small body, less even than the earth, and much less than Jupiter. We could not expect Mars to have large moons, but why should it be unlike its two neighbours, and not have any moon at all? So reasoned astronomers, but until modern times no satellite of Mars could be found. For centuries the planet has been diligently examined with this special object, and as failure after failure came to be recorded, the conclusion seemed almost to be justified that the chain of analogical reasoning had broken down. The moonless Mars was thought to be an exception to the rule that all the great planets outside Venus were dignified by an attendant retinue of satellites. It seemed almost hopeless to begin again a research which had often been tried, and had invariably led to disappointment; yet, fortunately, the present generation has witnessed still one more attack, conducted with perfect equipment and with consummate skill This attempt has obtained the success it so well merited, and the result has been the memorable detection of two satellites of Mars. This discovery was made by Professor Asaph Hall, the distinguished astronomer at the observatory of Washington. Mr. Hall was provided with an instrument of colossal [Pg 226]proportions and of exquisite workmanship, known as the great Washington refractor. It is the product of the celebrated workshop of Messrs. Alvan Clark and Sons, from which so many large telescopes have proceeded, and in its noble proportions far surpassed any other telescope ever devoted to the same research. The object-glass measures twenty-six inches in diameter, and is hardly less remarkable for the perfection of its definition than for its size. But even the skill of Mr. Hall, and the space-penetrating power of his telescope, would not have been able on ordinary occasions to discover the satellites of Mars. Advantage was accordingly taken of that memorable opposition of Mars in 1877, when, as we have already described, the planet came unusually near the earth. Had Mars been attended by a moon one-hundredth part of the bulk of our moon it must long ago have been discovered. Mr. Hall, therefore, knew that if there were any satellites they must be extremely small bodies, and he braced himself for a severe and diligent search. The circumstances were all favourable. Not only was Mars as near as it