The first marking detected upon Mars was the notable one called the Syrtis Major, also known, on account of its shape, as the Hour-Gla.s.s Sea. This observation was made by the famous Huyghens in 1659; and, from the movement of the marking in question across the disc, he inferred that the planet rotated on its axis in a period of about twenty-four hours.
There appears to be very little atmosphere upon Mars, the result being that we almost always obtain a clear view of the detail on its surface.
Indeed, it is only to be expected from the kinetic theory that Mars could not retain much of an atmosphere, as the force of gravity at its surface is less than one-half of what we experience upon the earth. It should here be mentioned that recent researches with the spectroscope seem to show that, whatever atmosphere there may be upon Mars, its density at the surface of the planet cannot be more than the one-fourth part of the density of the air at the surface of the earth. Professor Lowell, indeed, thinks it may be more rarefied than that upon our highest mountain-tops.
Seen with the naked eye, Mars appears of a red colour. Viewed in the telescope, its surface is found to be in general of a ruddy hue, varied here and there with darker patches of a bluish-green colour. These markings are permanent, and were supposed by the early telescopic observers to imply a distribution of the planet"s surface into land and water, the ruddy portions being considered as continental areas (perhaps sandy deserts), and the bluish-green as seas. The similarity to our earth thus suggested was further heightened by the fact that broad white caps, situated at the poles, were seen to vary with the planet"s seasons, diminishing greatly in extent during the Martian summer (the southern cap in 1894 even disappearing altogether), and developing again in the Martian winter.[18] Readers of Oliver Wendell Holmes will no doubt recollect that poet"s striking lines:--
"The snows that glittered on the disc of Mars Have melted, and the planet"s fiery orb Rolls in the crimson summer of its year."
A state of things so strongly a.n.a.logous to what we experience here, naturally fired the imaginations of men, and caused them to look on Mars as a world like ours, only upon a much smaller scale. Being smaller, it was concluded to have cooled quicker, and to be now long past its prime; and its "inhabitants" were, therefore, pictured as at a later stage of development than the inhabitants of our earth.
Notwithstanding the strong temptation to a.s.sume that the whiteness of the Martian polar caps is due to fallen snow, such a solution is, however, by no means so simple as it looks. The deposition of water in the form of snow, or even of h.o.a.r frost, would at least imply that the atmosphere of Mars should now and then display traces of aqueous vapour, which it does not appear to do.[19] It has, indeed, been suggested that the whiteness may not after all be due to this cause, but to carbonic acid gas (carbon dioxide), which is known to freeze at a _very low_ temperature. The suggestion is plainly based upon the a.s.sumption that, as Mars is so much further from the sun than we are, it would receive much less heat, and that the little thus received would be quickly radiated away into s.p.a.ce through lack of atmosphere to bottle it in.
We now come to those well-known markings, popularly known as the "ca.n.a.ls" of Mars, which have been the subject of so much discussion since their discovery thirty years ago.
It was, in fact, in the year 1877, when Mars was in opposition, and thus at its nearest to us, that the famous Italian astronomer, Schiaparelli, announced to the world that he had found that the ruddy areas, thought to be continents, were intersected by a network of straight dark lines.
These lines, he reported, appeared in many cases to be of great length, so long, indeed, as several thousands of miles, and from about twenty to sixty miles in width. He christened the lines _channels_, the Italian word for which, "ca.n.a.li," was unfortunately translated into English as "ca.n.a.ls." The a.n.a.logy, thus accidentally suggested, gave rise to the idea that they might be actual waterways.[20]
In the winter of 1881-1882, when Mars was again in opposition, Schiaparelli further announced that he had found some of these lines doubled; that is to say, certain of them were accompanied by similar lines running exactly parallel at no great distance away. There was at first a good deal of scepticism on the subject of Schiaparelli"s discoveries, but gradually other observers found themselves seeing both the lines and their doublings. We have in this a good example of a curious circ.u.mstance in astronomical observation, namely, the fact that when fine detail has once been noted by a competent observer, it is not long before other observers see the same detail with ease.
An immense amount of close attention has been paid to the planet Mars during recent years by the American observer, Professor Percival Lowell, at his famous observatory, 7300 feet above the sea, near the town of Flagstaff, Arizona, U.S.A. His observations have not, like those of most astronomers, been confined merely to "oppositions," but he has systematically kept the planet in view, so far as possible, since the year 1894.
The instrumental equipment of his observatory is of the very best, and the "seeing" at Flagstaff is described as excellent. In support of the latter statement, Mr. Lampland, of the Lowell Observatory, maintains that the faintest stars shown on charts made at the Lick Observatory with the 36-inch telescope there, are _perfectly visible_ with the 24-inch telescope at Flagstaff.
Professor Lowell is, indeed, generally at issue with the other observers of Mars. He finds the ca.n.a.ls extremely narrow and sharply defined, and he attributes the blurred and hazy appearance, which they have presented to other astronomers, to the unsteady and imperfect atmospheric conditions in which their observations have been made. He a.s.signs to the thinnest a width of two or three miles, and from fifteen to twenty to the larger. Relatively to their width, however, he finds their length enormous. Many of them are 2000 miles long, while one is even as much as 3540. Such lengths as these are very great in comparison with the smallness of the planet. He considers that the ca.n.a.ls stand in some peculiar relation to the polar cap, for they crowd together in its neighbourhood. In place, too, of ill-defined condensations, he sees sharp black spots where the ca.n.a.ls meet and intersect, and to these he gives the name of "Oases." He further lays particular stress upon a dark band of a blue tint, which is always seen closely to surround the edges of the polar caps all the time that they are disappearing; and this he takes to be a proof that the white material is something which actually _melts_. Of all substances which we know, water alone, he affirms, would act in such a manner.
The question of melting at all may seem strange in a planet which is situated so far from the sun, and possesses such a rarefied atmosphere.
But Professor Lowell considers that this very thinness of the atmosphere allows the direct solar rays to fall with great intensity upon the planet"s surface, and that this heating effect is accentuated by the great length of the Martian summer. In consequence he concludes that, although the general climate of Mars is decidedly cold, it is above the freezing point of water.
The observations at Flagstaff appear to do away with the old idea that the darkish areas are seas, for numerous lines belonging to the so-called "ca.n.a.l system" are seen to traverse them. Again, there is no star-like image of the sun reflected from them, as there would be, of course, from the surface of a great sheet of water. Lastly, they are observed to vary in tone and colour with the changing Martian seasons, the blue-green changing into ochre, and later on back again into blue-green. Professor Lowell regards these areas as great tracts of vegetation, which are brought into activity as the liquid reaches them from the melting snows.
[Ill.u.s.tration: PLATE XII. A MAP OF THE PLANET MARS
We see here the Syrtis Major (or "Hour-Gla.s.s Sea"), the polar caps, several "oases," and a large number of "ca.n.a.ls," some of which are double. The South is at the top of the picture, in accordance with the _inverted_ view given by an astronomical telescope. From a drawing by Professor Percival Lowell.
(Page 216)]
With respect to the ca.n.a.ls, the Lowell observations further inform us that these are invisible during the Martian winter, but begin to appear in the spring when the polar cap is disappearing. Professor Lowell, therefore, inclines to the view that in the middle of the so-called ca.n.a.ls there exist actual waterways which serve the purposes of irrigation, and that what we see is not the waterways themselves, for they are too narrow, but the fringe of vegetation which springs up along the banks as the liquid is borne through them from the melting of the polar snows. He supports this by his observation that the ca.n.a.ls begin to appear in the neighbourhood of the polar caps, and gradually grow, as it were, in the direction of the planet"s equator.
It is the idea of life on Mars which has given this planet such a fascination in the eyes of men. A great deal of nonsense has, however, been written in newspapers upon the subject, and many persons have thus been led to think that we have obtained some actual evidence of the existence of living beings upon Mars. It must be clearly understood, however, that Professor Lowell"s advocacy of the existence of life upon that planet is by no means of this wild order. At the best he merely indulges in such theories as his remarkable observations naturally call forth. His views are as follows:--He considers that the planet has reached a time when "water" has become so scarce that the "inhabitants"
are obliged to employ their utmost skill to make their scanty supply suffice for purposes of irrigation. The changes of tone and colour upon the Martian surface, as the irrigation produces its effects, are similar to what a telescopic observer--say, upon Venus--would notice on our earth when the harvest ripens over huge tracts of country; that is, of course, if the earth"s atmosphere allowed a clear view of the terrestrial surface--a very doubtful point indeed. Professor Lowell thinks that the perfect straightness of the lines, and the geometrical manner in which they are arranged, are clear evidences of artificiality.
On a globe, too, there is plainly no reason why the liquid which results from the melting of the polar caps should trend at all in the direction of the equator. Upon our earth, for instance, the transference of water, as in rivers, merely follows the slope of the ground, and nothing else.
The Lowell observations show, however, that the Martian liquid is apparently carried from one pole towards the equator, and then past it to the other pole, where it once more freezes, only to melt again in due season, and to reverse the process towards and across the equator as before. Professor Lowell therefore holds, and it seems a strong point in favour of his theory, that the liquid must, in some artificial manner, as by pumping, for instance, be _helped_ in its pa.s.sage across the surface of the planet.
A number of attempts have been made to explain the _doubling_ of the ca.n.a.ls merely as effects of refraction or reflection; and it has even been suggested that it may arise from the telescope not being accurately focussed.
The actual doubling of the ca.n.a.ls once having been doubted, it was an easy step to the casting of doubt on the reality of the ca.n.a.ls themselves. The idea, indeed, was put forward that the human eye, in dealing with detail so very close to the limit of visibility, may unconsciously treat as an actual line several point-like markings which merely happen to lie in a line. In order to test this theory, experiments were carried out in 1902 by Mr. E.W. Maunder of Greenwich Observatory, and Mr. J.E. Evans of the Royal Hospital School at Greenwich, in which certain schoolboys were set to make drawings of a white disc with some faint markings upon it. The boys were placed at various distances from the disc in question; and it was found that the drawings made by those who were just too far off to see distinctly, bore out the above theory in a remarkable manner. Recently, however, the plausibility of the _illusion_ view has been shaken by photographs of Mars taken during the opposition of 1905 by Mr. Lampland at the Lowell Observatory, in which a number of the more prominent ca.n.a.ls come out as straight dark lines. Further still, in some photographs made there quite lately, several ca.n.a.ls are said to appear visibly double.
Following up the idea alluded to in Chapter XVI., that the moon may be covered with a layer of ice, Mr. W.T. Lynn has recently suggested that this may be the case on Mars; and that, at certain seasons, the water may break through along definite lines, and even along lines parallel to these. This, he maintains, would account for the ca.n.a.ls becoming gradually visible across the disc, without the necessity of Professor Lowell"s "pumping" theory.
And now for the views of Professor Lowell himself with regard to the doubling of the ca.n.a.ls. From his observations, he considers that no pairs of railway lines could apparently be laid down with greater parallelism. He draws attention to the fact that the doubling does not take place by any means in every ca.n.a.l; indeed, out of 400 ca.n.a.ls seen at Flagstaff, only fifty-one--or, roughly, one-eighth--have at any time been seen double. He lays great stress upon this, which he considers points strongly against the duplication being an optical phenomenon. He finds that the distance separating pairs of ca.n.a.ls is much less in some doubles than in others, and varies on the whole from 75 to 200 miles.
According to him, the double ca.n.a.ls appear to be confined to within 40 degrees of the equator: or, to quote his own words, they are "an equatorial feature of the planet, confined to the tropic and temperate belts." Finally, he points out that they seem to _avoid_ the blue-green areas. But, strangely enough, Professor Lowell does not so far attempt to fit in the doubling with his body of theory. He makes the obvious remark that they may be "channels and return channels," and with that he leaves us.
The conclusions of Professor Lowell have recently been subjected to strenuous criticism by Professor W.H. Pickering and Dr. Alfred Russel Wallace. It was Professor Pickering who discovered the "oases," and who originated the idea that we did not see the so-called "ca.n.a.ls"
themselves, but only the growth of vegetation along their borders. He holds that the oases are craterlets, and that the ca.n.a.ls are cracks which radiate from them, as do the rifts and streaks from craters upon the moon. He goes on to suggest that vapours of water, or of carbonic acid gas, escaping from the interior, find their way out through these cracks, and promote the growth of a low form of vegetation on either side of them. In support of this view he draws attention to the existence of long "steam-cracks," bordered by vegetation, in the deserts of the highly volcanic island of Hawaii. We have already seen, in an earlier chapter, how he has applied this idea to the explanation of certain changes which are suspected to be taking place upon the moon.
In dealing with the Lowell ca.n.a.l system, Professor Pickering points out that under such a slight atmospheric pressure as exists on Mars, the evaporation of the polar caps--supposing them to be formed of snow--would take place with such extraordinary rapidity that the resulting water could never be made to travel along open channels, but that a system of gigantic tubes or water-mains would have to be employed!
As will be gathered from his theories regarding vegetation, Professor Pickering does not deny the existence of a form of life upon Mars. But he will not hear of civilisation, or of anything even approaching it. He thinks, however, that as Mars is intermediate physically between the moon and earth, the form of life which it supports may be higher than that on the moon and lower than that on the earth.
In a small book published in the latter part of 1907, and ent.i.tled _Is Mars Habitable?_ Dr. Alfred Russel Wallace sets himself, among other things, to combat the idea of a comparatively high temperature, such as Professor Lowell has allotted to Mars. He shows the immense service which the water-vapour in our atmosphere exercises, through keeping the solar heat from escaping from the earth"s surface. He then draws attention to the fact that there is no spectroscopic evidence of water-vapour on Mars[21]; and points out that its absence is only to be expected, as Dr. George Johnstone Stoney has shown that it will escape from a body whose ma.s.s is less than one-quarter the ma.s.s of the earth.
The ma.s.s of Mars is, in fact, much less than this, _i.e._ only one-ninth. Dr. Wallace considers, therefore, that the temperature of Mars ought to be extremely low, unless the const.i.tution of its atmosphere is very different from ours. With regard to the latter statement, it should be mentioned that the Swedish physicist, Arrhenius, has recently shown that the carbonic acid gas in our atmosphere has an important influence upon climate. The amount of it in our air is, as we have seen, extremely small; but Arrhenius shows that, if it were doubled, the temperature would be more uniform and much higher. We thus see how futile it is, with our present knowledge, to dogmatise on the existence or non-existence of life in other celestial orbs.
As to the ca.n.a.ls Dr. Wallace puts forward a theory of his own. He contends that after Mars had cooled to a state of solidity, a great swarm of meteorites and small asteroids fell in upon it, with the result that a thin molten layer was formed all over the planet. As this layer cooled, the imprisoned gases escaped, producing vents or craterlets; and as it attempted to contract further upon the solid interior, it split in fissures radiating from points of weakness, such, for instance, as the craterlets. And he goes on to suggest that the two tiny Martian satellites, with which we shall deal next, are the last survivors of his hypothetical swarm. Finally, with regard to the habitability of Mars, Dr. Wallace not only denies it, but a.s.serts that the planet is "absolutely uninhabitable."
For a long time it was supposed that Mars did not possess any satellites. In 1877, however, during that famous opposition in which Schiaparelli first saw the ca.n.a.ls, two tiny satellites were discovered at the Washington Observatory by an American astronomer, Professor Asaph Hall. These satellites are so minute, and so near to the planet, that they can only be seen with very large telescopes; and even then the bright disc of the planet must be shielded off. They have been christened Phobos and Deimos (Fear and Dread); these being the names of the two subordinate deities who, according to Homer, attended upon Mars, the G.o.d of war.
It is impossible to measure the exact sizes of these satellites, as they are too small to show any discs, but an estimate has been formed from their brightness. The diameter of Phobos was at first thought to be six miles, and that of Deimos, seven. As later estimates, however, considerably exceed this, it will, perhaps, be not far from the truth to state that they are each roughly about the size of the planetoid Eros.
Phobos revolves around Mars in about 7-1/2 hours, at a distance of about only 4000 miles from the planet"s surface, and Deimos in about 30 hours, at a distance of about 12,000 miles. As Mars rotates on its axis in about 24 hours, it will be seen that Phobos makes more than three revolutions while the planet is rotating once--a very interesting condition of things.
A strange foreshadowing of the discovery of the satellites of Mars will be familiar to readers of _Gulliver"s Travels_. According to Dean Swift"s hero, the astronomers on the Flying Island of Laputa had found two tiny satellites to Mars, one of which revolved around the planet in ten hours. The correctness of this guess is extraordinarily close, though at best it is, of course, nothing more than a pure coincidence.
It need not be at all surprising that much uncertainty should exist with regard to the actual condition of the surface of Mars. The circ.u.mstances in which we are able to see that planet at the best are, indeed, hardly sufficient to warrant us in propounding any hard and fast theories. One of the most experienced of living observers, the American astronomer, Professor E.E. Barnard, considers that the view we get of Mars with the best telescope may be fairly compared with our naked eye view of the moon. Since we have seen that a view with quite a small telescope entirely alters our original idea of the lunar surface, a slight magnification revealing features of whose existence we had not previously the slightest conception, it does not seem too much to say that a further improvement in optical power might entirely subvert the present notions with regard to the Martian ca.n.a.ls. Therefore, until we get a still nearer view of these strange markings, it seems somewhat futile to theorise. The lines which we see are perhaps, indeed, a foreshortened and all too dim view of some type of formation entirely novel to us, and possibly peculiar to Mars. Differences of gravity and other conditions, such as obtain upon different planets, may perhaps produce very diverse results. The earth, the moon, and Mars differ greatly from one another in size, gravitation, and other such characteristics. Mountain-ranges so far appear typical of our globe, and ring-mountains typical of the moon. May not the so-called "ca.n.a.ls" be merely some special formation peculiar to Mars, though quite a natural result of its particular conditions and of its past history?
THE ASTEROIDS (OR MINOR PLANETS)
We now come to that belt of small planets which are known by the name of asteroids. In the general survey of the solar system given in Chapter II., we saw how it was long ago noticed that the distances of the planetary orbits from the sun would have presented a marked appearance of orderly sequence, were it not for a gap between the orbits of Mars and Jupiter where no large planet was known to circulate. The suspicion thus aroused that some planet might, after all, be moving in this seemingly empty s.p.a.ce, gave rise to the gradual discovery of a great number of small bodies; the largest of which, Ceres, is less than 500 miles in diameter. Up to the present day some 600 of these bodies have been discovered; the four leading ones, in order of size, being named Ceres, Pallas, Juno, and Vesta. All the asteroids are invisible to the naked eye, with the exception of Vesta, which, though by no means the largest, happens to be the brightest. It is, however, only just visible to the eye under favourable conditions. No trace of an atmosphere has been noted upon any of the asteroids, but such a state of things is only to be expected from the kinetic theory.
For a good many years the discoveries of asteroids were made by means of the telescope. When, in the course of searching the heavens, an object was noticed which did not appear upon any of the recognised star charts, it was kept under observation for several nights to see whether it changed its place in the sky. Since asteroids move around the sun in orbits, just as planets do, they, of course, quickly reveal themselves by their change of position against the starry background.
The year 1891 started a new era in the discovery of asteroids. It occurred to the Heidelberg observer, Dr. Max Wolf, one of the most famous of the hunters of these tiny planets, that photography might be employed in the quest with success. This photographic method, to which allusion has already been made in dealing with Eros, is an extremely simple one. If a photograph of a portion of the heavens be taken through an "equatorial"--that is, a telescope, moving by machinery, so as to keep the stars, at which it is pointed, always exactly in the field of view during their apparent movement across the sky--the images of these stars will naturally come out in the photograph as sharply defined points. If, however, there happens to be an asteroid, or other planetary body, in the same field of view, its image will come out as a short white streak; because the body has a comparatively rapid motion of its own, and will, during the period of exposure, have moved sufficiently against the background of the stars to leave a short trail, instead of a dot, upon the photographic plate. By this method Wolf himself has succeeded in discovering more than a hundred asteroids (see Plate XIII., p. 226). It was, indeed, a little streak of this kind, appearing upon a photograph taken by the astronomer Witt, at Berlin, in 1898, which first informed the world of the existence of Eros.
[Ill.u.s.tration: PLATE XIII. MINOR PLANET TRAILS
Two trails of minor planets (asteroids) imprinted _at the same time_ upon one photographic plate. In the white streak on the left-hand side of the picture we witness the _discovery_ of a new minor planet. The streak on the right was made by a body already known--the minor planet "Fiducia." This photograph was taken by Dr. Max Wolf, at Heidelberg, on the 4th of November, 1901, with the aid of a 16-inch telescope. The time of exposure was two hours.
(Page 227)]
It has been calculated that the total ma.s.s of the asteroids must be much less than one-quarter that of the earth. They circulate as a rule within a s.p.a.ce of some 30,000,000 miles in breadth, lying about midway between the paths of Mars and Jupiter. Two or three, however, of the most recently discovered of these small bodies have been found to pa.s.s quite close to Jupiter. The orbits of the asteroids are by no means in the one plane, that of Pallas being the most inclined to the plane of the earth"s...o...b..t. It is actually three times as much inclined as that of Eros.
Two notable theories have been put forward to account for the origin of the asteroids. The first is that of the celebrated German astronomer, Olbers, who was the discoverer of Pallas and Vesta. He suggested that they were the fragments of an exploded planet. This theory was for a time generally accepted, but has now been abandoned in consequence of certain definite objections. The most important of these objections is that, in accordance with the theory of gravitation, the orbits of such fragments would all have to pa.s.s through the place where the explosion originally occurred. But the wide area over which the asteroids are spread points rather against the notion that they all set out originally from one particular spot. Another objection is that it does not appear possible that, within a planet already formed, forces could originate sufficiently powerful to tear the body asunder.
The second theory is that for some reason a planet here failed in the making. Possibly the powerful gravitational action of the huge body of Jupiter hard by, disturbed this region so much that the matter distributed through it was never able to collect itself into a single ma.s.s.
[18] Sir William Herschel was the first to note these polar changes.
[19] Quite recently, however, Professor Lowell has announced that his observer, Mr. E.C. Slipher, finds with the spectroscope faint traces of water vapour in the Martian atmosphere.