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