Other Worlds<br />Their Nature, Possibilities and Habitability in the Light of the Latest Discoveries, Garrett Putman Serviss [reading diary .txt] 📗

we have found so much of brilliant promise in the earth's twin sister that[Pg 76] I almost hesitate to approach another phase of the subject which may tend to weaken the faith of some readers in the habitability of Venus. It may have been observed that heretofore nothing has been said as to the planet's rotation period, but, without specifically mentioning it, I have tacitly assumed the correctness of the generally accepted period of about twenty-four hours, determined by De Vico and other observers. This period, closely accordant with the earth's, is, as far as it goes, another argument for the habitability of Venus.

But now it must be stated that no less eminent an authority than Schiaparelli holds that Venus, as well as Mercury, makes but a single turn on its axis in the course of a revolution about the sun, and, consequently, is a two-faced world, one side staring eternally at the sun and the other side wearing the black mask of endless night.

Schiaparelli made this announcement concerning Venus but a few weeks after publishing his discovery of Mercury's peculiar rotation. He himself appears to be[Pg 77] equally confident in both cases of the correctness of his conclusions and the certainty of his observation. As with Mercury, several other observers have corroborated him, and particularly Percival Lowell in this country. Mr. Lowell, indeed, seems unwilling to admit that any doubt can be entertained. Nevertheless, very grave doubt is entertained, and that by many, and probably by the majority, of the leading professional astronomers and observers. In fact, some observers of great ability, equipped with powerful instruments, have directly contradicted the results of Schiaparelli and his supporters.

The reader may ask: "Why so readily accept Schiaparelli's conclusions with regard to Mercury while rejecting them in the case of Venus?"

The reply is twofold. In the first place the markings on Venus, although Mr. Lowell sketched them with perfect confidence in 1896, are, by the almost unanimous testimony of those who have searched for them with telescopes, both large and small, ex[Pg 78]tremely difficult to see, indistinct in outline, and perhaps evanescent in character. The sketches of no two observers agree, and often they are remarkably unlike. The fact has already been mentioned that Mr. Lowell noticed a kind of veil partially obscuring the markings, and which he ascribed, no doubt correctly, to the planet's atmosphere. But he thinks that, notwithstanding the atmospheric veil, the markings noted by him were unquestionably permanent features of the planet's real surface. Inasmuch, however, as his drawings represent things entirely different from what others have seen, there seems to be weight in the suggestion that the radiating bands and shadings noticed by him were in some manner illusory, and perhaps of atmospheric origin.

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 mo[Pg 79]tion 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[Pg 80] 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 effect[Pg 81]iveness 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[Pg 82] 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[Pg 83] 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,[Pg 84] 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.[Pg 85]

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[Pg 86] 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[Pg 87] 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 dis[Pg 88]appearing, 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