Mars/Chapter 2

From Wikisource

←Mars

Mars/Chapter 2 by Percival Lowell

1895

Chapter 2

I. Evidence of it

To all forms of life of which we have any conception, two things in nature are vital, air and water. A planet must possess these two requisites to be able to support any life at all upon its surface. For there is no creature, no plant, no anything endowed with the possibility of that kind of change we call life, which is not in some measure dependent upon both of them. How, then, is Mars off for air?

Fortunately for an answer to this question, air, in the post-chaotic part of a planet's career, plays as vital a role in the inorganic processes of nature as in the organic ones. By the post-chaotic period of a planet's history we may designate that time in its evolutionary existence which follows the parting with its own inherent heat. After its heat has gone from it, atmosphere becomes essential, not only to any form of life upon its surface, but to the production of any change whatever there. Without atmosphere all development, even the development of decay, must come to a stand-still, when once what was friable had crumbled to pieces under the alternate roasting and refrigerating, relatively speaking, to which the body's surface would be exposed as it turned round on its axis into and out of the Sun's rays. Such disintegration once accomplished, the planet would roll thenceforth a mummy world through space.

An instance of this death in life we have exemplified by the nearest of the heavenly bodies, our own Moon. That cataclysmic changes once occurred there is still legible on her face, while the present well- nigh complete immutability of that face shows that next to nothing happens there now. Except for the possible tumbling in of a crater wall, such as seems to have taken place in the case of Linne a few years ago, all is now deathly still. But atmosphere is as absent as change. Whatever it may have had in the past, there is at present no perceptible air upon the surface of the Moon. And change pro tanto knows it no more.

With Mars it is otherwise. Over the surface of that planet changes do occur, changes upon a scale vast enough to be visible from the Earth. To appreciate the character and extent of these changes we will begin with the appearance of the planet last June. From the drawings it will be seen that the general aspect of the planet's surface at that time was tripartite. Upon the top part of the disk, round what we know to be the planet's pole, appeared to be a great white cap. This was the planet's south polar cap. The south lay at the top, because all astronomical views are, for optical reasons, upside down; but, inasmuch as we never see the features otherwise, to have them right side up is not vital to the effect. Below the white cap lay a region chiefly bluish-green, interspersed, however, with portions more or less reddish-ochre. Below this, again, came a vast reddish-ochre stretch. The first sign of change occurred in the polar cap. It proceeded slowly to dwindle in size. Such self- obliteration it has, with praiseworthy regularity, been seen to undergo once every two years since it was first seen by man. For nearly two hundred years now, it has been observed to wax and wane with clock- like precision, a precision timed to the change of season in the planet's year. During the spring, these snow-fields, as analogy at once guesses them to be, and as beyond doubt they really are, stretch in the southern hemisphere, the one presented to us at this last opposition, down to latitude sixty-five south and even further, covering thus more than the whole of the planet's frigid zone. As summer comes on, they dwindle gradually away, till by early autumn they present but tiny patches a few hundred miles across. This year, for the first time in human experience, they melted, apparently, completely.

The history of the cap's vicissitudes we shall take up farther on in connection with the question of water. It is only necessary here to note that changes occurred in it.

The disappearance of the polar snows is by no means the only change discernible upon the surface of the planet. Several years ago Schiaparelli noticed differences in tint at successive oppositions both in the dark areas and in the bright ones. These, he suggested, might be due to seasons. At the last opposition, that of 1894, it was possible at Flagstaff, owing to the length of time the planet was kept under observation, to watch the changes occur; thus conclusively proving them to be changes of a seasonal character.

From early in June, which corresponded to the Martian last of April, to the end of November, which corresponded to the Martian last of August, the bluish-green areas underwent a marked transformation. During the summer of the Martian southern hemisphere, a wave of seasonal change swept down from the pole over the face of the planet. What and why it was we will examine in detail when we take up the question of water. Like the changes in the polar cap, it suffices here to chronicle the fact that it took place; for the fact of its occurrence constitutes proof positive of the presence of an atmosphere.

A moment's consideration will show how absolutely positive this proof is. It is the inevitable deduction from the simplest of observed facts. Its cogency gains from its very simplicity. For it is independent of difficult detail or of doubtful interpretation. It is not concerned with what may be the constitution of the polar caps, nor with the character of the transformation that sweeps, wave-like, over the rest of the planet's face. It merely takes note that change occurs, and that note is final.

Now, since this was originally written, certain observations made at this observatory by Mr. Douglass have resulted apparently, most unexpectedly, in actually revealing this atmosphere to sight. Although the existence of an atmosphere is absolutely established by the above considerations, it is interesting to have ocular demonstration of it to boot; and this the more, that it would not have been thought possible to detect what, so to speak, disclosed itself. For the discovery was quite unconsciously made, being of the nature of a by-product to the outcome of another investigation. So systematically was his general search conducted that when the results came to be worked out it appeared not only that he had seen an atmosphere, but actually measured it, although he was quite unaware of doing so at the time. The occasion was the measuring of the diameters of the planet, polar and equatorial. Micrometric measures of these were begun as early as the beginning of July, and kept up at intervals till the latter part of November. But the ones that proved specially tell-tale were those made from September 20th to November 22nd, a set of polar and a set of equatorial ones having been taken throughout that interval on twenty-six nights.

Now, when these measures came to be worked out by me, corrected for all known sources of error and reduced to distance unity, a curious result made its appearance. As they stood arranged in their table chronologically, it was at once evident, even before taking the means, that, as time went on, something had affected the equatorial diameter which had not affected the polar one.

The values for the polar diameter were nearly the same from first to last. The equatorial values, on the other hand, showed, apparently, a systematic increase as the eye followed down the column. Something, therefore, had been at work on the one, which had not been at work on the other. Almost as instantaneously, it was evident what this something was, to wit, a visible twilight unconsciously measured for a part of the planet's surface. Like the Downeaster who shingled fifty feet on to the fog, Mr. Douglass had measured several miles into the Martian air.

A word or two will explain this. The planet came to opposition on October 20. The mid-measures of the series, therefore, were taken within a few days of opposition, just before and just after that event. The subsequent ones, on the other hand, were made at a gradually increasing distance from this position, as the planet passed toward quadrature. Now, at opposition, the disk of the planet is full, like the full Moon; while, as it passes to quadrature, it loses something of itself, becoming gibbous, as the Moon does two or three days after the full. This loss from phase chiefly affects the equatorial diameter, the polar one remaining substantially unchanged by it. It would remain absolutely unchanged if the planet moved in the plane of the ecliptic. It does not so move, but the quantity resulting from lack of accordance is so small that for the present explanation it may be neglected. Now, this question of phase was the only point, practically, in which the equatorial and polar diameters differed during the interval under consideration. This, then, was the clue to the discrepancy.

This work is in the public domain in the United States because it was published before January 1, 1923. The author died in 1916, so this work is also in the public domain in countries and areas where the copyright term is the author's life plus 80 years or less. This work may also be in the public domain in countries and areas with longer native copyright terms that apply the rule of the shorter term to foreign works.