Mars and Its Canals

by Percival Lowell

In gazing at the successive phases presented by the polar caps as their annual history unrolls itself to view, beginning with vast white cloaks that in winter hide so effectively the planet’s shoulders, to little round knobs that in summer sit like guardsmen’s caps more or less askew upon the poles, the bodily eye sees only the glisten of far-off snow. The mind’s eye, however, perceives something more: the conviction they carry of the presence of an atmosphere surrounding the planet. Elusive as water vapor is to sight for its transparency and to spectroscopic determination for its earthly omnipresence, recognition of its existence elsewhere by deduction raises such reasoning at once to a more conspicuous plane than it might otherwise assume. Especially is this true where the deduction is itself conclusive, as is here the case. For it depends on phenomena not its own, but which are in their turn dependent on it. We are not even beholden to any knowledge of the substance composing the caps for the fundamental inference that an atmosphere surrounds them. Whatever that substance were, the fact that the caps dissipate and reform shows us with absolute certainty that they pass into the gaseous state, to be later solidified afresh. This gas constitutes of itself an atmosphere; while another phenomenon, to wit, their blue girdles as they melt, affirming their substance to be snow and ice, enables us to precise the fact that this gas is water vapor.

From such premise given us by the polar caps we are able to infer much more by the help of the kinetic theory of gases. But the speed of parting by a planet with its gases is conditioned by the mean speed of each gas. Water vapor will, therefore, go before nitrogen, oxygen or carbonic acid gas. If, then, we find it present over the surface of a planet we are assured of the possibility that the other three may be there too, and from the similarity of matter in space strong reason to suspect that they actually are.

Corroborative evidence of the accuracy of the deduction as to the presence of a Martian air is shown in several other ways; in the existence of clouds to begin with. Rare as they are, these certainly float at times over parts of the planet, although it is doubtful whether they can then be seen. Fortunately for assurance we have other ways of ascertaining their presence than that of obscuration. Nor is it of account to the argument that they should be few and far between, as they unquestionably are. One single instance of such mediumistic support is enough to support the theory of a medium; and that instance has been more than once observed.

Direct evidence of atmosphere is further forthcoming in the limb-light. This phenomenon might be described as a brilliant obscuration. It is a circlet of illumination that swamps the features as they near the full edge of the disk, the limb of the planet as it is called. Obliteration of the sort is evident, more or less markedly, at all times, and is not due to foreshortening, as the broadest areas are affected. The fading out of the detail at the limb suggests nothing so much as a veil drawn between us and it, lighter in tint than what it covers. Such a veil can be none other than air or the haze and cloud that air supports. From its effect, impartial in place and partial in character, cloud is inadmissible as a cause and we are left with air charged with dust or vapor in explanation. Obscuration due to it should prove most dense at the limb, since there the eye has to penetrate a greater depth of it; just as on the earth our own air gives azure dimness to the distance in deepened tinting as the mountains lie remote.

Another bit of evidence lies in the apparent detection of a twilight arc. In 1894 measures made of the polar and equatorial diameters of the planet showed certain systematic residuals left after all known corrections had been applied. The only thing which would account for them was the supposition that a twilight arc had been unconsciously seen and as unconsciously measured. In delicate quantities of the sort too great reliance cannot be put, but if the residuals be not referable to other cause they give us not only further evidence of an atmosphere, but at the same time our only hint of that atmosphere’s extent. From them it would seem that the air must be rare, not more than about four inches of barometric pressure, as we reckon it, and probably less; a thin, high air more rarefied than prevails upon our highest mountain tops.

Corroborative of this is the aspect of the planet. From the general look of the disk a scant covering of air is inferable. For one of the striking things about the planet’s features is their patent exposure to our sight. Except in the winter time of its hemisphere or in the spring after the greatest melting of the polar cap, nothing seems to stand in our way of an uninterrupted view of the surface, whether in the arctic, temperate, or tropic zones. From the openness of its expression, however, too much case should not be made as we really know but little of how an atmosphere-enshrouded planet would look. We find no difficulty in seeing objects a hundred miles away across the surface of the earth and yet the thickness of the air strata in such horizontal traversing is many fold what it is when we look directly up. It is also out of all proportion laden with dust and smoke. In the purer regions of the earth, a clear air imposes but little bar to sight, and conjures up far things startlingly distinct.

Nevertheless, every evidence points to a thin air upon Mars: a priori reasoning, indirect deduction and direct sight. Now, from a thinness of atmosphere it would follow, other things equal, that the climate was cold. About this there has been much question, but less of answering reply. From the distance of the planet from the sun it is certain less heat is received by it than falls upon the earth in something like the ratio of one to two. But that the amount effective is as the amount received is far from sure. The available heat is much affected by the manner of its reception. A blanket of air acts like the glass of a conservatory, letting the light rays in, but hindering the heat rays out. The light rays falling on the ground or the air are transformed into heat rays that, finding the return journey less easy, are consequently trapped. All substances are thus calorifiers, but water vapor is many times more potent than ordinary air to heat-ensnaring. A humid air has a hothouse tang to it most perceptible. Now, what the relative percentage of water vapor in the Martian atmosphere may be we do not know.

The thinness of the Martian air has caused it to be likened to that upon our highest mountain peaks which are in large part covered with perpetual snow. But the comparison is not well founded. A peak differs materially from a plateau in the countenance it gives to the heat falling upon it. On a plateau each warmed acre of ground helps the retention of heat by its neighbor; while in addition to being destitute of side support the higher winds generated about an isolated peak blow its own caloric away. Still less does any analogy hold between the two when the plateau is a world-wide one.

From these considerations it is evident glosses are possible upon the bald idea of a much lower temperature prevailing on the Martian surface than on the earth’s. Doubtless the theoretic cold has been greatly overdone. Reversely, recent observations tend to lower the apparent temperature disclosed by the features of the disk, and between the rising of the theoretic and the falling of the observed we are left with a very reasonable compromise and reconcilement as the result.

The various look and behavior of the surface of Mars point to a mean temperature colder than that of the earth, but above the freezing-point of water; for regions, at least, outside of the polar caps and during all but the winter months. Except at certain special spots, and possibly even there, frost is unknown at all times within the tropics and except in winter in temperate latitudes. These anomalous localities, mentioned in the preceding chapter, may be said to be the exceptions that prove the rule of general non-glaciation. For if they be snow, they stand witness to its absence elsewhere upon the disk, and if they are not, they testify the more emphatically to the same effect.