The first sign of change occurred in the polar cap. It proceeded slowly to dwindle in size. Such obliteration it has, with praiseworthy regularity, undergone once every two years for the last two hundred. Since the polar cap was first seen it has waxed and waned 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 into the southern hemisphere, the one presented to us at this last opposition, down to latitude seventy, and even sixty-five south; covering thus more than the whole of the planet's south frigid zone. As summer comes on they dwindle gradually away, till be 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. This unprecedented event happened on October 13, or forty-three days after the summer solstice of the southern hemisphere, a date corresponding to about the middle of July on Earth. Evidently, it was a phenomenally hot season on Mars, for the minimum of the polar patch is reached usually about three months after Martian midsummer. It will be noticed how nearly such melting parallels what takes place with our arctic ice-cap on earth.
But the appearance of the polar snows is by no means the only change discernable upon the surface of the planet. Several years ago Shiaparelli noticed differences in tint at successive oppositions, both in the dark areas and in the bright ones. These, he suggested, might be due to the seasons. This year it has been possible to watch the change take place. From the Martian middle of August, the bluish-green areas have been steadily undergoing a most marked transformation. These proves, in fact, to be a wave of seasonal change that sweeps over the face of the planet from pole to pole. We will examine this more in detail when we take up the question of water. For the present point it suffices that it takes place; for it constitutes proof positive of the presence of an atmosphere.
A moment's consideration will show how absolutely positive this proof is; for it is the inevitable deduction from the simplest of observed facts. Its cogency consists in its simplicity. It is independent of difficult detail or of doubtful interpretation. It is not concerned with that may be the constitution of the polar caps, nor with the character of the transformation that sweeps, wavelike, over the rest of the planet. It merely states that changes occurs, and that statement is conclusive.
Having thus seen with the brain as much as with the eye, and in the simplest possible manner, that a Martian atmosphere exists, we will go on to consider what it is like.
The first and most conspicuous of its characteristics is its cloudlessness. A cloud is an event on Mars, a rare and unusual phenomenon, which should make it more fittingly appreciated there than Ruskin lamented was the case on earth. For it is almost perpetually fine weather on our neighbor in space. From the day's beginning to its close, and from one end of a year to the other, nothing appears to veil the greater part of the planet's surface.
This is more completely the case than has hitherto been supposed. We read sometimes in astronomical books and articles picturesque accounts of clouds and mist gathering over certain regions of the disc, hiding the coast lines and continents from view, and then, some hours later, clearing off again. No instance of such blotting out of detail has been seen this year at Flagstaff. Though the planet's face been scanned there almost every night, from the last day of May to the end of November, not a case of obscuration of any part of the central portions of the planet, from any Martian cause, has been detected by any one of three observers. Certain peculiar brightish patches have from time to time been noted, but, with a courtesy uncommon in clouds, they have carefully refrained from obscuring in the slightest degree any detail the observer might be engaged in looking at.
The only dimming of detail upon the Martian disc has been along its bright edge, what is technically called its limb. Fringing this is a permanent lune of light that swamps all except the very darkest markings in its glare. This limb-light has commonly been taken as evidence of sunrise or sunset mists on Mars. But observations of mine during last June show that such cannot be the case. In June Mars was gibbous,—that is, he showed a face like the moon between the quarter and the full,—and along his limb, then upon his own western side, lay the bright-limb-light, stretching inward about thirty degrees. Since the face turned toward us was the only in part illumined by the sun, the centre of it did not stand at noon, but some hours later, and the middle of the limb consequently not at sunrise, but at about nine o'clock of a Martian morning. As the limb-light extended in from this thirty degrees, or two hours in time, the mist, if mist it was, must have lasted till eleven o'clock in the day. Furthermore, it must have been mist of a singularly mathematical turn of mind, for it made a perfect semi-ellipse from one pole to the other, quite oblivious of the fact that every hour from sunrise to sunset lay represented along its edge, including high noon. What is more, as the disc passed, in course of time, from gibbous form to the full, and then to the gibbous form on the other side, the limb-light obligingly clung to the limb, regardless of everything except its geometric curve. But as it did so, the eleven o'clock meridian swung from one side of the centre of the disc to the other. As it crossed the centre its regions showed perfectly clear; not a trace of obscuration as it passed directly under the eye. It was evident, therefore, that Martian morning mists were not responsible for the phenomenon.
To what, then, was the limb-light due? At first sight, it would seem as if the moon might help us; for the moon's limb is similarly ringed by a lune of light. In her case the effect has been attributed to mountain slopes catching the sun's light at angles beyond the possibilities of plains. But Mars has few mountains worthy the name. His terminator—that is, the part of the disc which is just passing in or out of sunlight, and discloses mountains by the way in which they catch the coming light before the plains at their feet are illuminated—shows irregularities quite inferior to the lunar ones, proving that his elevations and depressions are relatively insignificant.
On the whole, the best explanation of the phenomenon seems to be that the Martian atmosphere itself is somewhat of a veil, and this veiling effect, though practically imperceptible in the centre of the disc, becomes noticeable as we go from the centre to the edge, owing to the greater thickness of the stratum through which we look. At thirty degrees in from the limb the observer would look through twice as much of it as when he looked plumb down upon the centre of the disc; in consequence, what would be diaphanous at the centre might well seem opaque toward the edge. The effect we are familiar with on earth in the haze that always borders the horizon,—a haze most noticeable in places where there is much water in the air. Here, then, we have a hint of what is the matter on Mars. Were his atmosphere charged with water-vapor, just such an effect as is observed should take place.
This first hint receives an independent support from another Martian phenomenon. Contrary to what the distance of the planet from the sun and the thinness of its atmospheric envelope would lead us to expect, the climate of Mars proves astonishingly mild. Whereas calculation from distance and atmospheric density puts its average temperature below freezing, thus relegating it to perpetual ice, the planet's surface features show that the temperature is relatively high. Observation reveals the fact that the mean temperature must actually be above that of the earth; for not only is there practically no snow or ice outside the frigid zone at any time, but the polar snow-caps melt to a minimum quite beyond that of our own, affording the Martians rare chance for quixotic polar expeditions. Such pleasing amelioration of the climate must be accounted for, and the aqueous vapor is quite specific as a planetary comforter, being the very best of blankets. It acts, indeed, like the glass of a conservatory, letting the light rays in, and opposing the passage of the heat rays out.
The state of things thus disclosed by observation, the cloudlessness and the rim of limb-light, turns out to agree in a most happy manner with what probability would lead us to expect: for the most natural supposition to make a priori about the Martian atmosphere is the following. When each planet was produced by fission from the parent nebula, we may suppose that it took with it as its birthright its proportion of chemical constituents; that is, that its amount of oxygen, nitrogen, and so forth was proportional to its mass. Doubtless its place in the primal nebula would to a certain extent modify the ratio, just as the size of the planet would to a certain extent modify the relative amount of these elements that would thereupon enter into combination. Supposing, however, that the ratio of free oxygen and so forth to the other elements remained substantially the same, we should have in the case of any two planets the same relative quantity of atmosphere. But the size of the planet would entirely alter the distribution of this air.
Three causes would all combine to rob the smaller planet of efficient covering, on the general principle that he that hath little shall have less.
In the first place, the smaller the planet, the greater would be its volume in proportion to its mass, because the materials of which it was composed, being subjected to less pressure owing to a lesser pull, would not be crowded so closely together. This is one reason why Mars should have a thinner atmosphere than is the case with our earth.
Secondly, of two similar bodies, spheres or others, the smaller has the greater surface for its volume, since the one quantity is of two dimensions only, the other of three. An onion will give us a good instance of this. By stripping off layer after layer we reach eventually a last layer which is all surface, inclosing nothing. We may, if we please, observe something analogous in men, among whom the most superficial have the least in them. In consequence of this principle, the atmosphere of the smaller body finds itself obliged to cover relatively more surface, which still further thins it out.
Lastly, gravity being less on the surface of the smaller body, the atmosphere is less compressed, and, being a gas, seizes that opportunity to spread out to a greater height, which renders it still less dense at the planet's surface.
Thus for three reasons Mars should have a thinner air at his surface than is found on the surface of the earth.