Calculating the effect of the above causes numerically, we find that on this a priori supposition; for the cloudless character of the Martian skies is precisely what we should look for in a rare air. Clouds are congeries of globules of water or particles of ice buoyed up by the air about them. The smaller these are, the more easily are they buoyed up, because gravity, which tends to pull them down, acts upon their mass, while the resistance they oppose to it varies as their surface, and this, as we saw just now, is relatively greater in the smaller particles. The result is that the smaller particles can float in thinner air. We see the principle exemplified in our terrestrial clouds; the low nimbus being formed of comparatively large globules, while the high cirrus is made up of very minute particles. If we go yet higher, we reach a region incapable of supporting clouds of any kind, so rarefied is its air. This occurs about five miles above the earth's surface; and yet even at this height the density of our air is greater than is the probably density of the air at the surface of Mars. We see, therefore, that the Martian atmosphere should from its rarity prove cloudless, just as we observe it to be.
So far in this our investigation of the Martian atmosphere we have been indebted solely to the principles of mathematics and molar physics for help, and these have told us something about the probable quantity of that atmosphere, though silent as to its possible quality. On this latter point, however, molecular physics turns out to have something to say; for an Irish gentleman, Dr. G. Johnstone Stoney, has recently made an ingenious deduction from the kinetic theory of gases bearing upon the atmospheric envelope which any planet can retain. His deduction is as acute as it appears from observation to be in keeping with the facts. It is this:—
The molecular theory of gases supposes them to be made up of myriads of molecules in incessant motion. What a molecule may be nobody knows; some scientists supposing it to be a vortex ring in miniature,—something like the swirl produced by a teaspoon when drawn through a cup of tea. But whatever it be, the idea of it accounts for the facts. The motion of the molecules is almost inconceivably swift as they dart hither and thither throughout the space occupied by the gas, and their speed differs for different gases. It is calculated that the molecules of oxygen travel, on the average, at the rate of fifteen miles a minute, and those of hydrogen, which are the fastest known, at the enormous speed of a mile a second. But this average velocity may, in any particular case, be increased by collisions of the molecules among themselves something like sevenfold. What is more, each molecule of the gas is bound, sooner or later, to attain this maximum velocity of its kind merely on the doctrine of chances. When it is attained, the molecule of water vapor at the rate of two and one third miles a second, and the molecule of hydrogen actually at seven miles at second, six hundred times as fast as our fastest express train.
Now, if a body, whether it be a molecule or a cannon-ball, be projected away from the earth's surface, the earth will at once try to pull it down again: this instinctive holding on of Mother Earth to what she has we call gravity. In the cases with which we are personally familiar, her endeavor is eminently successful; what goes up usually coming down again, either on the thrower or on some other person. But even the earth is not omnipotent. As the velocity with which the body is projected increases, it takes the earth longer and longer to overcome it and compel the body's return. Finally there comes a speed which the earth is just able to overcome, if she take an infinite time about it. In that case, the body would continue to travel away from her, at a constantly diminishing rate, but still at some rate, on and on into the depths of space, till it attained infinity, at which point the truant would stop, and reluctantly begin to return again. This velocity we may call the critical velocity. It is the velocity which the earth would cause in a body falling to it from an infinite distance, since gravity is able to destroy on the way up just the speed it is able to create on the way down. But now, if the body's departure were even hastier than this, the earth would never be able wholly to annihilate its speed, and the body would travel forever away out and out, till it fell, probably, under the sway of some distant star. In any case, the earth would know the vagabond no more.
As gravity, depends upon mass, the larger the attracting planet, the greater is its critical velocity, the velocity it can just control; and, reversely, the smaller the planet, the less its restraining power. With the earth the critical velocity is between six and seven miles a second. If any of us, therefore, could manage to become faster than this, socially or otherwise, we could bid defiance to the whole earth, and begin to voyage on our own account through space.
This is actually what happens, as we have seen, to the molecules of hydrogen. If, therefore, free hydrogen were present at the surface of the earth, and met with no other gas attractive enough to tie it down by uniting with it, the rover would, in course of time, attain a speed sufficient to allow it to bid good-by to earth, and start on interspacial travels of its own. That it should reach its maximum speed is all that is essential to liberty, the direction of its motion being immaterial. To each molecule in turn would come this happy dispatch, till the earth stood deprived of every atom of free hydrogen she possessed.
It is a highly significant fact that there is no free hydrogen found in the earth's atmosphere. With oxygen and water vapor, and indeed all the other gases we know, the case if different; for their maximum speed falls far short of the possibility of escape. So they have stayed with us solely because they must. And, as a matter of fact, the earth's atmosphere contains plenty of free oxygen, nitrogen, and the like. The actions of the heavenly bodies confirm this conclusion. The moon, for example, possesses no atmosphere, and calculation shows that the velocity it can control falls short of the maximum of any of these gases. All were, therefore, at liberty to leave it, and all have promptly done so. Whatever the moon's attraction for lovers, no gas was sufficiently attracted by it to stay. On the other hand, the giant planets give evidence of very dense atmospheres. They have kept all that they ever had.
But the most striking confirmation of the theory comes from the cusps of Venus and Mercury; for an atmosphere would prolong, by its refraction, the cusps of a crescent beyond their true limits. Length of cusp becomes, consequently, a criterion of the presence of an atmosphere. Now, in the appearance of their cusps there is a notable difference between Venus and Mercury. The cusps of Venus extend beyond the semi-circle; Mercury's do not. We see, therefore, that Mercury has no appreciable atmospheric envelope.
Turning to the case of Mars, we find with him the critical velocity to be about three miles a second. This is, like the earth's, below the maximum for the molecules of hydrogen, but also, like the earth's, above that of any other gas; from which we have reason to suppose that, except for possible chemical combinations, his atmosphere is in quality not unlike our own.
Having seen what the atmosphere of Mars is probably like, we may draw certain interesting inferences from it as to its capabilities for making life comfortable. The first consequence is that Mars is blissfully destitute of weather. Unlike New England, which has more than it can accommodate, Mars has none of the article. What takes its place as the staple topic of conversation for empty-headed folk remains one of the Martian mysteries yet to be solved. What takes its place in fact is a perpetual serenity, such as we can scarcely conceive of. Although over what we shall later see to be the great continental deserts the air must at midday be highly rarefied, and cause vacuums into which the surrounding air must rush, the actual difference gradient owing to the initial thinness of the air must be very slight. With a normal barometer of four and a half inches, a very great relative fall is a very slight actual one. In consequence, storms would such mild-mannered things that, for objectionable purposes, they might as well not be. In the first place, if we are right, there can be no rain, nor hail, nor snow in them, for the particles would be deposited before they gained the dignity of such separate existence. Dew or frost would be the maximum of precipitation that Mars could support. The polar snow-cap or ice-cap, therefore, is doubtless formed, not by the falling of snow, but by successive depositions of dew. Secondly, there would be about the Martian storms no very palpable wind. Though the gale might blow at fairly respectable rates, so flimsy is the substance moved that it might buffet a man unmercifully without reproach.
Another interesting result of the rarity of the air would be its effect upon the boiling-point of water. Reynault's experiments have shown that, in air at a density 14/100 of our own, water would boil at about 127o Fahrenheit. This, then, would be the temperature at which water would be converted into steam on Mars. So low a boiling-point would make it impossible to cook anything in the open air. Boiled eggs could be prepared only under cover, and such people as liked their meat boiled would probably find it convenient to prefer it done differently. Fortunately, roasts would still remain possible. The lowering of the boiling-point would raise the relative amount of aqueous vapor held in suspension by the air at any temperature. At about 127o the air would be saturated, and even at lower temperatures much more of it would evaporate and load the surrounding air than happens at similar temperatures on earth. Thus at the heels of similarity treads contrast. We may now go on to such phenomena bearing on the Martian atmosphere as show it to differ from ours. Some of them we were able more or less imperfectly to explain; some we are not.
Although no case of obscuration has been seen at Flagstaff this summer, certain bright patches have been observed on special portions of the planet's disc. That they are not storm-clouds, like those which, by a wavelike process of generation, travel across the American continent, for example, is shown by the fact that they do not travel, but are local fixtures. Commonly, they appear day after day, and even year after year, in the same spots; for identical patches have been observed by different astronomers at successive oppositions. To this category belong the regions known as Elysium, Ophir, Memnonia, Eridania, and Tempe. Still smaller patches, apparently, more fugitive in character, have been seen this year by Professor W. H. Pickering. But the most marked instance of variability was detected in September last by Mr. Douglass, in the western part of Elysium. On September 22 and 23 he found this blissfully named region, as usual, equally bright throughout. But on September 24 he noticed that the western half of it had suddenly increased in brightness and far outshone the eastern half, being almost as brilliant as the polar cap. When he looked at it again the next night, September 25, the effect of the night before had vanished, the western half being now actually the darker of the two. So fugitive an effect suggests cloud, forming presumably over high ground, and subsequently dissipating; it also suggests a deposition of frost that melted on the next day. It is specially noteworthy that the canals inclosing the region, Galaxias and Hyblaeus, were not in any way obscured by the bright apparition. On the contrary, Mr. Douglass found them perceptibly darker than they had been, an effect attributable perhaps to contrast.
Although not storm-clouds, it is possible that these appearances may have been due to cloud capping high land. There are objections, however, to this view, as in the first place, there is evidence that the Martian mountains are low; in the second place that they would have to be phenomenally high to produce a change in temperature sufficient to condense the air about them and so cap them with cloud; and in the third place that the air is not dense enough to support clouds, anyway. Nevertheless a most singular phenomenon was seen by Mr. Douglass on November 24, a bright detached projection, for which from measurement he deduced a height of thirty miles. This would seem to have been a cloud. With regard to its enormous height, it is not to be forgotten that a few years ago, on the earth, phenomenal dust-clouds were observed as high as one hundred miles.
Something more in the line of the explicable was a phenomenon observed in 1879 and in 1881 by Schiaparelli. From October, 1879, to January, 1880, he noticed certain bright patches which appeared to surround the north pole in a sort of crown, the pole itself being invisible. In 1881 he saw the same ramifications again, in apparently the same place. At this latter opposition the north pole was much better placed for observation, and he was able to mark a curious subsequent action in these spots; for as time went on they gradually contracted toward the pole, till finally they consolidated into the north polar patch, which up to that time had been absent. The polar patch proper did not thus appear till more than a month till more than a month after the vernal equinox of the northern hemisphere.
Here, then, we have a very curious phenomenon, a phenomenon which seems to indicate that the seasonal wave of change acts as a unit across the planet's face; that instead of a more or less continuous deposit of moisture at the pole, such as occurs on earth, Martian atmospheric conditions oblige such deposit to creep gradually with the season up into polar latitudes, where it appears first as a crown of frost, and does not envelop the pole and become a polar cap till it has got higher. No sooner has this happened than the advance of following warmer isotherms causes it to begin to melt. One deduction from this thin air we must, however, be careful not to make: that because it is thin it is incapable of supporting intelligent life. That beings constituted physically as we are would find it a most uncomfortable habitat is pretty certain. But lungs are not wedded to logic, and there is nothing in the world or beyond it to prevent, so far as we know, a being with gills, for example, from being a most superior person. A fish doubtless imagines life out of water to be impossible; and similarly, to argue that life of an order as high as our own, or higher, is impossible, because of less air to breathe than that to which we are locally accustomed, is, as Flammarion happily expresses it, to argue, not as a philosopher, but as a fish.
To sum up, now, what we know about the atmosphere of Mars: we have proof positive that Mars has an atmosphere; we have reason to believe that this atmosphere is very thing,—thinner at least by half than the air upon the summit of the Himalayas,—that in constitution it does not differ greatly from our own, and that it is relatively heavily charged with water vapor.
In the next paper I shall take up the question of water upon the planet.