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Chapter II. Atmosphere

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.

  • See Plates 15 16 17Uppermost figure.
  • 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.

    It was not, however, the loss of phase that was in question. That would have decreased the values of the equatorial diameter instead of increasing them, and, what is more immediately to the point, the correction for it had already been made. This correction is easily ascertained, for it depends chiefly upon the position of the planet in its orbit, which is known with great accuracy. The resulting values, therefore, had nothing to do with the phase correction as such, but they did, nevertheless, have to do with the phase itself.

    To see exactly how this is possible, let us consider the effect an illuminated atmosphere would have upon the measurements in question. To make matters more obvious we will introduce a diagram. The inner circle represents a section of the planet in the plane of the ecliptic; the arrows, the directions in the same plane of the Sun and Earth from the centre of the planet, in the different positions to be considered; and the outer circle, an atmosphere surrounding the planet, at the limit at which it is dense enough to reflect light.

    At opposition the Earth lay very nearly in the same line from the planet as the Sun. This is shown by the left-hand arrow. The illuminated semi-circumference of the planet's surface, at that time also the semi-circumference seen from the Earth, was gabp , and gop was the equatorial diameter; g'a'b'p' and g'op' the semi-circumference and equatorial diameter, upon the supposition of an atmospheric envelope encircling the surface. As the Earth and Mars passed along their orbits, the line from Mars to the Earth shifted into its second position, the Sun remaining as before. The illuminated part of the surface of Mars continued, therefore, to be gabp ; but the portion of this illuminated surface visible from the Earth was only dbp , the part gd being invisible from the Earth, and the part ph lying in shadow. If, however, there were an atmosphere capable of reflecting light up to a height represented by the greater circle, the Sun's rays would strike the upper visible limit of this atmosphere, not at p' but at s , sr being drawn parallel to the line from o to the Sun. The measured equatorial diameter, which is, of course, the projection of the arc d'b's on the line d'h' , would be d'f instead of de , which it would be were there no atmosphere. It thus appears that owing to side-lengthening, as we may perhaps style this reverse of foreshortening, the fringe of atmosphere increases in apparent width with increase of phase, to an apparent increase of the equatorial diameter.

    If, now, we take a third position for the Earth where Mars shows a yet greater phase, the third arrow, we find that in this case the resulting apparent increase in the equatorial diameter is mn , and we notice that mn is greater than, ef , just as ef was greater than pp' or cc'. That is, we see that the apparent increase in the size of the equatorial diameter varies directly, according to some law, with the increase in phase, or, as it is technically put, is a function of the phase.

    This increase, being an increase in the measure itself, would in due course come in for its share of all the corrections applied to the diameter. In consequence, that diameter, instead of coming out simply the full equatorial diameter, would come out too big in proportion to the amount added by the twilight arc.

    Pursuant, therefore, to the supposition that such was the cause of the increase, I took the means of the polar and of the equatorial diameters with regard to the time from opposition, at which the measures were made, to find myself confronted by a series of values counterparting what we have just seen would be given by the presence of a visible twilight arc. The resulting values are: --

    Polar Diameters:

  • October 15 to 23 inc. 9".35
  • October 12 and 24 to 30 inc. 9".35
  • November 2 to 21 inc. 9".36

    Equatorial Diameters:

  • October 15 to 23 inc. 9".40
  • October 12 and 24 to 30 inc. 9".43
  • November 2 to 21 inc. 9".53
  • The measure of the 12th of October and those of the 24th to 30th are taken together, because equidistant from opposition on October 20.

    The agreement of this table with that deducible by theory from the effect of an atmosphere is striking. But the agreement is even more exact than appears. For, as the polar axis was not in the same line as the axis of phase, the twilight arc to some extent affected the polar diameter at all times, but specially during November. This becomes evident, numerically, on applying the correction for an atmosphere, which gives the following values:

    Polar Diameters:

  • October 15 to 23 inc. 9".32
  • October 12 and 24 to 30 inc. 9".31
  • November 2 to 21 inc. 9".32

    Equatorial Diameters:

  • October 15 to 23 inc. 9".37
  • October 12 and 24 to 30 inc. 9".36
  • November 2 to 21 inc. 9".37
  • The middle values are evidently somewhat too small, since they affect both the polar and equatorial diameters alike. Otherwise the variation in the values of the same diameter is less than the probable errors of observation. Taking the mean of all but the middle ones, we deduce the value for the polar flattening given above, 1/190 of the equatorial diameter.

    From the correction for the effect of the atmosphere, we find the amount of the twilight arc upon the planet visible from the Earth to be about 10 degrees. That of the Earth, as seen from the Earth's surface, is 18 degrees but it is to be noticed that here the point of view is important From the topmost layer of our air of sufficient density to be capable of reflecting light we are but forty miles away; from the corresponding layer of the Martian air we are forty millions of miles off. We cannot, therefore, expect to detect the one to the same extent that we can the other. The value, then, for the Martian twilight arc of 10 degrees is simply a minimal value, not an absolute one. The twilight arc cannot, from the observations, be less than this, but it may be much more.

    The large number of measures from which the above means were deduced not only renders error in the result less likely, but shows that result to be due to air pure and simple. This appears from the fact that the observed increase is systematic. For its systematic character proves it due to something largely transparent. It is because it is chiefly not seen that it is seen at all. At first sight this deduction seems paradoxically surprising. But, in considering the problem, we shall realize that it must be so.

    If what was seen were opaque, as, for example, a mountain, then in certain positions it would indeed be seen projecting beyond the terminator,--for example, if it were at s in the diagram; if, on the other hand, it were in the position r , it would, instead of apparently increasing, decrease the diameter. Now, as the rotation of the planet would bring it eventually into all possible positions, it would be as likely on any one occasion to be measured in a position to decrease the diameter as to increase it. From but a few measures, therefore, it might appear that there was an increase in the calculated diameter, or it might seem that there was a decrease from it, and either would be equally likely to happen. If, however, many measures were made, and just in proportion as they were many, those decreasing the diameter would offset those increasing it, and the mean of all would show no trace of either. In the mean the minus quantity would wipe out the plus. Indeed, owing to the fact that both the Sun and the Earth are not infinitely far off from Mars, and in consequence that all the lines to them are not strictly parallel to one another, the decreasing effect would actually slightly exceed the increasing effect, but this would be too small to be perceptible.

    The same argument that applies to mountains applies to clouds, or to any opaque substance. Sporadic increase might be due to them; but for the increase to be systematic, it is necessary that the substance seen should also be seen through. It must be in part transparent. The measures, therefore, not only disclose the presence of an atmosphere, but do so directly.

    Having thus seen first with the brain and then with the eye, and both in the simplest possible manner, that a Martian atmosphere exists, we will go on to consider what it may be like.

    The first and most conspicuous of its characteristics is 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 the year to the other, nothing appears to veil the greater part of the planet's surface.

    This would seem to be even more completely the case than has hitherto been supposed. We read sometimes in astronomical books and articles picturesque accounts of clouds and mists gathering over certain regions of the disk, hiding the coast-lines and continents from view, and then, some hours later, clearing off again. Very possibly this takes place, but not with the certainty imputed to it. It is also doubtful if certain effects of longer duration are really attributable to such cause. For closer study reveals another cause at work, as we shall see later, and the better our own air the more the Martian skies seem to clear. Certainly no instance of the blotting out of detail upon the surface of Mars has been seen this year at Flagstaff. Though the planet's face has been scanned there almost every night, from the last day of May to the end of November, not a single case of undoubted 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 feature the observer might be engaged in looking at.

    The only certain dimming of detail upon the Martian disk has been along its bright semicircular edge or edges, as the case may be,--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 at Flagstaff 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 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 stretched from one pole to the other, quite oblivious of the fact that every hour from sunrise to sunset lay represented along the limb, including high noon. What is more, as the disk passed, in course of time, from the 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 across it from one side of the disk to the other. As it passed the centre the regions there showed perfectly clear; not a trace of obscuration visible as it lay beneath the observer's eye.

    From the first observation it is evident that Martian sunrise and sunset had nothing to do with the phenomenon, since it was not either Martian sunrise or sunset at the spot where it was seen; and, from both observations taken together, it is evident that the phenomenon did have to do with the position of the observer. For nothing on Mars had changed in the mean time, but only the point of view of the observer on Earth. It is clear, therefore, that it was not a case of Martian diurnal meteorological change, but a case of foreshortening of some sort.

    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 rim is similarly ringed by a lune of light. In her case the effect has been attributed to mountain slopes holding the Sun's light at angles beyond the possibility of plains. But Mars has few mountains worthy the name. His terminator--that is, the part of the disk 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.

    Not due, then, to either mountains or mist, there is something we know that would produce the effect we see,--dust or water particles in the Martian air; that is, just as the Earth's atmosphere is somewhat of a veil, so is the Martian one, and this veiling effect, though practically imperceptible in the centre of the disk, becomes noticeable as we pass from the centre to the edge, owing to the greater thickness of the stratum through which we look. At thirty degrees from the edge, our line of sight pierces twice as much of it as when we look plumb down upon the centre of the disk and more yet as we approach the edge itself; in consequence, what would be diaphanous at the centre might well seem opaque toward the limb. 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 dust, or ice, or water in the air. Here, then, we have a hint of the state of things on Mars. Ice particles both are probable and would give the brilliancy required.

    This first hint receives 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 appears to be astonishingly mild. Whereas calculation from distance and atmospheric density would put its average temperature below freezing, thus relegating it to perpetual ice, the planet's surface features imply that the temperature is relatively high. Observation gives every evidence that the mean temperature must actually be above that of the Earth; for not only is there practically no sign of 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 rare chance for quixotic polar expeditions. Such pleasing amelioration of the climate must be accounted for, and aqueous vapor seems the most likely thing to do it; for 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 the free gases 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 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 contain the least. 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.

    Calculating the effect of the above causes numerically we find that on this a priori supposition Mars would have at his surface an atmosphere of about fourteen hundredths, or one seventh, of the density of our terrestrial one.

    Observation supports this general supposition; for the cloudless character of the Martian skies is precisely what we should look for in 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 offer varies as the surface they present to the air, and this 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 probable 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 made by a teaspoon drawn through a cup of tea. But, whatever it be, the idea of it accounts very creditably 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. From the observed relations of the volumes and weights of gases to the pressures to which they are subjected is deduced the fact of this speed and its amount. It appears 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 more than a mile a second. But this average velocity may, for any particular molecule, be increased by collisions with its neighbors. The maximum speed it may thus attain Clerk-Maxwell deduced from the doctrine of chances to be sevenfold the average. What may thus happen to one, must eventually happen to all. Sooner or later, on the doctrine of chances, each molecule of the gas is bound to attain this maximum velocity of its kind. When it is attained, the molecule of oxygen travels at the rate of one and eight tenths miles a second, the molecule of water vapor at the rate of two and one half miles a second, and the molecule of hydrogen at over seven miles a second, or four hundred and fifty 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 coming down again. But even the Earth is not omnipotent. As the velocity with which the body is projected increases, longer and longer time is needed for the Earth to overcome it and compel the body's return. Finally there would be reached a speed which the Earth would just be able to overcome if she took 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, if there were no other bodies in the universe but the Earth and the molecule, till it attained infinity, at which point the truant would stop, and then reluctantly return. This velocity we may call the critical velocity. It is also known as the parabolic velocity, because it is at any point the velocity of a body moving in a parabola about the Earth, under the Earth's attraction; the parabola being the curve of a fall from infinity. The critical velocity is the parabolic velocity, inasmuch as gravity is able to destroy on the way up just the speed it is able to impart 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 out and out forever. If its speed at starting were less than twenty-seven miles a second, it would become thenceforth a satellite of the Sun; if its speed were yet greater, it would become an independent rover through space, paying brief visits only to star after 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 six and nine tenths miles a second. If any of us, therefore, could manage to acquire a speed greater than this, socially or otherwise, we could bid defiance to the whole Earth, and begin to voyage on our own account through space. [See Appendix]

    This speed is actually attained, as we have seen, by the molecules of hydrogen. If, therefore, a molecule of 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 molecule after molecule would come this happy dispatch, till the Earth stood deprived of every atom of free hydrogen.

    Now, it is a highly significant fact that there is no free hydrogen found in the Earth's atmosphere. There is plenty of it in the captivity of chemical combination, but none in the free state. This coincidence of lack of hydrogen with lack of liberty takes on yet more significance from the further fact that the same is not true of oxygen, water vapor, or indeed of any of the other gases we know. With them, freedom is not synonymous with absence. The Earth's atmosphere contains plenty of free oxygen, nitrogen, and the like. But, as we have just seen, [See Appendix] the maximum speed of all these gases falls short of the possibility of escape. This accounts for their presence. They have stayed with us solely because they must.

    The appearance of the other heavenly bodies seems to 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 our atmospheric gases, that velocity being but one and one half miles a second. All were, therefore, at liberty to leave it, and all have promptly done so. On the other hand, the giant planets give evidence of very dense atmospheres. They have kept all 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 apparently little or no atmospheric envelope, and we find that his critical velocity is only 2.2 miles per second,--below that of water vapor, and perilously near that of nitrogen and oxygen.

    Turning to the case of Mars, we find with him the critical velocity to be three and one tenths miles a second. Now, curiously enough, 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 very 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 of it 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 there, 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 of 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 be such mild-mannered things that, for objectionable purposes, they might as well not be. In the first place, there can be but little rain, or hail, or snow, for the particles would be likely to be deposited before they gained the dignity of such separate existence. Dew or frost would be the common precipitation on Mars. 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 un-mercifully 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 127 degrees Farenheit. This, then, would be the temperature at which water would be converted into steam on Mars. So low a boiling-point would raise the relative amount of aqueous vapor held in suspension by the air at any temperature. At about 127 degrees 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 some phenomena of the Martian atmosphere of a more specific character.

    II. Clouds

    Although no case of obscuration has been seen at Flagstaff this summer, certain parts of the planet's disk have appeared unaccountably bright at certain times. That these are not storm-clouds, like those which, by a wave-like 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, the same places appear bright continuously day after day and recurrently year after year, different astronomers at successive oppositions having so observed them. To this category belong the regions known as Elysium, Ophir, Memnonia, Eridania, and Tempe, which at certain seasons of the Martian year are phenomenally brilliant. They stay so for some time, and then the brightness fades out to appear again at the next opposition. Still smaller bright spots, apparently more fugitive, have been seen this year by Professor W.H. Pickering, notably just north of the Mare Sirenum. None of the phenomena look distinctively like cloud. There are, however, phenomena that do.

    Toward the end of August there were seen several times, first by Professor Pickering and then by me, strange flocculent collections of white patches, about fifteen degrees from the pole, in the place where the snow-cap had been, the cap itself having retreated farther south. In look they were unlike the snow-cap; and also unlike the land. But they did have very much the look of clouds. Possibly they were clouds, formed from the vapor left in the air by the melting of the cap. It was then but a few days to the summer solstice.

    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 bright 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, melting on the next day. It is specially noteworthy that the canals inclosing the region, the Galaxias, the Boreas, and the Eunostos, 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 thin cloud, capping high land. There are objections to this view, but as there are graver ones to any other it may stand provisionally, the more so that there are appearances not easily reconcilable with other cause. For example, a most singular phenomenon was seen by Mr Douglass on November 25, a bright detached projection, for which, from measurement, he deduced a height of thirty miles. This would seem to have been cloud, for the details of its changes in appearance seem quite incompatible with a mountainous character. 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.

    We now come to a highly interesting class of observations bearing upon the question of clouds,--Mr. Douglass's terminator observations. During the last opposition, seven hundred and thirty-six irregularities upon the terminator of the planet were detected at Flagstaff. They were seen by one or more of three observers, but chiefly by Mr. Douglass, who made a systematic scrutiny of the terminator for almost every degree of Martian longitude. Their full presentation would be both too tabular and too technical for this book. The paper embodying them will be found among the published annals of this observatory. I shall here give only certain deductions from it.

    Of the 736 irregularities observed, 694 were not only recorded but measured. Of these 403 were depressions. It is singular, in view of their easy visibility, that they never should have been noticed before. Schroeter, indeed, saw three appearances of the sort,--on September 21, 1798, November 12, 1800, and December 18, 1802,--but all on the limb, not the terminator, which shows them not to have been of those here meant. Nevertheless they are not difficult to see, and anything but rare. When the phase is large enough, several may be seen every night.

    The projections number 291. As their number shows, they are less common than the depressions, but they are even less of a feature of the surface than their number would indicate, for the depressions extend as a rule much further both in latitude and longitude.

    Usually the depressions look like parings from the planet's rind, and almost always appear upon that part of the terminator where the dark regions are passing out of sight; commonly therefore, in the case of the southern hemisphere, they are met with between latitudes 30 degrees to 60 degrees south. Not so common is it for them to occur over a part of the planet which is bright. Furthermore, they appear to occur more or less continuously. This would not be the case were they real depressions.

    As this may not at once be evident to the reader, and yet is easily made evident, we will consider the diagram on page 38. It will there be seen that an elevation like s or r--and the same reasoning applies mutatis mutandis to a depression--appears projected a relatively long way without or within the terminator, as compared with its actual length, owing to the angles under which it is respectively illuminated by the Sun and seen from the Earth. The relation between its height and its distance from the edge is that between the height of a hill and the shadow it casts at sunrise or sunset. What, therefore, is not high enough to be seen in profile on the limb, becomes vicariously visible on the terminator. But a hill could not continue long to appear as an elevation, as the rotation of the planet would carry it in due course from the position r to the position s, and there it would be forced to masquerade as a depression. The same, reversely, would happen to a valley. In order that a depression should appear continuously, there must be a belt of lower level along its circle, and this could not be made visible as in the former case by projection, since projection depends upon difference of level along the same surface contour, not as between adjacent ones. It could, therefore, only be noted by its actual profile,--a very small affair, still further diminished by reason of the angle under which that profile was viewed. The resulting quantity in the case of Mars would be exceedingly minute. We perceive therefore, on the very threshold of our inquiry, reason to doubt the mountainous character of the irregularities. Such inference becomes the more probable on a more detailed investigation, into which we will now enter. This investigation depends upon a very important principle; namely, that if we have, as in this case, a great number of observations, it is possible, by dividing them into classes according to their kind and then taking the mean value of each class, to discover characteristics not otherwise exposed.

    Means are very telling things. They are so from the fact of simplifying the effects of the factors at work. By taking the average of the series of observed values according to some definite principle, not only do we eliminate a very large class of errors, but we allow by so doing the various causes to unmask their separate results. The importance of reasoning upon averages could hardly be more strikingly exemplified than in the very case before us,--that of these depressions and projections seen on the terminator of Mars.

    Of the 694 irregularities measured, 291 were projections and 403 were depressions. Here at the very outset, then, we perceive an objection to the theory that they are due to mountains, to wit, because the number of depressions so greatly exceeds the number of projections. As previously explained on page 64, mountains would produce on the average as many projections as depressions, for they would project the light on the one side as much as they would cut it off on the other.

    Now let us classify these irregularities, and see if we can gain further information about them. There were two kinds of them,--the long and low, and the short and sharp. Each kind had its representatives among both the projections and the depressions. Of the short and sharp variety there were 95 projections. These averaged 0.276 seconds of arc in height. Of the same kind there were similarly 57 depressions which averaged 0.368 seconds of arc in depth. It will be noticed then, first, that the projections of this character exceeded in number the depressions of the same; secondly, that the average depth of the depressions exceeded the average height of the projections. Now, if the appearances had been due to mountains, both the number and size of the projections and of the depressions should have been substantially the same. They were emphatically neither. Consequently mountains fail to explain them. But there is another possible set of phenomena that will; namely, clouds. For, in the first place, clouds would cause apparent depressions and projections, since the light would linger on them as it does on mountain tops, and they would cast shadows as mountains do. But furthermore their two effects, of extending or curtailing the limit of vision along the terminator, would not necessarily be equal, as would be the case with hills. Because it is a peculiarity of mountains that they are attached to the soil, and are commonly permanencies; while clouds are not. The latter form and dissipate, dissipate and reform, and their metamorphoses are phenomena depending upon the time of day. Consequently they may appear in one place at one time, in another the next; and what is no less important, they may form at different heights at different times. They therefore not only account for irregularities on the terminator, but they account also for irregularity in the plus or minus character of these irregularities. Clouds, therefore, are capable of explaining the case before us, although mountains are not.

    From what we have just shown let us mark now just what clouds are here required to account for what we see. The clouds that cause depressions are those within the terminator,--those, that is, that form before sunset or after sunrise; while those that cause projections are those that gather after sunset or before sunrise. As the observed projections in this case exceed the depressions in number, we infer, then, that there are more clouds after nightfall than before it, and similarly more before daybreak than after it; next, as the average depression is greater than the average projection, we likewise infer that the day clouds lie at a higher altitude. Now, this is precisely what we should expect would be the case, just as it is the case on the Earth.

    Of the other class of irregularities, the long and low, there were observed 196 projections and 346 depressions. The projections averaged 0".136 in height; the depressions, 0".125 in depth. Here, then, we have an opposite state of things from that with which we were confronted in the short and sharp class. Here, as compared with the projections, instead of relatively few depressions of greater height, we have relatively many depressions of less height. Furthermore, there are a great many more of both projections and depressions than there were of the former variety, and they are both of much less height or depth. Evidently, therefore, we have here, in part at least, a different class of phenomena from what we have previously considered. Now we perceive at once that two factors enter here which did not enter in the case of the short and sharp irregularities. The long and low depressions occur, as we shall recall, almost always over the dark areas, while the short and sharp ones do not. In the next place, the average height or depth of the long and low irregularities is much nearer the value of the irradiation constant, that is, the amount by which a bright object seems bigger on account of its brightness; which would cause the dark areas to seem depressed. From these facts we infer that most of the depressions of this class are due to the character, not to the contour, of the surface where they occur; partly to the direct effect of lack of irradiation, partly to sombreness of the surface, which would cause the light to fade from them at a greater relative distance from the terminator. On eliminating these depressions, therefore, we find ourselves left with very few depressions as against nearly 200 projections. The excess in number of the latter shows, as in the case of the other variety, that we are here dealing chiefly with long and relatively low clouds formed after sunset or before sunrise; those so formed during daylight being few if any.

    One more observation made at Flagstaff, on the subject of cloud, is as peculiar as it is important. It was made by Mr. Douglass, and I shall give it in his own words. A more detailed account of it, together with his tables of figures, will appear in his paper upon it in the Observatory annals: --

    "On November 25 and 26 a bright spot was seen in the unilluminated portion of Mars, to which, in my opinion, no other name than cloud can be applied. Its great height, size, and brilliancy, and, on the second evening, its singular fluctuations, render it of importance in the study of the Martian atmosphere.

    "I first saw it at 16h. 35m., G.M.T., of November 25, and made an estimate of its height. It seemed to be rapidly increasing in length in a direction parallel to the terminator at that point. Subsequent estimates of its height gave a different and greater value than at first; until its sudden disappearance at 17h. 6m., or perhaps a minute later. After once attaining its size, it seemed to remain with little change, presenting the appearance of a line 115 miles long by 33 miles wide at the centre and lying parallel to the terminator, but separated from it by an apparent space of over 80 miles. It was generally yellowish in color, like the limb, but of less brilliancy than the centre of the disk, though distinctly surpassing in that respect the adjacent terminator. I estimated it to have the brilliancy of the bright areas of the disk at a distance of 9 degrees from the terminator. In one view it appeared to be a very small whitish point, and I am inclined to think that there may have been a real diminution in its size at that moment. This idea is partly sustained by the following night's observations. At 16h. 54m. it was observed by Professor Pickering, whose estimate gave 11 miles for its height. At 17h. 5m., after obtaining two readings of the micrometer screw for latitude, the seeing, which had been quite steadily at the figure 7 (on a scale of 10), dropped to 4, and in attempting the next setting I could not find the 'cloud,' although once before it had remained visible when the seeing dropped instantaneously to that figure. Nor did it reappear in the next half hour. This sudden disappearance, without any previous lessening of its height above the terminator or of its size, made its cloud character unmistakable, since a mountain beyond the sunrise terminator must either constantly decrease in height, or soon join the illuminated disk.

    "A subsequent computation showed that this phenomenon took place over the southern part of Schiaparelli's Protei Regio. Other reasons lead me to think, however, that he has placed that island some 5 degrees too far south.

    "On November 26 the cloud promptly appeared at 17h. 15m., G. M. T., but about 12 degrees farther north. Instead of remaining continuously visible, it dissipated and reformed at irregular intervals. The first appearance lasted sixteen minutes. After somewhat over four minutes had passed, it reappeared momentarily, and six minutes elapsed before it appeared again, lasting then but two and one half minutes. Then followed an absence of three minutes, presence for two minutes, absence for three minutes, presence for one minute, and a final brief appearance eight minutes later at 18h. 1m. Its presence was suspected five minutes before that hour, and again at 18h. 11m., but with great uncertainty.

    "At this time it presented in general the same characteristics as the night before, though its appearances were too brief to permit such careful observations as were hoped for. The seeing, too, was not so good as before, varying from 4 to 7; and if the cloud happened to appear under the former figure, its observation was difficult. It is needless to remark that under such conditions it was impossible to observe its appearance or disappearance to the second. In general, it seemed to exhibit a less elevation than the night before. A careful estimate of its latitude placed it precisely at the centre of the terminator. I believe these latitude observations, though made rapidly, cannot be subject to an error greater than 2 degrees, at 18h., probably less than 1 degree. On November 27, at 18h., I searched for the cloud, but was not rewarded by finding any trace of it.

    "Estimates of the size and height of this cloud were made with reference to a glass thread in the micrometer, whose diameter is 0".6. One tenth of the thread was found to represent on Mars a little less than twenty miles. This gives us an elevation above the surface of between 10 and 11 miles. In this process we have taken the apparent centre of the cloud, and have assumed the seeing to have no influence. We obtain, therefore, the smallest possible mean height of the centre of the cloud. If we assume that the seeing was not perfect, its effect would be to lessen the separation, but not to change the total height. Supposing, for example, that the apparent extension of the cloud was due to poor seeing enlarging a point, then our terminator distance would be 245 miles, and our minimum elevation 15 miles. Therefore we can assume 15 miles to be the smallest probable mean elevation of this cloud. The average height of our cirrus clouds is five and one half miles.

    "One more idea requires mention, namely, the movement of this cloud in latitude. From the extreme rarity of clouds on Mars I am inclined to connect intimately the appearances of the two evenings, and consider them as due to one source, presumably a large body of air moving northward. Such an advance would be at the rate of 18.7 miles per hour."

    I may add to this that the height of the cloud--relatively to those of the Earth--is what direct deduction from the less rapid thinning out of the air above the Martian surface, which must result from the smaller mass of Mars, would lead us to expect. The air at the surface would be thinner than at the surface of the Earth, but the rate at which it diminished with the height above that surface would not be so great. At no very great elevation the two densities would come to be the same.

    One deduction from this thin air we must 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, as public speeches show, 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 this atmosphere to be very thin,--thinner at least by half than the air upon the summit of the Himalayas,--and in constitution not to differ greatly from our own.

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