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Polar expeditions exert an extreme attraction on certain minds, perhaps because they combine the maximum of hardship with the minimum of headway. Inconclusiveness certainly enables them to be constantly renewed, without loss either of purpose or prestige. The fact that the pole has never been trod by man constitutes the lodestone to such undertakings; and that it continues to defy him only whets his endeavor the more. Except for the demonstration of the polar drift-current conceived of and then verified by Nansen, very little has been added by them to our knowledge of the globe. Nor is there specific reason to suppose that what they might add would be particularly vital. Nothing out of the way is suspected of the pole beyond the simple fact of being so positioned. Yet for their patent inconclusion they continue to be sent in sublime superiority to failure.

Martian polar expeditions, as undertaken by the astronomer, are the antipodes of these pleasingly perilous excursions in three important regards, which if less appealing to the gallery commend themselves to the philosopher. They involve comparatively little hardship; they have accomplished what they set out to do; and the knowledge they have gleaned has proved fundamental to an understanding of the present physical condition of the planet.

The antithesis in pole-pursuing between the two planets manifests itself at the threshold of the inquiry, in the relative feasibility with which the phenomena on Mars may be scanned. For, curiously enough, instead of being the pole and its surrounding paleocrystic ice which remains hidden on Mars, it is rather the extreme extent of its extension and the lowest latitudinal deposit of frost which lies shrouded in mystery. The difficulty there is not to see the pole but to see in winter the regions from which our own expeditions set out. And this because the poles are well displayed to us at times which are neither few nor very far between; while favorable occasions for marking the edge of the caps when at their greatest have neither proved so numerous nor so favorable. The tilt of the planet’s axis when conveniently placed for human observation has been the cause of the one drawback; the planet’s meteorological condition in those latitudes at that season the reason for the other.

What knowledge we have of the size of the caps in degrees upon the surface of the planet at this their extreme equatorward extension has been given in the last chapter. Their aspect at the time together with what that aspect betokens was not there touched upon. With it, therefore, and the peculiarities it presents to view we shall begin our account of the caps’ annual history.

South Polar Cap in winter.

When first the hemisphere, the pole of which has for half a Martian year been turned away from the sun, begins to emerge from its long hibernation, the snow-cap which covers it down even to temperate regions presents an undelimited expanse of white, the edges of which merge indistinguishably into the groundwork color of the regions round about. Of a dull opaque hue along its border, its contour is not sharp but fades off in a fleecy fringe without hard and fast line of demarcation. Such notably was the aspect of the north temperate zone in 1896 when, tilted as it then was away from us into a mere northern horizon of the planet’s limb, it showed prior to the definite recognition of the north polar cap in August of that year, and such too was the look of the disk’s southern edge both before and after the first certain detection of the southern cap in 1903 and 1905. Each was then in the depth of winter. For in Martian chronology the season corresponded in each at the time to what we know in our northern hemisphere as the latter part of February and the early part of March and the appearance of the planet’s surface in both was not unlike what we know at the same season in latitude 45°. Indeed, there is reason to suppose bad weather there then and the extreme fringe, from the pale tint it exhibited, to have been cloud rather than snow.

It is quite in keeping with what we know on earth or can conceive of elsewhere that such aspect should characterize the cap at or near the attainment of its greatest development. Whether it were not yet quite arrived at this turning-point of its career or had but slightly passed it a vagueness of outline would in either event proclaim the fact. For were the frost still depositing, the cap’s edge would show indefinite; and on the other hand had it just begun to melt, evaporation would give it an undefined edge before the melting water had gathered in sufficient quantities to be itself noticeable.

Its behavior subsequent to recognition bore out the inference from its aspect when it first appeared. While for many days prior to its coming unmistakably into view it was impossible to say whether what was seen of the southern cap in 1903 and 1905 was cloud or snow; so even after it had definitely disclosed itself it continued to play at odds with the observer. Showing sharp at the edges one day it would appear but hazily defined the next, thus clearly demonstrating itself to be at the then unstable acme of its spread. Such a state of things we are only too familiar with in our own March weather when after days of sunshine that have melted off the winter’s white and fringed it with rivulets and awakening grass, a snow-storm falling upon it powders the ground again that was beginning to be bare and at one stroke extends the domain of the snow while mystifying the actual limits it may be said to occupy. The same condition of things, then, is not unknown on Mars, and to fix the precise date of so wavering a phenomenon is not so much matter of difficult observation as of physical impossibility.

Hellas in winter.

Nor is the southern cap, at this the height of its winter expansion, confined strictly to its own proper limits. Faint extensions, now so connected with its main body as to form part and parcel of it, now so detached and dull of tint as to make the observer doubtful of the exact relationship, are generally to be seen attendant on it. Hellas in winter is much given to such questionable garb, and has in consequence been mistaken by more than one observer for the cap itself, appearing as it does well upon the southern limb and being often the only region to show white. Indeed, frost-bound as it then is, to consider it the polar cap, though possibly geographically incorrect, may climatologically be sustainable. Its northern extremity extends down to latitude 30°, a pretty low latitude for frost. Still such equatorward extension is not without corroborating parallel. In 1903, at what was in Martian dates April 26, the whole of the region south of the Solis Lacus and the Nectar showed white, with a whiteness which may as well have been hoarfrost as cloud. Now, the Nectar runs east and west in latitude 28°. So that in this instance, too, it is possible that arctic conditions knocked at the very doors of the tropics. Encroachment of the sort is equivalent to snow in Cairo and permanent snow at that; not an occasional snow flurry, but something to linger on the ground and stay visible sixty millions of miles away.

White south of Nectar and Solis Lacus.

Knowledge of either cap in this the midwinter of its year has been a matter of the most recent oppositions of the planet. Up to within the last few years our acquaintance with either cap was chiefly confined to the months,—one might almost say the weeks,—immediately surrounding the summer solstice of its respective hemisphere. The behavior of the caps during the rest of their career was largely unknown to us, from the very disadvantageous positions they occupied at the times the planet was nearest to the earth. Beginning with 1894, however, our knowledge of both has been much extended, by a proportionate extension of the period covered by the observations. It used to be thought impracticable to observe the planet far on either side of opposition; now it is observed from as much as four months before that event to the same period after it. The result is a systematic series of observations which in many ways has given unexpected insight into Martian conditions. One of the benefits secured has been the lengthening of the period of study of the cap’s career, a pushing of inquiry farther back into its spring history and a longer lingering with it in its autumnal rebuilding. Yet up to the very last opposition a gap in its chronology still remained between February 25 and April 1. The opposition of 1905 has bridged this hiatus and brought us down to the latter date, at which the melting of the cap begins in earnest.

From this point, April 1 on, we have abundant evidence of the cap’s behavior. Its career now for some time is one long chronicle of contraction. Like Balzac’s Peau de Chagrin it simply shrinks, giving out of its virtue in the process. The cap proceeds to dwindle almost under the observer’s eye till, from an enormous white counterpane spread over all the polar and a large part of the temperate zone, its area contracts to but the veriest nightcap of what it was before. From seventy degrees across it becomes sixty, then fifty, then forty, till by the middle of the Martian May it has become not more than thirty degrees in diameter. During this time, from the moment the melting began in good earnest, the retreating white is girdled by a dark band, of a blue tint, which keeps pace with the edge of the cap, shrinking as it shrinks, and diminishing in width as the volume of the melting decreases.

After the melting has been for some time under way and the cap has become permanently bordered by its dark blue band a peculiar phenomenon makes its appearance in the cap itself. This is its fission into one or more parts. The process begins by the appearance of dark rifts which, starting in from the cap’s exterior, penetrate into its heart until at last they cleave it in two. Rifts have been seen by several observers and in both caps; and what is most suggestive they always appear in the same places, year after year. Sometimes oppositions elapse between their several detections for they are not the least difficult of detail; but when they are caught, they prove to lie just where they did before.

The permanency in place of the rifts, a characteristic true of them all, shows them to be of local habit. Thus the rift of 1884 and 1897 reappeared again to another observer in the same position in 1901. They are, therefore, features of, or directly dependent on, the surface of the planet. But it will not do from this fact to infer that they are expressive of depressions there. The evidence is conclusive that great irregularities of surface do not exist on Mars. As we shall see when we come to consider the orology of the planet it is certain that elevations there of over two or three thousand feet in altitude are absent. Differences of temperature, able to explain a melting of the ice in one locality coincidentally with its retention in an adjacent one, must in consequence be unknown. And this much more conclusively than at first appears, for the reason that the smaller the planet’s mass the less rapidly does its blanket of air thin out in ascent above the surface. This is in consequence of the greater pull the larger body exerts and the greater density it imparts to a compressible gas like our atmosphere. Gravity acts like any force producing pressure and by it the envelope of air is squeezed into a smaller compass. But as this is done throughout the atmospheric layer it means a more rapid rarefaction as one leaves the body. The action is such that the height necessary to reach an analogic density varies inversely as the gravity of the mass. In consequence of this, to compass a relative thermometric fall for which a moderate difference of elevation would suffice on Earth, an immoderate one must be made on Mars. For gravity there being but three eighths what it is here, eight thirds the rise must be made to attain a proportionate lowering of temperature. This fact renders the above argument against elevation and depression being the cause of the phenomenon three times as cogent as it otherwise would be.

With so gradual a gradient in barometric pressure there and so low a set of contour lines, altitude must be a negligible factor in Martian surface meteorologic phenomena. Both density and temperature can be but little affected by such cause, and we must search elsewhere for explanation of what surface peculiarities we detect.

Meanwhile the rifts themselves, from being lines which penetrate the cap from its periphery in toward its centre, end by traversing it in its entirety and separating portions which, becoming outlying subsidiary patches, themselves proceed to dwindle and eventually disappear. The rifts usually take their rise from such broader parts of the cap-encircling blue belt as make beads upon that cordon and are clearly spots where the product of the melting of the cap is either specially collected, or produces its most visible effect.

So far the description might apply with substantial accuracy to either cap. Yet the conduct of the two is in some ways diverse and begins to accentuate itself from this point on.

Northern Cap hooded with vapor.

From the time that the north polar cap reaches a diameter of about twenty-five degrees, a singular change steals over it. From having been up to then of a well-defined outline it now proceeds to grow hazy and indistinct all along its edge. This change in its character at the same period of its career has been quite noticeable at each of the three last oppositions, so that small doubt remains that the metamorphosis is a regularly recurrent one in the history of the cap. Coincident with the obliteration of its contour, its dimensions seemingly enlarge. It is as if a hood had been drawn over the cap of a dull white different from the dazzling brilliance of the cap itself and covering more ground. Such is probably what occurs; with vapor for veil. The excessive melting of the cap produces an extensive evaporation which then in part condenses to be deposited afresh, in part remains as a covering, shutting off from our view the outlines of the cap itself. It would seem that at this time the cap melts faster than the air can carry it off. A sort of steaming appears to be going on, taking place in situ. For it clearly is not wafted away. The time of its coming too is significant. For the season is May 15, the height of time for a spring haze to set in. Then later it dissipates with the same quiet indefiniteness with which it gathered.

Northern Cap unmasked.

It is some time in Martian June before the spring haze clears away, and when it does go, only a tiny polar cap stands revealed beneath it, from six to eight degrees across, or from a tenth to a fifteenth of what it was when it passed into its curious spring chrysalis. The date of emergence varies. In 1903 it occurred early, the haze not being marked after June 3, though recurring again at intervals for a day or so. In 1905 it was later; perceptibly thin after June 21 it did not certainly clear away till June 9 and came back again on July 16 and possibly on the 25th.

These vicissitudes of aspect give us glimpses into a sweet unreasonableness in Martian weather which makes it seem more akin to our own. And this on two counts, diurnal and annual. From day to day atmospheric conditions shift for purely local cause; while, furthermore, successive Martian years are not alike. In some the season is early; in others late. So that Mars is no more exempt than are we from the wantonness of weather.

Clearly disclosed thus reduced to its smallest possible terms it remains for some months of our days, for six weeks of its own. During that period it continues practically unchanged, neither increasing nor decreasing significantly in size, nor altering notably in aspect. Measures of the drawings of it then make it from five to eight degrees across and it is possible that it really fluctuates between narrow limits, though its clear-cut outline at all times renders the variation difficult to explain. We are not so near it as we could wish; for on these occasions even at their best it is over two hundred times as distant as the moon and the greatest magnification possible still leaves it a hundred thousand miles away.

To the south polar cap a somewhat similar history attaches, but with a difference. In its case no such regularly recurrent spring haze has yet been noted. The melting of this cap would seem to be of a more orderly nature than its fellow and not to outdo what can conveniently be carried off.

That an excess of evaporation should not take place is the more peculiar from the fact that at its maximum it is the larger of the two and therefore has the greater quantity of matter to get rid of. Its summer, also, is shorter than the arctic one, so that it has the less time to dispose of its accumulations. The only other respect in which it seems to be differently circumstanced from its antipodes is in the character of its surroundings. About it are large blue-green areas which with intermissions stretch down in places to within less than ten degrees of the equator; whereas the other pole is continuously encircled for long distances by practically uninterrupted ochre. The character of the environment seems thus the only thing that can account for the difference in behavior and this proves the more plausible when we come to consider what those two classes of regions respectively represent.

In other ways as well the southern cap is the more self-contained. The rifts, indeed, break it up into separate portions and these in part remain as outlying detachments of the main body, as was notably the case in 1877 and in 1894, but they hardly have the permanency and importance of those similarly formed about the arctic pole. Nothing antarctic for instance compares with the subsidiary patch of the north polar regions lying in longitude 206°, which both in Schiaparelli’s time, and during the late oppositions as well was almost as fixed a feature of the arctic zone as the cap proper. Not quite so constant, however, and not so solid-looking a landmark is this patch for all its extent, which nearly equals the area of the more legitimate portion. It bears on its face a more pallid complexion as if it were thinner, and this is borne out by the fact that it occasionally disappears, an event which so far at least has never befallen the northern cap itself.

Less constant the southern one is to its own minimum than the northern. In some seasons, in most in fact, it reaches like the other a more or less definite limit of diminution which it does not pass. But this is not always the case. In 1894 it disappeared entirely at the height of its midsummer. The season was probably unusually hot then in the southern hemisphere of Mars.

In position the caps have something to say about physiographic conditions. Both caps at their minima are then irregular and the centre of the south one is markedly eccentric to the areographic pole. It lies some six degrees north along the thirtieth meridian. The northern one is also probably eccentric, but much less so, with a divergence not much exceeding a degree and of doubtful orientation. Not only are both caps not upon their respective poles but they are not opposite each other, the one lying in longitude 30°, the other in 290°. This speaks, of course, for local action. In some wise this must depend on the configuration of the surface, yet so far as markings go there is nothing to show what the dependence is.

The eccentring of the caps is paralleled by the like state of things on earth. The pole of cold does not coincide in either hemisphere with the geographic pole. On the earth its position is largely determined by the distribution of the land-masses. Continents are not such equalizers of heat as oceans because of their conductivity on the one hand and their immobility on the other. In winter they part with their heat more quickly and convection currents cannot supply the loss. This accounting for thermal pole eccentricity is inapplicable to Mars because of the absence there of bodies of water. And it is significant that the degree the earthly poles of cold are out much exceeds what is the case on Mars. Possibly areas of vegetation there replace to some effect areas of water. It is certainly in favor of this view that the arctic regions there are more desert than the antarctic and that the north pole of cold occupies more squarely the geographic pole.

Not till 1903 did the actual starting again of either cap chance to be seen. Nor was this, indeed, a matter of hazard but of persistent inquiry by observation prolonged after the planet had got so far away that its scanning had hitherto been discontinued. Such search beyond the customary limits of observation was essential to success, because of the relation of the axial tilt to the position of the planet in its orbit. At an opposition well placed for nearness, the tilt is such as largely to hide the pole and to present the polar regions too obliquely to view for effective scanning. This is true both of the arctic and the antarctic regions in turn. For the Martian axis being inclined somewhat as our own is to the plane of the planet’s orbit, we at times see well and at times but poorly the arctic or antarctic zones.

The cap, the starting to form of which was thus caught, was the arctic one; the date 128 days after the northern summer solstice, or thereabouts, for as is perhaps natural the advent of the phenomenon partook of the wavelike advance of such things familiar on earth, an advance succeeded by a recession and then followed by another advance. So much is proof of local weather there as here. Hoar-frost was successively deposited and then melted off.

Deposition of frost.

What is significant, the deposition of the frost took place simultaneously over large areas. The very first patch of it, in about longitude 320°, extended at one stroke down to latitude 55°. For it actually crossed the Pierius somewhat to the south. A second patch stretched to the east of the cap. Two wings these made to the kernel of cap itself. Through the wings could be marked the line of the canal: the Pierius upon the one side, the Enipeus upon the other. Such visibility of the canals through the white stretches proved the white not to be due to cloud suspended between us and them, but a surface deposit which found no lodgment upon the canals themselves. The same avoidance of dark markings was evidenced by the showing of the dark rim round the cap’s kernel. Now, if the deposit were indeed hoarfrost, this failure to find permanent foothold on the dark markings is what we should expect to witness. For whether they were vegetation or water, equally in either case the frost would melt from them first. Probably they were both vegetal, though some doubt might exist about the latter, the band around the kernel. It was then August 20 in that hemisphere.

Such deposition over great stretches of country is perhaps not so surprising as it appears at first sight when seen from without in its totality. After all, something not unlike it occurs in our snow-storms when hundreds of square miles are whitened at once. Furthermore, with an atmosphere as thin as Mars seems to possess the temperature must be perilously near the freezing-point in the arctic and subarctic regions at the close of summer.

Steadily, with intermissions, the white sheet increased until even the dark border to the cap became obliterate, the kernel showing at first through the veil like the ghost of what it had been, and then ceasing to be visible at all, its delimitations being buried under deeper and deeper depositions of frost.

First northern snow.

The perennial portion of the cap was thus merged in the new-fallen snow. This marked the on-coming of the arctic winter in full force and happened even before the polar sun had wholly set. For the pole did not enter into the shadow till two of our months later, the autumnal equinox occurring 183 days after the summer solstice or 55 days after the first fall of frost. Then the pole passed into its star-strewn arctic night, a polar night of twice the duration of our own and the circumpolar regions entered upon their long hibernation of ten of our months.