Mars/Chapter 3
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| 1895 |
- Chapter 3
- I. The Polar Cap
After air, water. If Mars be capable of supporting life, there must be water upon his surface; for, to all forms of life, water is as vital a matter as air. On the question of habitability, therefore, it becomes all- important to know whether there be water on Mars. To the solution of this inquiry, also, the planet's polar cap turns out to hold the key. For just as the fact of change in the cap proves the presence of air, so the manner of that change implies the presence of water. It not only does this; it turns out to do a deal more. For to the whole water question it appears to play the part not only of occasion but of cause. In more senses than one, it is in that great glistening white patch that our water problem takes its rise.
On the 3d of June, 1894, the south polar cap stretched, almost one unbroken waste of white, over about 55 degrees of latitude. A degree on Mars measures 37 miles; consequently the cap was 2,035 miles across. Inasmuch as the inclination of the Martian equator to the plane of the Martian orbit is, according to Schiaparelli, 24 degrees 52', it must have then covered more than the whole south frigid zone of the planet.
Now, to take in the full meaning of the condition of the cap at this time and of the changes that ensued, we must begin by determining the Martian time of year. This is done by fixing the dates at which the Martian pole reached its maximum tilt toward or from the Sun, and the dates at which it was not tilted either to or from, but sideways to, the Sun; the former gives us the Martian solstices, and the latter the Martian equinoxes. It thus appears that on April 7, 1894, occurred the vernal equinox of the Martian southern hemisphere, on August 31, its summer solstice, and on February 7, 1895, its autumnal equinox. From these dates it is easy to transform the one calendar into the other. On the 3d of June, 1894, therefore, it was about May 1 on the southern hemisphere of Mars.
On May 1, then, Martian time, the cap was already in rapid process of melting; and the speed with which it proceeded to dwindle showed that hundreds of square miles of it were disappearing daily. As it melted, a dark band appeared surrounding it on all sides. Except, as I have since learned, at Arequipa, this band has never, I believe, been distinctively noted or commented on before, which is singular, considering how conspicuous it was at Flagstaff. It is specially remarkable that it should never have been remarked upon elsewhere, in that a similar one girdling the north polar cap was seen by Beer and Madler as far back as 1830. For it is, as we shall shortly see, a most significant phenomenon. In the first place, it was the darkest marking upon the disk, and was of a blue color. It was of different widths at different longitudes, and was especially pronounced in tint where it was widest, notably in two spots where it expanded into great bays, one in longitude 270 degrees and one in longitude 330 degrees. The former of these was very striking for its color, a deep blue, like some other-world grotto of Capri. The band was bounded on the north, that is, on the side toward the equator, by the bluish-green areas of the disk. It was contrasted with those both in tone and tint. It was both darker and more blue.
The band not only varied in width at different longitudes, but its width corresponded to the amount of the blue-green areas of the disk visible at these longitudes below it. It was widest where these were greatest in extent, and narrowest where they were least. If we consult the map of Mars we shall see that below the bay in longitude 330 degrees lies the great dark area, the Syrtis Major, and, below the one in longitude 270 degrees, the Syrtis Minor. This correlation was highly suggestive in itself. As if, however, to remove all question as to possible coincidence having a hand in the matter, the agreement in position was emphasized by visible connection. Two long dark streaks appeared joining respectively each bay to its corresponding Syrtis.
But the most significant fact about the band was that it kept pace with the polar cap's retreat toward the pole. As the white cap shrank it followed pari passu so as always to border the edge of the snow. It thus showed itself not to be a permanent marking of the planet's surface, since it changed its place, but a temporary one, dependent directly upon the waning of the cap itself. In short, it was an associated detail, and itself instantly suggested its character, namely, that it was water at the edge of the cap due to the melting of the polar snow.
Not only did the band conform with the cap in position; it did so in size. As the snows dwindled, the blue band about them shrank in width to correspond. By August it was a barely discernible thread drawn round the tiny white patch which was all that remained of the enormous snow-fields of some months before.
Finally, on October 13, when the snow entirely disappeared, as we shall presently see, the spot where it and its girdle, long since grown too small for detection, had been became one yellow stretch.
That the blue was water at the edge of the melting snow seems unquestionable. That it was the color of water; that it so persistently bordered the melting snow; and that it subsequently vanished, are three facts mutually confirmatory to this deduction. But a fourth bit of proof, due to the ingenuity of Professor W. H. Pickering, adds its weight to the other three. For he made the polariscope tell the same tale. On scrutinizing the great bay through an Arago polariscope, he found the light coming from the bay to be polarized. Now, to polarize the light it reflects is a property, as we know, of a smooth surface such as that of water is.
Before going further we will take up here at the outset the question of the constitution of these polar caps, which in their general behavior so strikingly suggest our own ice-caps as they would appear could they be seen from a distance of forty millions of miles. That they so instantly suggest snow has suggested, to that class of mind which likes to make of molehills of question mountains of doubt, the possibility that instead of ice we have here snow-caps of solid carbonic acid gas (carbon dioxide). The occasion of the suggestion is the fact that carbonic dioxide under certain conditions becomes a colorless liquid, and then a solid of a floccular, snow-like character. It assumes, in short, under proper conditions of pressure and cold, the various appearances presented by water under higher temperatures, although it does so with very different degrees of ease. Superficially, therefore, the idea seems plausible. Let us see if it still seems so when critically examined.
Faraday made experiments on the relation of the congealing point of carbonic acid gas to the pressure, and found that at 0 degrees C. it took a pressure of 36 atmospheres, that is, 540 pounds to the square inch, to solidify the gas, and that at -99 degrees C., the lowest temperature with which he experimented, it took 1.14 atmospheres. At this point the curve representing the relation was becoming apparently asymptotic, that is, a slight decrease in pressure involved a great falling off of temperature. Under a pressure of one atmosphere, therefore, the temperature would be about -170 degrees F., that is, on the surface of the Earth this would be the congealing point of the gas.
He found further that the curve for the liquefaction point lay very close to that for the congealing point, and approached yet closer as the pressure decreased. In other words, the gas passed almost immediately from the gaseous to the solid state.
In the light of these facts let us consider the condition of Mars. Three points arise which we will take in the inverse order of their importance. First: the appearance of the planet shows conclusively that, if the polar caps be composed of solid carbonic acid gas, then either there is no water at all on Mars in any form whatsoever, or what there is is ice so overlaid with detritus as to be invisible. For if the two substances were there together, and the cold at the surface of the planet of so extreme a character as to congeal the carbon dioxide, the water must a fortiori be frozen, and would continue so long after the temperature rose above the melting point of the former substance. We should therefore still have snow- fields of snow after the melting of those formed of carbonic acid gas, either visible as white patches or so covered up with dirt as to pass for land. Now there are no such additional white patches to be seen, nor, so far as we can judge, does any part of the planet behave as if it were glacier-bound.
Second: carbonic dioxide passes, as we saw, almost simultaneously into the liquid and solid states, especially under slight pressure. Now, the pressure is certainly very slight on the surface of Mars; not probably more than, and probably less than, one seventh of an atmosphere. In consequence, on a rise of temperature the frozen carbonic acid gas would there pass practically straight from the solid into the gaseous state. Now, from the existence of the surrounding polar sea, we remark that in the substance composing the polar caps of Mars this does not occur. A considerable portion of it is always in the transition state of a liquid. Carbonic dioxide would not thus tarry: water would.
Third: from the curve of metamorphosis, it is evident that the temperature necessary to freeze the gas under the pressure of one seventh of an atmosphere must lie between -100 degrees C. and -200 degrees C., if not lower. -200 degrees C. is, so far as we can judge, about the temperature of inter-planetary space, or what would be the temperature of the night side of Mars were the planet destitute of atmosphere. But there is an atmosphere on Mars, and, even if there were not, on melting the carbonic dioxide would itself make an atmosphere. This would instantly raise the temperature, and under any rise in temperature the congealing of the gas at once becomes an impossibility. The gas itself thus suggests its own refutation.
There is no such apparent objection to water. With an atmosphere properly constituted (and there is nothing to show that the Martian atmosphere is not so constituted), the temperature might easily rise high enough to melt ice. We may therefore conclude water to be the most probable solution of the question.
With such more or less solid ground to stand on, we may now go on to describe the behavior of the cap as constituted of snow. Whether we call it snow-cap or ice-cap is immaterial, as, although it would probably be deposited as hoar-frost rather than as snow in the first instance, owing to the thinness of the Martian air, the latter end of either form of the substance would be much the same,--glacier-ice.
It will, be interesting to examine more in detail the annual history of the ice-cap, especially as this history was unrolled before us last year more minutely than has been the case for the last fifteen years, and than will be the case for fifteen years to come. This was due not only to the relative proximity of the planet during the last opposition, but to the further fact that its south pole was tilted toward us at maximum angle. The vicissitudes which the polar cap underwent stood, in consequence, remarkably well displayed. To such advantage were they seen that it has been possible to construct a map of the Martian south circumpolar regions to a degree of detail such as has never been possible before, and which I have accordingly done. It will be seen from it (on the opposite page) how much farther advanced is our knowledge of the Martian south pole, and the regions about it, than is our knowledge of either of our own.
It is also pleasing to remember that during this our polar expedition we were not frost-bitten for life, nor did we have to be rescued by a search party. We lived not unlike civilized beings during it all, and we actually brought back some of the information we went out to acquire.
On examining the chart in which the successive appearances of the southern ice-cap are depicted at different times, from June 3 to October 13, or, in terms of the Martian time of year, from May 1 to July 15, the first point to strike one is that the cap was during its whole existence eccentrically placed with regard to the geographical pole of the planet. In other words, the pole of rotation and the pole of cold did not coincide. The latter lay on the average some six degrees distant from the former. This shows that the isotherms in the southern hemisphere of Mars do not coincide with the parallels of latitude.
The manner of the cap's melting further shows that differences of level exist in it. For, in addition to melting round its edge, the cap proceeded to melt asymmetrically. On the first night that Professor W. H. Pickering observed it, on May 22, with the six-inch telescope, he suspected a rift crossing the cap from longitude 330 degrees to longitude 170 degrees. This rift grew more and more evident, until, in the early part of June, it was unmistakable. It grew in visibility chiefly from actual growth in size. On June 6 it was estimated, on a scale of ruled lines made for the purpose, to be about 100 miles wide. On June 15 it was similarly found to measure 220 miles.
Meanwhile an interesting phenomenon occurred in the cap on June 7. On that morning, at about a quarter of six (or, more precisely, on June 8, 1h. 17m., G. M. T.), as I was watching the planet, I saw suddenly two points like stars flash out in the midst of the polar cap. Dazzlingly bright upon the duller white background of the snow, these stars shone for a few moments and then slowly disappeared. The seeing at the time was very good. It is at once evident what the other-world apparitions were,--not the fabled signal-lights of Martian folk, but the glint of ice-slopes flashing for a moment earthward as the rotation of the planet turned the slope to the proper angle; just as, in sailing by some glass-windowed house near set of sun, you shall for a moment or two catch a dazzling glint of glory from its panes, which then vanishes as it came. But though no intelligence lay behind the action of these lights, they were none the less startling for being Nature's own flash-lights across one hundred millions of miles of space. It had taken them nine minutes to make the journey; nine minutes before they reached Earth they had ceased to be on Mars, and, after their travel of one hundred millions of miles, found to note them but one watcher, alone on a hill-top with the dawn.
Calculation showed the position of the star-points to be in longitude 280 degrees and 290 degrees and in latitude 76 degrees south. At this place on the planet, then, there was a range of slopes sufficiently tilted to reflect the Sun from their ice-clad sides. On comparing its position with Green's map of his observations upon the cap at Madeira in 1877, it appeared that this was the identical position of the spot where he had seen star-points then, and where Mitchell had seen them in 1846, to whom they had suggested the same conclusion. Green christened them the "Mitchell Mountains." At the times both these observers saw them, they were detached from the rest of the cap. At the time of this observation in June, they were still in the midst of the cap. We shall see that they eventually became islands, just as Green saw them, and that the observation in June marked an earlier stage in their history.
On June 10 Mr. Douglass detected a second rift in the cap backing the range of slopes. And on June 13 I noticed that behind the bright points the snow fell off shaded to this rift. Meanwhile a third rift had been made out by him, running from longitude 170 degrees to longitude 90 degrees,--very nearly, therefore, at right angles to the first rift and debouching into it. Bright points continued to be seen at various points to the westward round the cap. They are marked by crosses on the chart. Throughout these days, the cap was wont to appear shaded upon the terminator side, as one might expect of a snow or ice slope. During June, also, the contour of the cap was apparently elliptical. But on June 25 Professor W. H. Pickering noted, for the first time, that it no longer looked so. The melting had resulted in making its asymmetry perceptible.
On July 1 our Martian polar expedition disclosed what used to be the supreme quest of earthly expeditions,-- that dream of arctic explorers, an open polar sea. On that day Professor Pickering perceived, in the midst of the cap, in longitude 260 degrees and latitude 80 degrees, a sheet of water about 250 miles long by 150 broad. It was in fact the spreading of the first rift about midway across the cap, and lay not far from the geographical pole of the planet, though not, it is to be noticed, near the pole of cold for it lay on the further side of the geographical pole from it. There is a touch of the irony of fate in this detection of an open polar sea on Mars before explorers have succeeded in doing so on the Earth.
In addition to these rifts and other irregularities of melting, small detached bits of the cap showed from time to time, one being seen by Professor Pickering on July 9 in longitude 284 degrees, and another by him on July 23 in about longitude 160 degrees.
Meanwhile the cap had been steadily decreasing in size, its progressive diminutions being shown on the map in the successive contour lines. The polar sea faithfully followed it in its shrinkage, even the bays keeping their longitudes unchanged. But, whereas early in June the bay at longitude 270 degrees had been blue, it now appeared brown; of that mud-color land does from which the water has recently been drained off.
After various vicissitudes, too numerous to mention in detail, on August 6 a separate patch of snow showed very conspicuous, to the left of the main body. The smaller detachment lay in longitude 290 degrees, and in latitude 75-78 degrees. Now, on turning to the record of the star-points that had appeared two months before, it will be seen that this was their position. Here, then, was proof of the identity of the star-points seen in June with the islands recorded by Mitchell and Green. The detached patch was in fact the range of slopes left in isolated insularity after all about it had melted away. From this we have an interesting bit of corroborative testimony that it stood on higher ground.
On August 11 the detached patch was yet farther separated from the main body of the cap, the smaller patch being many degrees distant to the north of either the geographical pole or the pole of cold, with water and even dry land to the south of it. It will be remembered, for the points of the compass, that this is the southern hemisphere of which we are speaking, and that, for climatic purposes, north and south here stand interchanged. On August 13 the detached patch is recorded for the last time, or, in other words, about this time it melted away. The larger one remained, contracting in size, however, as time went on. So it continued through August, September, and well into October.
On October 12, at 10h. 40m., I made the following entry about it: "Polar cap has been very faint for some time; barely visible." At 13h. 26m., or, in other words, at about half past one that night, Mr. Douglass measured its position and estimated its size, as was his wont every few days. He found it to be six degrees distant from the planet's pole, in longitude 54 degrees The patch was very small, covering about one hundred and fifty miles square. On looking at the planet on October 13, at 8h. 15m., to his surprise he found the cap gone. Not a trace of it could be seen; nor could either he or I detect it during the rest of that night although such was the longitude of the central meridian throughout it as to bring the cap on the nearer side of the pole, and therefore show it to best advantage. What had certainly been there on the 12th was not there on the 13th. The ice-cap had disappeared.
No such occurrence has ever been chronicled before. It is the first time since man began to observe the planet that the ice-cap has completely disappeared. Hitherto it has been seen to diminish to a minimum of from 7 to 4 degrees, and then begin to increase again. This last autumn, for the first time, it vanished entirely. The date of this occurrence was, in Martian chronology, about July 20. Evidently, for some reason unknown to us, it was a phenomenally hot season in the southern hemisphere of the planet.
Practically it never reappeared again during the season. That it did return occasionally, as a very small speck, was from time to time suspected, and doubtless did take place. Certainly it left for some time behind it a glimmer where it had been, due presumably to the moisture from its melting, still tarrying on the ground or lingering in the air. Otherwise, to all intents and purposes, where the polar ice-cap and polar sea had been was now one ochre stretch of desert.
Having thus followed to its vanishing point the polar cap, we will now return to it in the heyday of its youth, in June, 1894, when it was girdled by its broad blue belt. We have seen that we have reason to believe this to be in all probability a polar sea, a real body of water. There is, therefore, water on the surface of Mars. We also mark that this body of water is ephemeral. It exists while the ice-cap is melting, and then it somehow vanishes. What becomes of it, and whether there be other bodies of water on the planet, either permanent or temporary, we will now go on to inquire.
II. Areography
As in the course of our inquiry we shall have occasion to refer familiarly to different Martian features, we had best begin it with some slight exposition of Martian geography, or of areography, as it may by analogy be called. To get this we will, by the help of Plates III. to XIV., suppose ourselves to be viewing the planet from some standpoint in space, and watching the surface features pass in procession under our gaze as the rotation of the planet brings them successively round into view. In the matter of names the map of the planet toward the end of the book, with its accompanying index, will give identification. We may thus make a far journey without leaving home, and from the depths of our arm-chairs travel in spirit to lands we have no hope of ever reaching in body. We may add to this the natural delight of the explorer, for we shall be gazing upon details of Martian geography never till last summer seen by man.
Areography is a true geography, as real as our own. Quite unlike the markings upon Jupiter or Saturn, where all we see is cloud, in the markings on Mars we gaze upon the actual surface features of the Martian globe. That we do so we know from the permanency of the spots and patches thus revealed to us. They change in appearance, indeed, according to times and seasons, but they alter as true surface features would, not like cloud-belts that gather to-day and vanish forever to-morrow. That the markings are essentially permanent has been known ever since Cassini in 1666 definitely discovered, what Huyghens had thought to detect in 1659, the rotation of the planet, by means of their periodic presentations.
The twelve views we shall here scan are of the nature of a map, made in November, 1894. They represent the ensemble of the drawings from this observatory, for about that date. The details from these drawings were plotted upon a globe, which was then tilted toward the observer at the angle at which the Martian south pole itself was tilted toward--the Earth during November, and photographed at intervals of 30 degrees. The negatives were then made to conform as near as might be to the actual look of the planet. To photograph minute planetary markings directly is, for reasons too long to state here, impossible. The views give between them the whole surface of the planet shown us at what corresponds to our first of August. Thus, neither the polar cap nor the polar sea appear in the pictures, for both had then disappeared. Nor do the southern parts of the so called straits show, for a similar reason. But from a knowledge of the features here presented the reader will find interpolation of any others referred to easy.
Previous to the present chart, the most detailed map of the planet was Schiaparelli's, made in 1888. On comparison with his, it will be seen that the present one substantially confirms all his detail, and adds to it about as much more. I have adopted his nomenclature, and in the naming of the newly found features have selected names conformable to his scheme, which commends itself both on practical and on poetic grounds.
We will begin our journey at the origin of Martian longitudes and travel west, taking the points of the compass as they would appear were we standing upon the planet. As all astronomical pictures are, for optical reasons, upside down, south lies at the top of the pictures, west to the right, north at the bottom, and east to the left. Mars rotates as the Earth does, from west to east, so that day as it advances across the face of the planet follows the order here shown in Plates III. to XIV., the order in which we shall observe them. Places on the right of the picture are in the morning of their Martian day; places on the left, in its afternoon. To facilitate reference by longitude and latitude, the globe has been belted by meridians and parallels each 10 degrees apart, and the meridians have been numbered along the equator. This premised, we will suppose ourselves to be standing on the equator at its intersection with the 0 degree meridian.
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This work is in the public domain in the United States because it was published before January 1, 1923.
The author died in 1916, so this work is also in the public domain in countries and areas where the copyright term is the author's life plus 80 years or less. This work may also be in the public domain in countries and areas with longer native copyright terms that apply the rule of the shorter term to foreign works. |
