Mars (Lowell)/Chapter 1
MARS
I
GENERAL CHARACTERISTICS
I. AS A STAR
Once in about every fifteen years a startling visitant makes his appearance upon our midnight skies,—a great red star that rises at sunset through the haze about the eastern horizon, and then, mounting higher with the deepening night, blazes forth against the dark background of space with a splendor that outshines Sirius and rivals the giant Jupiter himself. Startling for its size, the stranger looks the more fateful for being a fiery red. Small wonder that by many folk it is taken for a portent. Certainly, no one who had not followed in their courses what the Greeks so picturesquely called “the wanderers” (οἱ πλανῆται) would recognize in the apparition an orderly member of our own solar family. Nevertheless, one of the wanderers it is, for that star is the planet Mars, large because for the moment near, having in due course again been overtaken by the Earth, in her swifter circling about the Sun, at that point in space where his orbit and hers make their closest approach.
Although the apparent new-comer is neither new nor intrinsically great, he possesses for us an interest out of all proportion to his size or his relative importance in the universe; and this for two reasons: first, because he is of our own cosmic kin; and secondly, because no other heavenly body, Venus and the Moon alone excepted, ever approaches us so near. What is more, we see him at such times better than we ever do Venus, for the latter, contrary to what her name might lead one to expect, keeps her self so constantly cloaked in cloud that we are permitted only the most meagre peeps at her actual surface; while Mars, on the other hand, lets us see him as he is, no cloud-veil of his, as a rule, hiding him from view. He thus offers us opportunities for study at closer range than does any other body in the universe except the Moon. And the Moon balks inquiry at the outset. For that body, from which we might hope to learn much, appears upon inspection to be, cosmically speaking, dead. Upon her silent surface next to nothing now takes place save for the possible crumbling in of a crater wall. For all practical purposes Mars is our nearest neighbor in space. Of all the orbs about us, therefore, he holds out most promise of response to that question which man instinctively makes as he gazes up at the stars: What goes on upon all those distant globes? Are they worlds, or are they mere masses of matter? Are physical forces alone at work there, or has evolution begotten something more complex, something not unakin to what we know on Earth as life? It is in this that lies the peculiar interest of Mars.
That just as there are other masses of matter than our globe, so there are among them other worlds than ours is an instant and inevitable inference from what we see about us. That we are the only part of the cosmos possessing what we are pleased to call mind is so earth-centred a supposition, that it recalls the other earth-centred view once so devoutly held, that our little globe was the point about which the whole company of heaven was good enough to turn. Indeed, there was much more reason to think that then, than to think this now, for there was at least the appearance of turning, whereas there is no indication that we are sole denizens of all we survey, and every inference that we are not.
That we are in some wise kin to all the rest of the cosmos, science has been steadily demonstrating more and more clearly. The essential oneness of the universe is the goal to which all learning tends. Just as Newton proved all the planets to obey a common force, the Sun; just as Laplace showed it to be probable that we were all evolved from one and the same primal nebula; so more recently the spectroscope has revealed unsuspected relationship betwixt us and the stars. Matter turns out to be but common property; and the very same substances with which we are so familiar on the Earth, iron, magnesium, sodium, and so forth, prove present on those far-off suns that strew the depths of space. Only in detail does everything differ.
So much for matter. As for that manifestation of it known as mind, modesty, if not intelligence, forbids the thought that we are sole thinkers in all we see. Indeed, we seldom stop in our locally engrossing pursuits to realize how small the part we play in the universal drama. Let us consider for a moment how we should appear, or, more exactly, not appear, could we get off our world and scan it from without. If distance could thus reduce for us the scale upon which the universe is fashioned to one we could take in, that on which the Earth should be represented by a good-sized pea, with a grain of mustard seed, the Moon, circling about it at a distance of seven inches, the Sun would be a globe two feet in diameter, two hundred and fifteen feet away. Mars, a much smaller pea, would circle around the two-foot globe three hundred and twenty-five feet from its surface; Jupiter, an orange, at a distance of a fifth of a mile; Saturn, a small orange, at two fifths of a mile; and Uranus and Neptune, good-sized plums, three quarters of a mile and a mile and a quarter away, respectively. On this same scale the nearest star would lie eight thousand miles off, and an average third-magnitude star at about the present distance of our Moon; that is, on a scale upon which the Moon should be but seven inches off, the average star would still be as far from us as the Moon is now. Now when we think that each of these stars is probably the centre of a solar system grander than our own, we cannot seriously take ourselves to be the only minds in it all.
Probable, however, as extra-terrestrial life in general is, it is another matter to predicate it in any particular case. Nevertheless, if it exist it must exist somewhere, and the first place to scan is the place we can scan best. Now the Moon appears to be hopelessly dead. Mars, therefore, becomes of peculiar interest, and it was in hope of learning something on the subject that the observations about to be described in this book were made. Before proceeding, however, to an account of what in consequence we have learned about our neighbor, a couple of misapprehensions upon the subject,—not confined, I am sorry to say, wholly to the lay mind,—must first be corrected. One of these is that extra-terrestrial life means extra-terrestrial human life. Such an inference recalls to my mind the exclamation of an innocent globetrotter to a friend of mine in Japan once, a connoisseur of Japanese painting, upon being told that the Japanese pictures were exceedingly fine. "What!" the globe-trotter exclaimed in surprise, "do the Japanese have pictures,—real pictures, I mean, in gilt frames?" The existence of extra-terrestrial life does not involve "real life in trousers," or any other particular form of it with which we are locally conversant. Under changed conditions, life itself must take on other forms.
The next point is as to what constitutes proof. Now, between the truths we take for granted because of their age, and those we question because of their youth, we are apt to forget that in both proof is nothing but preponderance of probability. The law of gravitation, for example, than which we believe nothing to be more true, depends eventually, as recognized by us, upon a question of probability; and so do the thousand and one problems of daily life upon so many of which we act unhesitatingly and should be philosophic fools if we did not. All deduction rests ultimately upon the data derived from experience. This is the tortoise that supports our conception of the cosmos. For us, therefore, the point at issue in any theory is not whether there be a possibility of its being false, but whether there be a probability of its being true. This, which is evident enough when squarely envisaged, is too often lost sight of in discussing theories on their road to recognition. Negative evidence is no evidence at all, and the possibility that a thing might be otherwise, no proof whatever that it is not so. The test of a theory is, first, that it shall not be directly contradicted by any facts, and secondly, that the probabilities in its favor shall be sufficiently great.
As to what constitutes sufficiency it is important to bear in mind one point, namely, that the odds that a thing is true from the fact that two or more witnesses agree on the same statement is not the sum of the odds that each tells the truth, but the product of those odds.[1] Therefore, if the chances for the truth of a theory, in consequence of its explaining a certain set of details, be three to one, and because of its explaining another set,—for the purposes of argument unrelated to the first,—four to one, then the chances in its favor from its explaining both sets are not seven to one but twelve to one. If it explains a third set whose independently resulting odds are of five to one, the chances in its favor, from its explaining all three sets, not twelve to one but sixty to one; if a fourth set be added, with further odds of five to one, the sum total from the four becomes not seventeen to one but three hundred to one in favor of its being true. It will be seen how rapidly the probability of the truth of a theory mounts up from the amount of detail it explains. This law is to be remembered throughout the coming exposition, for whatever the cogency of each detail of the argument in itself, the concurrence of all renders them not simply additionally but multiplicitly effective. That different lines of induction all converge to one point proves that point to be the radiant point of the result.
II. ORBIT
To determine whether a planet be the abode of life in at least resembling that with which we are acquainted, two questions about it must be answered in turn: first, are its physical conditions such as render it, in our general sense, habitable; and secondly, are there any signs of its actual habitation? These problems must be attacked in their order, for unless we can answer the first satisfactorily, it were largely futile to seek for evidence of the second.
Thoroughly to appreciate, then, the physical condition of Mars, we must begin at the beginning of our knowledge of the planet, since every detail will be found to play its part in the final result. I shall therefore give in a word or two the general facts known about the planet, before taking up the observations which make the subject matter of this book. The first of these general facts is the path the planet describes about the Sun. Who first found out that the ruddy star we call Mars was not like the rest of the company about him we do not know; possibly some, to fame unknown, Chaldean shepherd alone with the night upon the great Chaldean plains. With the stars for sole companions while his sheep slept, he must, as he watched them night after night, have early recognized that they always kept the same configuration. They rose and set, but they all rose and set together. But one night he thought he noticed that one of them had changed its place with reference to the rest. A few nights later he became sure of it. One of the immovable had patently moved. That memorable though unremembered night marked the birth of our acquaintance with the rest of the universe.
Whether the midnight pioneer was Chaldean or Assyrian or of some other race, certain it is that to the Egyptians we owe the first systematic study of the motions of this and of four other roving stars, and to the Greeks whom they taught, the name by which we know them, that of planets, meaning merely wanderers. Since then, as we know, many others of like habit have been added to the list.
Now, from observations of the apparent places of a planet, it is possible to determine the relative path of the planet in space as compared with the path of the Earth. This Kepler did from observations of Tycho Brahe's, and showed the wanderers to belong to a system of bodies, all revolving about the Sun in various elliptic orbits, the Sun being at the focus of each ellipse. He also found that the line connecting each planet with the Sun passed over equal areas in equal times, and thirdly, that the squares of the times were as the cubes of the major axes of the orbits. From these three "laws" Newton deduced the fact that the force controlling the planets was directed toward the Sun, that it varied inversely as the square of the distance, and that it was the same in origin for all. This is the so-called law of gravitation, and this is the way in which it was discovered. We do not yet know why gravity so acts, but it is interesting to note that it follows the simple law of geometrical expansion, diminishing in exact ratio to the space it fills, just like electricity or light. It may, therefore, also be a wave motion.
Thus all the wanderers proved to be associated in common dependence on the Sun, and among the members of the solar family thus recognized Mars was found to hold the position next exterior to the Earth, and the path he followed in his circuit of the Sun to be situated with regard to the Earth's as in the following diagram.
_-_Diagram_of_the_Orbits_of_Mars_and_the_Earth.jpg)
Diagram of the Orbits of Mars and the Earth.
On consulting the diagram we shall at once perceive why it is that every fifteen years Mars becomes so unusually bright as to seem, to one who has not kept track of him, a new and startling star. His orbit, it will be seen, is an ellipse of some eccentricity, and deviates in consequence considerably from a circle. The point marked Perihelion denotes the point where the planet is nearest the Sun; the point marked Aphelion, the point where the planet is the most remote from the Sun. In like manner the points marked Perihelion and Aphelion on the inner circle show the corresponding points of the Earth's orbit, which is much more nearly circular. Now as the two planets revolve in different periods of time, Mars taking 686.98 of our days to complete his circuit, and the Earth 365.26 days to complete hers, the one planet will overtake the other only once every two years and two months or so. Meanwhile they are at great distances apart. But even when they do meet, they do not always meet equally near. For the one orbital period is not an exact multiple of the other, and as the orbits are both ellipses, it is evident that these meetings of the two planets will occur at different points of their orbits, and, therefore, at different distances. If the meeting occur when Mars is in perihelion the planets approach one another within 35,050,000 miles; if in aphelion, only within 61,000,000 miles.
But even this difference in distance does not measure the full extent of the variation in brilliancy. As the brightness of an illuminated body varies inversely as the square of its distance from the source of light, and as the total amount of light it reflects to an observer varies inversely as the square of his distance from it, it makes every difference in the apparent brilliancy of a body how the body is situated, both with regard to the source of light and with regard to the observer. Now it so chances that at the meetings of Mars with the Earth these two factors attain their maximum effects nearly together, and similarly with their minimum. For at the times when we are closest to Mars, Mars is nearly at his closest to the Sun, and reversely when we meet him at the opposite part of his orbit. It thus comes about that at some meetings,—oppositions, they are called, because Mars then is in the opposite part of the sky from the Sun,—the planet appears four and one half times as bright as at others. Here, then, we have the explanation of the planet's great changes in appearance, changes so great as to deceive any one who has not followed its wanderings, into the belief that it is some new and portentous apparition.
Important as is the ellipse in which Mars moves with regard to his visibility by us, it is considerably more important as regards the physical condition of the planet itself. For the Sun being situated at one of the foci of his orbit, the motion of the planet sweeps him now near to, now far from that dispense of light and warmth; and the amount of both which the planet receives varies just like gravity with his distance from their source. Now the eccentricity of the orbit of Mars is such that when nearest the Sun his distance is 129,500,000 miles, when at his mean distance 141,500,000 miles, and when most remote 154,500,000 miles. The proportion of light and heat he receives respectively is therefore roughly as 16 to 20 to 24; or half as much again at certain times as at others.
So much in our knowledge of Mars is pretelescopic. Men might have and practically did learn this much without ever seeing the planet other than as a point of light. Its orbit was tolerably accurately known and could have been known still more accurately without telescopic aid; not so, until we become much more nearly omniscient than we at present are, the planet's self.
III. SIZE AND SHAPE
With the telescope we enter upon a new phase in our knowledge of the planet: the determination of its shape and size.
The relative plan of the solar system can be learned with great accuracy from observations of the motions of its members; not so easily learned is the scale upon which it is constructed. Although the former is intrinsically a very complicated, the latter a very simple problem, two characteristics of the actual system makes it possible to solve the former much more nearly than the latter. One of these characteristics is the fact that the distances between the masses which compose the system are very much greater than the dimensions of the masses themselves, of quite a higher order of magnitude. The diameters of the planets are measured by thousands of miles, the distances between them by tens of millions. The second characteristic consists in the approximately spherical shape of the planets themselves, and in the fact that by a mathematical consequence of the actual law of gravitation a sphere acts upon any outside body as if all its mass were concentrated at its centre, a most interesting peculiarity not true under many other supposable laws. These two facts very materially simplify the problem of the motions of celestial mechanics.
But just as the first of these peculiarities helps us to comprehension of the relative dimensions of the solar system, so does it hinder us in determining its actual dimensions. For this determination depends upon a problem in celestial surveying, the finding the distance to a body by measuring the angle it subtends from the two ends of a base-line. Now, as unfortunately we cannot get off the earth for the purpose, our base-line is at most the diameter of the earth itself, and as the distance to the other body immensely exceeds our own size, the angle to be measured becomes so excessively small as to be very difficult to determine with accuracy. Fortunately this is matter chiefly of theoretic regret, as we now know the actual sizes to within a degree of exactness practically sufficient for most purposes but perturbations; to within about 1/300 part of the whole, so far as our ultimate measure is concerned, the distance we are off from the Sun.
A good idea of the method and some appreciation of the difficulty involved in it can be got by considering a precisely similar case, that of determining the distance of a spire a mile and three fifths away by shutting first one eye and then the other and noting the shift of the spire against its background. It is needless to add that without telescopic aid the determination is impossible, and that it is exceeding difficult with it.
Nevertheless, from the distance of the Sun determined in this manner, we find from measurements of the apparent disk of the planet made at Flagstaff that Mars is about 4,215 miles in diameter. This makes his surface a little more than a quarter that of the Earth and his volume about one seventh of hers.
The next point to find out is his mass, that is, the amount of matter he contains. This is very easy to determine when a planet has a satellite, and very difficult to determine when a planet has not. The reason is this: the mass of a body is known from the pull it exerts, inasmuch as this pull depends, by the law of gravitation, upon its mass and the square of its distance. If then we know the pull and the distance from which it is exerted, we can find the mass. Now we gauge the pull from its effects in causing some other body to move. By measuring, therefore, the motion of this other body, we learn the mass of the first one. To get this accurately the motion must be large enough to admit of satisfactory measurement in the first place, and be as uncomplicated with motions due to pulls of other bodies as possible, in the second. As each body pulls every other, and it is only their relative displacement we can measure, as we have no foothold in space, even the case of only two bodies presents difficulties of apportionment. We can learn the aggregate mass of the two, but not the separate mass of either alone unless it so happen that the mass of one is so insignificant compared with the other that the mass of that other may be taken as the mass of both. Now this is substantially realized in the case of the solar system. Owing to the greatly disproportionate size of primary and secondary bodies in it, the great size of the Sun as compared with that of any of the planets, and the great size of the planets as compared with their satellites (with the exception of the Moon, and she, fortunately, is an only child), the determination of the mass of the smaller by measurement of its motion about the larger,—as if only the pair of bodies under consideration existed, and the mass of both were concentrated in the greater of the two,—is very nearly exact. In consequence each planet discloses with some accuracy the mass of the Sun, but tells next to nothing about its own mass; and in the same way each satellite reveals the mass of its primary. The mass of a planet possessing a satellite is, therefore, easy of determination. Not so that of one which travels unattended. The only way to obtain its mass is from the perturbations or disturbing pulls it exerts upon the other planets, or upon stray comets from time to time, and these disturbances are, by the nature of the case, of a much smaller order of magnitude, to say nothing of the fact that all act coincidently to increased difficulty of disentanglement. The practical outcome of this in the case of Mars was that before his satellites were discovered the values obtained for his mass ranged all the way from 1/3700000 to 1/2500000 of the mass of the Sun, or, in other words, varied fifty per cent. His insignificant satellites, however, and just because they are insignificant, have made it possible to learn his mass with great exactness. It turns out to be 1/3093500 of that of the Sun, or 10/94 of that of the Earth.
Knowing his mass, we know his average density, since to find it we have but to divide his mass by his volume. It proves to be 72/100 of that of the Earth. We also learn the force of gravity at his surface, inasmuch as this is directly as his mass and inversely as the square of his radius. It comes out 38/100 of that of the Earth. In consequence, all things there would weigh but 38/100 of their weight on earth; a man, for example, weighing 150 pounds here would weigh but 55 pounds if transported to the surface of Mars, and all manual labor would be lightened threefold.
So soon as the planet was scanned telescopically, he was seen to present a disk, round at times, at other times lacking somewhat of a perfect circle, showing like the Moon when two days off from full. Such appearance visibly demonstrated, first, that he was not a self-luminous body, and secondly, that he revolved about the Sun outside of the Earth. A glance at the diagram of the orbit will make time latter point clearer. If we draw a line from the Sun to the centre of Mars and pass a plane through the planet perpendicular to this line and to time plane of his orbit, this plane will divide the illumined half of him from the unillumined half. If now we draw another line from any point of the Earth’s orbit to Mars’ centre, and pass a plane similarly perpendicular to that, it will cut off the hemisphere we see at any moment from the one we do not. As the two lines do not in general coincide, it will appear that in certain positions, in fact in all but two, Mars must present to us a face partly steeped in daylight, partly shrouded in night; in short, that he shows gibbous like the Moon when she is between the half and the full. This accounts for the look of the drawings made during June, 1894, in which from a seventh to a sixth of the disk is wanting on the left.[2] By drawing lines from his centre to more than one position occupied by the Earth it will be seen that this lacking lune reaches a maximum when the Earth as viewed from Mars is at extreme elongation from the Sun, and that the amount of the phase at such time exactly equals the number of degrees of this elongation. For example, on the sixteenth of last June the lacking lune amounted to 47°, that is, the Earth was then evening star upon the Martian twilight skies at an angular distance of 47° from the Sun, about what Venus seems to us at her extreme elongation, in fact, to Mars we occupy much the same astronomical position that Venus does to us.
To Huyghens we owe the first really important telescopic observation upon the planet. On November 28, 1659, at 7 p.m., he made the first drawing of the planet worthy the name, for on it is the first identifiable feature ever made out by man on the surface of Mars. This feature is the Hourglass Sea, now more commonly known as the Syrtis Major. The accompanying cut of it is reproduced from Flammarion. If the dark patch in it be compared with the markings in time other pictures of the planet, shown later in this book, it will be seen that the patch can be none other than the Hourglass Sea.
_-_Huyghens%27_drawing_of_the_Syrtis_Major,_Nov._28,_1659.jpg)
Now, innocent as it looks of much detail, Huyghens’ drawing is perhaps the most important one of Mars that has ever been made. For, from his observations of the spot it depicts at successive dates, he was able to prove that Mars rotated on his own axis, and to determine the time of that rotation, about 24 hours. As he subsequently came to doubt his results, the honor of the discovery rests with Cassini, who, in 1666, definitely determined that the planet rotated in 24 hours 40 minutes. Thus was it first learned that Mars had a day, and that its length was not far from the length of our own.
The importance of these earliest pictures of Mars has not lapsed with the lapse of time. By comparison of this and other early drawings with modern ones, has been deduced a very accurate value of the length of the Martian day (its sidereal day), a determination accurate to the tenth of a second. It amounts to 24 hours, 37 minutes, 22.7 seconds. Our sidereal day, that is, the day reckoned by the stars, not by the Sun, is roughly 23 hours, 56 minutes; so that the Martian day is about 40 minutes longer than our own. The result is not given here closer than the tenth of a second, because the Flagstaff observations have shown that the value of the length of the Martian day hitherto accepted is probably a trifle too small.
From the discovery of the rotation followed the approximate position of the planet's poles. Round about the poles so determined appeared two white patches, the first study of which we owe to Maraldi. They are the planet’s polar caps. They are to be detected with the smallest modern telescope.
The apparent position of the planet poles as presented to the Earth gives the tilt of the planet’s axis to the plane of its orbit. It turns out to be about 25°. This is very nearly the same as the Earth’s axial tilt to the plane of her orbit, which is 23° 24′. As the inclination of the axis to the plane of the orbit determines the seasons, we see that not only has Mars its spring, summer, autumn, and winter, but that these are not very unlike our own.
It is not uninteresting to inquire in what the difference consists. The slight difference of tilt in the Martian axis would slightly extend the breadth of the tropical and the polar regions at the expense of the temperate ones, and thus accentuate the seasons, but the chief seasonal contrast between Mars and the Earth would come in in consequence of the much greater eccentricity of Mars’ orbit. For the more eccentric the ellipse, the greater the variation in the planet’s velocity at different parts of it, inasmuch as the Sun pulls the planet toward himself with a force depending on his distance. The less this distance, the greater the angular velocity. But the angular velocity determines the length of the seasons upon a planet whose pole of rotation is tilted to the plane of its orbit, like the Earth or Mars. The greater the eccentricity of the ellipse, therefore, the greater the difference in the length of the seasons. In the case of the Earth the difference is about eight days, winter in the northern hemisphere being eight days shorter than summer. In the case of Mars, owing to the much greater eccentricity of his orbit combined with his longer period, the difference amounts to 74 days. In one hemisphere winter is long and cold, summer short and hot; in the other winter and summer interchange. Owing to the present position of the line of apsides, the line connecting the points of Mars’ nearest approach to and farthest recession from the Sun, the former hemisphere happens to be the southern one; the latter, the northern. The lengths of their respective seasons are as follows:—
In the northern hemisphere, winter lasts 147 of his own days; spring, 191 days; summer, 181 days; autumn, 149 days; while in the southern hemisphere, winter lasts 181 days; spring, 149 days; summer, 147 days; autumn, 191 days.
Curiously enough, an analogous distribution of heat and cold occurs also at the present time in the case of the Earth; its axis and line of apsides holding the same relation to each other that the Martian ones do. This similarity of aspect is, as we shall see later, apparently very curiously reproduced in certain peculiarities of the surfaces of the two planets. But with Mars the result is much more marked on account of the greater eccentricity of his orbit, which is .0931 as against the Earth’s .0168.
As even under these exaggerated conditions his two polar regions show much alike, modern theories about our glacial epochs are considerably shaken.
The last of the preliminary points to be taken up is the form of the planet. Consideration of it makes in some sort a bridge from the planet’s past to its present. For its deviation from a perfect sphere tells us something of its history.
Between the shapes of the large planets, Jupiter, Saturn, Uranus, and probably Neptune, and those of the small ones, Mercury, Venus, the Earth, and Mars, there is a striking dissimilarity, the former being markedly oblate spheroids, the latter almost perfect spheres.
Into the cause of this, very interesting as it is, we have not here space to go. The effect, however, is so noticeable that while the most casual glance at the disk of Jupiter will reveal its ellipticity, the most careful scrutiny would fail to show Mars other than perfectly round.
Nevertheless, the planet is slightly flattened at the poles. Measures have repeatedly been made to determine the extent of this flattening, with surprisingly discordant results, most of the values being much too large.
Observations at Flagstaff during this last opposition have not only shown that most of the values were too large, but have revealed the cause of their discrepancy. There turns out to be a factor in the case, hitherto unsuspected, whose presence proves to be precisely such as would cause the observed variations in measurements. It not only accounts for the fact of discrepancy, but for the further fact that the discrepancies should usually be on the side of an increase of the apparent polar flattening. This factor is the recognition of a perceptible twilight upon the planet, not only of enough account to be visible, but to have been actually measured, quite unconsciously, by Mr. Douglass, and disclosed only when the measures came to be compared with each other. Of this I shall speak more at length when we reach the subject of atmosphere. Here it is only necessary to say that the presence of a twilight fringing the surface of the planet would have the effect of increasing the apparent size of the equatorial diameter at all times, but to a different degree at different times, and almost always more than it would the polar one. In consequence, the polar flattening, which is the ratio borne by the difference of the equatorial and polar diameters to the equatorial diameter, would be seemingly increased.
The value of Mr. Douglass’ measures is heightened by a certain happy event of an unprecedented nature,—the first observed disappearance of the polar cap, and that at the very time the most important measures were made. The presence of the polar cap enters as a disturbing element into measures of the planet’s disk, on account of the increased irradiation it causes at the extremity of the polar diameter, which makes the polar diameter measure more than it otherwise would. For the polar cap is the most brilliant part of the disk; and for the same reason that any bright body seems larger than a dark one of the same size, it dilates the planet unduly in that direction. The resulting effect is further complicated by the fact that the polar cap is eccentrically situated with regard to the pole of rotation, as we shall see later; and as the pole is tilted, the cap is sometimes on the edge of the disk and the irradiation in consequence large, and sometimes well on the disk itself where its irradiation is little or nothing. As the amount of its magnifying effect is not accurately known, there enters with it an unknown error. Now, last autumn Nature herself kindly eliminated this source of error.
The measures made by Mr. Douglass are thus entitled to special regard, not only because of their number (a great many of them were taken), but chiefly because Nature made the disturbing influence of the polar cap nil. When, in addition, the twilight arc is allowed for, the measures show a most satisfactory accordance and give for the value of the polar flattening 1/190 of the equatorial diameter.
Now, it is interesting that this value should receive corroborative support from two quite different directions. The first of these is that 1/190 is just about the flattening which would result from the most probable supposition we can make as to the past history of the planet. To show this we may take the case of the Earth. Investigations along several different lines all result in showing that the polar flattening of the Earth is almost exactly such as would result in a fluid body whose density from surface to centre increased according to the pressure and temperature of our Earth in the past, and which rotated with its present angular velocity. In the case of Mars, Tisserand has shown that the polar flattening under the influence of his present rotation would, if the increase of density in his strata were similar to the Earth’s, be 1/227 of his equatorial diameter. If, on the other hand, his mass were homogeneous, his polar flattening would be 1/178. Now, in a fluid condition a body could not remain homogeneous, owing to the pressure exerted by the outer strata upon the inner ones, unless the matter of which it was composed were rigorously incompressible, which is certainly not the case with the Earth, and with quite equal certainty not the case with Mars. On the other hand, the increase of density from surface to centre is undoubtedly less in Mars than in the Earth, since the pressure depends upon the mass and the Earth’s mass is nearly ten times that of Mars. Consequently, from this cause, the polar flattening should be somewhere between 1/178 and 1/227, not far therefore from the value found above, 1/190.
The second bit of corroborative testimony comes from the behavior of the satellites of the planet. Unlike a sphere, a spheroid acts unequally upon a body revolving about it in an ellipse inclined to its equator. The ring pulls the satellite now this way, now that, thus altering its nodes, that is, the points where the plane of its orbit crosses the planet’s equator, and also its apsides, or the points in which the satellite’s orbit is nearest and farthest from the planet. The effect of an equatorial protuberance tilted thus is to shift these points round the orbit, the line of nodes retrograding, while contrarily the line of apsides advances. From the speed with which these revolutions take place, it is possible to calculate the size of the bulge. Hermaun Struve has just done this for the lines of apsides of the two satellites of Mars, and finds for the value for the consequent polar flattening of the planet 1/190 of its equatorial diameter. From these two independent determinations we may conclude that the value found at Flagstaff is pretty nearly correct.
We find, then, that Mars is a little flatter than our Earth, though not noticeably so, the polar flattening amounting to about 22 miles.
The value, 1/190, for his polar flattening, hints that at some past time Mars was in a fluid— that is, a molten—condition, just as the Earth’s polar flattening of 1/303 similarly shows her to have been, and that in both cases the flattening was then impressed. Now, inasmuch as the tides, lunar and solar in the case of the Earth, solar practically alone in the case of Mars, have been slowing up the planet’s rotation ever since this refrigeration happened, but as their respective rates of rotation still agree substantially with what a fluid condition demands, it is evident that in the case of neither planet could the cooling have begun so very long ago, but that it began longer ago for Mars than for the Earth.
In so far, then, we trace a certain similarity of development in the early chaotic stage of evolution of the two planets, a stage pre-natal to their career as worlds.
From these basic facts of size and shape we will now go on to more latter-day detail.