The Solar System/Chapter 3

by Percival Lowell

III

MARS

Mercury old;
Mars in middle age.Mercury presents us one phase of planetary development; Mars another, quite different. The two represent stages in world-life as distinct as those of gray hair and brown in human life.

Whatever the absolute ages of the several planets, their relative ages, as measured intrinsically, decrease pretty steadily with their distance from the Sun. Mercury is old; Mars, middle aged; Jupiter young.

World-life has its earmarks of time as human life has, and betrays them quite as patently.

Lack of atmosphere, colorlessness, changeless attitude toward the Sun, are the signs of old age in a planet. Mercury shows all these tokens of senility. Mars presents a very different picture.

Color a conclusive criterion.Color is a telltale trait; for it is a sign that surface development still goes on. Lack of atmosphere alone prevents vegetation, and this, coupled with unalterableness of face presented to the Sun, weathers the surface to a neutral gray. Such a body shows but the bleached bones of a once living world.

Now color is conspicuously wanting on Mercury. The disk of the planet is a chiaroscuro of black and white, tones devoid of tints.

Mars a life supporting world Mars is an opal. Colors comparable only to that stone variegate its disk. At top and bottom, collars of pearl-white contrast vividly with light areas of rose-saffron and darker ones of robin's-egg blue. Daylight reveals these colors much better than night, because the contrast of the blue-black sky clothes the disk with yellow it does not really possess, diluting the true tints.

Mars has polar regions, temperate zones, and tropicsThe markings enable the rotation of the planet to be found - The markings move under the observer's eye and yet keep their relative configurations the same, day after day and year after year. They thus reveal the fact that the planet rotates, and by the course of their motion disclose the axis about which the rotation takes place. From the observed data, spherical trigonometry enables us to fix this axis in space and determine its tilt to the plane of the planet's orbit. We thus find that it is inclined to the Martian ecliptic by an angle of 25°, and that the solar day there is 24 hours and 40 minutes long. Thus Mars has both days and seasons, and both days and seasons are practically Seasons accentuated much like ours and day of about same length counterparts of our own. The days are a little longer and the seasons nearly twice as long, reckoned either by Earthly or by Martian days. The orderly succession of day and night, spring, summer, autumn, and winter, are the same there as here.

Now this is no accident. It is a direct consequence of the planet's size and of its position in the solar family. That, however, the circumstances of the Earth and Mars should chance to agree so nearly in quantity as well as quality, we as yet lack the data to explain.

Scant atmosphere Size, or rather lack of it, has done something else for Mars. It has reduced the atmospheric blanket that covers the planet's body. It did this both at the start and subsequently. If the planets set out with atmospheres in proportion to their masses, a small planet having a greater surface in proportion to its mass would not have this surface so thickly covered, and its lesser gravity would further spread this out skywards.

Surface being as $\scriptstyle 4\pi r^{2}$ , while mass is as $\scriptstyle {\frac {4}{3}}\pi r^{3}$, the one is to the other, surface to mass, as

$\scriptstyle {\frac {4\pi r^{2}}{{\frac {4}{3}}\pi r^{3}}}={\frac {3}{r}}.$

The ratio of surface to mass increases, therefore, inversely as the radius of the body.

In the next place, its gravity could control only a much smaller velocity at its surface, thus making the critical velocity beyond which a particle would pass off into space much less. By the kinetic theory of gases, a certain number of particles will in a given time attain the critical velocity, and the more the lower the critical velocity. Thus, from the planet that hath not shall be taken away even that which it hath.

In consequence, on Mars the density of the air at the surface of the planet at the start was probably not denser than one-seventh of our own, or more rare than that at the top of our loftiest mountains, and now probably is rarer even than this, owing to the greater speed with which it has been lost.

Rate of loss differs with different gases. The rate at which the different gases would be differs. The curve of probability shows that they would disappear much more rapidly than the ratio of their speeds. Water vapor would go long before atmospheric air.

MAXWELL'S LAW.

The possible values which the components of the molecular velocities can assume are distributed among the molecules in question, according to the same law by which the possible errors of observation are by the method of least squares distributed among the observations.

The number of molecules traveling at speed u is given by the equation,—

$\textstyle N{\sqrt {\frac {km}{\pi }}}e^{-kmu^{2}}du=dy$

just as the probability of an error is given by the equation,

$y={\frac {h}{\sqrt {\pi }}}e^{-h^{2}x^{2}}.$

VALUES OF THE SPEEDS.

G = mean value of speed in metres per second.
G'= mean value of speed in miles per second.

Hydrogen		.	.	.	.	.	1838	1.14
Water vapor		.	.	.	.	.	614	0.38
Nitrogen		.	.	.	.	.	492	0.31
Atmospheric air			.	.	.	.	485	0.30
Oxygen	.	.	.	.	.	.	461	0.29
Carbon dioxide			.	.	.	.	392	0.24
Cyanogen		.	.	.	.	.	361	0.22

These speeds are got from the consideration that the energy, from which follows the temperature, is the same in the two gases; and, therefore, that

$\textstyle {\frac {1}{2}}mv^{2}={\frac {1}{2}}m_{1}v_{1}^{2};$

and, therefore, the speed of the molecule is inversely as the square root of the atomic weight.

Air on Mars. So far theory. Now it is not a little interesting that observation supports this. That air still exists on Mars, oxygen, nitrogen, and carbonic acid, is certain because of the changes which we can see going on in the surface markings; for without air no change could take place, and changes are indisputable. Water is relatively scarce.

Change in polar caps. That change goes on upon the planet's surface has been known for a long time. The polar caps were the first telltale. Sir William Herschel, at the end of the eighteenth century, observed that they waxed and waned periodically, and that their period was timed to that of the planet's year. They were therefore seasonal phenomena.

They behaved like ice and snow, and this they are generally supposed to be. Some astronomers find difficulty in conceiving of enough heat on Mars to permit them to be water, and carbonic acid has been suggested instead. But certain phenomena connected with the melting prove that carbonic acid cannot be the substance. The evidence is now very strong that they are what they look to be, and that the necessary heat will some- how be explained.

Pre-Schiaparellian knowledge and ideas.

Up to the time of Schiaparelli, not much beyond this behavior of the polar caps and the general permanency of the dark and light markings was known about the planet. Its physical condition was likened to the Earth's, the white patches being polar snows, the dark markings oceans and seas, and the light markings land.

Mars intrinsicly older than the Earth.In fundamentals, indeed, Mars shows a general similarity to the Earth; but in subsequent characteristics it betrays a most interesting dissimilarity. It is the dissimilarity that modern study has specially brought out.

The cause of the dissimilarity springs from the planet's size. The less mass of Mars did not permit it initially to present so fertile a field for development. Mere size entirely alters physical possibilities. In the next place, its dwarfing caused it to age quicker than the Earth.

Schiaparelli's discoveryOur knowledge of the planets, and especially of Mars, has advanced greatly within the last quarter of a century. The first steps of this advance we owe, not to instruments, but to the genius of one man, the Italian astronomer Schiaparelli. In 1877 he began to observe Mars, and at once showed a keenness of vision surpassing that of any previous observer and a susceptibility to impressions surpassing even his acuteness of sight. It was not so much a matter of eye as of brain. For it turns out now, after the fact, that several of his phenomena had been dimly seen and recorded before, but without that understanding which made of them stepping-stones to further results.

His object was to map the planet mircometrically. But in the course of his mapping he became aware of some curious markings: dark bands seaming the surface of the light areas, or so-called continents. These he named canali, or channels; for he, in company with every one else, at the time believed the dark regions to be seas.

Having got the hint, for it was scarcely more than that, during his first season, the opposition of 1877, he then showed that element of genius without which very little is ever accomplished, the persistence to follow up a clue. As Mars came round again he attacked the planet in the light of what he had already learnt, and first confirmed and then extended his discovery. This he continued to do at each succeeding opposition. The more he studied, the stranger grew the phenomena he detected. And it is to his everlasting credit that he did this in the face of the skepticism and denial of practically the whole astronomic world. He won. The voices that ridiculed him are all silent now. To-day the canals of Mars are well-recognized astronomic facts, and constitute one of the most epoch-making astronomic discoveries of the nineteenth century.

Through a complete cycle of oppositions, that is, from the nearest to the most remote and round to the nearest again, a period of fifteen years, Schiaparelli continued to study these curious phenomena, having them practically all to himself, Indeed, his grand isolation in the quest makes one of the finest and saddest chapters in the history of discovery. In the course of these solitary years he came to see the canals better, and they grew, on improving acquaintance, steadily more strange. He found that they were far more regular than he had at first thought, and he noted that they were dependent in appearance upon the season of the planet's year. So, likewise, were the large dark markings, and he attributed the behavior of both to a seasonal shift of water over the surface.

His theory of the planet's physical condition, derived from his observations, was as follows : that the polar caps were ice and snow ; that the blue-green areas were seas and the reddish-ochre ones land; that the canals were natural water-channels or straits honeycombing the land and cutting it up into a patchwork of large islands, a sort of natural Venice on a world-wide scale ; and finally that the surface was subject to annual or semiannual inundations and dryings-up, timed to the melting of the polar caps.

Schiaparelli retired practically in 1892, though not formally till a little later. His work was taken up by other hands, and the impetus he gave the matter has resulted in a knowledge of Mars which has quite revolutionized even the conception he bequeathed of the planet.

Methods of Observation. Before proceeding to post-Schiaparellian work, it may interest you to know how the phenomena in question have been detected, and what they look like when seen.

Contrary to what the layman thinks, the size of the instrument is the least important factor in the process. As in most things, the man is the essential machine; and next in desirability to the presence of man is the absence of atmosphere. In good air, with fair attention, the canals are not very difficult objects. Indeed, the surprise is that they were not detected long ago. Under suitable atmospheric conditions a four-inch glass will show them perfectly. Steady air is one essential; steady study another.

The canals. In appearance they are unlike any other phenomena presented in the heavens. Pale pencil lines, deepening on occasion to India ink, seem to cobweb the continents. Their tone depends on the seeing, in the first place, and on the season, in the second. Their width is invariable throughout, and their directness something striking. Measurable width they have not; it is only by

Fig. VIII. Map of Mars.

depth of tint that their importance is inferred. But their most amazing attribute is their geometric character. They seem to be generally arcs of great circles drawn from certain salient points on the planet's surface to certain other equally salient ones.

Their number appears to be legion. Schiaparelli discovered 104. But the better the planet is seen the more of them come out. About 350 have now been mapped at Flagstaff, and the number is only limited by our penetration. Like the asteroids, the larger ones have already been detected.

Each opposition now brings out smaller and smaller specimens.

Their Seasonal character But now comes a most interesting fact connected with them which was discovered by Schiaparelli and found equally true at Flagstaff. They are not always equally visible. Sometimes they are conspicuous, sometimes scarcely discernible even to a practiced eye. And this is not mere matter of distance. The best time for seeing the planet is not the best time for detecting the canals.

At certain oppositions we pass the planet at close quarters, at certain others a good way off. The close approaches are called favorable oppositions, the distant encounters unfavorable ones. But the latter are not so unfavorable as they are thought. For another factor beside nearness affects the reckoning. The planet's axis is tilted to the plane of its orbit at an angle of 25°, and is so faced that the southern hemisphere is presented to us at the time of closest approach. Now the canals lie chiefly in the northern hemisphere. In the next place, it is then the northern winter, and careful comparison reveals the fact that the conspicuousness of a canal is a function of the Martian time of year, becoming pronounced in summer and fading out in winter.

This is one reason why the canals so long eluded astronomers. They were not looked for at the proper time.

The first important post-Schiaparellian advance“Seas” not seas. was made in the dark regions of the planet.

For two centuries the dark regions were held to be seas. It became evident, however, from Pickering's observations in 1892 that the great part of them could not be such. In 1894, at Flagstaff, it further became evident that no part of them could be water. From the way in which the clarification of the dark regions progressed with the planet's seasons, it had become patent that the bodily transference of substance, such, for instance, as water, from one place to another, could not account for the phenomena. For the decrease in one locality was not offset by the increase in others. As the quantity of the change, positive and negative, did not balance, the change could not be due to a shift of matter. It must, therefore, be ascribable to a transformation of matter. And the only thing of suitable conduct and proper local color to show the phenomena was vegetation. The "seas" were not seas, but probably areas of vegetation.

Oases. The next significant discovery was the detection of the oases, or small round black spots that dot the planet's surface. These were initially seen as such by W. H. Pickering, at Arequipa, in 1892. Pickering called them lakes, but for a reason which will appear later it seems more proper to consider them oases. Quite as singular a feature as the canals, they prove to be as universal a one. They are the more difficult of detection ; which is the reason they were recognized later. Schiaparelli told the writer that he had himself suspected them, but could not make sure.

Just as the canals form a mesh over the disk, so the oases make the knots where the lines of the network cross. To them, in short, the canals rendezvous. The number of lines which thus come together at one and the same point is sometimes considerable. Nine meet at the Phoenix lake, eleven at the Trivium Charontis, and no less than seventeen at the Ascraeus Lacus at the top of Ceraunius. Nor, so far as can be seen, is any important junction without its spot. Their bearing upon the explanation of the canals is at once evident.

In character the oases are, when well seen, very small and very dark. Too small to disclose distinctive color, they are the most deeply complexioned detail upon the disk, and presumably blue. It is only in poor air that they show large and diffuse. About three degrees in diameter and seemingly quite round as a rule, they must be 100 miles across, and, for all their minuteness, cover a goodly area of ground.

They seem to share the same seasonal transformation with all the other markings.

The next step was the discovery of canals in Canals and oases in the the dark regions the dark regions of the planet. Streaks in these dark regions, regions were seen in 1892 at Arequipa and at the Lick Observatory, much as Dawes had seen streaks in the light ones thirty years before. But in 1894, at Flagstaff, Mr. Douglass found that the streaks were not irregular markings, but a system of lines possessing the same singular characteristics which distinguish and differentiate the "canals" in the light regions from other celestial phenomena. In short, he detected in the dark regions what Schiaparelli had detected in the light. Counterparting exactly the network over the light areas, a mesh of similar lines overspread the blue-green areas. The lines were of uniform width, of unswerving directness, and went from definite points to other equally determinate ones. These points were always of geographic importance. They were at the ends of "seas," at the bottom of "bays," or at points on the "coast-line" where canals debouched. The lines connected these topographical centres, crossing one another in the process, and at the junctions there showed, just as in the light areas, dark round spots.

Instantly to be deduced from such engraving was that the "seas" were not bodies of water. We knew this already, as I have shown; but the evidence was valuable in completely convincing those who require more than mediate proof. Permanent lines cannot be writ on water. The seas lost their character forever.

The absence of any bodies of water outside of the temporary polar sea introduces a far-reaching difference between Mars and the Earth. On Earth three quarters of the surface is water; on Mars all is land. Instead of having more sea than it can use, the planet must be in straits for the article. Its whole supply comes from the annual melting of the polar caps.

The canal system of the dark regions not only

Fig. IX.

Two systems.resembles the system in the light; the one joins on to the other. The points where the system in the light areas strike the dark are the points from which the canals in the dark regions set out. The two are thus but parts of one world-wide whole. Whatever purpose the one subserves is thus taken up and extended by the other.

Nor does the communication come to an end in the dark regions. From the southern portions of these, in the southern hemisphere, other canals run straight into the polar cap; in the northern hemisphere, similarly, canals penetrate to the most northern limit of the snow.

Lastly, the rifts which appear in the caps during the process of melting turn out to be where subsequently are seen canals. Now, as there are no mountains on Mars, differences of level cannot be a cause of melting; areas of vegetation could.

Summation. We may sum up our present knowledge of the surface conditions of the planet as follows:

(1) Change takes place upon the planet's surface; this proves the presence there of an atmosphere.

(2) The limb-light, the apparent evidence of a twilight, and the albedo, all point to a density for this atmosphere very much less than our own.

(3) The polar caps melt in their summer and accumulate in their winter, thus showing themselves to be seasonal in character.

(4) As they melt, they are bordered by a blue belt, which retreats with them. This negatives carbonic acid as the substance composing them, and leaves to our knowledge only water as a possible explanation.

(5) Their extensive melting shows their quantity to be inconsiderable, and points to a dearth of water.

(6) Comparison with previous observations shows the melting to occur in the same consecutive places year after year. The melting is thus a thing which can be locally counted on.

(7) The greatest local melting is just south of the largest dark (blue-green) regions, the bays in the polar sea in these longitudes being the largest.

(8) The dark regions are subject to a wave of seasonal changes;

(9) which follows upon the melting of the cap. They darken in early summer and fade out in their autumn.

(10) The dark regions are not seas: first, because in Professor W. H. Pickering's experiments their light showed no trace of polarization, while that of the polar sea did;

(11) second, because the quantity of the darkening is not offset by the synchronous lightening elsewhere. It cannot therefore be due to shift of substance;

(12) third, because they are seamed by a canal system counterparting that of the light areas, permanent in place.

(13) Extension of this shows that there are no permanent bodies of water on the planet.

(14) All the phenomena are accounted for by supposing them to be areas of vegetation.

(15) The polar sea being a temporary affair, the water from it is fresh.

(16) Observations on the terminator reveal no mountains on Mars, the details of the observations being incompatible with such supposition;

(17) but do reveal apparently clouds, which, however, are rare, and are chiefly visible at sunrise and sunset,

(18) and seem connected with the heat equator.

(19) The bright areas look and behave like deserts.

(20) In their winter, the south temperate light regions are covered by a white veil, which may be hoar-frost or may be cloud.

(21) Very brilliant patches appear also in the equatorial light regions that last for weeks, and seem independent of diurnal conditions.

(22) They appear always in the same places.

(23) A spring haze surrounds the polar caps during certain months, outside of and distinct from the cap itself.

(24) A progressive change of darkening sweeps over the planet's face from pole to pole semiannually, beginning with the cap, and developing as vegetation would down the disk.

Conclusions as to physical condition These phenomena lead to the conclusion that the polar caps are masses of snow and ice; that the light areas are deserts; that the blue-green areas are tracts of vegetation; that there are no permanent bodies of water on the planet, and very little water in any form; that the surface is remarkably flat; that the temperature is moderately high by day but low at night; that it is fairly warm in summer but cold in winter; and that the seasonal change of the vegetation is marked even at our distance away.

To these conclusions we are led by the general aspect and behavior of the planet's disk. We have reached them without reference to the canals considered in themselves, and we should continue to put faith in them were the canals, with all their strange characteristics, blotted from existence. Unbeholden, then, to the canals for this conclusion, we are the more impressed to find that the supposition that the "canals" are not the result of chance falls completely in line with our result.

Water is very scarce on the planet, and is absolutely essential to life. Vegetation exists there, and it is therefore highly probable that organic life is to be found there, too. This becomes a posteriori probable, when we behold a system of lines inexplicable on any other ground and precisely what would be needed for the diffusion of water over the planet's surface.

What we find is this:—

(25) A network of fine dark lines meshing the deserts.

(26) The lines are uniform throughout and from five to thirty-five miles in width,^[1]

(27) and hundreds, sometimes thousands of miles long,

(28) usually, if not always, following arcs of great circles,

(29) starting from topographically important points in the dark regions,

(30) and traveling to other equally conspicuous points;

(31) both terminals show dark spots, a caret in the coastline and what seems around spot in the desert; (32) all the way from three to seventeen "canals" will converge upon the same spot;

(33) the spots are perhaps a hundred miles in diameter, and their number is very great;

(34) the dark regions are meshed by a similar network;

(35) the points of departure of both are the same;

(36) similar centring spots show in the dark areas, darker than their background;

(37) with the dark network "canals" others connect, running to the edge of the extreme melting limits of both caps;

(38) the lines are seasonal phenomena, developing after the melting of their respective polar cap and fading out later;

(39) those in the polar regions occupy the place of earlier rifts in the snow-field, as if the ground were there thawed by vegetation.

They are of uniform width; that is, they waste nothing in breadth. Whatever breadth is necessary is used, and no more, and that is retained throughout. They go directly from certain conspicuously probable points to certain others. If we were obliged to connect the planet by a system of intercommunication, it is precisely those points we should ourselves select. In addition to the departure points on the borders of the dark regions which are provided by nature are a host of others not apparently so originated. These are the round black dots, the oases. They are found at the intersections of the lines. How important they are in the planet's economy is to be inferred from the host of canals each of them receives. Four, very rarely three, is the minimum number of approaches or departures from them, and this number rises in the case of Ceraunius to seventeen. Even London hardly has this number of railway lines entering and leaving it. It is not too much to suppose, though as yet we cannot count it more than a conjecture, that the oases serve some such purpose as our cities and are centres of population.

From this, we add to our list of conclusions, that the canals are artificial, and therefore imply organic intelligent life upon the planet.

Our synthesis leads, then, to the conclusion that Mars is circumstanced like ourselves in the midway of planetary existence, but that the planet has advanced further on the road to old age and death than we have yet done.

That its world-life was, in any but the broadest sense, an analogue of our own, is certainly not the case. Its career began under different physical conditions, owing to its size, ran more rapidly through its successive stages, again owing to its size, and will come to an end sooner for the same reason.

As a detail of this, life on Mars must take on a very different guise from what it wears on Earth. It is certain that there can be no men there; that is as certain as anything well can be. But this does not preclude a local intelligence equal to, and perhaps easily superior to, our own. We seem to have evidence that something of the sort does exist there at the present moment, and has made imprint there of its existence far exceeding anything we have yet left upon mother Earth.

In conclusion, let me warn you to beware of two opposite errors; of letting your imagination soar unballasted by fact, and, on the other hand, of shackling it so stolidly that it loses all incentive to rise. You may come to grief through the first process; you will never get anywhere by the second. Take general mechanical principles for compass and then follow your observations. Imagination is as vital to any advance in science as learning and precision are essential for starting points.

1. Tests by the writer on telegraph lines show that a line can be seen, owing to its length, when its width is 2".5, to the naked eye. This would mean about 5 miles on Mars.