Astronomy: The Science of the Heavenly Bodies Part 16

CHAPTER x.x.xIII

THE Ca.n.a.lS OF MARS

Then there are the so-called ca.n.a.ls of Mars, about which so much is written and relatively little known. Faint markings which resemble them in character were first drawn in 1840 and later in 1864, but Schiaparelli, the famous Italian astronomer, is probably their original discoverer, when Mars was at its least distance from the earth in 1877.

He made the first accurate detailed map of Mars at this time, and most of the important or more conspicuous ca.n.a.ls (_ca.n.a.li_, he called them in Italian, that is, channels merely, without any reference whatever to their being watercourses) were accurately charted by him.

At all the subsequent close approaches of Mars, the ca.n.a.ls have been critically studied by a wide range of astronomical observers, and their conclusions as to the nature and visibility of the ca.n.a.ls have been equally wide and varied. The most favorable oppositions have occurred in 1892 and 1894, also in 1907 and 1909. On these occasions a close minimum distance of Mars was reached, that is, about 35 millions of miles; but in 1924 the planet makes the closest approach in a period of nearly a thousand years. Its distance will not much exceed 34 millions of miles.

But although this is a minimum distance for Mars, it must not be forgotten that it is a really vast distance, absolutely speaking; it is something like 150 times greater than the distance of the moon. With no telescopic power at our command could we possibly see anything on the moon of the size of the largest buildings or other works of human intelligence; so that we seem forever barred from detecting anything of the sort on Mars.

Nevertheless, the closest scrutiny of the ruddy planet by observers of great enthusiasm and intelligence, coupled with imagination and persistence, have built up a system of ca.n.a.ls on Mars, covering the surface of the planet like spider webs over a printed page, crossing each other at intersecting spots known as "lakes," and embodying a wealth of detail which challenges criticism and explanation.

To see the ca.n.a.ls at all requires a favorable presentation of Mars, a steady atmosphere and a perfect telescope, with a trained eye behind it.

Not even then are they sure to be visible. The training of the eye has no doubt much to do with it. So photography has been called in, and very excellent pictures of Mars have already been taken, some nearly half as large as a dime, showing plainly the lights and shades of the grander divisions of the Martian surface, but only in a few instances revealing the actual ca.n.a.ls more unmistakably than they are seen at the eyepiece.

The appearance and degree of visibility of the ca.n.a.ls are variable: possibly clouds temporarily obscure them. But there is a certain capriciousness about their visibility that is little understood. In consequence of the changing physical aspects, as to season, on Mars and his...o...b..tal position with reference to the earth, some of the ca.n.a.ls remain for a long time invisible, adding to the intricacy of the puzzle.

For the most part the ca.n.a.ls are straight in their course and do not swerve much from a great circle on the planet. But their lengths are very different, some as short as 250 miles, some as long as 4,000 miles; and they often join one another like spokes in the hub of a wheel, though at various angles. As depicted by Lowell and his corps of observers at Flagstaff, Arizona, the ca.n.a.l system is a truly marvelous network of fine darkish stripes. Their color is represented as a bluish green.

Each marking maintains its own breadth throughout its entire length, but the breadth of all the ca.n.a.ls is by no means the same: the narrowest are perhaps fifteen to twenty miles wide, and the broadest probably ten times that. At least that must be the breadth of the Nilosyrtis, which is generally regarded as the most conspicuous of all the ca.n.a.ls. The Lowell Observatory has outstripped all others in the number of ca.n.a.ls seen and charted, now about 500.

What may be the true significance of this remarkable system of markings it is impossible to conclude at present. Schiaparelli from his long and critical study of them, their changes of width and color, was led to think that they may be a veritable hydrographic system for distributing the liquid from the melting polar snows. In this case it would be difficult to escape the conviction that the ca.n.a.ls have, at least in part, been designed and executed with a definite end in view.

Lowell went even farther and built upon their behavior an elaborate theory of life on the planet, with intelligent beings constructing and opening new ca.n.a.ls on Mars at the present epoch. Pickering propounded the theory that the ca.n.a.ls are not water-bearing channels at all, but that they are due to vegetation, starting in the spring when first seen and vitalized by the progress of the season poleward, the intensity of color of the vegetation coinciding with the progress of the season as we observe it.

Extensive irrigation schemes for conducting agricultural operations on a large scale seem a very plausible explanation of the ca.n.a.ls, especially if we regard Mars as a world farther advanced in its life history than our own. Erosion may have worn the continents down to their minimum elevation, rendering artificial waterways not difficult to build; while with the vanishing Martian atmosphere and absence of rains, the necessity of water for the support of animal and vegetal life could only be met by conducting it in artificial channels from one region of the planet to another.

Interesting as this speculative interpretation is, however, we cannot pa.s.s by the fact that many competent astronomers with excellent instruments finely located have been unable to see the ca.n.a.ls, and therefore think the astronomers who do see them are deceived in some way. Also many other astronomers, perhaps on insufficient grounds, deny their existence _in toto_.

Many patient years of labor would be required to consult all the literature of investigation of the planet Mars, but much of the detail has been critically embodied in maps at different epochs, by Kayser, Proctor, Green, and Dreyer. And Flammarion in two cla.s.sic volumes on Mars has presented all the observations from the earliest time, together with his own interpretation of them. Areography is a term sometimes applied to a description of the surface of Mars, and it is scarcely an exaggeration to say that areography is now better known than the geography of immense tracts of the earth.

For some reason well recognized, though not at all well understood, Mars although the nearest of all the planets, Venus alone excepted, is an object by no means easy to observe with the telescope. Possibly its unusual tint has something to do with this. With an ordinary opera gla.s.s examine the moon very closely, and try to settle precise markings, colors, and the nature of objects on her surface; Mars under the best conditions, scrutinized with our largest and best telescope, presents a problem of about the same order of difficulty. There are delicate and changing local colors that add much uncertainty. Nevertheless, the planet's leading features are well made out, and their stability since the time of the earliest observers leaves no room to doubt their reality as parts of a permanent planetary crust.

The border of the Martian disk is brighter than the interior, but this brightness is far from uniform. Variations in the color of the markings often depend on the planet's turning round on its axis, and the relation of the surface to our angle of vision. If we keep in mind these obstacles to perfect vision in our own day, it is easy to see why the early users of very imperfect telescopes failed to see very much, and were misled by much that they thought they saw. Then, too, they had to contend, as we do, with unsteadiness of atmosphere, which is least troublesome near the zenith.

As their telescopes were all located in the northern hemisphere, the northern hemisphere of Mars is the one best circ.u.mstanced for their investigation; because at the remote oppositions of Mars, which always happen in our northern winter with the planet in high north declination, it is always the north pole of Mars which is presented to our view.

Whereas the close oppositions of the planet always come in our northern midsummer, with Mars in south declination and therefore pa.s.sing through the zenith of places in corresponding south lat.i.tude.

With Mars near opposition, high up from the horizon, a fairly steady atmosphere, and a magnifying power of at least 200 diameters, even the most casual observer could not fail to notice the striking difference in brightness of the two hemispheres: the northern chiefly bright and the southern markedly dark. Formerly this was thought to indicate that the southern hemisphere of Mars was chiefly water and the northern land, much as is the case on the earth: with this difference, however, that water and land on the earth are proportioned about as eleven to four.

But Mars in its general topography presents no a.n.a.logy with the present relation of land and water on the earth. There seems no reason to doubt that the northern regions with their prevailing orange tint, in some places a dark red and in others fading to yellow and white, are really continental in character. Other vast regions of the Martian surface are possibly marshy, the varying depth of water causing the diversity of color. If we could ever catch a reflection of sunlight from any part of the surface of Mars, we might conclude that deep water exists on the planet; but the farther research progresses, the more complete becomes the evidence that permanent water areas on Mars, if they exist at all, are extremely limited.

Since 1877 Mars has been known to possess two satellites, which were discovered in August of that year by Hall at Washington. Moons of this planet had long been suspected to exist and on one or two previous occasions critically looked for, though without success. In the writings of Dean Swift there is a fanciful allusion to the two moons of Mars; and if astronomers had chanced to give serious attention to this, Phobos and Deimos, as Hall named them, might have been discovered long before.

They are very small bodies, not only faint in the telescope, but actually of only ten or twenty miles diameter; and from the strange relation that Phobos, the inner moon, moves round Mars three times while the planet itself is turning round only once on its axis, some astronomers incline to the hypothesis that this moon at least was never part of Mars itself, but that it was originally an inner or very eccentric member of the asteroid group, which ventured within the sphere of gravitation of Mars, was captured by that planet, and has ever since been tributary to it as a secondary body or satellite.

CHAPTER x.x.xIV

LIFE IN OTHER WORLDS

Popular interest in astronomy is exceedingly wide, but it is very largely confined to the idea of resemblances and differences between our earth and the bodies of the sky. The question most frequently asked the astronomer is, "Have any of the stars got people on them?" Or more specifically, "Is Mars inhabited?" The average questioner will not readily be turned off with yes or no for an answer. He may or may not know that it is quite impossible for astronomers to ascertain anything definite in this matter, most interesting as it is. What he wants to find out is the view of the individual astronomer on this absorbing and ever recurring inquiry.

We ought first to understand what is meant by the manifestation here on the earth called life, and agree concerning the conditions that render it possible. Apparently they are very simple. We may or may not agree that a counterpart of life, or life of a wholly different type from ours, may exist on other planets under conditions wholly diverse from those recognized as essential to its existence here. The problem of the origin of life is, in the present state of knowledge, highly speculative and hardly within the domain of science. Here on earth, life is intimately a.s.sociated with certain chemical compounds, in which carbon is the common element without which life would not exist. Also hydrogen, oxygen, and nitrogen are present, with iron, sulphur, phosphorus, magnesium and a few less important elements besides. But carbon is the only substance absolutely essential. Protoplasm cannot be built without it, and protoplasm makes up the most of the living cell.

Closely related to carbon is silica also, as a subst.i.tution in certain organic compounds. Protoplasm is able to stand very low temperatures, but its properties as a living cell cease when the temperature reaches 150 Fahrenheit.

Animal life as it exists on the earth to-day appears to have been here many million years. The palaeontologists agree that all life originated in the waters of the earth. It has pa.s.sed through evolutionary stages from the lowest to the highest. Throughout this vast period the astronomer is able to say that the conditions of the earth which appear to be essential to the maintenance of life have been pretty constantly what they are to-day. The higher the type of life, the narrower the range of conditions under which it thrives. Man can exist at the frigid poles even if the temperature is 75 degrees below Fahrenheit zero; and in the deserts and the tropics, he swelters under temperatures of 115 degrees, but he still lives. At these extremes, however, he can scarcely be said to thrive.

We have, then, a relatively narrow range of temperatures which seems to be essential to his comfortable existence and development: we may call it 150 degrees in extent. Had not the surface temperature of the earth been maintained within this range for indefinite ages, in the regions where the human race has developed, quite certainly man would not be here. How this equability of temperature has been maintained does not now matter. Clearly the earth must have existed through indefinite ages in the process of cooling down from temperatures of at least 6,000 degrees.

During this stage the temperature of the surface was earth-controlled.

Then this period merged very gradually into the stage where life became possible, and the temperature of the surface became, as it now is, sun-controlled. How many years are embraced in this span of periods, or ages, we have no means of knowing. But of the sequence of periods and the secular diminution of temperature, we may be certain.

Then there is the equally important consideration of water necessary for the origination, support, and development of life. We cannot conceive of life existing without it. On the earth water is superabundant, and has been for indefinite ages in the past. There is little evidence that the oceans are drying up; although the commonly accepted view is that the waters of the earth will very gradually disappear. Water can exist in the fluid state, which is essential to life, at all temperatures between 32 degrees and 680 degrees F.

Air to breathe is essential to life also. The atmosphere which envelops the earth is at least 100 miles in depth, and its own weight compresses it to a tension of nearly 15 pounds to the square inch at sea level.

This atmosphere and its physical properties have had everything to do with the development of animal life on the planet. Without it and its remarkable property of selective absorption, which imprisons and diffuses the solar heat, it is inconceivable that the necessary equability of surface temperature could be maintained. This appears to be quite independent of the chemical const.i.tuents of the atmosphere, and is perhaps the most important single consideration affecting the existence of life on a planet. If the surface of a planet is partly covered with water, it will possess also an atmosphere containing aqueous vapor.

Heat, water, and air: these three essentials determine whether there is life on a planet or not. Of course there must be nutrition suitable to the organism; mineral for the vegetal, and vegetal for the animal. But the narrow range of variation appears to be the striking thing: relatively but a few degrees of temperature, and a narrow margin of atmospheric pressure. If this pressure is doubled or trebled, as in submarine caissons, life becomes insupportable. If, on the other hand, it is reduced even one-third, as on mountains even 13,000 feet high, the human mechanism fails to function, partly from lack of oxygen necessary in vitalizing the blood, but mainly because of simple reduction of mechanical pressure.

If, then, we conceive of life in other worlds and it is agreed that life there must manifest itself much as it does here, our answer to the question of habitability of the planets must follow upon an investigation of what we know, or can reasonably surmise, about the surface temperatures of these bodies, whether they have water, and what are the probable physical characteristics of their atmospheres.

We may inquire about each planet, then, concerning each of these details.

The case of Mercury is not difficult. At an average distance of only 36 million miles from the sun, and with a large eccentricity of orbit which brings it a fifth part nearer, conditions of temperature alone must be such as to forbid the existence of life. The solar heat received is seven times greater than at the earth, and this is perhaps sufficient reason for a minimum of atmosphere, as indicated by observation. If no air, then quite certainly no water, as evaporation would supply a slight atmosphere. But according to the kinetic theory of gases, the ma.s.s of Mercury, only a very small fraction of that of the sun, is inadequate to retain an atmospheric envelope. If, however, the planet's day and year are equal, so that it turns a constant face to the sun, surface conditions would be greatly complicated, so that we cannot regard the planet as absolutely uninhabitable on the hemisphere that is always turned away from the sun.

Venus at 67 millions of miles from the sun presents conditions that are quite different. She receives double the solar heat that we do, but possessing an atmosphere perhaps threefold denser than ours, as reliably indicated by observations of transits of Venus, the intensity of the heat and its diffusion may be greatly modified. What the selective absorption of the atmosphere of Venus may be, we do not know. Nor is the rotation time of the planet definitely ascertained: if equal to her year, as many observations show and as indicated by the theory of tidal evolution, there may well be certain regions on the hemisphere perpetually turned away from the sun where temperature conditions are identical with those on the tropical earth, and where every condition for the origin and development of life is more fully met than anywhere else in the solar system. Whether Venus has water distributed as on the earth we do not know, as her surface is never seen, owing to dense clouds under which she is always enshrouded. Her cloudy condition possibly indicates an overplus of water.

Is the moon inhabited? Quite certainly not: no appreciable air, no water, and a surface temperature unmodified by atmosphere--rising perhaps to 100 degrees F. during the day, which is a fortnight in length, and falling at night to 300 degrees below zero, if not lower.

Is Mars inhabited? The probable surface temperature is much lower than the earth's, because Mars receives only half as much solar heat as we do; and more important still, the atmosphere of Mars is neither so dense nor so extensive as our own. Seasons on Mars are established, much the same as here, except that they are nearly twice as long as ours; and alternate shrinking and enlarging of the polar caps keeps even pace with the seasons, thereby indicating a certainty of atmosphere whose equatorial and polar circulation transports the moisture poleward to form the snow and ice of which the polar caps no doubt consist.

There is a variety of evidence pointing to an atmosphere on Mars of one-third to one-half the density of our own: an atmosphere in which free hydrogen could not exist, although other gases might. The spectroscopic evidence of water vapor in the Martian atmosphere is not very strong. It is very doubtful whether water exists on Mars in large bodies: quite certainly not as oceans, though the evidence of many small "lakes" is pretty well made out. With very little water, a thin atmosphere and a zero temperature, is Mars likely to be inhabited at the present time? The chances are rather against it. If, however, the past development of the planet has progressed in the way usually considered as probable, we may be practically certain that Mars has been inhabited in the past, when water was more abundant, and the atmosphere more dense so as to retain and diffuse the solar heat.

Biologists tell me that they hardly know enough regarding the extreme adaptability of organisms to environment to enable them to say whether life on such a planet as Mars would or would not keep on functioning with secular changes of moisture and temperature. The survival of a race might be insured against extremely low temperatures by dwelling in sub-Martian caves, and sufficient water might be preserved by conceivable engineering and mechanical schemes; but the secular reduction of the quant.i.ty and pressure of atmosphere--it is not easy to see how a race even more advanced than ourselves could maintain itself alive under serious lack of an element so vital to existence. Both Wallace, the great biologist, and Arrhenius, the eminent chemist (but biologist, astronomer, and physicist as well), both reject the habitation theory of Mars, regarding the so-called ca.n.a.ls as quite like the luminous streaks on the moon; that is, cracks in the volcanic crust caused by internal strains due to the heated interior. Wallace, indeed, argues that the planet is absolutely uninhabitable.

The asteroids, or minor planets? We may dismiss them with the simple consideration that their individual ma.s.ses are so insignificant and their gravity so slight that no atmosphere can possibly surround them.

Their temperatures must be exceedingly low, and water, if present at all, can only exist in the form of ice.

Jupiter, the giant planet, presents the opposite extreme. His ma.s.s is nearly a thousandth part of the sun's, and is sufficient to retain a very high temperature, probably approximating to the condition we call red-hot. This precludes the possibility of life at the outset, although the indications of a very dense atmosphere many thousand miles in depth are unmistakable.

Of Saturn, one thirty-five hundredth the ma.s.s of the sun, practically the same may be said. Proctor thought it quite likely that Saturn might be habitable for living creatures of some sort, but he regarded the planet as on many accounts unsuitable as a habitation for beings const.i.tuted like ourselves. Mere consideration of surface temperature precludes the possibility of life in the present stage of Saturn's development; but the consensus of opinion is to the effect that life may make its appearance on these great planets at some inconceivably remote epoch in the future when the surface temperature is sufficiently reduced for life processes to begin. Discoveries of algae flourishing in hot springs approaching 200 degrees Fahrenheit make it possible that these beginnings may take place earlier and at much higher temperatures than have hitherto been thought possible.