Within the orbit of Venus circles a planet of which, until lately, even less was known than of Venus herself. Of Venus we knew practically nothing; of this other, less than nothing, what we thought we knew turning out not to have been so. This body, which was thus not only a riddle, but a riddle misguessed, is the planet Mercury.
Nature makes no jumps; but science, our knowledge of nature, does. By one of these, we have come recently into possession of information about Mercury as interesting as regards the planet personally as it is of moment as regards planetary revolution in general; for while it tells us of the present condition of Mercury, it tells us something about the life-history of our solar system.
To most people Mercury is known chiefly as being very difficult to see; and to be seen at all he must be looked for low down in the twilight sky, at certain specified times, during certain equally specified seasons of the year. Seeing him is enhanced by the tradition that the great Copernicus died without ever having done so.
He is, however, not so difficult to detect as this probably true story about Copernicus has led many to suppose. Two impediments to the observation of Mercury stood in Copernicus’s way: one, that Copernicus lived very far north; the other, that the mists at the mouth of the Vistula rose nightly to obscure the twilight sky. The latter obstacle is as evident as it made Mercury the reverse; the former will be none the less apparent when we reflect that in northern latitudes the path in which all the major planets travel is greatly bowed to the horizon. In consequence, that path is subject for a long distance from the sun to all those atmospheric disturbances peculiar to the horizon, — disturbances which make observations near it practically impossible; and the farther north, the greater the difficulty.
Fortunately, at certain times and places Mercury is more removed from this all-obliterating influence than he is at others, and at such times he may be very distinctly seen, shortly after sunset, twinkling through the gloaming in the west. The whole difficulty lies in the sky, for the planet himself is much brighter than the background upon which he twinkles would lead one to think. If the observer chance to have a bright star—Arcturus or Altair, for example—in the west at the time, he may note, by comparing the planet with the star, how very much brighter the planet really is than he looks to be. As a matter of fact, Mercury shines with a lustre surpassing that of a first magnitude star, outshining, when projected against an equally illuminated sky, almost every fixed star in the firmament. But to detect Mercury one must be quick, in keeping with the planet’s name; for a few minutes suffice to hide him as he settles into the horizon vapors, there to vanish from view, while a few days cause such a change in his distance from the sun as to make him invisible even at his most propitious hour. The best chance of detecting the planet is when he attains his greatest elongation in the early spring, inasmuch as then the ecliptic, or path in or near which all the planets move, has its greatest northerly inclination to the horizon. He is in consequence higher then, for the same distance from the sun, than at any other season, and so is raised out of the low-lying mists and vapors. This height is all important, for a slight difference of background malkes every difference in the planet’s visibility.
That the ancients detected him we know from records dating from before the Christian era. What is more, they detected that he was a planet; that is, a wandering star, one that moved among the host of heaven. They did not, however, recognize him as one and the same body on both sides of the sun, for they gave him two names, according as he was morning or evening star. The Greeks, for instance, called him Hesperus when he appeared in the west after sunset, and Mercury when he was seen in the east before sunrise. In the case of the Greeks this was but a poetic survival of archaic notions, for the Greeks knew very well—that is, the Greek philosophers did—that both apparitions belonged to one body, and that body an attendant of the sun.
With Copernicus came the recognition that Mercury was a body revolving round the sun inside of the orbit of the earth. So soon as this rectification of the solar system took place, by which the earth was relegated to her true subordinate position in it, the path Mercury pursued became known; for all the facts had been gathered before, and needed only to be arranged, to be understood. It was thus made evident that Mercury was the nearest planet to the sun, and that he revolved about that body in approximately eighty-eight days, in an orbit highly eccentric for a planet. The eccentricity of his ellipse amounts, indeed, to two tenths of his mean distance, so that at certain times he is nearer the sun than at others in the proportion of two to three.
With the invention of the telescope and the progress of science it was learned that he was a small body, with a diameter of something more than three thousand miles. (The earth, it will be remembered, is a little under eight thousand miles through.) And this, curious as it may seem, was all that was positively known of him until within the memory of men still young. Men are yet being brought up on this somewhat meagre knowledge, plus some which is not so; for they are taught to believe that he is a very dense planet, and this with an assurance greater in proportion to the poorness of the textbook they study. Now, a planet’s mean density depends upon his mass; and the mass of Mercury is not definitely known. The mass of some of the planets is very well known, — that of Jupiter or Saturn, for example; of all such, in fact, as possess satellites. The reason is that the possession of an attendant enables the mass of the primary to be very well weighed by the motion caused by him in that attendant. But, on the other hand, an unattended planet has nothing to betray his avoirdupois; the disturbances he may cause in passing comets, or the slight swayings he may induce in neighbor planets, alone affording any criterion of his cosmic size. Mercury is singularly ill situated, for he is so small as to produce little effect in either case. From observations of a certain comet, however, that chanced once to come near him, a mass was deduced for him which made his density quite great, greater than that of the earth. The density was really that of the deduction; for the data in the case were known to be of great uncertainty, and a little thought would have shown that general principles pointed the other way.
Common sense is as uncommon in science, unfortunately, as it is in everything else. Now, the prime factor in common sense is the sense of the importance of general principles. The cosmos is but the material manifestation of law; not meaning by this either divine or human laws, but the relations and reactions of things. And these laws, or principles, are perfectly general in their application. None of them ever fails; the appearance of failure being due simply to another’s paramount influence. If we apply general principles to the case before us, it is plainly apparent that, whatever mode of genesis we admit for the coming into existence of the planets, the probable density of the matter composing them must have been approximately the same. We can best conceive this by considering what must have been the tenuity of the parent nebula even when Mercury was born. So much for the probable initial density. The final density assumed by a planet depends not only upon the initial density of the matter composing it, but upon the volume of matter the planet contains; for each particle of this matter attracts every other particle of it, and the greater the whole number of particles in the aggregation, the greater the crushing in of all and the denser the resulting body. The smaller the size of the planet, therefore, the less its density, other things being equal. We see, then, a priori, reason for inferring that Mercury cannot be dense.
Determinations of Mercury’s mass subsequent to those upon which the usually quoted density of the planet depends have shown themselves more in accord with this a priori inference; the most recent deduction from these same perturbations having greatly reduced the resulting mass of the planet, and with it his density. But even now general principles are a safer guide than any of these particular determinations.
This is one of the points to which I referred as our minus knowledge of Mercury. It is much more far reaching than appears at first sight; for upon the mass of any planet depend that planet’s physical characteristics, notably its power to hold an atmosphere, and, with its capability of holding an atmosphere, its ability to sustain life. We shall now see that the capabilities of Mercury in this respect are below the possibilities of life.
The probable mass of Mercury is about .039 of the mass of the earth; his probable diameter, 3100 miles; for he is probably a little larger than supposed. To measure the full disk of Mercury is possible only when he is seen in transit projected on the face of the sun. When he is so seen, the measures made of him are certain to be too small, inasmuch as irradiation always makes a dark object seen against a bright one seem smaller than it is. If we could ever see Mercury full when off the sun, we could strike a mean and deduce a pretty accurate result. But we cannot do this. We are constrained to take as basis his transit measures, but it is absolutely essential that we should add to them a correction.
His probable mass and presumable size give us for his density about .65 of the earth’s. Gravity, therefore, at his surface would be about one quarter of hers, and his critical velocity, or the velocity he could just restrain, 2.1 miles a second. Now, from the kinetic theory of gases we see that this velocity falls below that of water vapor, which is 2.5, and just above that of oxygen, which is 2.0, and nitrogen, which is 1.8. Hence Mercury cannot keep water vapor about him, and it is doubtful whether he could hold oxygen or nitrogen. As for the denser vapors, to vaporize these would require a temperature at his surface enormously high and perpetually sustained. We shall see in a moment that the conditions upon the planet’s surface are such as to preclude even this possibility. Mercury, therefore, probably has no appreciable atmosphere. We shall find that this deduction is fully sustained by the look of the planet’s surface.
Having thus reviewed all we knew, and much that we did not know, but should have known, of the planet, up to within the last few years, we will proceed to the fundamental addition those years have given us. This addition has been the work of two sets of observations: those of Schiaparelli in 1889, and those at Flagstaff, Arizona, last summer.
In 1889 Schiaparelli began observations of the planet, which up to that time had presented to terrestrial observers a disk devoid of markings of any definiteness, a blank sheet upon which no certain characters were written. But Schiaparelli soon discovered that this lack of expression was no fault of the planet. Markings were found by him on the disk, — perfectly recognizable and delineable ones. As time went by he noticed that these markings slowly shifted in position upon the face of the planet. Minutes and hours, indeed, brought no change of place in them, but days did. Such shift showed that the planet rotated, and furthermore that the rotation was very slow. As he continued his search the period of this rotation was before long disclosed, and a very singular one it turned out to be; for it appeared that, quite unlike the earth, Mercury turned on his axis once only in the course of his circuit about the sun.
It is perhaps supererogatory to remark that the discovery remained largely unappreciated. Schiaparelli communicated it at once to the astronomical world, but scientific men refused to credit it with more than half-confidence. We have seen how they had accepted previously the erroneous determinations of Mercury’s mass, which recalls the story of the simple old soul and her son Jack when the latter returned from his voyages and poured his tales into her ear. She accepted the mountains of sugar and rivers of wine with perfect composure, but balked at admitting the flying fish. “No, Jack,” she said, “you will never make your old mother believe that fish can fly.” The astronomical world reminds us in this of what Tame wrote of Prosper Mérimée: he was so resolved not to be taken in that he ended by becoming the dupe of his own distrust. Skepticism in moderation is a beneficial thing, but extreme skepticism betrays extreme ignorance.
Matters rested here until last summer, when the observations which resulted in the following knowledge of the planet were made.
Next to as good air as the observer can get, the most important prerequisite, if he would see anything of Mercury, is that his observations should all be made by day. It will not do to wait for twilight to disclose the planet, as the tremors of the air at very low altitudes blur all detail upon his disk. He must be sought in broad daylight, while he is quite invisible to the naked eye. This is done by pointing the telescope toward his calculated position; if the instrumental adjustments are correct, he appears punctual to the law of gravitation, and looks for all the world like a little moon swimming alone in the vast blue sky.
The first characteristic he presents is pallor. He looks as wan as the moon herself when seen under like circumstances. His disk is of that pale white ashen hue with which we are so familiar in the moon by day. To an observer who observes aright every detail means something. Thus even this pallor is important. We note, to start with, that it is like the aspect of the moon. We next note that if we compare Mercury’s disk with that of Venus there is a most marked contrast between the two. While Mercury is pale white, Venus is a brilliant straw-color; and her disk is ever so much brighter, square unit for square unit, than his. Now, when we consider the relative distance of the two bodies from the sun, we see that the contrast should be the other way if the surfaces of the two were alike; for the illumination Mercury receives from the sun in consequence of his proximity is more than two and a half times that which Venus receives; and yet, in spite of this, when both are seen against the same background, the surface of Venus far outshines that of Mercury. To what can this difference be due? A second thought will suggest the answer: Atmosphere. Venus, we know, has an atmosphere; Mercury, we have every reason to believe, has none. An atmosphere would produce just the effect we mark in the case of Venus, a brightening of her disk all over. Mercury, being without such light-reflecting veil, shows as lie really is. Here, then, from the surface look of the planet we have corroboration of the deduction from its probable density of its lack of atmosphere. Our moon is like Mercury, devoid of atmosphere, and she like Mercury looks pale. In both the pallor is what the actual surface of a globe of material not unlike our earth should present if bare of atmospheric covering. We shall see this more distinctly by comparing the relative reflecting power of the moon, of Mercury, and of certain known rocks. The moon reflects about .11 of the incident light, sandstone .237, and quartz porphyry .108. We see, then, from consideration of his pallor alone, that we probably look upon the actual surface of the planet, and that that surface is probably rock, sand, or soil, of a color between sandstone and quartz porphyry.
We now come to another point. As Mercury passes from the full to the crescent phase his surface diminishes visibly in lustre. I do not now refer to his loss of light as a whole, but to the loss in brilliancy of the illuminated part, square area for square area. His surface fades out, as it were. Just this fading out occurs with the moon. She too loses lustre as she wanes to a crescent. In the case of the moon, the loss has been attributed, and doubtless correctly, to the mountainous or craterous character of her surface. As the mountain peaks of the crater walls pass toward the sunrise or sunset edge of the disk—the edge which makes the phase—they cast longer and longer shadows, which, indistinguishable as such to the naked eye, result in cutting off just so much of the light, and give the effect of a paling of the surface. Mercury’s like behavior may be due to a like cause, and the planet may possess a surface which is covered more or less with mountain ranges or with crater walls.
Thirdly, we may note that with absence of air goes absence of water. Even were there water without air to start with, it could not remain in the absence of air; for as it evaporated to vapor, which in the course of time must happen to all of it, the same conditions which caused the air itself to leave the planet would cause the water vapor to follow suit, and thus eventually leave the planet water-bare.
Thus the absence of air on Mercury precludes the possibility of seas or oceans or rivers there. Furthermore, the absence of water prevents the existence of any vegetation upon the planet. The surface of Mercury is therefore, in all probability, one vast desert.
We will now turn to the markings and see what they disclosed. As soon as the planet was scanned at Flagstaff markings were apparent upon the disk. The markings were dark, very much darker than those of Venus. In consequence, they were proportionally easier to make out. In good air they were remarkably distinct, and even in bad air they were quite recognizable. They were numerous and permanent in place. Curiously enough, they were lines rather than patches. In this they differed noticeably from the moon’s markings, which to the naked eye have the look of blotches. The two sets of markings agreed only in being the darker portions of their respective bodies. The patches on the moon, when examined telescopically, have the appearance of having been old sea-bottoms. This may be illusive inference, as it is more than questionable whether the moon ever had the necessary water; but it is certain that the dark patches are the smooth, plain parts of our satellite, while the brighter parts of her are the mountain and crater regions. We may perhaps infer from this that on Mercury, too, the dark areas represent the flatter country.
The first point to chronicle about the markings, because the most general one, is their entire absence of color. The whole disk, like that of Venus, was a chiaroscuro of markings, a picture in black and white unrelieved by colored tint of any kind. Here we have a telltale appearance, as in the case of Venus; for the absence of color shows the absence of both water and vegetation. We know that did water or vegetation exist on Mercury’s surface we could tell it by its tint, for this is precisely what we do with Mars. The hues of Mars are perfectly distinct even across so many millions of miles of space. His vegetation makes itself apparent by its beautiful blue-green tint.
Here, then, we have corroboration from another source of the absence of water on Mercury.
As the markings were so distinct, it was speedily possible to see whether or not they moved across the face of the disk; if motion occurred, it would give the rotation period of the planet, and the position of the axis around which such rotation took place. It was very soon evident that the markings did not change their place from hour to hour, nor perceptibly from day to day. Whenever the observer looked, the same markings appeared in the same positions. Now, as the intervals between the observations were of all lengths from very short to very long ones, no rotation of short period was compatible with such immutability of place.
This showed what the rotation was not. As time went on it became possible to say what the rotation was; for as the edge of the illuminated portion of the planet shifted across the disk in consequence of the planet’s revolution in his orbit about the sun, all the markings slowly shifted with it. As I explained in my paper on Venus, this showed immediately that the planet rotated once in the course of his journey round the sun; that is, that his periods of axial rotation and orbital revolution coincided.
But here a very neat little variation appeared in the consecutive appearances of the planet’s face, — a variation not visible in the case of Venus. During a portion of his orbital revolution, in a certain part of his path, the markings proceeded to lag behind the place they should have occupied, on the supposition of isochronism of rotation and revolution. The illumination crept faster across the face of the planet than the markings were able to follow. At first sight this will seem to have been conclusive evidence that the rotation period and the orbital one were not the same. As a matter of fact it really proved the reverse, the apparent variation from synchronousness evidencing the absolute synchronism of the two; for the variation was nothing more nor less than the planet’s libration in longitude made visible.
Libration in longitude, or the apparent swing of the centre of the disk one way or the other, is the inevitable consequence of eccentricity of orbit. To understand it, we need only consider the planet’s motion in its path. If a planet travel in a circle about the sun, its angular change must be the same for equal intervals of time. If, on the other hand, it move in an ellipse, the equality is no longer preserved. For when it is nearest the sun it moves much faster than when far away, being more attracted in the one case than in the other; furthermore, when near, the angle it describes is greater even for the same speed. Doubly fast, therefore, will be its sweep round the sun when it is near, compared with its velocity when it is remote.
Now, Mercury’s orbit, unlike Venus’s, is, for a planetary orbit, very eccentric, the planet being at times half as far again from the sun as at others. In consequence, when near perihelion Mercury sweeps through six and a third degrees a day; in aphelion he only manages to compass two and three quarters degrees. His rotation on his axis is uniform because of the great momentum of that rotation, and is equal to about four degrees a day. If, therefore, we suppose the planet to start from perihelion, the angle of revolution will proceed to gain on the angle of rotation, and this gain will continue until the planet has reached that point in his orbit where his angular movement round the sun has so far diminished as just to equal his daily angle of rotation. In the case of Mercury, the equality is brought about 18 days and 9 hours after the date of the planet’s passing perihelion. As the angle of revolution has throughout this time been gaining upon the angle of rotation, the difference between them will here be at its greatest, and will amount to the very considerable divergence of twenty-three and two thirds degrees. This has the effect of swinging all the markings to one side through the same number of degrees.
So soon as the above point is passed the conditions will be reversed, the angle of rotation now proceeding to gain on the angle of revolution and slowly catching up with its arrears. When the planet reaches aphelion the loss has all been made up, and the two angles start together again. But as the angular revolution is here least, the angle of rotation continues its gain until, at a point corresponding to the one in the first half of its orbit where the two angular movements were the same, they again become equal, the angular rotation having accumulated its maximum advance, — an advance it then proceeds to lose to equality again at perihelion. There is thus brought about a swing of the planet’s face, first one way and then the other, and this is what is known as libration in longitude. In the case of Mercury the whole effect of the swing is forty-seven and a third degrees; twice, that is, twenty-three and two thirds.
Now, it so chances that upon the eastern side of Mercury there are markings which make this libration very strikingly and interestingly evident. When the planet is in his mean position two dark lines are visible cutting off the cusps, — one at a slight distance from the southern pole of the planet, the other nearly the same distance from the northern one. As the libration swings, the markings round these two lines proceed to curve more toward one another, until when the libration has reached about fifteen degrees they are seen to join, forming one continuous band from top to bottom, with bright surface beyond them. After this time the bright area beyond widens. This gives the markings the look of a lyre, the time-honored instrument of the god, from which I have accordingly named the lines.
Mercury, therefore, like Venus, rotates but once during the time he takes to make his circuit of the sun. The phenomena of eternal day on one side of him and of everlasting night on the other, together with all the resulting physical effects of such a state of things upon the body’s surface conditions, are the same for both planets except in so far as they are affected by two considerations: the presence of air and lack of libration on Venus, and the lack of air and presence of libration on Mercury; for as Mercury is practically devoid of air, so Venus is practically without libration. Venus’s orbit is so nearly circular that from the extreme point of her libratory swing on one side to the extreme point on the other is only a degree and a half, or ninety miles measured along her surface, a debatable territory of day and night not worth debating. Now, both air and libration are needed for change, and each planet wants one or the other of these prerequisites. If Venus had a sensible libration or Mercury a sensible atmosphere, the resulting climatic conditions upon the strips of their surfaces which alone experience the recurrent alternations of sunshine and shade would be most interesting to consider. As it is, the two planets are equally impossibly circumstanced for any resulting effect, inasmuch as the lack of either of the above attributes, air or libration, is fatal.
The isochronism of rotation and revolution which the markings disclose not only corroborates the physical state of Mercury’s surface which the very look of the surface implies; it explains how such a state of things came about. We saw this in the case of Venus, and precisely the same argument applies to Mercury except in so far as libration affects the latter. But even in the presence of air it seems doubtful if libration could alter the eventual meteorologic conditions; for any water must of necessity seek the extreme point of the dark side, and leave the debatable strip of territory in the end as arid as the sun-baked portion of the planet. Finally, it is more than likely that Mercury has neither water nor air, and so lacks even the premises to any other conclusion.
Physically, the effects of the libration and of the eccentricity of the planets orbit are probably nil; phenomenally, they are rather interesting. Over the three eighths of the surface which are exposed forever to the sun, that body appears to oscillate back and forth through forty-seven degrees of sky, taking 51 days and 5 hours to go from his extreme eastern point to his extreme western one, and then 36 days and 18 hours to get back again, his westward swing being made nearly half as fast again as his eastward one. While the sun is thus oscillating he is changing in apparent size. At his extreme western position he appears as a sun about seven times as large as ours, proportionately bright and hot. From this he increases to nearly ten times in the middle of his motion east, and then decreases to seven times again at his eastern extreme. As he swings back he decreases further to about four times ours in the middle of his path, to increase again to seven times at his western limit and begin over again. The Mercurials, were there any such folk, would thus see in their sun a very palpable negative of their own orbit, as we may express it, projected on the sky.
In the debatable strip of territory the sun would rise and go through a part of this path according to the position of the observer on the strip in question, and then set again at a different rate. To observers on the eastern side of the planet he would rise fast and set slow; to those on the western, the reverse.
From what I have here sketched it will be seen that we are now in possession of evidence of the physical condition of Mercury to a not inconsiderable degree of detail. That condition may be summarized as follows: Mercury is a body devoid, practically if not absolutely, of air, of water, and of vegetation; consequently incapable of supporting any of those higher organisms which we know as living beings. His surface is a vast desert. It is rough rather than smooth. Whether this roughness be due to mountains proper or to craters we are too far away from him to be able yet to say. The latter cause is the more probable. Over the greater part of his surface, change, either diurnal or seasonal, is unknown. Three eighths of his surface are steeped in perpetual glare, three eighths shrouded in perpetual gloom, while the remaining quarter slowly turns between the two. The planet itself, as a world, is dead.
Interesting as Mercury thus proves to be, the interest as regards the planet himself is of a rather corpselike character. Less deterrent, perhaps, is the interest he possesses as a part of the life-history of the solar system; for tidal friction, the closing act in the cosmic drama, has brought him where he is. The machine has run down. Whether he ever supported life upon his surface or not, the power to do so has now forever passed away. Like Venus and for like cause, he is now a dead world. And he was the first thus to reach the end of his evolutionary career, — earlier to do so than Venus, inasmuch as tidal action was very much greater on him than on her, and consequently produced its effect more quickly. Mercury has long been dead, — how long, measured by centuries, we cannot say, but practically for a very long time. Venus must have become so comparatively recently. Both, however, now have finished their course, and have in a most literal sense entered into their rest. They are the only planets that have yet done so. They are the first, but not the last; for the same fate is doubtless in store for all the others, each in its turn. Each foretells it by having already reached a stage in the process almost exactly proportionate to its position. It is not a little curious, indeed, that the several stages should be as precisely represented by the several planets as they are. Our recent knowledge of the condition of Mercury and Venus has made this apparent. These planets have supplied the missing links in the chain of evidence; or rather, it is these that have made something more than missing links, for they stand at one end of the line, and by so doing furnish specimens of the final act in the process, without which the whole process would not have been evident.
As the matter of planetary decrepitude and death turns upon the cessation of planetary rotation, let us compare the rotation periods of the several planets, proceeding outward from the sun. As, however, it is isochronism of axial rotation and orbital revolution that determines this death, and not the actual rotation period, what we must compare is the rotations of the several planets as regards the sun, not as regards space. Represented in tabular form, these periods, planetary days as they are, stand as follows: —
Earth: 24 hours.
Mars: 24 hours, 39.5 minutes.
Jupiter: 9 hours, 55 minutes.
Saturn: 10 hours, 14 minutes.
Uranus: unknown, but probably rapid.
Neptune: unknown, but probably rapid.
This table is striking. We see from it that the slowing up of planetary rotation is almost precisely timed to the distance of the planet from the sun. We could not possibly expect the accordance to come out closer than it does; for, other things being equal, the size of a planet must certainly affect the speed of its rotation, and the sizes of the planets are very diverse, ranging all the way from the giant Jupiter to Mars the pygmy or to Mercury the dwarf; Jupiter being actually more comparable to the sun itself in size than to Mars or Mercury. Now, as, in spite of such diversity, distance from the sun apparently determines the effect, we see how paramount a factor in the process of planetary decrepitude distance from the sun must be.
No less suggestive are the colors of the several planets. As the planets are commonly observed only by night, the full effect of their contrasting tints escapes recognition. If one have the opportunity of observing them telescopically by day, and avail himself of the chance, he will be surprised to find how striking this contrast is; for daylight brings out the colors in a way one would not suppose possible beforehand. Whatever differences are seen by night are intensified by day. For the four inner planets and the moon these colors are as follows: —
Mercury: white and black chiaroscuro.
Venus: straw-color chiaroscuro.
Earth: ochre and blue-green.
Mars: rose-ochre and blue-green.
Moon: white and black chiaroscuro.
It is at once evident from this that Mercury, Venus, and the moon fall into one category, the earth and Mars into another. The former have the hues of death; the latter, of life.
As to the major planets seen by night, Jupiter and Saturn are brick-red between their cloud-belts, while Uranus and Neptune seem to be pale green; but we get so little light from the latter two that we may suspend definite judgment as to their tints.
In both the above classifications—rotation period as regards the sun, and color—it will be noticed how curiously the planets go in pairs: —
Mercury and Venus: Rotation, infinitely long; color, arid.
Earth and Mars: Rotation, about a day; color, vegetation.
Jupiter and Saturn: Rotation, about ten hours; color, heat glow.
Uranus and Neptune: Rotation, probably rapid; color, doubtful.
While this coupling is doubtless fortuitous, the apparent progression underlying it is doubtless not so.
Thus do the several planets combine to give us a consecutive picture of the career of each. Through the telescope we look not only at the present, but back into the past and on into the future. From study of all we can read the main events in the life-history of each; for each must have passed or must be passing from a formless infancy through a plastic youth to a rigid old age.
In detail, the life of each must differ from that of its neighbor; for size and distance from the sun would each cause a difference in physical characteristics into which—very interesting as the subject is—we have not space here to go. Suffice it that from the fact that the matter composing the cosmos seems to be of common character, and that physical forces, so far as we know them, must be universal in their application, we can make some deduction as to the conditions prevailing upon each of these globes. On some points we can affirm pretty positively; on others, as yet, little or nothing. Whether, for example, there be other forms of life in the universe of which we have neither cognizance nor conception, we cannot be sure. But we can say that in certain cases life such as we know it cannot exist. We can affirm with something like certainty that no life like ours can now be possible on either Mercury or Venus. Whatever they may once have been, these two planets are now ghastly parodies of worlds, — globes having the semblance of possible abodes, but being really pitilessly the reverse.
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