Other Worlds · Part 2

This preliminary judgment, arising from the evident unity of the planetary system, can only be varied by an examination of its members in detail.

One striking fact that commands our attention as soon as we have entered the narrow precincts of the solar system is the isolation of the sun and its attendants in space. The solar system occupies a disk-shaped, or flat circular, expanse, about 5,580,000,000 miles across and relatively very thin, the sun being in the center. From the sun to the nearest star, or other sun, the distance is approximately five thousand times the entire diameter of the solar system. But the vast majority of the stars are probably a hundred times yet more remote. In other words, if the Solar system be represented by a circular flower-bed ten feet across, the nearest star must be placed at a distance of nine and a half miles, and the great multitude of the stars at a distance of nine hundred miles!

Or, to put it in another way, let us suppose the sun and his planets to be represented by a fleet of ships at sea, all included within a space about half a mile across; then, in order that there might be no shore relatively nearer than the nearest fixed star is to the sun, we should have to place our fleet in the middle of the Pacific Ocean, while the distance of the main shore of the starry universe would be so immense that the whole surface of the earth would be far too small to hold the expanse of ocean needed to represent it!

From these general considerations we next proceed to recall some of the details of the system of worlds amid which we dwell. Besides the earth, the sun has seven other principal planets in attendance. These eight planets fall into two classes--the terrestrial planets and the major, or jovian, planets. The former class comprises Mercury, Venus, the earth, and Mars, and the latter Jupiter, Saturn, Uranus, and Neptune. I have named them all in the order of their distance from the sun, beginning with the nearest.

The terrestrial planets, taking their class name from terra, the earth, are relatively close to the sun and comparatively small. The major planets--or the jovian planets, if we give them a common title based upon the name of their chief, Jupiter or Jove--are relatively distant from the sun and are characterized both by great comparative size and slight mean density. The terrestrial planets are all included within a circle, having the sun for a center, about 140,000,000 miles in radius. The space, or gap, between the outermost of them, Mars, and the innermost of the jovian planets, Jupiter, is nearly two and a half times as broad as the entire radius of the circle within which they are included. And not only is the jovian group of planets widely separated from the terrestrial group, but the distances between the orbits of its four members are likewise very great and progressively increasing. Between Jupiter and Saturn is a gap 400,000,000 miles across, and this becomes 900,000,000 miles between Saturn and Uranus, and more than 1,000,000,000 miles between Uranus and Neptune. All of these distances are given in round numbers.

Finally, we come to some very extraordinary worlds--if we can call them worlds at all--the asteroids. They form a third group, characterized by the extreme smallness of its individual members, their astonishing number, and the unusual eccentricities and inclinations of their orbits. They are situated in the gap between the terrestrial and the jovian planets, and about 500 of them have been discovered, while there is reason to think that their real number may be many thousands. The largest of them is less than 500 miles in diameter, and many of those recently discovered may be not more than ten or twenty miles in diameter. What marvelous places of abode such little planets would be if it were possible to believe them inhabited, we shall see more clearly when we come to consider them in their turn. But without regard to the question of habitability, the asteroids will be found extremely interesting.

In the next chapter we proceed to take up the planets for study as individuals, beginning with Mercury, the one nearest the sun.

CHAPTER II

MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS

Mercury, the first of the other worlds that we are going to consider, fascinates by its grotesqueness, like a piece of Chinese ivory carving, so small is it for its kind and so finished in its eccentric details. In a little while we shall see how singular Mercury is in many of the particulars of planetary existence, but first of all let us endeavor to obtain a clear idea of the actual size and mass of this strange little planet. Compared with the earth it is so diminutive that it looks as if it had been cut out on the pattern of a satellite rather than that of an independent planet. Its diameter, 3,000 miles, only exceeds the moon's by less than one half, while both Jupiter and Saturn, among their remarkable collections of moons, have each at least one that is considerably larger than the planet Mercury. But, insignificant though it be in size, it holds the place of honor, nearest to the sun.

It was formerly thought that Mercury possessed a mass greatly in excess of that which its size would seem to imply, and some estimates, based upon the apparent effect of its attraction on comets, made it equal in mean density to lead, or even to the metal mercury. This led to curious speculations concerning its probable metallic composition, and the possible existence of vast quantities of such heavy elements as gold in the frame of the planet. But more recent, and probably more correct, computations place Mercury third in the order of density among the members of the solar system, the earth ranking as first and Venus as second. Mercury's density is now believed to be less than the earth's in the ratio of 85 to 100. Accepting this estimate, we find that the force of gravity upon the surface of Mercury is only one third as great as upon the surface of the earth--i.e., a body weighing 300 pounds on the earth would weigh only 100 pounds on Mercury.

This is an important matter, because not only the weight of bodies, but the density of the atmosphere and even the nature of its gaseous constituents, are affected by the force of gravity, and if we could journey from world to world, in our bodily form, it would make a great difference to us to find gravity considerably greater or less upon other planets than it is upon our own. This alone might suffice to render some of the planets impossible places of abode for us, unless a decided change were effected in our present physical organization.

One of the first questions that we should ask about a foreign world to which we proposed to pay a visit, would relate to its atmosphere. We should wish to know in advance if it had air and water, and in what proportions and quantities. However its own peculiar inhabitants might be supposed able to dispense with these things, to us their presence would be essential, and if we did not find them, even a planet that blazed with gold and diamonds only waiting to be seized would remain perfectly safe from our invasion. Now, in the case of Mercury, some doubt on this point exists.

Messrs. Huggins, Vogel, and others have believed that they found spectroscopic proof of the existence of both air and the vapor of water on Mercury. But the necessary observations are of a very delicate nature, and difficult to make, and some astronomers doubt whether we possess sufficient proof that Mercury has an atmosphere. At any rate, its atmosphere is very rare as compared with the earth's, but we need not, on that account, conclude that Mercury is lifeless. Possibly, in view of certain other peculiarities soon to be explained, a rare atmosphere would be decidedly advantageous.

Being much nearer the sun than the earth is, Mercury can be seen by us only in the same quarter of the sky where the sun itself appears. As it revolves in its orbit about the sun it is visible, alternately, in the evening for a short time after sunset and in the morning for a short time before sunrise, but it can never be seen, as the outer planets are seen, in the mid-heaven or late at night. When seen low in the twilight, at evening or morning, it glows with the brilliance of a bright first-magnitude star, and is a beautiful object, though few casual watchers of the stars ever catch sight of it. When it is nearest the earth and is about to pass between the earth and the sun, it temporarily disappears in the glare of the sunlight; and likewise, when it it is farthest from the earth and passing around in its orbit on the opposite side of the sun, it is concealed by the blinding solar rays. Consequently, except with the instruments of an observatory, which are able to show it in broad day, Mercury is never visible save during the comparatively brief periods of time when it is near its greatest apparent distance east or west from the sun.

The nearer a planet is to the sun the more rapidly it is compelled to move in its orbit, and Mercury, being the nearest to the sun of all the planets, is by far the swiftest footed among them. But its velocity is subject to remarkable variation, owing to the peculiar form of the orbit in which the planet travels. This is more eccentric than the orbit of any other planet, except some of the asteroids. The sun being situated in one focus of the elliptical orbit, when Mercury is at perihelion, or nearest to the sun, its distance from that body is 28,500,000 miles, but when it is at aphelion, or farthest from the sun, its distance is 43,500,000 miles. The difference is no less than 14,000,000 miles! When nearest the sun Mercury darts forward in its orbit at the rate of twenty-nine miles in a second, while when farthest from the sun the speed is reduced to twenty-three miles.

Now, let us return for a moment to the consideration of the wonderful variations in Mercury's distance from the sun, for we shall find that their effects are absolutely startling, and that they alone suffice to mark a wide difference between Mercury and the earth, considered as the abodes of sentient creatures. The total change of distance amounts, as already remarked, to 14,000,000 miles, which is almost half the entire distance separating the planet from the sun at perihelion. This immense variation of distance is emphasized by the rapidity with which it takes place. Mercury's periodic time, i.e., the period required for it to make a single revolution about the sun--or, in other words, the length of its year--is eighty-eight of our days. In just one half of that time, or in about six weeks, it passes from aphelion to perihelion; that is to say, in six weeks the whole change in its distance from the sun takes place. In six weeks Mercury falls 14,000,000 miles--for it is a fall, though in a curve instead of a straight line--falls 14,000,000 miles toward the sun! And, as it falls, like any other falling body it gains in speed, until, having reached the perihelion point, its terrific velocity counteracts its approach and it begins to recede. At the end of the next six weeks it once more attains its greatest distance, and turns again to plunge sunward.

Of course it may be said of every planet having an elliptical orbit that between aphelion and perihelion it is falling toward the sun, but no other planet than Mercury travels in an orbit sufficiently eccentric, and approaches sufficiently near to the sun, to give to the mind so vivid an impression of an actual, stupendous fall!

Next let us consider the effects of this rapid fall, or approach, toward the sun, which is so foreign to our terrestrial experience, and so appalling to the imagination.

First, we must remember that the nearer a planet is to the sun the greater is the amount of heat and light that it receives, the variation being proportional to the inverse square of the distance. The earth's distance from the sun being 93,000,000 miles, while Mercury's is only 36,000,000, it follows, to begin with, that Mercury gets, on the average, more than six and a half times as much heat from the sun as the earth does. That alone is enough to make it seem impossible that Mercury can be the home of living forms resembling those of the earth, for imagine the heat of the sun in the middle of a summer's day increased six or seven fold! If there were no mitigating influences, the face of the earth would shrivel as in the blast of a furnace, the very stones would become incandescent, and the oceans would turn into steam.

Still, notwithstanding the tremendous heat poured upon Mercury as compared with that which our planet receives, we can possibly, and for the sake of a clearer understanding of the effects of the varying distance, which is the object of our present inquiry, find a loophole to admit the chance that yet there may be living beings there. We might, for instance, suppose that, owing to the rarity of its atmosphere, the excessive heat was quickly radiated away, or that there was something in the constitution of the atmosphere that greatly modified the effective temperature of the sun's rays. But, having satisfied our imagination on this point, and placed our supposititious inhabitants in the hot world of Mercury, how are we going to meet the conditions imposed by the rapid changes of distance--the swift fall of the planet toward the sun, followed by the equally swift rush away from it? For change of distance implies change of heat and temperature.

It is true that we have a slight effect of this kind on the earth. Between midsummer (of the northern hemisphere) and midwinter our planet draws 3,000,000 miles nearer the sun, but the change occupies six months, and, at the earth's great average distance, the effect of this change is too slight to be ordinarily observable, and only the astronomer is aware of the consequent increase in the apparent size of the sun. It is not to this variation of the sun's distance, but rather to the changes of the seasons, depending on the inclination of the earth's axis, that we owe the differences of temperature that we experience. In other words, the total supply of heat from the sun is not far from uniform at all times of the year, and the variations of temperature depend upon the distribution of that supply between the northern and southern hemispheres, which are alternately inclined sunward.

But on Mercury the supply of solar heat is itself variable to an enormous extent. In six weeks, as we have seen, Mercury diminishes its distance from the sun about one third, which is proportionally ten times as great a change of distance as the earth experiences in six months. The inhabitants of Mercury in those six pregnant weeks see the sun expand in the sky to more than two and a half times its former magnitude, while the solar heat poured upon them swiftly augments from something more than four and a half times to above eleven times the amount received upon the earth! Then, immediately, the retreat of the planet begins, the sun visibly shrinks, as a receding balloon becomes smaller in the eyes of its watchers, the heat falls off as rapidly as it had previously increased, until, the aphelion point being reached, the process is again reversed. And thus it goes on unceasingly, the sun growing and diminishing in the sky, and the heat increasing and decreasing by enormous amounts with astonishing rapidity. It is difficult to imagine any way in which atmospheric influences could equalize the effects of such violent changes, or any adjustments in the physical organization of living beings that could make such changes endurable.

But we have only just begun the story of Mercury's peculiarities. We come next to an even more remarkable contrast between that planet and our own. During the Paris Exposition of 1889 a little company of astronomers was assembled at the Juvisy observatory of M. Flammarion, near the French capital, listening to one of the most surprising disclosures of a secret of nature that any savant ever confided to a few trustworthy friends while awaiting a suitable time to make it public. It was a secret as full of significance as that which Galileo concealed for a time in his celebrated anagram, which, when at length he furnished the key, still remained a riddle, for then it read: "The Mother of the Loves imitates the Shapes of Cynthia," meaning that the planet Venus, when viewed with a telescope, shows phases like those of the moon. The secret imparted in confidence to the knot of astronomers at Juvisy came from a countryman of Galileo's, Signor G. V. Schiaparelli, the Director of the Observatory of Milan, and its purport was that the planet Mercury always keeps the same face directed toward the sun. Schiaparelli had satisfied himself, by a careful series of observations, of the truth of his strange announcement, but before giving it to the world he determined to make doubly sure. Early in 1890 he withdrew the pledge of secrecy from his friends and published his discovery.

No one can wonder that the statement was generally received with incredulity, for it was in direct contradiction to the conclusions of other astronomers, who had long believed that Mercury rotated on its axis in a period closely corresponding with that of the earth's rotation--that is to say, once every twenty-four hours. Schiaparelli's discovery, if it were received as correct, would put Mercury, as a planet, in a class by itself, and would distinguish it by a peculiarity which had always been recognized as a special feature of the moon, viz., that of rotating on its axis in the same period of time required to perform a revolution in its orbit, and, while this seemed natural enough for a satellite, almost nobody was prepared for the ascription of such eccentric conduct to a planet.

The Italian astronomer based his discovery upon the observation that certain markings visible on the disk of Mercury remained in such a position with reference to the direction of the sun as to prove that the planet's rotation was extremely slow, and he finally satisfied himself that there was but one rotation in the course of a revolution about the sun. That, of course, means that one side of Mercury always faces toward the sun while the opposite side always faces away from it, and neither side experiences the alternation of day and night, one having perpetual day and the other perpetual night. The older observations, from which had been deduced the long accepted opinion that Mercury rotated, like the earth, once in about twenty-four hours, had also been made upon the markings on the planet's disk, but these are not easily seen, and their appearances had evidently been misinterpreted.

The very fact of the difficulty of seeing any details on Mercury tended to prevent or delay corroboration of Schiaparelli's discovery. But there were two circumstances that contributed to the final acceptance of his results. One of these was his well-known experience as an observer and the high reputation that he enjoyed among astronomers, and the other was the development by Prof. George Darwin of the theory of tidal friction in its application to planetary evolution, for this furnished a satisfactory explanation of the manner in which a body, situated as near the sun as Mercury is, could have its axial rotation gradually reduced by the tidal attraction of the sun until it coincided in period with its orbital revolution.

Accepting the accuracy of Schiaparelli's discovery, which was corroborated in every particular in 1896 by Percival Lowell in a special series of observations on Mercury made with his 24-inch telescope at Flagstaff, Arizona, and which has also been corroborated by others, we see at once how important is its bearing on the habitability of the planet. It adds another difficulty to that offered by the remarkable changes of distance from the sun, and consequent variations of heat, which we have already discussed. In order to bring the situation home to our experience, let us, for a moment, imagine the earth fallen into Mercury's dilemma. There would then be no succession of day and night, such as we at present enjoy, and upon which not alone our comfort but perhaps our very existence depends, but, instead, one side of our globe--it might be the Asiatic or the American half--would be continually in the sunlight, and the other side would lie buried in endless night. And this condition, so suggestive of the play of pure imagination, this plight of being a two-faced world, like the god Janus, one face light and the other face dark, must be the actual state of things on Mercury.

There is one interesting qualification. In the case just imagined for the earth, supposing it to retain the present inclination of its axis while parting with its differential rotation, there would be an interchange of day and night once a year in the polar regions. On Mercury, whose axis appears to be perpendicular, a similar phenomenon, affecting not the polar regions but the eastern and western sides of the planet, is produced by the extraordinary eccentricity of its orbit. As the planet alternately approaches and recedes from the sun its orbital velocity, as we have already remarked, varies between the limits of twenty-three and thirty-five miles per second, being most rapid at the point nearest the sun. But this variation in the speed of its revolution about the sun does not, in any manner, affect the rate of rotation on its axis. The latter is perfectly uniform and just fast enough to complete one axial turn in the course of a single revolution about the sun. The accompanying figure may assist the explanation.