When we are thinking of worlds, and trying to exalt the imagination with them, it is well to turn to Jupiter, for there is a planet worth pondering upon! A world thirteen hundred times as voluminous as the earth is a phenomenon calculated to make us feel somewhat as the inhabitant of a rural village does when his amazed vision ranges across the million roofs of a metropolis. Jupiter is the first of the outer and greater planets, the major, or Jovian, group. His mean diameter is 86,500 miles, and his average girth more than 270,000 miles. An inhabitant of Jupiter, in making a trip around his planet, along any great circle of the sphere, would have to travel more than 30,000 miles farther than the distance between the earth and the moon. The polar compression of Jupiter, owing to his rapid rotation, amounts in the aggregate to more than 5,000 miles, the equatorial diameter being 88,200 miles and the polar diameter 83,000 miles.

Jupiter's mean distance from the sun is 483,000,000 miles, and the eccentricity of his...o...b..t is sufficient to make this distance variable to the extent of 21,000,000 miles; but, in view of his great average distance, the consequent variation in the amount of solar light and heat received by the planet is not of serious importance.

When he is in opposition to the sun as seen from the earth Jupiter's mean distance from us is about 390,000,000 miles. His year, or period of revolution about the sun, is somewhat less than twelve of our years (11.86 years). His axis is very nearly upright to the plane of his...o...b..t, so that, as upon Venus, there is practically no variation of seasons. Gigantic though he is in dimensions, Jupiter is the swiftest of all the planets in axial rotation. While the earth requires twenty-four hours to make a complete turn, Jupiter takes less than ten hours (nine hours fifty-five minutes), and a point on his equator moves, in consequence of axial rotation, between 27,000 and 28,000 miles in an hour.

The density of the mighty planet is slight, only about one quarter of the mean density of the earth and virtually the same as that of the sun.

This fact at once calls attention to a contrast between Jupiter and our globe that is even more significant than their immense difference in size. The force of gravity upon Jupiter's surface is more than two and a half times greater than upon the earth's surface (more accurately 2.65 times), so that a hundred-pound weight removed from the planet on which we live to Jupiter would there weigh 265 pounds, and an average man, similarly transported, would be oppressed with a weight of at least 400 pounds. But, as a result of the rapid rotation of the great planet, and the ellipticity of its figure, the unfortunate visitor could find a perceptible relief from his troublesome weight by seeking the planet's equator, where the centrifugal tendency would remove about twenty pounds from every one hundred as compared with his weight at the poles.

If we could go to the moon, or to Mercury, Venus, or Mars, we may be certain that upon reaching any of those globes we should find ourselves upon a solid surface, probably composed of rock not unlike the rocky crust of the earth; but with Jupiter the case would evidently be very different. As already remarked, the mean density of that planet is only one quarter of the earth's density, or only one third greater than the density of water. Consequently the visitor, in attempting to set foot upon Jupiter, might find no solid supporting surface, but would be in a situation as embarra.s.sing as that of Milton's Satan when he undertook to cross the domain of Chaos:

"Fluttering his pinions vain, plumb down he drops, Ten thousand fathom deep, and to this hour Down had been falling had not, by ill chance, The strong rebuff of some tumultuous cloud.

Instinct with fire and niter, hurried him As many miles aloft; that fury stayed, Quenched in a boggy Syrtis, neither sea Nor good dry land, nigh foundered, as he fares, Treading the crude consistence, half on foot, Half flying."

The probability that nothing resembling a solid crust, nor, perhaps, even a liquid sh.e.l.l, would be found at the visible surface of Jupiter, is increased by considering that the surface density must be much less than the mean density of the planet taken as a whole, and since the latter but little exceeds the density of water, it is likely that at the surface everything is in a state resembling that of cloud or smoke. Our imaginary visitor upon reaching Jupiter would, under the influence of the planet's strong force of gravity, drop out of sight, with the speed of a shot, swallowed up in the vast atmosphere of probably hot, and perhaps partially incandescent, gases. When he had sunk--supposing his ident.i.ty could be preserved--to a depth of thousands of miles he might not yet have found any solid part of the planet; and, perchance, there is no solid nucleus even at the very center.

The cloudy aspect of Jupiter immediately strikes the telescopic observer. The huge planet is filled with color, and with the animation of constant movement, but there is no appearance of markings, like those on Mars, recalling the look of the earth. There are no white polar caps, and no shadings that suggest the outlines of continents and oceans. What every observer, even with the smallest telescope, perceives at once is a pair of strongly defined dark belts, one on either side of, and both parallel to, the planet's equator. These belts are dark compared with the equatorial band between them and with the general surface of the planet toward the north and the south, but they are not of a gray or neutral shade. On the contrary, they show decided, and, at times, brilliant colors, usually of a reddish tone. More delicate tints, sometimes a fine pink, salmon, or even light green, are occasionally to be seen about the equatorial zone, and the borders of the belts, while near the poles the surface is shadowed with bluish gray, imperceptibly deepening from the lighter hues of the equator.

All this variety of tone and color makes of a telescopic view of Jupiter a picture that will not quickly fade from the memory; while if an instrument of considerable power is used, so that the wonderful details of the belts, with their scalloped edges, their diagonal filaments, their many divisions, and their curious light and dark spots, are made plain, the observer is deeply impressed with the strangeness of the spectacle, and the more so as he reflects upon the enormous real magnitude of that which is spread before his eye. The whole earth flattened out would be but a small blotch on that gigantic disk!

Then, the visible rotation of the great Jovian globe, whose effects become evident to a practised eye after but a few minutes' watching, heightens the impression. And the presence of the four satellites, whose motions in their orbits are also evident, through the change in their positions, during the course of a single not prolonged observation, adds its influence to the effectiveness of the scene. Indeed, color and motion are so conspicuous in the immense spectacle presented by Jupiter that they impart to it a powerful suggestion of life, which the mind does not readily divest itself of when compelled to face the evidence that Jupiter is as widely different from the earth, and as diametrically opposed to lifelike conditions, as we comprehend them, as a planet possibly could be.

The great belts lie in lat.i.tudes about corresponding to those in which the trade-winds blow upon the earth, and it has often been suggested that their existence indicates a similarity between the atmospheric circulation of Jupiter and that of the world in which we live. No doubt there are times when the earth, seen with a telescope from a distant planet, would present a belted appearance somewhat resembling that of Jupiter, but there would almost certainly be no similar display of colors in the clouds, and the latter would exhibit no such persistence in general form and position as characterizes those of Jupiter. Our clouds are formed by the action of the sun, producing evaporation of water; on Jupiter, whose mean distance from the sun is more than five times as great as ours, the intensity of the solar rays is reduced to less than one twenty-fifth part of their intensity on the earth, so that the evaporation can not be equally active there, and the tendency to form aerial currents and great systems of winds must be proportionally slight. In brief, the clouds of Jupiter are probably of an entirely different origin from that of terrestrial clouds, and rather resemble the chaotic ma.s.ses of vapor that enveloped the earth when it was still in a seminebulous condition, and before its crust had formed.

Although the strongest features of the disk of Jupiter are the great cloud belts, and the white or colored spots in the equatorial zone, yet the telescope shows many markings north and south of the belts, including a number of narrower and fainter belts, and small light or dark spots. None of them is absolutely fixed in position with reference to others. In other words, all of the spots, belts, and markings shift their places to a perceptible extent, the changes being generally very slow and regular, but occasionally quite rapid. The main belts never entirely disappear, and never depart very far from their mean positions with respect to the equator, but the smaller belts toward the north and south are more or less evanescent. Round or oblong spots, as distinguished from belts, are still more variable and transient. The main belts themselves show great internal commotion, frequently splitting up, through a considerable part of their length, and sometimes apparently throwing out projections into the lighter equatorial zone, which occasionally resemble bridges, diagonally spanning the broad s.p.a.ce between the belts.

[Ill.u.s.tration: JUPITER AS SEEN AT THE LICK OBSERVATORY IN 1889. THE GREAT RED SPOT IS VISIBLE, TOGETHER WITH THE INDENTATION IN THE SOUTH BELT.]

Perhaps the most puzzling phenomenon that has ever made its appearance on Jupiter is the celebrated "great red spot," which was first noticed in 1878, although it has since been shown to be probably identical with a similar spot seen in 1869, and possibly with one noticed in 1857.

This spot, soon after its discovery in 1878, became a clearly defined red oval, lying near the southern edge of the south belt in lat.i.tude about 30. Its length was nearly one third of the diameter of the disk and its width almost one quarter as great as its length. Translated into terrestrial measure, it was about 30,000 miles long and 7,000 miles broad.

In 1879 it seemed to deepen in color until it became a truly wonderful object, its redness of hue irresistibly suggesting the idea that it was something hot and glowing. During the following years it underwent various changes of appearance, now fading almost to invisibility and now brightening again, but without ever completely vanishing, and it is still (1901) faintly visible.

n.o.body has yet suggested an altogether probable and acceptable theory as to its nature. Some have said that it might be a part of the red-hot crust of the planet elevated above the level of the clouds; others that its appearance might be due to the clearing off of the clouds above a heated region of the globe beneath, rendering the latter visible through the opening; others that it was perhaps a ma.s.s of smoke and vapor ejected from a gigantic volcano, or from the vents covering a broad area of volcanic action; others that it might be a vast incandescent slag floating upon the molten globe of the planet and visible through, or above, the enveloping clouds; and others have thought that it could be nothing but a cloud among clouds, differing, for unknown reasons, in composition and cohesion from its surroundings. All of these hypotheses except the last imply the existence, just beneath the visible cloud sh.e.l.l, of a more or less stable and continuous surface, either solid or liquid.

When the red spot began to lose distinctness a kind of veil seemed to be drawn over it, as if light clouds, floating at a superior elevation, had drifted across it. At times it has been reduced in this manner to a faint oval ring, the rim remaining visible after the central part has faded from sight.

One of the most remarkable phenomena connected with the mysterious spot is a great bend, or scallop, in the southern edge of the south belt adjacent to the spot. This looks as if it were produced by the spot, or by the same cause to which the spot owes its existence. If the spot were an immense mountainous elevation, and the belt a current of liquid, or of clouds, flowing past its base, one would expect to see some such bend in the stream. The visual evidence that the belt is driven, or forced, away from the neighborhood of the spot seems complete. The appearance of repulsion between them is very striking, and even when the spot fades nearly to invisibility the curve remains equally distinct, so that in using a telescope too small to reveal the spot itself one may discover its location by observing the bow in the south belt. The suggestion of a resemblance to the flowing of a stream past the foot of an elevated promontory, or mountain, is strengthened by the fact, which was observed early in the history of the spot, that markings involved in the south belt have a quicker rate of rotation about the planet's axis than that of the red spot, so that such markings, first seen in the rear of the red spot, gradually overtake and pa.s.s it, and eventually leave it behind, as boats in a river drift past a rock lying in the midst of the current.

This leads us to another significant fact concerning the peculiar condition of Jupiter's surface. Not only does the south belt move perceptibly faster than the red spot, but, generally speaking, the various markings on the surface of the planet move at different rates according as they are nearer to or farther from the equator. Between the equator and lat.i.tude 30 or 40 there is a difference of six minutes in the rotation period--i.e., the equatorial parts turn round the axis so much faster than the parts north and south of them, that in one rotation they gain six minutes of time. In other words, the clouds over Jupiter's equator flow past those in the middle lat.i.tudes with a relative velocity of 270 miles per hour. But there are no sharp lines of separation between the different velocities; on the contrary, the swiftness of rotation gradually diminishes from the equator toward the poles, as it manifestly could not do if the visible surface of Jupiter were solid.

In this respect Jupiter resembles the sun, whose surface also has different rates of rotation diminishing from the equator. Measured by the motion of spots on or near the equator, Jupiter's rotation period is about nine hours fifty minutes; measured by the motion of spots in the middle lat.i.tudes, it is about nine hours fifty-six minutes. The red spot completes a rotation in a little less than nine hours and fifty-six minutes, but its period can not be positively given for the singular reason that it is variable. The variation amounts to only a few seconds in the course of several years, but it is nevertheless certain. The phenomenon of variable motion is not, however, peculiar to the red spot.

Mr. W.F. Denning, who has studied Jupiter for a quarter of a century, says:

"It is well known that in different lat.i.tudes of Jupiter there are currents, forming the belts and zones, moving at various rates of speed.

In many instances the velocity changes from year to year. And it is a singular circ.u.mstance that in the same current a uniform motion is not maintained in all parts of the circ.u.mference. Certain spots move faster than others, so that if we would obtain a fair value for the rotation period of any current it is not sufficient to derive it from one marking alone; we must follow a number of objects distributed in different longitudes along the current and deduce a mean from the whole."[10]

[Footnote 10: The Observatory, No. 286, December, 1899.]

Nor is this all. Observation indicates that if we could look at a vertical section of Jupiter's atmosphere we should behold an equally remarkable contrast and conflict of motions. There is evidence that some of the visible spots, or clouds, lie at a greater elevation than others, and it has been observed that the deeper ones move more rapidly. This fact has led some observers to conclude that the deep-lying spots may be a part of the actual surface of the planet. But if we could think that there is any solid nucleus, or core, in the body of Jupiter, it would seem, on account of the slight mean density of the planet, that it can not lie so near the visible surface, but must be at a depth of thousands, perhaps tens of thousands, of miles. Since the telescope is unable to penetrate the cloudy envelope we can only guess at the actual const.i.tution of the interior of Jupiter's globe. In a spirit of mere speculative curiosity it has been suggested that deep under the clouds of the great planet there may be a comparatively small solid globe, even a habitable world, closed round by a firmament all its own, whose vault, raised 30,000 or 40,000 miles above the surface of the imprisoned planet, appears only an unbroken dome, too distant to reveal its real nature to watchers below, except, perhaps, under telescopic scrutiny; enclosing, as in a sh.e.l.l, a transparent atmosphere, and deriving its illumination partly from the sunlight that may filter through, but mainly from some luminous source within.

But is not Jupiter almost equally fascinating to the imagination, if we dismiss all attempts to picture a humanly impossible world shut up within it, and turn rather to consider what its future may be, guided by the not unreasonable hypothesis that, because of its immense size and ma.s.s, it is still in a chaotic condition? Mention has been made of the resemblance of Jupiter to the sun by virtue of their similar manner of rotation. This is not the only reason for looking upon Jupiter as being, in some respects, almost as much a solar as a planetary body. Its exceptional brightness rather favors the view that a small part of the light by which it shines comes from its own incandescence. In size and ma.s.s it is half-way between the earth and the sun. Jupiter is eleven times greater than the earth in diameter and thirteen hundred times greater in volume; the sun is ten times greater than Jupiter in diameter and a thousand times greater in volume. The mean density of Jupiter, as we have seen, is almost exactly the same as the sun's.

Now, the history of the solar system, according to the nebular hypothesis, is a history of cooling and condensation. The sun, a thousand times larger than Jupiter, has not yet sufficiently cooled and contracted to become incrusted, except with a sh.e.l.l of incandescent metallic clouds; Jupiter, a thousand times smaller than the sun, has cooled and contracted until it is but slightly, if at all, incandescent at its surface, while its thickening sh.e.l.l, although still composed of vapor and smoke, and still probably hot, has grown so dense that it entirely cuts off the luminous radiation from within; the earth, to carry the comparison one step further, being more than a thousand times smaller than Jupiter, has progressed so far in the process of cooling that its original sh.e.l.l of vapor has given place to one of solid rock.

A sudden outburst of light from Jupiter, such as occurs occasionally in a star that is losing its radiance through the condensation of absorbing vapors around it, would furnish strong corroboration of the theory that Jupiter is really an extinguished sun which is now on the way to become a planet in the terrestrial sense.

Not very long ago, as time is reckoned in astronomy, our sun, viewed from the distance of the nearer fixed stars, may have appeared as a binary star, the brighter component of the pair being the sun itself and the fainter one the body now called the planet Jupiter. Supposing the latter to have had the same intrinsic brilliance, surface for surface, as the sun, it would have radiated one hundred times less light than the sun. A difference of one hundredfold between the light of two stars means that they are six magnitudes apart; or, in other words, from a point in s.p.a.ce where the sun appeared as bright as what we call a first-magnitude star, its companion, Jupiter, would have shone as a sixth-magnitude star. Many stars have companions proportionally much fainter than that. The companion of Sirius, for instance, is at least ten thousand times less bright than its great comrade.

Looking at Jupiter in this way, it interests us not as the probable abode of intelligent life, but as a world in the making, a world, moreover, which, when it is completed--if it ever shall be after the terrestrial pattern--will dwarf our globe into insignificance. That stupendous miracle of world-making which is dimly painted in the grand figures employed by the writers of Genesis, and the composers of other cosmogonic legends, is here actually going on before our eyes. The telescope shows us in the cloudy face of Jupiter the moving of the spirit upon the face of the great deep. What the final result will be we can not tell, but clearly the end of the grand processes there in operation has not yet been reached.

The interesting suggestion was made and urged by Mr. Proctor that if Jupiter itself is in no condition at present to bear life, its satellites may be, in that respect, more happily circ.u.mstanced. It can not be said that very much has been learned about the satellites of Jupiter since Proctor's day, and his suggestion is no less and no more probable now than it was when first offered.

There has been c.u.mulative evidence that Jupiter's satellites obey the same law that governs the rotation of our moon, viz., that which compels them always to keep the same face turned toward their primary, and this would clearly affect, although it might not preclude, their habitability. With the exception of the minute fifth satellite discovered by Barnard in 1892, they are all of sufficient size to retain at least some traces of an atmosphere. In fact, one of them is larger than the planet Mars, and another is of nearly the same size as that planet, while the smallest of the four princ.i.p.al ones is about equal to our moon. Under the powerful attraction of Jupiter they travel rapidly, and viewed from the surface of that planet they would offer a wonderful spectacle.

They are continually causing solar eclipses and themselves undergoing eclipse in Jupiter's shadow, and their swiftly changing aspects and groupings would be watched by an astronomer on Jupiter with undying interest.

But far more wonderful would be the spectacle presented by Jupiter to inhabitants dwelling on his moons. From the nearer moon, in particular, which is situated less than 220,000 miles from Jupiter's surface, the great planet would be an overwhelming phenomenon in the sky.

Its immense disk, hanging overhead, would cover a circle of the firmament twenty degrees in diameter, or, in round numbers, forty times the diameter of the full moon as seen from the earth! It would shed a great amount of light and heat, and thus would more or less effectively supply the deficit of solar radiation, for we must remember that Jupiter and his satellites receive from the sun less than one twenty-fifth as much light and heat as the earth receives.

The maze of contending motions, the rapid flow and eddying of cloud belts, the outburst of strange fiery spots, the display of rich, varied, and constantly changing colors, which astonish and delight the telescopic observer on the earth, would be exhibited to the naked eye of an inhabitant of Jupiter's nearest moon far more clearly than the greatest telescope is able to reveal them to us.

Here, again, the mind is carried back to long past ages in the history of the planet on which we dwell. It is believed by some that our moon may have contained inhabitants when the earth was still hot and glowing, as Jupiter appears to be now, and that, as the earth cooled and became habitable, the moon gradually parted with its atmosphere and water so that its living races perished almost coincidently with the beginning of life on the earth. If we accept this view and apply it to the case of Jupiter we may conclude that when that enormous globe has cooled and settled down to a possibly habitable condition, its four attendant moons will suffer the fate that overtook the earth's satellite, and in their turn become barren and death-stricken, while the great orb that once nurtured them with its light and heat receives the Promethean fire and begins to bloom with life.

CHAPTER VII

SATURN, A PRODIGY AMONG PLANETS

One of the first things that persons unaccustomed to astronomical observations ask to see when they have an opportunity to look through a telescope is the planet Saturn. Many telescopic views in the heavens disappoint the beginner, but that of Saturn does not. Even though the planet may not look as large as he expects to see it from what he has been told of the magnifying power employed, the untrained observer is sure to be greatly impressed by the wonderful rings, suspended around it as if by a miracle. No previous inspection of pictures of these rings can rob them of their effect upon the eye and the mind. They are overwhelming in their inimitable singularity, and they leave every spectator truly amazed. Sir John Herschel has remarked that they have the appearance of an "elaborately artificial mechanism." They have even been regarded as habitable bodies! What we are to think of that proposition we shall see when we come to consider their composition and probable origin. In the meantime let us recall the main facts of Saturn's dimensions and situation in the solar system.

Saturn is the second of the major, or Jovian, group of planets, and is situated at a mean distance from the sun of 886,000,000 miles. We need not consider the eccentricity of its...o...b..t, which, although relatively not very great, produces a variation of 50,000,000 miles in its distance from the sun, because, at its immense mean distance, this change would not be of much importance with regard to the planet's habitability or non-habitability. Under the most favorable conditions Saturn can never be nearer than 744,000,000 miles to the earth, or eight times the sun's distance from us. It receives from the sun about one ninetieth of the light and heat that we get.

[Ill.u.s.tration: SATURN IN ITS THREE PRINc.i.p.aL PHASES AS SEEN FROM THE EARTH. From a drawing by Bond.]

Saturn takes twenty-nine and a half years to complete a journey about the sun. Like Jupiter, it rotates very rapidly on its axis, the period being ten hours and fourteen minutes. Its axis of rotation is inclined not far from the same angle as that of the earth's axis (26 49'), so that its seasons should resemble ours, although their alternations are extremely slow in consequence of the enormous length of Saturn's year.

Not including the rings in the calculation, Saturn exceeds the earth in size 760 times. The addition of the rings would not, however, greatly alter the result of the comparison, because, although the total surface of the rings, counting both faces, exceeds the earth's surface about 160 times, their volume, owing to their surprising thinness, is only about six times the volume of the earth, and their ma.s.s, in consequence of their slight density, is very much less than the earth's, perhaps, indeed, inappreciable in comparison.

Saturn's mean diameter is 73,000 miles, and its polar compression is even greater than that of Jupiter, a difference of 7,000 miles--almost comparable with the entire diameter of the earth--existing between its equatorial and its polar diameter, the former being 75,000 and the latter 68,000 miles.

We found the density of Jupiter astonishingly slight, but that of Saturn is slighter still. Jupiter would sink if thrown into water, but Saturn would actually float, if not "like a cork," yet quite as buoyantly as many kinds of wood, for its mean density is only three quarters that of water, or one eighth of the earth's. In fact, there is no known planet whose density is so slight as Saturn's. Thus it happens that, notwithstanding its vast size and ma.s.s, the force of gravity upon Saturn is nearly the same as upon our globe. Upon visiting Venus we should find ourselves weighing a little less than at home, and upon visiting Saturn a little more, but in neither case would the difference be very important. If the relative weight of bodies on the surfaces of planets formed the sole test of their habitability, Venus and Saturn would both rank with the earth as suitable abodes for men.

But the exceedingly slight density of Saturn seems to be most reasonably accounted for on the supposition that, like Jupiter, it is in a vaporous condition, still very hot within--although but slightly, if at all, incandescent at the surface--and, therefore, unsuited to contain life.