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 pass 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 latitude 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 latitudes 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 latitudes, 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 latitudes 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 circumstance that in the same current a uniform motion is not maintained in all parts of the circumference. 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]
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 constitution 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 shell, 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 mass, 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 mass 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 shell 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 shell, 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 shell 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 space 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.