Now, this exclusiveness on the part of the “great red spot” really offers us an insight to its character. Clearly it was no void, but occupied space with more than ordinary persistency. As it was neither above nor below the dark spot and shattered that spot on approach, which its former surroundings had not done, its force must have been due to motion. This can be explained by its being formed of a vast uprush of heated vapor from the interior. In short, it was a sort of baby elephant of a volcano, or geyser, occurring as befits its youth in fluid, not solid, conditions, but fairly permanent, nevertheless—a bit of kindergarten Jovian geology. This estimate of it is concurred in by Dr. Slipher’s spectrogram of the dark and light belts respectively. For in the spectrum of the dark one we see the distinctive Jovian bands intensified as if the light had traversed a greater depth of Jovian air. Its color, a cherry red, abets the conclusion—that in such places we look down into the fiery, chaotic turmoil so incessantly going on.
Photograph of a sun spot—after the late M. Janssen.
It is of interest to note that we have prototypes of this sort of extraterrestrial cyclone in the Sun. His spots are probably local upsettings of atmospheric equilibrium, using the word atmospheric in the widest possible sense. Just as our storms are the mildest examples of the like expostulation at the impossibility of keeping up a too long continued decorum. Only that with us the Earth is not so much to blame as the Sun; while both Jupiter and the Sun are themselves responsible for their condition.
Thus we have, in the very depth of their negation, warrant from the dark belts of Jupiter that the bright ones are cloud. But also that they are not clouds ordered as ours. The Jovian clouds pay no sort of regard to the Sun. In orbital matters Jupiter obeys the ruler of the system; but he suffers no interference from him in his domestic affairs. His cloud-belts behave as if the Sun did not exist. Day and night cause no difference in them; nor does the Jovian year. They come when they will; last for months, years, decades; and disappear in like manner. They are sui Jovis, caused by vertical currents from the heated core and strung out in longitudinal procession by Jupiter’s spin. They are self-raised, not sun-raised, condensations of what is vaporized below. Jove is indeed the cloud-compeller his name implies.
Yet Jupiter emits no light, unless the cherry red of his darker belts be considered its last lingering glow. He is thus on the road from Sun to world, and his present appearance informs us that this incubation takes place under cloud.
The like is true of Saturn, in fainter replica, even to the cherry hue. In one way Saturn visibly asserts his independence beyond that possible by Jupiter. For Jupiter’s equator lies almost in the plane of his orbit, and on a hasty view the Sun might be credited with the ordering of the belts, as was indeed long the case. But Saturn’s inclination to his orbital plane is 27°; yet his belts fit his figure as neatly as his rings, and never get displaced, no matter how his body be turned.
Uranus and Neptune are in the same self-centred attitude at present as the faint traces of belts on their disk, otherwise of the same albedo as cloud, lead us to conclude. Yet both their densities and their situation give us to believe them further advanced than the giant planets, and still they lie wrapped in cloud.
These planets, then, are quite unbeholden to the Sun for all their present internal economies. What goes on under that veil of clouds with which they discreetly hide their doings from the too curious astronomic eye—we can only conjecture. But we discern enough to know that it is no placid uneventfulness. That it will continue, too, we are assured. For whether these clouds are largely water-vapor now, or not, to watery ones they must come as the last of all the wrappers they will eventually put off.