Crossing the zone of asteroids on our journey outward from the sun, we meet with a group of bodies widely different from the "inferior" or terrestrial planets. Their gigantic size, low specific gravity, and rapid rotation, obviously from the first threw the "superior" planets into a class apart; and modern research has added qualities still more significant of a dissimilar physical constitution. Jupiter, a huge globe 86,000 miles in diameter, stands pre-eminent among them. He is, however, only primus inter pares; all the wider inferences regarding his condition may be extended, with little risk of error, to his fellows; and inferences in his case rest on surer grounds than in the case of the others, from the advantages offered for telescopic scrutiny by his comparative nearness.
Now the characteristic modern discovery concerning Jupiter is that he is a body midway between the solar and terrestrial stages of cosmical existence—a decaying sun or a developing earth, as we choose to put it—whose vast unexpended stores of internal heat are mainly, if not solely, efficient in producing the interior agitations betrayed by the changing features of his visible disc. This view, impressed upon modern readers by Mr. Proctor's popular works, was anticipated in the last century. Buffon wrote in his Époques de la Nature (1778):[1039]—"La surface de Jupiter est, comme l'on sait, sujette à des changemens sensibles, qui semblent indiquer que cette grosse planète est encore dans un état d'inconstance et de bouillonnement."
Primitive incandescence, attendant, in his fantastic view, on planetary origin by cometary impacts with the sun, combined, he concluded, with vast bulk to bring about this result. Jupiter has not yet had time to cool. Kant thought similarly in 1785;[1040] but the idea did not commend itself to the astronomers of the time, and dropped out of sight until Mr. Nasmyth arrived at it afresh in 1853.[1041] Even still, however, terrestrial analogies held their ground. The dark belts running parallel to the equator, first seen at Naples in 1630, continued to be associated—as Herschel had associated them in 1781—with Jovian trade-winds, in raising which the deficient power of the sun was supposed to be compensated by added swiftness of rotation. But opinion was not permitted to halt here.
In 1860 G. P. Bond of Cambridge (U.S.) derived some remarkable indications from experiments on the light of Jupiter.[1042] They showed that fourteen times more of the photographic rays striking it are reflected by the planet than by our moon, and that, unlike the moon, which sends its densest rays from the margin, Jupiter is brightest near the centre. But the most perplexing part of his results was that Jupiter actually seemed to give out more light than he received. Bond, however, rightly considered his data too uncertain for the support of so bold an assumption as that of original luminosity, and, even if the presence of native light were proved, thought that it might emanate from auroral clouds of the terrestrial kind. The conception of a sun-like planet was still a remote, and seemed an extravagant one.
Only since it was adopted and enforced by Zöllner in 1865,[1043] can it be regarded as permanently acquired to science. The rapid changes in the cloud-belts both of Jupiter and Saturn, he remarked, attest a high internal temperature. For we know that all atmospheric movements on the earth are sun-heat transformed into motion. But sun-heat at the distance of Jupiter possesses but 1/27, at that of Saturn 1/100 of its force here. The large amount of energy, then, obviously exerted in those remote firmaments must have some other source, to be found nowhere else than in their own active and all-pervading fires, not yet banked in with a thick solid crust.
The same acute investigator dwelt, in 1871,[1044] on the similarity between the modes of rotation of the great planets and of the sun, applying the same principles of explanation to each case. The fact of this similarity is undoubted. Cassini[1045] and Schröter both noticed that markings on Jupiter travelled quicker the nearer they were to his equator; and Cassini even hinted at their possible assimilation to sun-spots.[1046] It is now well ascertained that, as a rule (not without exceptions), equatorial spots give a period some 5-1/2 minutes shorter than those in latitudes of about 30°. But, as Mr. Denning has pointed out,[1047] no single period will satisfy the observations either of different markings at the same epoch, or of the same markings at different epochs. Accelerations and retardations, depending upon processes of growth or change, take place in very much the same kind of way as in solar maculæ, inevitably suggesting similarity of origin.
The interesting query as to Jupiter's surface incandescence has been studied since Bond's time with the aid of all the appliances furnished to physical inquirers by modern inventiveness, yet without bringing to it a categorical reply. Zöllner in 1865, Müller in 1893, estimated his albedo at 0·62 and 0·75 respectively, that of fresh-fallen snow being 0·78, and of white paper 0·70.[1048] But the disc of Jupiter is by no means purely white. The general ground is tinged with ochre; the polar zones are leaden or fawn coloured; large spaces are at times stained or suffused with chocolate-browns and rosy hues. It is occasionally seen ruled from pole to pole with dusky bars, and is never wholly free from obscure markings. The reflection, then, by it, as a whole, of about 70 per cent. of the rays impinging upon it, might well suggest some original reinforcement.
Nevertheless, the spectroscope gives little countenance to the supposition of any considerable permanent light-emission. The spectrum of Jupiter, as examined by Huggins, 1862-64, and by Vogel, 1871-73, shows the familiar Fraunhofer rays belonging to reflected sunlight. But it also shows lines of native absorption.