“Seen with this instrument on a fine night, the disc of Jupiter was a most beautiful object, covered with a wealth of detail which could not possibly be accurately represented in a drawing.... Scarcely any portion of Jupiter, except the Red Spot and the extreme polar regions, was of a uniform tint, the surface being mottled with flocculent and more or less irregular cloud masses.... The equatorial zone, occupying the space between the red belts, was marked in the centre by a salmon-coloured stripe, which was occasionally interrupted by an extension of the white clouds on the sides of the zone. The edges were brilliant white, and were formed of rounded cloud-like masses, which at certain places extended into the red belts as long streamers.... Near their junction with the equatorial zone, the streamers were white and definite in outline, but they became redder in tint toward their outer extremities, and more diffuse, until they were lost in the general red colour of the background. When the seeing was good they were seen to be formed of irregular rounded or feathery clouds, fading toward the outer ends, until the structure could no longer be distinguished.... The portions of the equatorial zone surrounding the roots of well-marked streamers were somewhat brighter than at other places, and it is a curious circumstance that they were almost invariably suffused with a pale olive-green colour, which seemed to be associated with great disturbance, and which was rarely seen elsewhere.... The red belts presented on all occasions the appearance of a passive medium, in which the phenomena of the streamers and other forms ... were manifested. The phenomena would be exactly reproduced by streamers of cloudy white matter floating in a semi-transparent reddish fluid, sometimes submerged and sometimes rising to the surface.... The dark spots frequently seen on the red belts usually occupied spaces left by sharp turns in the streamers, and they were of the same colour as the belts, but deeper in tint, as if the fluid medium could be seen to a greater depth.”[15]
In other words, Jupiter is a striped or banded planet, the bands lying along the direction of turning. These bands are coloured in varying tints, and the planet rotates very rapidly, for the details in the bands pass quickly from one limb to the other. And not only is the speed of rotation of the whole very rapid—Jupiter turns about its axis in a little less than ten hours, so that a particle at its equator moves through 466 miles in each minute—but the various items that form the bands rotate in different times. They may also alter their form and their colour. Jupiter seems, then, to be a planet with a great and rapidly changing atmosphere that extends above a shoreless sea formed of some liquified substance or substances—the whole in a state of flux.
But if we turn back to the Table, we see that Jupiter at its mean distance from the Sun is 5·2 times that of the Earth; that is to say, it receives only 1⁄27th of the light and heat that we receive. But in Chapter VIII, we learnt from Mars that as this receives only 3⁄7ths of the Earth’s light and heat, its mean temperature would sink to -30°C.; the Earth’s being 16°C. Mars is therefore almost always a frozen planet; frozen except on its mere surface when this is exposed to the full rays of the Sun. No sea there would ever be melted to a depth of more than a few inches, even at noonday in midsummer. And yet Mars has at least ten times the advantages of Jupiter. Jupiter, then, must be a frozen planet through and through; no liquid of any sort can exist on its surface; no vapour of any substance can exist in its atmosphere. It must be icebound even at its summer noonday.
Yet, from the description given by Prof. Keeler, it is manifestly not so; and another item in the Table emphasizes that it cannot be so. The density of the Sun is 1·4 that of water, Jupiter’s is 1·33, showing that but a very small proportion (if any) of its bulk can be solid; the rest must be vaporous, or at least fluid. How then can we reconcile these inconsistencies?
It is in the dimensions of Jupiter that we find the answer. The mass of the planet is 317 times that of the Earth; it is indeed nearly three times as great as that of all the other planets put together. But the aggregation of so vast an amount of material is of itself a source of heat; the chief source at the present time of the enormous output of heat from the Sun is ascribed to its gradual contraction; the slow falling of its substance, if we may so express it, a little nearer to its centre. The great mass of Jupiter points to its inherent store of heat being much greater than that of any other planet. And of two bodies equally hot, the larger must cool more slowly than the smaller. If, therefore, all the members of the solar system had at one and the same moment possessed the same surface temperature, that equality would have ceased directly they began to radiate their heat into space; the temperature of the smaller bodies falling more rapidly than those of the larger. This is another example of the principle that has already been noted, that the properties of a small world are not those of a large world divided by a constant factor. It is not possible to conceive a model of the solar system in which all the significant factors should be true to the same scale. If the diameters and distances were all made on a one-tenth scale, the surfaces would be one-hundredth of reality, the volumes one-thousandth.
But a radiating body radiates from its surface, while the store of heat from which that radiation is kept up is supplied by its volume. It follows, therefore, that a large and heavy world must differ from a small light world, not merely in scale, but also in kind.
The surface of a world is all that we see of it; it is, therefore, very commonly all that we consider. But unseen, and hence often unconsidered, beneath the surface lies its substance or mass, and it is this that determines the state and condition of the surface; it is the underlying power. Two men may be contending in a financial struggle; to the eye they may look alike, equally prosperous; both may have the same amount of money actually in their pockets; but the one has nothing else, the other has a great banking account and vast investments, and is, in fact, a millionaire; and it is his unseen power and resources that will make themselves felt.
Jupiter therefore introduces us to a new factor in world-condition; not all its heat is derived from the Sun; much is inherent to it. And though it is not possible at present to say that the mass of Jupiter being so much its inherent heat must be this or that quantity as a function of that mass, yet in general, and neglecting other considerations, we can say that of two worlds the one with the greater mass will be that with the higher inherent temperature. This factor of inherent temperature was one that did not require to be noticed in dealing with the Moon, or Venus, or Mars, for these and all the planets yet noticed are less in size, surface, volume, and mass than the Earth, and hence possess less inherent heat. It is only now that the greater planets are being considered that the question of a source of heat, other than the Sun, can arise.
But the evidence of such heat on Jupiter is not to be disputed. The albedo or reflective index of Jupiter has been put by the late Prof. G. Bond, of Harvard College Observatory, as higher than unity; in other words, that it emits more light than it receives. This is now generally regarded as an excessive estimate, but the albedo of the disc as a whole cannot be put lower than 0·72, or about that of white paper. But many of the “belts” or dark regions are of a dull copper tint, and the polar caps are dusky, so that Bond’s estimate must be realized for the most brilliant “zones,” as the brighter regions are called; certainly for the whitest of the white spots.
No direct evidence of inherent luminosity has been obtained, for the satellites disappear entirely in eclipse. But though their shadows in transit appear very dark, it is clear that they are not absolutely black, since sometimes such a shadow is not distinguishable in darkness from the satellite that casts it; a delicate proof that the background on which it falls has some intrinsic luminosity.