The Plurality of Worlds

by William Whewell

11. And in the first place, we may remark that the Earth is really much less heavy than we should expect, from what we know of the materials of which it consists. For, measuring the density, or specific gravity, of materials, (that is their comparative weight in the same bulk,) by their proportion to water, which is the usual way, the density of iron is 8, that of lead 11, that of gold 19: the ordinary rocks at the Earth's surface have a density of 3 or 4. Moreover, all the substances with which we are acquainted, contract into a smaller space, and have their density increased, by being subjected to pressure. Air does this, in an obvious manner; and hence it is, that the lower parts of our atmosphere are denser than the upper parts; being pressed by a greater superincumbent weight, the weight of the superior parts of the atmosphere itself. Air is thus obviously and eminently elastic. But all substances, though less obviously and eminently, are still, really, and in some degree, elastic. They all contract by compression. Water for instance, if pressed by a column of water 100000 feet high, would be reduced to a bulk one-tenth less than before. In the same manner iron, compressed by a column of iron 90000 feet high, loses one-tenth of its bulk, and of course gains so much in density. And the like takes place, in different amounts, with all material whatever. This is the rate at which compression produces its effect of increasing the density, in bodies which are in the condition of those which lie around us. But if this law were to go on at the same rate, when the compression is greatly increased, the density of bodies deep down towards the centre of the Earth must be immense. The Earth's radius is above 20 million feet. At a million feet depth we should have matter subjected to the pressure of a column of a million feet of superincumbent matter, heavier than water; and hence we should have a compression of water 10 times as great as we have mentioned; and, therefore, the bulk of the water would be reduced almost to nothing, its density increased almost indefinitely: and the same would be the case with other materials, as metals and stones. If, therefore, this law of compression were to hold for these great pressures, all materials whatever, contained in the depths of the Earth's mass, must be immensely denser, and immensely specifically heavier, than they are at the surface. And thus, the Earth consisting of these far denser materials towards the centre, but, nearer the surface, of lighter materials, such as rock, and metals, in their ordinary state, must, we should expect, be, on the whole, much heavier than if it consisted of the heaviest ordinary materials; heavier than iron, or than lead; hundreds of times perhaps heavier than stone.

12. This, however, is not found to be so. The expectation of the great density of the Earth, which we might have derived from the known laws of condensation of terrestrial substances, is not confirmed. The mass of the Earth being weighed, by means of such processes as we have already referred to, is found to be only five times heavier than so much water: less heavy than if it were made of iron: less than twice as heavy as if it were made of ordinary rock. This, of course, shows us that the condensation of the interior parts of the Earth's mass, is by no means so great as we should have expected it to be, from what we know of the laws of condensation here; and from considering the enormous pressure of superincumbent materials to which those interior parts are subjected. The laws of condensation, it would seem, do not go on operating for these enormous pressures, by the same progression as for smaller pressure. If a mass of a material is compressed into nine-tenths its bulk by the weight of a column of 100000 feet high, it does not follow that it will be again compressed into nine-tenths of its condensed bulk, by another column of 100000 feet high. The compression and condensation reach, or tend to, a limit; and probably, before they have gone very far. It may be possible to compress a piece of iron by one-thousandth part, even by such forces as we can use; and yet it may not be possible to compress the same piece of iron into one half its bulk, even by the weight of the whole Earth, if made to bear upon it. This appears to be probable: and this will explain, how it is, that the materials of the Earth are not so violently condensed as we should have supposed; and thus, why, the Earth is so light.

13. We must avoid drawing inferences too boldly, on a subject where our means of knowledge are so obscure as they are with regard to the interior of the Earth; but yet, perhaps, we may be allowed to say, that the result which we have just stated, that the Earth is so light, suggests to us the belief that the interior consists of the same materials as the exterior, slightly condensed by pressure.[4] We find no encouragement to believe that there is a nucleus within, of some material, different from what we have on the outside; some metal, for instance, heavier than lead. If the earth were of granite, or of lava, to the centre, it would, so far as we can judge, have much the same weight which it now has. Such a central mass, covered with the various layers of stone, which form the upper crust of the Earth, would naturally make this globe of at least the weight which it really has. And therefore, if we were to learn that a planet was much lighter than this, as to its materials,—much less dense, taking the whole mass together,—we should be compelled to infer that it was, throughout, or nearly so, formed of less compact matter than metal and stone; or else, that it had internal cavities, or some other complex structure, which it would be absurd to assume, without positive reasons.

14. Now having decided these views from an examination of the Earth, let us apply them to other planets, as bearing upon the question of their being inhabited; and in the first place, to Jupiter. We can, as we have said, easily compare the mass of Jupiter and of the Earth; for both of them have Satellites. It is ascertained, by this means, that the mass of weight of Jupiter is about 333 times the weight of the earth; but as his diameter is also 11 times that of the earth, his bulk is 1331 times that of the earth: (the cube of 11 is 1331); and, therefore, the density of Jupiter is to that of the earth, only as 333 to 1331, or about 1 to 4. Thus the density of Jupiter, taken as a whole, is about a quarter of the earth's density; less than that of any of the stones which form the crust of the earth; and not much greater than the density of water. Indeed, it is tolerably certain, that the density of Jupiter is not greater than it would be, if his entire globe were composed of water; making allowance for the compression which the interior parts would suffer by the pressure of those parts superincumbent. We might, therefore, offer it as a conjecture not quite arbitrary, that Jupiter is a mere sphere of water.

15. But is there anything further in the appearance of Jupiter, which may serve to contradict, or to confirm, this conjecture? There is one circumstance in Jupiter's form, which is, to say the least, perfectly consistent with the supposition, that he is a fluid mass; namely, that he is not an exact sphere, but oblate, like an orange. Such a form is produced, in a fluid sphere, by a rotation upon its axis. It is produced, even in a sphere which is (at present at least,) partly solid and partly fluid; and the oblateness of the earth is accounted for in this way. But Jupiter, who, while he is much larger than the earth, revolves much more rapidly, is much more oblate than the earth. His polar and equatorial diameters are in the proportion of 13 to 14. Now it is a remarkable circumstance, that this is the amount of oblateness, which, on mechanical principles, would result from his time of revolution, if he were entirely fluid, and of the same density throughout.[5] So far, then, we have some confirmation at least, of his being composed entirely of some fluid which in its density agrees with water.

16. But there are other circumstances in the appearances of Jupiter, which still further confirm this conjecture of his watery constitution. His belts,—certain bands of darker and lighter color, which run parallel to his equator, and which, in some degree, change their form, and breadth, and place, from time to time,—have been conjectured, by almost all astronomers, to arise from lines of cloud, alternating with tracts comparatively clear, and having their direction determined by currents analogous to our trade-winds, but of a much more steady and decided character, in consequence of the great rotatory velocity.[6] Now vapors, supplying the materials of such masses of cloud, would naturally be raised from such a watery sphere as we have supposed, by the action of the Sun; would form such lines; and would change their form from slight causes of irregularity, as the belts are seen to do. The existence of these lines of cloud does of itself show that there is much water on Jupiter's surface, and is quite consistent with our conjecture, that his whole mass is water.[7]

17. Perhaps some persons may be disposed to doubt whether, if Jupiter be, as we suppose, merely or principally a mass of water and of vapor, we are entitled to extend to him the law of universal gravitation, which is the basis of our speculations. But this doubt may be easily dismissed. We know that the waters of the earth are affected by gravitation; not only towards the earth, as shown by their weight, but towards those distant bodies, the Sun and the Moon; for this gravitation produces the tides of the ocean. And our atmosphere also has weight, as we know; and probably has also solar and lunar tides, though these are marked by many other causes of diurnal change. We have, then, the same reason for supposing that air and water, in other parts of the system, are governed by universal gravitation, and exercise themselves the attractive force of gravitation, which we have for making the like suppositions with regard to the most solid bodies. Whatever argument proves universal gravitation, proves it for all matter alike; and Newton, in the course of his magnificent generalization of the law, took care to demonstrate, by experiment, as well as by reasoning, that it might be so generalized.

18. As bearing upon the question of life in Jupiter, there is another point which requires to be considered; the force of gravity at his surface. Though, equal bulk for equal bulk, he is lighter than the earth, yet his bulk is so great that, as we have seen, he is altogether much heavier than the earth. This, his greater mass, makes bodies, at equal distances from the centres, ponderate proportionally more to him than they would do to the earth. And though his surface is 11 times further from his centre than the earth's is, and therefore the gravity at the surface is thereby diminished, yet, even after this deduction, gravity at the surface of Jupiter is nearly two and a half times that on the earth.[8] And thus a man transferred to the surface of Jupiter would feel a stone, carried in his hands, and would feel his own limbs also, (for his muscular power would not be altered by the transfer,) become 2¹/₂ times as heavy, as difficult to raise, as they were before. Under such circumstances animals of large dimensions would be oppressed with their own weight. In the smaller creatures on the earth, as in insects, the muscular power bears a great proportion to the weight, and they might continue to run and to leap, even if gravity were tripled or quadrupled. But an elephant could not trot with two or three elephants placed upon his back. A lion or tiger could not spring, with twice or thrice his own weight hung about his neck. Such an increase of gravity would be inconsistent then, with the present constitution and life of the largest terrestrial animals; and if we are to suppose planets inhabited, in which gravity is much more energetic than it is upon the earth, we must suppose classes of animals which are adapted to such a different mechanical condition.

19. Taking into account then, these circumstances in Jupiter's state; his (probably) bottomless waters; his light, if any, solid materials; the strong hand with which gravity presses down such materials as there are; the small amount of light and heat which reaches him, at 5 times the earth's distance from the sun; what kind of inhabitants shall we be led to assign to him? Can they have skeletons where no substance so dense as bone is found, at least in large masses? It would seem not probable.[9] And it would seem they must be dwellers in the waters, for against the existence there of solid land, we have much evidence. They must, with so little of light and heat, have a low degree of vitality. They must then, it would seem, be cartilaginous and glutinous masses; peopling the waters with minute forms: perhaps also with larger monsters; for the weight of a bulky creature, floating in the fluid, would be much more easily sustained than on solid ground. If we are resolved to have such a population, and that they shall live by food, we must suppose that the waters contain at least so much solid matter as is requisite for the sustenance of the lowest classes; for the higher classes of animals will probably find their food in consuming the lower. I do not know whether the advocates of peopled worlds will think such a population as this worth contending for: but I think the only doubt can be, between such a population, and none. If Jupiter be a mere mass of water, with perhaps a few cinders at the centre, and an envelope of clouds around it, it seems very possible that he may not be the seat of life at all. But if life be there, it does not seem in any way likely, that the living things can be anything higher in the scale of being, than such boneless, watery, pulpy creatures as I have imagined.