The Evolution of Worlds

by Percival Lowell

Tidal action accomplishes the end. In checking up a body rotating contrary to the general consensus of spin, its first effect is to start to turn the axis over. For the body is in dynamical unstable equilibrium with regard to the rest of the system. The righting would continue, practically to the exclusion of any diminution at first of the spin, until the body had turned over in its plane so that the spin became direct. As the force increases greatly with nearness to the Sun, the effect would be most marked on the nearer, and most so on the biggest, bodies. This would account for the otherwise strange gradation from retrograde to direct in the tilts of the axes of the outer planets, and also for the present tilts of all the inner ones.

Related to the initial retrograde rotations of the planets, and in a sense survivals from an earlier state of things, are two of the latest discoveries of motions in the solar system, the retrograde orbital movements of the ninth satellite of Saturn and the eighth of Jupiter. Considered so anomalous as scarcely at first to be believed, it has been stated that they directly contradict the theory of Laplace. This is true; in the same sense and no more in which they directly contradict the contradictor, one of the latest theories. For neither theory has anything to explain them as the result of law. That they cannot be the sport of indifferent chance seems evidenced by their occupying similar external positions in their respective systems. As the product of a law we must regard them, and to find that law we now turn. Suppose the planet originally to have been rotating backward, or in the direction of the hands of a clock. At this time the satellite, which may never have formed a part of its mass, was travelling backward too, according to what we have said. Then under the friction of the tides raised on the planet by the Sun, the planet proceeded to turn over. It continued to do so until it spun direct. During this process there was no passage through zero of its moment of momentum considered with regard to itself, and therefore no difficulty on that score of supposing that it successively generated satellites at all degrees of inclination. That its children are of the nature of adopted waifs, Babinet’s criterion (1861) would seem to imply. But it must be remembered that the Sun has been slowing up the planet’s rotation now for æons. As it turned over, its tidal bulges tended to carry over with it such satellites as it already had. This effect was much greater on the nearer ones, both because they were nearer and because they were much larger than the outer. So that the nearer kept with the planet, the others lagged proportionately behind. This suggests itself to account for the facts, but the subject involves so much that is uncertain that I submit the hypothesis with the distrust which Laplace has so eminently bespoken. I advance in its favor only the three striking facts: that a steady progression in their tilts of rotation is observable from Neptune to Jupiter and a substantially accordant one from Mars to Mercury; secondly, that the satellites turn their faces to their primaries, as likewise do Mercury and Venus to the Sun; and, thirdly, that the orbits of the satellites of all the planets are themselves tilted in accordance with what it would require [[see NOTE 7]].

After the axial spins have been made over to the same sense, the second consequence of tidal action in the case of two bodies revolving about their common centre of gravity is to slow down both spins until first the smaller and then the larger turn the same face to each other and remain thus constant ever after. Now such is precisely the pass to which we observe the satellites of the planets have come. All that we can be sure of now turn the same face always to their primary. The Moon was the first to betray her attitude, because the one we can best note. On scrutiny, however, Jupiter’s satellites, so far as we can make out, do the like; and Saturn’s, too. And a very proper attitude it is, this regard paid to compelling attraction. Thus one of the congruities we noticed stands accounted for. The satellites could hardly have been at first so observant; time has brought about this unfailing recognition of their lords.

Of the peculiar massing of the bodies in the family of the Sun, and the still stranger copying of it in their own domestic circles, little can as yet be said in interpretation. That the planetary families and their ancestral group should agree is not the least strange part of the affair. It shows that none of them was fortuitous, but that at the formation of all some common principle presided, apportioning the aggregations to their proper place. But it is such fine print of the system’s history as at present to preclude discernment.

So much for the details we may deduce of the method of our birth. We perceive unmistakably that our solar system grew to be what it is, and that it developed by agglomeration of its previously shattered fragments into the planets we behold to-day, but exactly how the process progressed we are as yet unable to precise. We are, however, as what I have mentioned and tabled show, every day accumulating data which will enable an eventual determination probably to be reached.

From the fact of agglomeration, the essence of the affair, we turn to the traces it has left upon its several offspring.

Just as the continued existence to-day of meteorites in statu quo informs us of a previous body from which our nebula sprang; so a physical characteristic of our own earth at the present time shows it to have evolved from that nebula—even though we cannot make out all the steps. Of its having done so, we are far more sure than of how it did.

That primitive man perceived that somewhere below him was a fiery region which was not an agreeable abode, is plain from his consigning to such Tophet those whose religious tenets did not square with his own. That his conception of it was not strictly scientific is evidenced by his not realizing that to bury his enemies was the way to make them take the first step of the journey thither. Indeed, the vindictive venting of his notions clearly indicates their source as volcanic, rather than bred of a general disapproval of a downward descent either in silicates or sin.

It was not till man began to bore into the Earth for metallic or potable purposes that he brought to light the generic fact that it was everywhere hotter as one went down. And this not only in a very regular, but in a most speedy, manner. The temperature increased in a really surprising way 1° F. for every sixty-five feet of descent. As the rise continued unabated to the limit of his borings, becoming very unpleasant at its end, it was clear that at a depth of thirty-five miles even so refractory a substance as platinum must melt, and practically all the Earth except a thin crust be molten or even gaseous.

Now heat, like money, is easy to dissipate but hard to acquire, as primitive man was the first to realize. It does not come without cause. Being a mode of motion, other motion must have preceded it from which it sprang. So much the doctrine of the conservation of energy teaches us, a doctrine considered now to have been the great scientific heirloom of the nineteenth century to the twentieth, yet which in its day caused the death of its first discoverer, Mayer, of a broken heart from non-recognition; its second, Helmholtz, was refused publication by the leading Berlin physical magazine of the time. So quick is man to delay his own advance.