The Evolution of Worlds

by Percival Lowell

The action observed was a corollary from the important principle of commensurability of orbital period. As we saw in the case of the asteroids, if two bodies be travelling round a third and their respective periods of revolution be commensurate, they will constantly meet one another in such a manner that great perturbation will ensue and the bodies be thrown out of commensurability of period.

What has happened to the asteroids has likewise occurred in Saturn’s rings. The disturber in this case has been, not Jupiter, as with them, but one or other of Saturn’s own satellites. For when we calculate the problem, we find that Mimas, Enceladus, and Tethys have periods exactly commensurate with the divisions of the rings; in other words, these three inner satellites, whose action because of proximity is the greatest, have fashioned the rings into the three parts we know, called A, the outermost; B, the middle one; and C, the crêpe ring, nearest to the body of the planet. Mimas has been the chief actor, though helped by the two others, while Enceladus has further subdivided ring A by what is known as Encke’s division.

Such has been the chief action of the satellites on the rings: it has made them into the system we see. But if we consider the matter, we shall realize that a secondary result must have ensued—when we remember that the particles composing the rings must be very crowded for the rings to show as bright as they do, and also that, though relatively thin, the rings are nevertheless some eighty miles through.

Now it is evident that any disturbance in so closely packed a system of small bodies as that constituting Saturn’s rings must result in collisions between the bodies concerned. Particles pulled out or in must come in contact with others pursuing their own paths, and as at each collision some energy is lost by the blow, a general falling in toward the planet results. At the same time, as the blow will not usually be exactly in the plane in which either particle was previously moving, both will be thrown more or less out of the general plane of their fellows, and the ring at that point, even if originally flat, will not remain so. For the ring, though very narrow relatively, has a real thickness, quite sufficient for slantwise collision, if the bodies impinge.

Saturn’s Rings.

November 1907.

Now the knots or beads on the rings appeared exactly inside the points where the satellites’ disturbing action is greatest, or, in other words, in precisely their theoretic place. We can hardly doubt that such, then, was their origin.[9]

The result must be gradually to force the particles as a rule nearer the planet, until they fall upon its surface, while a few are forced out to where they may coalesce into a satellite,—a result foreseen long ago by Maxwell. It is this process which in the knots we are actually witnessing take place, and which, like the corona about the eclipsed Sun, only comes out to view when the obliterating brightness of the main body of the rings is withdrawn by their edgewise presentation.