Flowers of the Sky

by Richard A. Proctor

Saturn has a globe about 70,000 miles in diameter, where it bulges out at the equator; but he is somewhat flattened at his poles, so that his polar diameter is about 7000 miles less than the equatorial diameter. In volume he exceeds our earth about 700 times; but in mass only about ninety times: for his mean density is but about 13/100 of the earth's. In fact, if we could imagine an ocean of water wide enough and deep enough for the planets to be all set in it, Saturn would float with about one-fourth of his bulk out of water,—always supposing that no change took place in his density directly after he was immersed. Saturn, indeed, would float highest of all the planets, or rather all of them would sink except Saturn and Neptune, and Saturn would float higher than Neptune. Uranus would just sink. Jupiter is half as heavy again as he should be to float. All the terrestrial planets, Mercury, Venus, the Earth, and Mars, would go to the bottom at once.

It is almost impossible to regard any feature of Saturn as better deserving to be considered first than his ring system. Yet for the sake of preserving a due sequence of ideas we must first consider his globe.

We find ourselves at once in presence of difficulties like those we encountered when we considered the planet Jupiter. How is it that the mighty mass of a planet like Saturn, constructed, we have every reason to believe, of materials resembling those which constitute our earth, has so failed to gather in its substance that the mean density is much less than that of the earth's globe? It must be remembered that gravity prevails throughout the frame of Saturn as throughout our earth's frame. Every particle of that enormous globe is drawn towards the centre with a force almost exactly the same as would be exerted by the attraction of the entire mass of that portion of the planet which lies nearer to the centre than the particle, if this mass were collected at the centre. But this is not all. It is not merely the attraction exerted on each particle of Saturn's mass which has to be considered, but the entire weight of all the superincumbent matter. The distinction between attraction and weight, by the way, is very commonly overlooked in considering the planets' interiors. I think it was Sir David Brewster who argued that as attraction can easily be shown to diminish downwards towards the centre, it is possible to conceive that the interior of a planet may be hollow. The error is readily perceived, if we take a familiar instance where the attraction is the same yet the effect of pressure very much greater. (Without voyaging to the centre of the earth, which is troublesome, and certainly not a familiar experience, we cannot reach places where the attraction of gravity is greatly less than at the surface.) Take a massive arch of brickwork: the bricks near the top are subject to the same attraction as those belonging to the foundation; but the pressure to which the foundation bricks are exposed is very much greater than that affecting the upper bricks. So again with a deep sea: the particles at the bottom of such a sea are subject to no greater attraction than the particles near the top; but we know that a strong hollow case of metal which near the top of such a sea would be scarcely pressed at all, and would suffer no change of shape, will be crushed perfectly flat under the tremendous pressure to which it will be exposed when sunk to the bottom.

There is, in fact, no escape from the conclusion that the interior portions of a planet like Saturn or Jupiter, nay, even of a body like our earth or the moon, must be subject to tremendous pressure, a pressure exceeding many hundred-fold the greatest which we can obtain experimentally, and that under that enormous pressure the density of the materials composing those central parts must be increased. How is it then that Saturn is of much smaller density than the earth? I can imagine no other explanation at once so natural and so complete as this, that an intense heat pervading the entire frame of the planet enables it to resist the tremendous pressure due to mere weight. The planet's mass is expanded by the heat; large portions which at ordinary temperatures would be solid are liquified or even vaporised; matters which are liquid on our earth are vaporised; and, in fine, the planet assumes (as seen from our distant station) the appearance of being very much larger than it really is. We measure not the true globe, which, for aught that is known, may be exceedingly dense, but the dimensions of cloud-layers floating high in the planet's atmosphere.

In describing Jupiter, I considered the changes which have been noticed in that planet's outline, and observed that it is impossible adequately to explain the evidence, without assuming that the changes of outline are real. The outline is not that of a solid globe, however, but of cloud-layers surrounding such a globe, and probably at a great distance from its surface.

Fig. 31.—Saturn's square-shouldered aspect.

In Saturn's case we have very singular evidence to the same purpose. It was observed by Sir W. Herschel in April, 1805, that Saturn occasionally appears distorted, as though bulging out in the latitudes midway between the pole and the equator of the planet. [Fig. 31] represents the appearance of the planet so far as shape is concerned, but the ring was not, when Sir W. Herschel observed it, so narrow as it is shown in [fig. 31]. In fact the ring had been turned edgewise to the earth two years before; and when Herschel noticed the abnormal appearance of Saturn, the rings had begun to open out, though their outermost outline was still far within the regions of the planet which seemed to project as shown. [Fig. 31] in fact represents Saturn as seen by Schröter in 1803, when, as he said, the planet did not seem to present a truly spheroidal figure. Herschel tested his observations by using several telescopes of different dimensions,—ten, seven, twenty, and forty feet in length. In 1818, when the rings were scarcely visible, Kitchener saw the planet as shown in [fig. 31], or "square-shouldered," as some have called it. On one occasion the present Astronomer-Royal saw the planet of that figure. In January, 1855, Coolidge, using the fine refractor of the Cambridge U.S. Observatory, noticed that the equatorial diameter was not the greatest; on the 9th the planet seemed of its usual shape; but on December 6, Coolidge writes, "I cannot persuade myself that it is an optical illusion which makes the maximum diameter of the ball intersect the limb half-way between the northern edge of the equatorial belt, and the inner ellipse of the inner bright ring." This was at a time when the rings were nearly at their greatest opening; so that, including Schröter's observation, we have Saturn out of shape when his ring has presented every shape between that shown in [fig. 30] and that shown in [fig. 31]. Again, in the report of the Greenwich Observatory for 1860-61, when the ring was nearly closed, it is stated that "Saturn has sometimes appeared to assume the square-shouldered aspect." Lastly, the eminent observers, G. Bond and G. P. Bond, father and son, have seen Saturn abnormally shaped, flattened unduly in the north polar regions in 1848, when the ring was turned edgewise towards us, and unsymmetrical in varying ways in 1855-57, when the ring was most widely opened.

Yet the planet's outline is usually a perfect oval, and has been shown to be so by careful measurements effected in some instances by the same observers, who, making equally careful measurements, have found the planet to be distorted.

Does it not seem abundantly clear that the great cloud-layers which float in the atmosphere of Saturn have a widely varying range in height, and that therefore as we see and measure the outline of the cloud-layers, we see and measure in effect a planet which is variable in figure? This seems so natural and complete an explanation of the observed peculiarities that it appears idle on the one hand to reject the evidence of some among the most skilful observers who have ever lived; or, on the other, to imagine that the solid frame of the planet has undergone changes so tremendous as would be involved by the observed variations of outline if they really signified that a solid planet had changed in shape.