SATURN is beyond the life line of the solar system -- 880,000,000 miles from the Sun. There the Sun's light and heat have been spread in space, diluted by distance; from one of the planet's 9 satellites, the Sun would be a tiny, heatless disk of brilliance.
But no living thing can be there to see it -- not on those airless, frozen worlds. The largest of them is little more than 2,000 miles in diameter, the smallest we can see, through any telescope we have now, is 160 miles in diameter. For smaller ones, so far from us and from the Sun, do not reflect enough light to be seen. All but Titan the largest, is far too small to have held gaseous atmosphere.
And uniformly, they are cold -- cold beyond any Earthly meaning of cold as applied to weather. What little air Titan may have must lie frozen or liquid at his poles. At the equator, in hot summer the temperature may soar to -170° C., but not at the poles. There, only hydrogen and helium could remain gaseous, and they have long, long been dissipated -- 9 dead, and useless worlds. The same logic that applies to the satellites of Jupiter applies here. Saturn, though smaller than Jupiter, is still 71,000 miles in diameter, 95 times more massive than Earth. It would never be a simple problem to escape the vast gravitational field of that planet, never be easy to visit and leave those 9 satellites.
Saturn itself is -- probably dead, almost certainly lifeless. But of all the planets, Saturn has given us more impossible, contradictory facts than any other. Since the early 17th century, when Galileo first turned a telescope on it, it has been a mystery. To-day it is still the most mystifying of the planets. We have "read the last chapter" of the 250-year-detective story that finally searched out the truth of the mystery of the rings, and to-day it seems obvious.
But remember that the men who solved that problem did not know the answers, and remember this, too: five of the greatest minds Earth has produced worked on that problem of the rings before it was solved; Galileo; Huygens, the great Dutch mathematician and contemporary rival of Newton; Newton himself; Laplace, the great French astronomer, who proposed one of the earliest and longest-lived theories of the origin of the solar system; and James Clerk Maxwell, the second great mathematical physicist of the modern world, the man, who, working by pure mathematical logic, predicted the hitherto unguessed radio waves. Ten generations of men were born between the night that Galileo first saw the mysterious "wings" of Saturn and the time that Maxwell, the last of that quintet, finally solved the problem.
Galileo's little telescope didn't give him more than a hint of the existence of some unknown structure floating in space close to the planet. He called the structure "wings," and other astronomers observed the strange phenomenon. The, 2 years later, they had disappeared. Nearly a half century passed before Huygens conceived the answer, and even the, in 1656, he was so unsure that his first publication was in the form of a cipher,* which he explained only after 3 years of thought and observation.
THIS RING THEORY so excellently explained the observed results that it was accepted fairly quickly. At that time, before Newton had announced his law of gravity, no serious objections were raised. To them it simply meant that Saturn was a sort of double world, a curious thing consisting of the usual sort of round, spherical world, and a second world of an unusual, but not impossible sort.
Newton's greatest contribution to the understanding of this first mystery of Saturn was wholly destructive. The astronomers had fallen into believing the idea of a ring -- naturally a solid ring made out of dirt and stones.
It was not. Newton's Principia, and the law of gravity, showed three great faults, and the faults were not explained away for a period longer than the history of the United States.
First, that flat, solid band might be thin -- actually less than 50 miles through -- but any structure 170,000 miles from side to side, even with an 80,000-mile diameter hole in the middle, was enormously massive. With gravity of a giant planet pulling, the solid ring would collapse, unless in swift rotation, so that centrifugal force would support it; but second, it could not be rotating in that way, because to balance the weight of the inner ring meant a rotation so swift that the centrifugal force would tear the outer rim, nearly 90,000 miles farther out, to pieces. Third, even if it were rotating in balance, a deviation of the minutest trace, such as the attraction of giant Jupiter, would precipitate it with a thundering crash on Saturn.
The obvious answer (not so obvious as we, who already know it, would suppose) was that it was not solid, but already in pieces, rotating in orbits. But an entirely new spirit was rife in astronomy. Yes, that might be so, but can you give mathematical proof that those wildly heterogeneous orbits would be stable either?
No one could. Laplace, though, did what might be termed the next best thing: he gave mathematical proof that the only possible way to balance the solid ring would be to load one side, make it actually, a fairly ordinary satellite, revolving about Saturn, with a huge central mass of about 41/2 times that of the ring. The ring, then, sort of sprouted from the equator of the satellite, like the shadow of a man with his arms circled out before him, fingers touching, as it would be projected by a light overhead. And that was not so, since the great mass of the satellite was not seen, so it could not be solid.
But Saturn is given to could nots.
IN THE MEANTIME, Saturn was displaying other mystifying irregularities. Some of them were explained when Uranus was discovered. Saturn's orbit had been varying, changing pace, in a way a good backfielder might envy. The astronomers, however, did not envy it. The discovery of Uranus, and the consequent explanation of some of the unknown, perturbing factors settled part of it; the Great Inequality, as it was called, remained.
Jupiter was perturbing Saturn, and being perturbed in return. The two mightiest planets of the solar system were attracting each other across the void with a force unimaginably immense. Jupiter's mass 1,200,000,000,000,000,000,000,000 tons, Saturn's nearly a third as great. Periodically, they pulled at each other in one direction, then for a while in the other. The mathematics of that was finally settled, mathematics of that was finally settled, but in the meantime, Cassini, and Italian observer, had noticed a new vaguery of the rings.
Saturn's rings are plural; there is an inner ring beginning only about 4,600 miles from the surface of the planet, then a break, another ring, another break, and finally the outer ring extending to a distance of 170,000 miles. The breaks now had to be accounted for in any mathematical explanation. Evidently, if the rings were made up of particles, the particles found orbits in these particular regions unstable. If there -- why not unstable elsewhere?
Roche contributed something, too. He had studied the factors involved in the stability of satellites, and had discovered an important and interesting principle; when a satellite is near a planet, there is a tidal strain developed, naturally. Gravity falls off as the square of the distance increases. Then the near side of the satellite is pulled harder, tends to fall toward the planet more rapidly than the far side. This resulting force tends to break the satellite in two. When the disrupting force equals the gravity of the satellite at its surface -- it is doomed.
Roche showed that a planet would disrupt any satellite that came within a distance less than 2.5 times the radius of the planet. The outermost edge of Saturn's ring is 85,000 miles from the center of Saturn; 21/2 times the radius of the planet is 95,000 miles. Once Saturn was circled by 10 dead worlds.
James Clerk Maxwell at last developed the mathematics that explained the problem. It was not in some ancient and bygone time. Those rings were not explained until a date so recent that men living to-day can remember the first appearance of the solution. The mathematics that the meteorlike particles of the rings could revolve in free, stable orbits anywhere but at a certain, specified distances. There should be no particles in those ranges. There should be -- Cassini's division.
Saturn's innermost satellite, Mimas, is a 400-mile world, revolving about Saturn in about 221/2 hours. Next is Enceladus, a 500-mile satellite revolving in 33 hours. Then comes Tethys, 800 miles in diameter, revolving in 45 and a fraction hours. If a mass were revolving about Saturn in the blank space called Cassini's division, it would make the trip in just about 11 hours. Twice 11 is 22, 3 times is 33 and 4 times is 44. How long, do you suppose, an orbit in free space could be stable when once every few hours 400-miles Mimas got in line and pulled; then Enceladus lined up and heaved, and every now and then Tethys laid violent hands on it? Maxwell showed it would not be stable. You will find that generally true: planets, asteroids, all such bodies, do not revolve in harmony; their periods never have a common factor so simple as 2, 3, or 4.
BUT when that had, at last, been settled, the orbits of the satellites had revealed another thing. Saturn had long been known for its remarkable density, 0.73. It has even been called the "pea-soup" planet, which, one might almost say, unfortunately, it is not. Good pea soup is a lot denser; Saturn comes nearer being a "gasoline-soup" planet. Saturn is the weirdest world of the system. No science-fictionest[ed. science-fictionist] in his wildest moments has proposed a planet with an atmosphere so incredibly contradictory. We now enter the land of "can be."
From the way the satellites move, they tell us the approximate distribution of Saturn's mass. Most of it is concentrated in the center; at the very heart the density appears to be somewhere around that of lead, not gasoline. The exact figures, we can't quite determine, but apparently there is an inner, solid core, 32,000 miles through, and about as dense as Earth. That core, then accounts for 70 per cent, or so, of the mass of the planet. The remaining 30 per cent is in the atmosphere, a reasonable-sounding figure. Remember, all the major planets have immense, deep atmospheres. This one, we know from spectroscopic research, consists largely of methane, ammonia in small amounts and a great deal of hydrogen and helium.
There is, however, just one slight difficulty. The average density of that atmosphere is 0.26 -- 1/4 as heavy as water. And its depth is 20,000 miles! That is something of an atmosphere. Earth, placed on that solid core, would make a fair mountain, but not much more. If our whole planet were placed on Saturn's core, and Venus lowered into position on top, and the whole planet Mars added to that, Mars' outermost edge would still be under an atmosphere denser than that planet had ever known! 3 worlds, one on top of the other, and they would still fall short! The pressures at the base of the incredible atmosphere amount to 10,000,000 pounds per square inch.
So -- it can't be the kind of gas we know. If the kind of gas we know, obeying the laws we ordinarily experience, were put under that pressure, you could put all the gas in the Hindenburg in a coat closet. Actually, gases don't obey the same laws when you approach those colossal pressures; entirely new laws must be taken into account. But -- gases compress, and you can't explain that low density by ordinary gases!
Liquids and solids don't compress. Perhaps, then, that atmosphere is actually a a vast ocean? Perhaps, but what liquid? There are only two liquids that are less than one third as heavy as water; liquid hydrogen and helium, 0.07 and 0.12 respectively.
Saturn is tremendously could, 180° C. below zero -- terribly hot, for hydrogen. Hydrogen cannot exist as a liquid at that temperature. The temperature must be nearly 50° C. lower, before hydrogen can exist as a liquid, and it must be even colder for helium. So -- that can't be a liquid ocean!
There are only 2 solid substances light enough to meet that density problem. Right; hydrogen and helium. And nobody has ever seen them solidify without turning liquid first. So, apparently it can't be glaciers of solid hydrogen and helium. No, by reductio ad absurdam reasoning, as the geometrician puts it, we are reduced to the total absurdity; it simply can't be.
BUT SATURN is the third largest fact in the immediate universe. We can't overlook it; there must be some explanation. The latest theory advanced in explanation is that while no man has ever seen solid hydrogen without the liquid first, it is possible, under immense pressures, to have the solid in equilibrium with the gas at any temperature! A hint of this is perhaps found in "Ice VI." Ice VI is ordinary, everyday water under extraordinary pressure. Under those conditions, water is solid, dense ice at over 157° C., far above its normal boiling point.
If that is the case, Saturn is a mighty, 32,000-mile globe of rock and metal such as Earth, but incased[ed. sic] in a colossal, overwhelming shell of solid, glassy hydrogen and helium, a frozen, infinite wilderness of mountainous masses of these "hot-solid" gases under immense pressure. Here and there dense outcroppings of bluish, heavy "rock" occur, crystals of solid ammonia. And, perhaps, an occasional mass of a rare, massively dense material more than 10 times heavier than the surrounding rocks -- a densely heavy, crystalline stuff known, on warmer worlds, as water.
And overhead, vast clouds of ammonia and methane gas shriek by -- vast gales roaring around a limitless world of frightful cold. Far overhead, beyond the dense, impenetrable clouds of the ultracompressed atmosphere, a vast arch of shining light belts the heavens -- the rings. No eye exists to see them. They would be as beautiful as an eternal rainbow, from the surface of Saturn. But Saturn is dead. No life can exist on the cold world. Even if Saturn were warmed, it seems impossible that it could support life. Saturn is a world spoiled in the making. It is made wrong.
Beyond the life line of the solar system -- 880,000,000 miles from the Sun -- Saturn and its 9 dead satellites circle. It must be dead; its composition is wrong. But -- is that picture of the planet right? Are those vast, 20,000-mile thick glaciers of solid hydrogen and helium real? Is that guess the true explanation?
Saturn -- the more we have learned, the less we have understood.
Article No. 12 by John W. Campbell, Jr.