Biography of Percival Lowell

by A. Lawrence Lowell

In both “Mars as the Abode of Life” and “The Evolution of Worlds,” he accepts the proposition that our present solar system began with a collision with some dark body from interstellar space, as had been suggested by Chamberlin and Moulton a few years before. He points out that stars which have finished contracting, grown cold and ceased to be luminous, must exist, and although we cannot see them directly we know about some of them,—such as the dark companion of Algol, revolving around it and cutting off two-thirds of its light every three days. Many dark wanderers there must be, and the novae, as he says, are sometimes, at least, due to a collision with such a body,—not necessarily an actual impact, but an approach so near that the star is sprung asunder by the tidal effect. In such a case the opposite sides of the victim would be driven away from it, and if it was rotating would form spirals. Now we know that the apparently empty spaces in our solar system still contain a vast number of little meteoric particles, which as judged from their velocity do not fall from outer space, but are members of our system travelling in their own orbits around the sun. As he puts it, “Could we rise a hundred miles above the Earth’s surface we should be highly sorry we came, for we should incontinently be killed by flying brickbats. Instead of masses of a sunlike size we should have to do with bits of matter on the average smaller than ourselves[24] but hardly on that account innocuous, as they would strike us with fifteen hundred times the speed of an express train.” That these meteorites are moving in the same direction as the Earth he shows by an ingenious calculation of the proportion that in such a case would be seen at sunrise and sunset, which accords with the observed facts. Moreover, their chemical composition shows that they were once parts of a great hot body from which they have been expelled.

The meteorites that are seen because they become hot and luminous in traversing our atmosphere, and occasionally fall upon the Earth, are the remnants of vastly larger numbers formerly circling about the sun, but which, by collision and attraction, were, as he describes, gathered into great masses, thus forming the planets. The force of gravity gradually compacted these fragments closer and closer together, thereby generating heat which if the body were homogeneous would be in proportion to the square of its mass. The larger the planet therefore the more heat it would generate, and owing to the fact that mass is in proportion to the cube and its radiating surface to the square of the diameter the slower it would radiate, and thus lose, its heat, so that the larger ones would be hotter and remain hot longer than the smaller ones.

Some of the planets may once have been white-hot, and luminous of themselves, some were certainly red-hot, some only darkly warm; all growing cooler after the amount radiated exceeded the amount generated. Now by the difference in the heat generated and retained by the larger and smaller bodies he explains the diverse appearance of those whose surfaces we know, the Earth, Mars and the Moon. As the surface cools it forms a crust, but if the interior still remains molten it will continue to contract, the crust will be too large for it and crinkle, like the skin of a dried apple; and this will be more true of a large than a small body. “In like manner is volcanic action relatively increased, and volcanoes arise, violent and widespread, in proportion; since these are vents by which the molten matter under pressure within finds exit abroad.” By a calculation, which agrees with the formula of Laplace, he finds that the effective internal heat of the Earth might be 10,000 degrees Fahrenheit, enough to account for all the phenomena; and for Mars only 2,000, which is below the melting point of iron, and would not cause volcanic action. Now the observations of Mars at Flagstaff show that there can be no mountains on it more than two or three thousand feet high, and that the surface is singularly flat.

But here he met a difficulty; for the Moon ought to be flatter still if it had evolved in the ordinary way, whereas it has enormous volcanic cones, craters 17,000 feet high, some exceeding 100 miles in diameter, and a range of mountains rising to nearly 30,000 feet. An explanation he finds in the analysis of the action of the tides in the Earth-Moon system by Sir George Darwin, who showed that when traced backward it “lands us at a time when the Moon might have formed a part of the Earth’s mass, the two rotating together as a single pear-shaped body in about five hours.... For in that event the internal heat which the Moon carried away with it must have been that of the parent body—the amount the Earth-Moon had been able to amass. Thus the Moon was endowed from the start of its separate existence with an amount of heat the falling together of its own mass could never have generated. Thus its great craters and huge volcanic cones stand explained. It did not originate as a separate body, but had its birth in a rib of Earth.”[25]

The Flagstaff site having been selected for the purpose of planetary observation yielded facts less easily detected elsewhere. Mercury, for instance, is so near the Sun that it could be observed in the dark only a short time after sunset and before sunrise, an obstacle that gave rise to errors of fact. Schiaparelli led the way to better results by observing this planet in broad daylight. Up to that time it had been supposed to rotate on its axis in about twenty-four hours, and therefore to have a day and night like those of the Earth, but daylight observation showed him markings constant on its illuminated face, and therefore that it turns nearly the same side to the Sun. Before knowing his conclusions, and therefore independently, the study of Mercury was taken up at Flagstaff in 1896, and the result was a complete corroboration of his work. It showed that, as in the case of the Moon with the Earth, tidal action on the still partially fluid mass had slowed its rotation until it has little with regard to the central body around which it revolves. He discovered also other facts about Mercury, which Schiaparelli had not, that its size, mass and density had not been accurately measured.

A similar discovery about the period of rotation had been made in the case of Venus. For more than two centuries astronomers had felt sure that this period was just under twenty-four hours, figured, indeed, to the minute. But again it was Schiaparelli who doubted, and once more by observing the planet at noon; when he noted that the markings on the disk did not change from day to day, and concluded that the same side was always pointed at the Sun. At Flagstaff in 1896 his observations were verified and the inference later confirmed by the spectroscope, which was, indeed, first brought to the Observatory for that purpose. Thus Venus, which from its distance from the Sun, its size and density, is most like the Earth, turns out to be in a totally different condition, one face baked by unending glare, the other chilled in interstellar night, and as he puts it: “To Venus the Sun stands substantially stock-still in the sky,— ... No day, no seasons, practically no year, diversifies existence or records the flight of time. Monotony eternalized,—such is Venus’ lot.”[26]

On the movements and physical condition of the Earth it was needless to dwell, and he passed to the asteroids. He describes how they began to be discovered at the beginning of the last century by searching for a planet that would fill a gap in Bode’s law. This, a formula of arithmetical progression for the distances of the planets from the Sun, has proved not to be a law at all, especially since the discovery of Neptune which is much nearer than the formula required; but for nearly a century it had a strong influence on astronomic thought, and the gap in the series between Mars and Jupiter was searched for the missing link. Two were found, then two more, about the middle of the last century another, and then many, smaller and smaller, until by the time Percival wrote six hundred were known, and their number seems limitless. Only the four first found, he remarks, exceed a hundred miles in diameter, the greater part being hardly over ten or twenty. But here he points out a notable fact, that they are not evenly distributed throughout this space; and although massed in a series growing thicker toward its centre there are many gaps, even close to the centre, where few or no asteroids are found. Now it is the large size and attraction of Jupiter by which Percival explains the presence of asteroids with gaps in their ranks, instead of a planet, in the space between it and Mars; but we shall hear much more of this subject when we come to his work on Saturn’s rings and the order in the distribution of the planets.

Jupiter, he tells us, having a mass 318 times that of the Earth, and a volume 1400 times as large, is much less dense, not much more than water, in short still fluid; and as it has a tremendous spin, rotating in less than ten hours, it is more oblate than the Earth; that is, the diameter at its equator is larger in proportion to that from pole to pole. The observations at Flagstaff brought out some interesting facts: first, that the dark belts of cloud that surround it are red, looking as if the planet within were still molten;[27] second, that the bright central belt lies exactly upon its equator, without regard to, and hence independent of, its tilt toward the Sun, and that the belts of cloud on each side appear at the planet’s morning just as they left it in the evening. All which shows that Jupiter’s cloud formation is not due to the Sun, but to its own internal heat, an interpretation of the phenomena that has a direct bearing on his explanation of the Earth’s carboniferous age.

Saturn is still less dense, even more oblate; but its most extraordinary feature is of course the rings. Assumed by the early astronomers to be solid and continuous, they were later shown to have concentric intervals, and to be composed of discrete particles. They have usually been supposed flat, but when the position of the planet was such that they were seen on edge knots or beads appeared upon them; and in 1907 these were studied critically at Flagstaff, when it was found that the shadows of the rings on the planet were not uniform, but had dark cores; these thicker places lying on the outer margin of each ring where it came to one of the intervals. These phenomena he explained in the same way as the distribution of the intervals among the asteroids.[28]

About Uranus and Neptune he tells us in this book little that was not known, and save for their orbits, masses and satellites not much was known of their condition. But later, in 1911, the spectroscope at Flagstaff determined the rotation period of Uranus, afterwards precisely duplicated at the Lick; and later still the spectral bands in the vast atmosphere of the giant planets were identified as due to methane, or marsh, gas.[29]