128 THE EARTH AS MAN'S ABODE As evidence piled up against it, this theory was finally given up even by its fondest adherents. The Planetesimal Hypothesis. The planetesimal hypothesis, advanced by Chamberlin and Moulton of the University of Chicago, superseded the nebular hypothesis. This theory, like all others, retains the central idea that the whole solar system began as one large body but differs from the nebular theory in the belief that the force which caused the separation of the planets and other smaller bodies came from without. Another star is thought to have come near the star of which our sun is the residue. It did not come close enough to produce a collision but did come close enough to exert such a force of attraction on the outer portions of the sun that large arms were shot out as it revolved near the passing star. These gases then cooled to form solid meteorites, or planetesimals. The planets and their satellites were originally just extra-large meteorites that steadily grew by attracting to themselves the still smaller particles. This theory differs from the nebular hypothesis in that it assumes that the earth was built up from smaller solid particles which were cold, that the earth grew hotter due to increasing pressures developed by the gradual accumulation of mass, and that the earth originally had no atmosphere. The Tidal Theory. The tidal theory, advanced by the British astronomers. Sir James Jeans and Harold Jeffreys, in 1919, is a variation of the planetesimal hypothesis that seems to be satisfactory today. According to this theory, the planets were torn from the sun, possessing masses approximately as we know them today, as the result of tides produced in the surfaces of the two suns as they came very close to each other. In any event, it is probable that the origin of our earth was no ordinary, everyday cosmic affair. Stars do not come near each other very often, and when they do it is unlikely that they do so in such a way as to form planets like those of our sun. But here again we may be all wrong. Perhaps many stars have planets too distant to be observed. For instance, a man on the nearest star. Alpha Centauri, would have to have a telescope a thousand times more powerful than our best telescope to be able to see the sun's largest planet, Jupiter. Undoubtedly the present hypotheses will be revised from time to time as additional knowledge becomes available. It is important to note that scientists are just as anxious to disprove these hypotheses as they are to prove them. Their acceptance of them is real, but tentative, because they are ready to reject them for better hypotheses at any time. There is no more fundamental test of the degree to which one is
THE ORIGIN AND AGE OF THE EARTH 129 influenced by the scientific hypotheses. attitude than one's attitudes toward his Fig. 34. The Colorado River and the Grand Canyon from the north rim at Cape Royal, Arizona. (Courtesy of the Union Pacific Railroad.) The Age of the Earth Has Been Estimated by Many Methods. At one time all of the water in the earth was probably fresh water. The salt in the oceans was brought there by the rivers from the land.