THE NAGGING QUESTION OF ORIGIN
Where did the Moon come from? This is the big puzzler of them all. There were three major theories. The Moon came from Earth, possibly wrenched from what is now the Pacific Ocean. Or the Moon formed in the vicinity of Earth at the same time that Earth was forming. Or the Moon formed somewhere else in the solar system, and was later captured by Earth. Most students of the subject reject the first possibility, because it proves very difficult to explain all the steps that must have occurred to bring this about. That leaves various versions of the last two as principal contenders. Apollo has done little to favor one over the other. Indeed, some feel that we will never be able to say for sure. But the question is a nagging one, and scientists will continue to argue over it.The deep significance of the Apollo investigations lies in the fact that these measurements and observations give us a detailed insight into a planetary body other than Earth, thereby helping us to understand better our own planet. Before space probes, lunar and planetary science had for a long time been inactive, due in part to lack of new data to spark serious thought. With the vast quantities of lunar data returned by Apollo and other lunar missions, together with rich new space- probe data from Mars, Venus, Mercury. and Jupiter, and new discoveries on Earth such as the slow spreading of ocean floors and the drifting of continents, a new field of comparative study of planets has virtually exploded into world science. In this comparative investigation of the planets, the Moon is an important link.
Already it is clear that bodies of planetary size will undergo considerable evolution after their formation. Most of this evolution takes place early, and it is probably less in the case of a Moon-sized planet, leaving the planet relatively quiet for most of its history. A planet the size of Mars, though substantially smaller than Earth, remains active for much longer than a Moon-sized body, as the Mariner 9 pictures clearly show. With its huge volcanoes, its giant canyon several times deeper than the Grand Canyon and long enough to span the United States, its variable polar caps, its suggestion of colliding crustal plates, Mars is clearly still an active planet. Venus, the size of the Earth, with its very hot surface and extremely dense and dynamic atmosphere, may well prove to be more active volcanically and tectonically than Earth. Mercury, intermediate between Mars and the Moon in size, is heavily cratered and seems to be much like the lunar highlands.
It will be fascinating when we can complete this perspective by studying the giant outer planets on the one hand, and the very small bodies like the comets and asteroids on the other. As Pioneer 10 and 11 have shown, Jupiter is extremely dynamic. We may expect the same to prove true of the other giant planets when we get a chance to see them close at hand. Moreover, the large planets, consisting, as they do, mainly of the lighter elements like hydrogen and helium, should provide a revealing insight into conditions in the solar nebula at the time the planets of the solar system were condensing out of the nebula. The same should be true of the comets and asteroids. The asteroids in particular are probably too small to have undergone any gravitational or radioactive melting, and therefore should in their interiors be as they were at the time of their formation, unless they, too, turn out to be fragments of what were once larger bodies.
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