Scientific Interest in the Moon

In the two and a half centuries since Galileo first turned his crude telescope on the moon, astronomers have mapped its surface features, measured the height of its mountains and the depth of its craters, and calculated its size, mass, and orbital parameters with increasing accuracy over the years. Even so, the best optical telescopes, under the best observing conditions, could not show any detail smaller than about 300 meters across (1,000 feet, about the size of the capitol building in Washington); so information concerning the nature and texture of the surface was limited.

To the telescopic observer, the moon's surface shows two distinct types of regions: 1. the maria (Latin = "seas"; singular mare) - dark, apparently smooth, and roughly circular areas extending over hundreds of kilometers; and 2. the highlands, mostly mountainous regions much lighter in color. Among the most striking features are the craters - tens of thousands of circular depressions ranging in diameter from 180 miles (290 kilometers) down to the limit of telescopic resolution. Most large craters have high walls and a depressed floor, and many have a central peak or ridge. Some are centers from which streaks of light-colored material (rays) extend for considerable distances. Besides the craters, the lunar surface shows domes, ridges, and rilles - long, narrow channels resembling dry watercourses - that run for several kilometers.1 Measurements of reflected light indicate that the lunar surface is covered with a layer of finely pulverized material. Airless and arid, the lunar surface is subjected to temperature changes of more than 200 degrees C (360 degrees F) during the course of a lunar day.2

For decades these features fascinated astronomers and cosmologists, who have generated volumes of speculation as to the moon's origin and history. Three hypotheses attracted adherents: one, that the moon was spun off from a molten proto-earth; a second, that the moon was formed in a separate event and later captured by the earth; and the third, that the earth and moon formed at about the same time in the same region of space (this hypothesis was considered somewhat less likely than the first two). Those who believed the moon was formed by accretion of smaller bodies generally supposed that during its evolution the moon, like the earth, went through a molten stage in which its components were chemically fractionated, with iron and nickel being separated from the rocky minerals as the moon cooled. A smaller body of opinion held that the moon never grew large enough to be completely melted by the heat generated as its component particles coalesced, and would not be chemically differentiated.3

All of these assumptions led to difficulties when examined in light of accepted celestial mechanics. If the moon separated from the earth at some early stage in its formation, its orbital plane should lie in the plane of the earth's equator, but it does not. If, on the other hand, the moon was formed elsewhere in the solar system, its capture by the earth would be highly improbable and its present orbit difficult to account for. Finally, if the earth and moon were formed out of the sane primordial matter by any mechanism, it is hard to explain the fact that the earth is nearly half again as dense as the moon.4

None of the hypotheses could be proved or disproved on the basis of the evidence available from visual observation alone. Both led to logical conclusions concerning the chemical composition and internal structure of the moon that could not be tested. Scientists who held that the moon was once molten regarded the mafia as massive lava flows from volcanoes or fissures in the lunar crust, similar to known examples on earth, and pointed to the domes and certain craters that are much like well known terrestrial volcanic structures. Advocates of the "cold moon" theory considered the maria to be the solidified remains of large bodies of molten rock created by collision with meteorites or asteroids, or perhaps by localized heating due to some other cause.5 The origin of lunar craters was a matter for debate. Some had undoubtedly been produced by impacts of cosmic debris, but strong arguments could be made that others were volcanic in origin. It was generally accepted that the moon's surface, unaffected by wind and water, preserved a record of cosmic events which the earth must also have undergone; on earth, however, the effects have been obliterated by erosion.6

Crucial to the confirmation of either hypothesis, or to the creation of an alternative, was information that could be obtained only by direct examination of the moon. Analysis of samples of the lunar surface would show whether the moon was chemically similar to the earth and whether the lunar material had ever been extensively melted. Measurement of the flow of heat from the moon's interior to the surface would show whether it was still cooling from a molten state. Other important investigations included the seismic properties of the moon, which could reveal its interior structure. Some of this information could be provided by instruments, perhaps including remotely controlled samplers capable of returning lunar material to earth. But some tasks, such as examining the moon's surface and selecting samples on the basis of that examination, could better be done by humans. With the advent of the space age, lunar scientists could look forward to sending instruments to the lunar surface; only after the decision to land people on the moon could they hope to send a trained explorer.

1. See Harold C. Urey, "The Contending Moons," Astronautics & Aeronautics, vol. 7, no. 1 (Jan. 1969), pp. 37-41, reproduced in "Lunar Science Prior to Apollo 11," Robert Jastrow, Vivien Gornitz, Paul W. Gast, and Robert A. Phinney, eds. (NASA Goddard Space Flight Center Institute for Space Studies, 1969), pp. I-1 to I- 6. This collection, a summary of discussions at a conference on problems in lunar science held in New York on June 5, 1969, contains papers and excerpts from books by the leading researchers in lunar science. Although it includes some results from Ranger and Surveyor that are not pertinent to the present discussion, this volume is a good single source of pre-Apollo information on the moon.

2. Eugene M. Shoemaker, "Exploration of the Moon's Surface," American Scientist 50 (1962):99- 129.

3. Urey, "The Contending Moons"; Harold C. Urey and G. J. F . MacDonald, "Origin and History of the Moon," in "Lunar Science Prior to Apollo 11," pp. I-14 to I-135.

4. Urey, "The Contending Moons."

5. "Principal Issues in Lunar Exploration," in "Lunar Science Prior to Apollo 11," pp. 1-19.

6. Ibid., p. 4.

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