


The New Moon

In 1974, following the fifth lunar science conference, Robert Jastrow* attempted a brief synthesis of what science had learned from the Apollo studies for the New York Times. After outlining the gross features of the moon's evolution, he acknowledged that "Apollo has left at least one great cosmological question unanswered: Where did the moon come from?" For years before Apollo 11 landed, Jastrow said,

scientists argued the merits of various theories with great intensity, but the battle ended in a stalemate. Each theory suffered from at least one major defect. Everyone expected that the lunar landings would promptly settle the debate. It seemed obvious that as soon as we found out what the moon was made of, we would be able to tell where it came from: it would have either the same or different chemistry from that of the earth.

These hopes were not realized. Analysis of the moon rocks has shown that . . . the moon did not come from the earth. But it didn't suggest any alternatives.

The origin of the moon remains as much a mystery as it was before Apollo. . . .13

But if the moon's origin remained mysterious, the early stages in its evolution became clearer as the results of Apollo science accumulated.

Arriving at a model of lunar evolution was not easy, since so many different and often unrelated scientific disciplines contributed essential information. Interpretation of the data from geological studies concerning volcanic activity, for example, had to be consistent with the seismic scientists' findings about the moon's internal structure. Heat flow measurements had to be interpreted in light of the geochemical abundances of radioactive elements. Gerald J. Wasserburg, professor of geology and geophysics at Caltech, who participated in the lunar studies from the beginning, commented that this required most investigators to change their approach to science. For the most part, Wasserburg said, they had come to the first lunar science conference as specialists, "without any conception of the relationship of their particular observations to the global problems of a planet." As the work proceeded, however, "specialists began to recognize the interrelationships between their own work and studies done by other individuals in completely different fields. So a broader, truer planetary science [began] to emerge."14

Before Apollo 11, considerable information about the moon had been available - principally its physical characteristics, but including some basic chemical facts as well.15 For many years it had been known that the moon was only 60 percent as dense as the earth (3.3 grams per cubic centimeter as against 5.5 for the earth) and that its rotational properties were almost identical with those of a homogeneous sphere. In the 1960s, space probes yielded more information. Surveyors V, VI, and VII carried instruments that measured the proportions of key chemical elements in the lunar surface. They showed that the maria were different from the highlands and that the moon was chemically different from both the earth and the (presumed) primordial material of the solar system. The terrestrial material most closely resembling the maria was basalt, an igneous rock, indicating the maria had been flooded with molten rock that had later solidified. Geochemists deduced from its density that the lunar basalt could not represent the average composition of the entire moon, which allowed the inference that the moon was not homogeneous.

Later, the Lunar Orbiters returned photographs that showed the moon's hidden side to be topographically quite different from the visible side. Besides providing information essential to the selection of Apollo landing sites, Orbiter photographs contained a wealth of geological clues. They convinced geologists that at least some of the moon's surface features had been produced by volcanic activity. Analysis of the spacecrafts' orbits provided evidence for anomalous concentrations of mass ("mascons") under some of the maria, suggesting that the lunar crust was thicker and more rigid than was previously supposed. Had the crust been more plastic, the mascons would have settled deeper into it over geologic time, eliminating the gravitational anomalies that affected Lunar Orbiter V. Thus the outer layer of the moon had been cold and rigid for a very long period, yet lava flows large enough to fill the basins had occurred as well. This too implied a nonuniform structure for the moon and probably a complex evolutionary history.

To this sketchy body of knowledge about the moon Apollo contributed an overwhelming flood of new information, out of which scientists began to construct new models, some to be discarded quickly, others to be modified and retained. The geophysical data may have been the easiest to integrate into a coherent picture. Apollo's four seismometers detected infrequent, weak tremors originating within the moon, at depths of 800 to 1,100 kilometers (500 to 700 miles), much deeper than those on earth. Forty-three moonquake zones were identified, each showing periodic activity correlated with lunar tides. Artificial seismic events (impacts of LM ascent stages and spent S-IVB stages) - plus the fortuitous collisions of a large (1,100 kilograms, 2,400 pounds) meteorite with the moon16 - produced signals that suggested a crust about 60 kilometers (37 miles) thick, overlying a different homogeneous layer extending down to about 1,000 kilometers (625 miles). Deeper still a partially molten core may exist, which, assuming it to be a silicate rock, would be at a temperature of about 1,500 degrees C (2,700 degrees F).

Results from the laser altimeters carried on Apollo 15 and 16 confirmed that the moon's center of mass does not coincide with its geometrical center. Some scientists suggested this was due to the presence of the low-lying maria on the near side. Others suggested that the crust on the far side was thicker than that on the near side, which would make the absence of maria on the far side easier to explain: a thicker crust could have prevented the extrusion of molten material except in the very deepest craters, such as Tsiolkovsky.17

Geological and geochemical data were harder to interpret; only the broadest general conclusions could be stated after Apollo 17. The returned samples showed that the moon was chemically different from the earth, containing a smaller proportion of volatile elements (those that are driven off at low to moderate temperatures) and more radioactive elements than the cosmic average. Geochemical evidence indicated that three types of rocks predominate on the lunar surface: basalts rich in iron covering the maria; plagioclase or aluminum-rich anorthosites characteristic of the highlands; and uranium- and thorium-rich basalts that also contain high proportions of potassium, rare-earth elements, and phosphorus ("KREEP" basalts). Some scientists felt, however, that these could not represent the structure and texture of the primordial lunar rocks; it seemed likely that those characteristics had been virtually obliterated in the massive bombardment that the moon has obviously undergone.18

In spite of the difficulties, scientists - as is their habit - began to formulate models for lunar evolution as soon as the first results became available. Each successive mission brought a surprise, however, and it was only after Apollo 15 that the broad outlines of the moon's history emerged fairly clearly.19

The oldest rocks found on the moon appear to have been chemically assembled around 4.5 billion years ago, in the late stages of formation of the solar system. How the materials of the moon came together is still an unanswered question; the evidence indicates that the moon aggregated out of the debris left over from the formation of the sun. According to a widely held view, it was never completely molten; only its outer layer, perhaps to a depth of 320 kilometers (200 miles, roughly one-fifth of its radius) was melted. This sea of molten rock, agitated by a continuing rain of fragments, lost its heat to space and began to solidify. As it did so, different minerals crystallized at different temperatures. Convection currents set up by cooling at the surface brought deeper, hotter material to the surface and at least partially remelted the surface crust. As cooling continued, crystals of different composition separated, giving rise to the chemical segregation observed in the lunar crust. Eventually, probably after some 200 million years, a rigid crust of considerable thickness formed, composed mostly of light-colored minerals rich in calcium and aluminum (plagioclase). Beneath the crust a mantle of iron- and magnesium-rich material settled, consisting predominantly of the minerals pyroxene and olivine. At the center of the moon a core of dense, partially melted material may have formed, rich in iron and sulfur. The presence of a semiliquid core is indicated by the behavior of seismic waves passing through the moon and by the presence of residual magnetism in lunar rocks: at some time in the past, it appears, the moon had a magnetic field (thought to be produced by a liquid metallic core) which has since almost entirely vanished.

For perhaps 300 million years after the crust formed, fragments of primordial material, some of them 50 to 100 kilometers (30 to 60 miles) in diameter, pelted the moon. The impacts caused local melting of the crust and chemical alteration of the original material, scattered debris over thousands of square miles of the moon's surface, and fractured the crust to a considerable depth.

Below the solidified crust the moon's mantle remained partially molten, owing to the insulating properties of the crust and also probably to considerable heating by disintegration of radioactive elements (uranium and thorium). Toward the end of the massive bombardment period, from around 4.1 to 3.9 billion years ago, the large basins (Imbrium, Serenitatis, Crisium) were gouged out, probably by objects almost as large as planets. This was followed by episodic flows of basaltic material into the basins, filling them to approximately their present levels. The youngest igneous (heat-formed) rock so far found on the moon is 3.16 billion years old, but major surface manifestations of internal lunar heat may not have stopped for another two billion years.

This generalized picture of the moon's history has changed little in the years since the end of Apollo. Subsequent study has extended the onset of mare volcanism backward in time somewhat, and its duration is now thought to have been considerably longer. It has been recognized that many of the craters saturating the highlands are secondary, having been produced by large fragments thrown out by impacting objects. The need to understand lunar craters (and, in subsequent years, those on other celestial bodies, e.g., Mercury, Mars, and the satellites of Jupiter and Saturn) has stimulated interest in the mechanics of crater formation, leading to laboratory studies using high-velocity particles. Finally, the impact theory of the moon's origin has recently been revived, with modifications. Collision of an object the size of Mars with earth has been computer modeled and found to be not unattractive. Newer thinking about the origin of the moon considers that more than one process may have contributed significantly, that is, collision followed by accretion of additional material by both bodies. As yet, the available evidence does not permit a clear-cut choice among the various postulates.20

So far as the Apollo results have shown, the moon's surface has undergone no large-scale changes for something like 3 billion years. Continuous slow erosion of large features by the impacts of small meteoroids has added to the dust layer on its surface, now and then a substantial meteoroid has produced another crater, and occasionally a boulder, dislodged by an internal tremor or an external shock, has rolled down a slope, leaving its track in the dust. Today the moon looks very much as it looked to the first humans, and it must have changed very little since the age of the dinosaurs.

* Jastrow, the first director of the Theoretical Division in the Office of Space Sciences [see Chapter 2], later headed NASA's Goddard Institute of Space Studies in New York City.

13. Robert Jastrow, "Moon Still Is A Generally Silent Witness," New York Times, Mar. 24, 1974.

14. G. J. Wasserburg, "The Moon and Sixpence of Science," Astronautics and Aeronautics 10(4) (Apr. 1972):16-21.

15. Material in this and the next paragraph is a synthesis of information from several sources: "Planetology," L. S. Walter, B. M. French, and P. D. Lowman, eds., in Significant Achievements in Space Science, 1967, NASA SP-167 (Washington, 1968), pp. 326-52; Harold Urey, "The Contending Moons," Astronautics and Aeronautics 7(1) (Jan. 1969):37-41; A. L. Turkevich, W. A. Anderson, T. E. Economou, E. J. Franzgrote, H. E. Griffin, S. L. Grotch, J. H. Patterson, and K. P. Sowinski, "The Alpha-Scattering Chemical Analysis Experiment on the Surveyor Lunar Missions," in Surveyor Program Results, NASA SP-184 (Washington, 1969), pp. 271-350; Wasserburg, "The Moon and Sixpence of Science."

16. Gary V. Latham, Maurice Ewing, Frank Press, George Sutton, James Dorman, Yosio Nakamura, Nafi Toksoz, David Lammlein, and Fred Duennebier, Passive Seismic Experiment, in Apollo 16 Preliminary Science Report, NASA SP-315 (Washington, 1972), p. 9-1. This massive chunk of rock struck approximately 145 kilometers (90 miles) north of the Apollo 14 station on May 13, 1972.

17. Hammond, "Lunar Science: Analyzing the Apollo Legacy."

18. Ibid.; Wasserburg, "The Moon and Sixpence of Science."

19. Material in the following paragraphs is based primarily on Harrison H. Schmitt's summary, "Evolution of the Moon: The 1974 Model," in The Soviet-American Conference on Cosmochemistry of the Moon and Planets, John H. Pomero and Norman J. Hubbard, eds., NASA SP-370 (Washington, 1977), pp. 63-80, and S. Ross Taylor's Lunar Science: A Post-Apollo View (New York: Pergamon Press, 1975).

20. The Lunar Geoscience Working Group, Status and Future of Lunar Geoscience, NASA SP-484 (Washington, 1986), pp. 3-32. Members of this working group were: P. D. Spudis, U.S. Geological Survey, chairman; B. R. Hawke, Univ. of Hawaii; L. L. Hood, Univ. of Arizona; P. H. Schultz, Brown Univ.; G. J. Taylor, Univ. of New Mexico; and D. E. Wilhelms, U.S. Geological Survey. This 54-page booklet provides a brief summary of the current understanding of the moon and a prospectus for future lunar exploration (manned and unmanned). It also contains a 5-page scientific bibliography covering recent work in lunar science.