LUNAR THEORY BEFORE 1964
The moon has long been a natural subject of inquiry. It is our closest celestial neighbor, and, next to the sun, the most prominent of heavenly objects viewed from earth. But the urge to examine the moon firsthand is heightened by other conditions. Although the moon is one of thirty-two known satellites that circle about the planets in our solar system, and other satellites are more massive, the moon is unique among them: it obtains an orbital angular momentum about the earth that exceeds the earth's rotational angular momentum, and no other planet possesses a satellite whose mass is so great a fraction of the mass of the primary body. One-quarter the size of the earth, the moon has one-eightieth as much mass, and possesses one-sixth the gravity of the earth. It rotates and revolves around the earth in equal periods, in a nearly circular orbit at a distance of approximately a quarter million miles, and on a plane inclined 5 degrees to the plane of the elliptic. Three principal theories have attempted to connect these intriguing conditions and explain the moon's origin.
The oldest hypothesis postulated a binary earth and moon system in which the two planets condensed from a single mass of interstellar gas and dust. This process of formation implied a similar chemical and physical constitution, and early in this century most scientists abandoned the idea because the relatively large size of the moon did not correspond to the difference in density between the two bodies, or to other important dynamical considerations. In 1954 Gerard Kuiper modified and revived this theory. 1 He held that the earth and moon originated as a double planet within a common gaseous envelope, and, since the heat generated by the decay of radioactive elements had been much greater early in the life of the solar system, the moon had undergone some internal melting, likely had a small iron core, and had swept up dust and debris remaining near the earth afterwards.
Before Kuiper revived the double planet thesis, George Darwin, son of the famous British naturalist, had in 1878 advanced an attractive alternative. Drawing on the pronounced tidal effects exerted between the earth and moon, Darwin speculated that the moon's mass had been ejected from a fluid and rapidly spinning protoearth when centrifugal force and solar tides, acting on matter in the earth's equatorial plane, exceeded the force of gravity. In time, the moon moved out to its present orbit and attained its coincident period of rotation and revolution as a result of tidal interactions between the two bodies. 2 In 1892 the Rev. Osmond Fisher suggested that the Pacific Basin marked the point of this separation, and that this material, having been drawn from the earth's mantle, explained the lower density of the moon. 3 The hypothesis, however, could not satisfactorily explain the cause for the moon's orbit at an inclination to the earth's equator. In 1930, moreover, Harold Jeffreys mathematically demonstrated that, given the earth's internal friction, damping of the required excessive tidal bulge would take place, preventing any separation of material. 4
In the 1950s Harold Urey and Horst Gerstenkorn developed a third theory that rapidly gained wide acceptance. They proposed that the moon had accreted from gas and dust elsewhere in the solar system, was later captured by the earth at a close distance, and then moved out to its present radius from the radius of first capture. 5 More plausible than the Darwinian postulate, this theory also held the moon to be a primary body in the solar system, rather than a chunk of the earth's mantle, and it attracted many planetologists who hoped to find undisturbed on the moon clues to the formation of the solar system. Nevertheless, proponents of the capture model faced unresolved time-scale questions relating to the moon's present location and its gradual recession from the earth when contrasted with the proposed time and manner of its acquisition. Grove Karl Gilbert and Alfred Wegener, 6 and more recently Gordon MacDonald and others, suggested that the capture process involved a number of smaller moons, and that the orbital elements of the system changed at a rate proportional to the mass of the protomoons. Ultimately the largest moon increased in mass and moved outward as it swept up the smaller moonlets, around the earth. 7
Each of the preceding theories postulated an overall structure of our satellite that was hot-concentric and similar in form (though probably not in proportion) to that of the earth, or warm-essentially undifferentiated, or cold-partially heterogeneous but not layered. Corresponding scientific interest in surface morphology, reflecting one or another of these viewpoints, likewise led to differing explanations of the moon's features.
The most pronounced features on the visible sunbaked face of the moon consist of the great circular ringed maria, or lunar plains, clustered about and to the north of the lunar equator, and the intensely cratered ancient highlands to the south. Craters of varying sizes are everywhere in evidence. In addition, crater rays or streak systems can be seen associated with such major craters as Tycho and Copernicus, radiating out in all directions across the surface. Secondary features that appear under increased optical resolution include meandering cracks or valleys known as rilles, domical structures, and various wrinkle-ridges in the plains regions (frontispiece).
Two hypotheses sought to explain these conditions. The first, generally accepted for many years, accorded plutonic processes a central role. 8 Internal vulcanism, it was argued, shaped the twisted surface, formed calderas, and filled the lunar plains with darkened ash or lava flows. The second theory, first considered by Robert Hooke in 1665, credited meteoric impact as the principal agent responsible for sculpturing the surface morphology. Hooke conducted experiments with a mixture of pipe clay and water into which he dropped musket balls to form impact craters. But meteoric craters remained virtually unknown in contemporary terrestrial experience, and the impact hypothesis did not prevail.
In the 19th century, several years after Darwin proposed his origin of the moon, Grove Karl Gilbert reevaluated the impact hypothesis. Gilbert, the Chief Geologist of the United States Geological Survey, observed in 1892 that, lunar craters simply did not conform in physical characteristics or in sheer numbers to terrestrial volcanic craters, and he reasoned that the moon's primary features, including the extensive maria, must be accounted for by impact cratering. 9 Alfred Wegener in 1921 10 and L. J. Spencer in 1932 and 1933 11 performed more detailed comparative studies that lent increased support to the impact thesis. These efforts culminated in the work of the industrialist-astronomer Ralph Baldwin in the 1940s. 12 Baldwin demonstrated conclusively that not only do lunar craters bear little or no physical resemblance to volcanic craters, but most lunar crater diameters as a function of their depth correspond closely to those of known terrestrial meteoric craters and artificial craters formed by explosive charges. Many proponents of the Baldwin thesis also believed that the material of the mare plains, rather than volcanic ash or magma, would be found largely composed of eroded dust. 13 Whatever the composition of the lunar soil, by 1960 the impact thesis was generally accepted among planetary astronomers and geophysicists as describing the primary process that shaped the lunar surface.
Beyond scientific advocacy purporting to explain the history of the moon and its surface features, a secondary current of interest eddied about the possible existence of life forms on the moon. This question, first seriously discussed after the advent of the telescope in the 17th century, found scientific opinion favorably disposed to the proposition. Some, such as Giovanni Riccioli, maintained, however, that the inhospitable environment-the apparent absence of water and any noticeable atmosphere-precluded higher life forms; he contended forcefully that the moon must resemble an arid desert. Sentiment in favor of sentient life on the moon, nevertheless, continued among authorities well into the 19th century. In 1780, for example, Sir William Herschel directed a communication to the fourth Astronomer Royal, Nevil Maskelyne, asserting that "there is almost an absolute certainty of the moon's being inhabited…" 14 The Director of the Vienna
Astronomical Observatory, J. von Littrow, proposed in 1830 to establish communications with Selenites on the moon by constructing large geometric symbols on the Siberian steppes. 15
Opinion disposed to intelligent life on the moon began to ebb after the "Great Moon Hoax" of 1835, 16 and reversed completely in 1837 with the publication of the definitive selenographic study Der Mond by Beer and Madler. But speculation concerninng possible life forms, if not on the order of "reasoning Selenites" at least at a lower organic level, continued intermittently. In 1876 the British astronomer Edmund Neilson attempted to establish that the moon did in fact possess an atmosphere of sufficient density to support life. 17 And in 1924 W. H. Pickering, a Harvard College Observatory astronomer, reported observing possible flora and fauna. 18
When space exploration began in the 1950s, scientific thinking generally concurred that organic matter probably was not present on the moon's surface, though it might possibly be found as spores in certain protected locations. Perhaps it could exist below the surface as a residue of some prehistoric period when the moon possessed a reducing atmosphere or was contaminated by material from the earth. The moon, after all, had acted as a gravitational trap for meteoroidal material accumulated from space over many eons. Detection of terrestrial-like organisms, for instance, would furnish strong support for the hypothesis of panspermia-that reproductive bodies of living organisms exist throughout the universe and develop wherever an environment is favorable. Opinion held, in any case, that care should be exercised not to introduce organisms from the earth that might accompany man-made instruments sent to the moon, and thus spoil a major scientific opportunity to discover and examine extraterrestrial life. 19 By the same token, assuming that lunar organisms might exist, Apollo astronauts returning from the moon would have to be quarantined to prevent "back contamination" of the earth.
The closeup pictures of the moon taken by Rangers 7, 8, and 9 could be and were appropriated to support each of the theories of the surface morphology, sharply escalating the scientific debate. It remained for other unmanned and manned lunar missions in the 1960s and 1970s to furnish tentative answers to many of these questions and settle the issue of life on the moon. Many more years and more missions will be necessary before a firm explanation of the moon's history is at hand. 20
1. Gerard P. Kuiper, "On the Origin of the Lunar Surface Features," National Academy of Sciences, Proceedings, Vol. 40, 1954, pp. 1096-1112.
2. George H. Darwin, "On the Precession of a Viscous Spheroid," Nature,Vol. 18, 1878, pp. 580-582. Recent consideration in D.U. Wise, "Origin of the Moon by Fission," in B. G. Marsden and A. G. W. Cameron (eds.), The Earth-Moon System (New York: Plenum Press, 1966), p. 213.
3. Rev. Osmond Fisher, "On the Physical Cause of the Ocean Basins," Nature, January 12, 1882; and, Communication, "Hypothesis of a Liquid Condition of the Earth's Interior Considered in Connexion with Professor Darwin's Theory of the Genesis of the Moon," Cambridge Philosophical Society Proceedings, Vol. 7, 1892, p. 335.
4. Harold Jeffreys, "Resonance Theory of the Origin of the Moon, II," Royal
Astronomical Society Monthly Notices, Vol. 91, November 1930, pp. 169-173.
5. Horst Gerstenkorn, "Uber Gezeitenreibung beim Zweikorperproblem" [Effect of Tide Friction on the Two Body Problem], Zeitschrift für Astrophysik Vol. 36, 1955, pp. 245-274 (capture in a retrograde orbit); and Harold C. Urey, The Planets: Their Origin and Development (New Haven: Yale University Press, 1952), p. 25; also, "The Origin of the Moon's Surface Features," Parts I and II, Sky and Telescope, January and February 1956; and, "The Chemistry of the Moon," Proceedings of the Lunar and Planetary Exploration Colloquium Vol. 1, No. 3, October 29, 1958, p. 1 (capture in a direct orbit).
6. G. K. Gilbert, "The Moon's Face, A Study of the Origin of its Features," Bulletin of the Philosophical Society, Washington, D. C., Vol. 12, 1893, p. 262; Alfred Wegener, Die Entstehung der Mondkrater [The Origin of the Lunar Craters] (Braunschweig, Germany: Friedr. Vieweg & Son, 1921). About this time Wegener's theory of continental drift was published in English, and drew heavy fire from uniformitarian geologists. (Wegener, The Origin of Continents and Oceans, translation of the 3rd German Edition by J. A. G. Skerl, London: Methuen & Co., 1924).
7. Gordon J. F. MacDonald, "Origin of the Moon: Dynamical Considerations, " in The Earth-Moon System, op. cit., p. 198.
8. The most recent detailed exposition of this theory is in J. E. Spurr, Geology Applied to Selenology, Vol. I (Lancaster, Pa.: Science Press, 1944).
9. G. K. Gilbert, op. cit., p. 241; abstract in American Naturalist, Vol. 26, 1892, p. 1056.
10. Alfred Wegener, Die Entstehung der Mondkrater, op. cit. Wegener's important work, published in Germany, apparently remained little known in the United States. It was not cited by Baldwin, Urey, or other impact theorists in the 1940s and 1950s.
11. L. J. Spencer, "Meteorite Craters," Nature, Vol. 129, 1932, pp. 781-784; and "Meteoric Craters as Topographical Features on the Earth Surface," Geographical Journal, Vol. 56, March 1933, pp. 205-206.
12. First consideration in Ralph B. Baldwin, "The Meteoritic Origin of Lunar Craters," Popular Astronomy, Vol. 50, August 1942, pp. 365-369; comprehensive treatment in The Face of the Moon (Chicago: University of Chicago Press, 1949); and The Measure of the Moon (Chicago: University of Chicago Press, 1963).
13. See, for example, Thomas Gold, "The Lunar Surface," Monthly Notices of the Royal Astronomical Society, Vol. 115, No. 6, 1955, p. 585.
14. Letter from Sir William Herschel to the Rev. Dr. Nevil Maskelyne, June 12, 1780, as reprinted in Zdenek Kopal, The Moon (New York: Academic Press Inc., Publishers, 1964), pp. 119-120.
15. Account as reprinted in David Lasser, The Conquest of Space (New York: Penguin Press, 1931), p. 34.
16. This elaborate hoax, perpetrated by an enterprising American reporter, Richard Locke, ran as a serial in the New York Sun during August-September 1835 under the impressive title "Great Astronomical Discoveries Lately Made by Sir John Herschel, L.L.D., F.R.S., etc., at the Cape of Good Hope." The articles reported scientific observations supposedly made by Herschel (the son of William Herschel) with a new telescope in Africa that permitted a lunar resolution of about 2 feet. Locke vividly described exotic lunar vegetation and simian life forms. Carefully laced with scientific "facts," the story was widely accepted and reprinted in Europe and America.
17. Edmund Neilson, The Moon, and the Condition and Configurations of its Surface (London: Longmans, Green and Co., 1876), Chapter II.
18. W. H. Pickering, "Eratosthenes No. 4," pp. 69-78; "Eratosthenes No. 5,"pp. 302-312; and, "Eratosthenes No. 6, Migration of the Plats," pp. 393-404, in Popular Astronomy, Vol. 32, 1924.
19. Cf, Joshua Lederberg, "Moondust," Science, Vol. 127, p. 1473, January1958; and "Resolution of the Research Council, National Academy of Sciences," February 8, 1958, cited in A Review of Space Research (Report of the Summer Study conducted under the auspices of the Space Science Board of the National Academy of Sciences at the State University of Iowa, June 17-August 10, 1962), pp. 10-11. Opinion was by no means unanimous. Harold Urey, for one, argued that any contamination of the moon would be localized and not likely to affect other areas in the harsh environment: "This talk has caused a lot of trouble. The effort ... to get the last bacterium off a missile or space ship is enormous, and probably futile. In the second place, it is very difficult to contaminate the moon, for the reason there is so much moon and the amount of chemical contamination that can occur there is so small." (Urey, Proceedings of the Lunar and Planetary Exploration Colloquium, op. cit., p. 31 ); see also CETEX report in ICSU Review, Vol. 1, 195 9.
20. A succinct review of the state of these theories immediately following the Apollo missions is contained in Richard S. Lewis, The Voyages of Apollo: The Exploration of the Moon (New York: Quadrangle/The New York Times Book Co., 1974); see also, Stuart Ross Taylor, Lunar Science: A Post-Apollo View (New York: Pergamon Press Inc., 1975).