Abstract
The preceding section of this survey contained a general description of the more common types of formations encountered on the lunar surface. What can we say at present about the origin of such a relief- and, in particular, of its characteristically dominant feature, the so-called craters? A glance at the almost bewildering array of such formations of all sizes — as shown on many photographs accompanying this chapter -makes it perhaps unlikely that all of them originated in the same way, or at the same time; and a more detailed analysis of their features suggests that a suspicion of different origin is probably well founded. In fact, the most reasonable approach to this problem can be made if we ask ourselves the following question : what are all the processes which could have conceivably cooperated in shaping up the surface of our satellite? And once we thus formulate our problem, we find ourselves facing two principal contending theories of crater origin : namely, the external theory — invoking the effects produced by impacts of other celestial bodies (asteroids, meteorites, or comets) on the lunar surface — and the alternative theory relying on the internal processes connected with convection, gradual defluidization and degassing of the lunar globe consequent upon its build-up of internal heat due to spontaneous decay of radioactive elements, or any other activity which could be loosely termed as “volcanic”.
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Bibliographical Notes
A more comprehensive presentation of the crater-forming processes by external impacts than that given in this section has been made by Shoemaker (1962) in a contribution which has since become
standard, and which contains an almost exhaustive list of references to previous literature. A similar (though in places less critical) presentation of the case for the internal origin of lunar formations has recently been summarized by sixty-one different authors in the ‘Geological Problems in Lunar Research’, published as Vol. 123 (pp. 367–1257) of the Annals of the New York Academy of Sciences in 1965.
Of individual investigations of more recent date concerned with the mechanism of the origin of the lunar craters by impacts of external bodies cf., e.g., Daly (1946), Dietz (1946), Baldwin (1949, 1963), Kuiper (1954, 1955, 1959), Urey (1955, 1956b, c, d), Gilvarry and Hill (1965a, b) Alter (1956, 1957, 1958), Gilvarry (1960), Levin (1964), Jean-Pierre (1964), Kviz (1964), Cross (1965), Salisbury, Smalley and Ronca (1965), Sida (1965), Brinkmann (1966), Miller (1966), and many others.
For more specific aspects of the mechanism of crater formation cf., e.g., Stanyukovich and Fedynsky (1947), Patridge and Van Fleet (1958), et al. Of investigations concerned with empirical size-energy relation for impact craters cf. Baldwin (1949, Appendix D; or 1963, Chapter 8); Hill and Gilvarry (1956); Hess and Nordyke (1961); Nordyke (1962); Hawkins (1963); etc. For the topographic profile of the impact formations cf., e.g., Gold (1955); or, more recently, Fulmer and Roberts (1963), Roberts (1964), Carlson and Jones (1965), etc. Investigations of the displacement and loss of mass accompanying the formation of primary impact craters have been undertaken by Warner (1961) or Gilvarry (1964).
Collisions of the Moon with the comets were first considered by Kopal (1959a) as a possible explanation of the origin of certain types of flat-floored craters (like Plato) or maria; and, more recently, Kuiper (1965) as well as Urey (1965) invoked the same mechanism to account for the origin of ray-craters. But all these tentative identifications of differences between meteoritic and cometary impacts must still be viewed with considerable reserve.
The problem of the origin of the lunar bright rays, and the ballistics of their ejection, have in more recent years been considered by Hacker and Stewart (1935), Lenham (1955), Alter (1955), O’Keefe (1957), Giamboni (1959), Fielder (1961, 1962), Devadas (1962) and, most recently, by Kopal (1966) in connection with the Tychonic ejecta photographed by Ranger 7 (cf., Figure 17–16). This latter work has shown that such ray material could have been deposited only by ejection in nearly circular orbits grazing the lunar surface.
Of recent literature concerned with descriptive properties and possible origin of lunar rilles, domes or wrinkle ridges cf., e.g., Fielder (1960, 1962a), Salisbury (1961), Rae (1963), Cameron (1964), Pither (1964), Brungart (1965) or Quaide (1965).
For diverse views of the nature of lunar maria expressed in recent years cf., e.g., Gold (1955) with comments by Urey (1956d), Kuiper (1954, 1959), Urey (1956b, c), Gilvarry (1957, 1958), Kopal (1959a), Warner (1961a, b), Wilson (1962), Nash (1963) or Fielder (1963). This latter paper errs, however, in its basic assumptions; for the Moon could not have been heated to its melting point by the action of long-lived radioactive elements in the past — if it vere had been so molten, it would be so at the present, and would continue to grow hotter still for at least 3 x 109 years in the future (cf., Kopal, 1962a).
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© 1966 Springer Science+Business Media Dordrecht
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Kopal, Z. (1966). Origin of the Lunar Formations. In: An Introduction to the Study of the Moon. Astrophysics and Space Science Library. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-6320-2_17
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