The Geologic History of the Moon

The Moon held little interest for most scientists after its basic astronomic properties had been determined and before direct exploration appeared likely. Speculations about its internal structure, composition, and origin were only broadly constrained by cosmochemical data from meteorites and solar spectra, and by astronomic data about its size, shape, motions, and surficial properties. Most investigators who were active before the space age began in 1957 believed that significant new advances in lunar knowledge required acquisition of additional data. One analytical technique, however, was insufficiently exploited before the 1960's. Few scientists since the geologist Gilbert had studied the lunar surface systematically from the historical point of view. Those who did immediately obtained important new insights about the Moon's postaccretion evolution. Then, the pioneering work of E.M. Shoemaker and R.J. Hackman focused the powerful methods of stratigraphy on lunar problems. Stratigraphy is the study of the spatial distribution, chronologic relations, and formative processes of layered rocks. Its application to the Moon came relatively late and met resistance, but the fundamental stratigraphic approach was, in fact, readily transferable to the partly familiar, partly exotic deposits visible on the lunar surface. Stratigraphic methods were applied systematically during the 1960's in a program of geologic mapping that aimed at reconstructing the evolution of the Moon's nearside. Order was discovered among the seemingly diverse and random landforms of the lunar surface by determining the sequence in which they were emplaced. The stratigraphic sequence and the emplacement processes deduced therefrom provided a framework for exploration by the Apollo program and for the task of analyzing the returned samples. During the 19703, the sophisticated labor of hundreds of analysts was brought to bear on the wealth of material returned by the American Apollo and the Soviet Luna spacecraft. Our present perception of the Moon has emerged from the interplay between sampling studies and stratigraphically based photogeology. These two approaches are complementary: Photogeology contributes a historical context by viewing the whole Moon from a distant vantage point, whereas the samples contain information on rock types and absolute ages unobtainable by remote methods. Neither approach by itself, even the most elaborate program of direct surface exploration, could have yielded the current advanced state of knowledge within the relatively short time of two decades. This volume presents a model for the geologic evolution of the Moon that has emerged mainly from this integration of photogeologic stratigraphy and sample analysis. Other aspects of the vast field of lunar science are discussed here only insofar as they pertain to the evolution of visible surface features. Chemical data obtained by remote sensing supplement the photogeologic interpretations of some geologic units, and geophysical data obtained both from lunar orbit and on the surface constrain hypotheses of the origin of many internally generated structures and deposits. Studies of the same data that treat the Moon as a whole, including speculations about the intriguing but unsolved problem of its origin, have been adequately covered in other reviews. This volume is written primarily for geoscientists and other planetologists who have examined some aspect of lunar or planetary science and who want a review of lunar science from the viewpoint of historical geology. It should also provide a useful summary for the advanced student who is conversant with common geologic terms. It may, furthermore, interest the geologist who has not studied the Moon but who wishes to see how his methodology has been applied to another planet.