Reviews in Mineralogy & Geochemistry Vol. 60, pp. 221-364, 2006 3 Copyright © Mineralogical Society of America The Constitution and Structure of the Lunar Interior Mark A. Wieczorek1, Bradley L. Jolliff2, Amir Khan3, Matthew E. Pritchard4, Benjamin P. Weiss5, James G. Williams6, Lon L. Hood7, Kevin Righter8, Clive R. Neal9, Charles K. Shearer10, I. Stewart McCallum11, Stephanie Tompkins12, B. Ray Hawke13, Chris Peterson13, Jeffrey J. Gillis13, Ben Bussey14 1Institut de Physique du Globe de Paris, Saint Maur, France 2Washington University, St. Louis, Missouri, U.S.A. 3Niels Bohr Institute, University of Copenhagen, Denmark 4Cornell University, Ithaca, New York, U.S.A. 5Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A. 6Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, U.S.A. 7University of Arizona, Tucson, Arizona, U.S.A. 8Astromaterials Branch, Johnson Space Center, Houston, Texas, U.S.A. 9University of Notre Dame, Notre Dame, Indiana, U.S.A. 10University of New Mexico, Albuquerque, New Mexico, U.S.A. 11University of Washington, Seattle, Washington, U.S.A. 12Science Applications International Corporation, Chantilly, Virginia, U.S.A. 13University of Hawaii, Honolulu, Hawaii, U.S.A. 14Applied Physics Laboratory, Laurel, Maryland, U.S.A. e-mail: [email protected] “This picture, though internally consistent, is subject to revision or rejection on the basis of better data.” Don Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348. 1. INTRODUCTION The current state of understanding of the lunar interior is the sum of nearly four decades of work and a range of exploration programs spanning that same time period. Missions of the 1960s including the Rangers, Surveyors, and Lunar Orbiters, as well as Earth-based telescopic studies, laid the groundwork for the Apollo program and provided a basic understanding of the surface, its stratigraphy, and chronology. Through a combination of remote sensing, surface exploration, and sample return, the Apollo missions provided a general picture of the lunar interior and spawned the concept of the lunar magma ocean. In particular, the discovery of anorthite clasts in the returned samples led to the view that a large portion of the Moon was initially molten, and that crystallization of this magma ocean gave rise to mafic cumulates that make up the mantle, and plagioclase flotation cumulates that make up the crust (Smith et al. 1970; Wood et al. 1970). This model is now generally accepted and is the framework that unifies our knowledge of the structure and composition of the Moon. The intention of this chapter is to review the major advances that have been made over the past decade regarding the constitution of the Moon’s interior. Much of this new knowledge is a direct result of data acquired from the successful Clementine and Lunar Prospector missions, as well as the analysis of new lunar meteorites. As will be seen, results from these studies have led to many fundamental amendments to the magma ocean model. 1529-6466/06/0060-0003$15.00 DOI: 10.2138/rmg.2006.60.3 222 Wieczorek et al. Lunar Interior Constitution & Structure 223 Much of what we know from sample analyses has been previously summarized elsewhere, and only their most important aspects will be discussed in this chapter. The reader is referred to the relevant chapters in the books Basaltic Volcanism on the Terrestrial Planets (Basaltic Volcanism Study Project 1981), The Lunar Sourcebook (Heiken et al. 1991), and Planetary Materials (Papike et al. 1998) for more in-depth assessments. Fortunately, the lunar samples were curated carefully and with forethought as to the possibility that there might be no sample- return missions for a long time after Apollo. Thus, as new analytical methods are developed and old ones are improved, analyses of the samples continue to yield important results. Much of our geophysical knowledge of the Moon comes from instruments that were deployed during the manned Apollo landings. As part of the Apollo Lunar Surface Experiments Package (ALSEP), Surface magnetometers, heat-flow probes, and seismometers operated until 1977 when the transmission of data was terminated. While re-analyses of these data occasionally yield surprises (such as the Apollo seismic data), many aspects of the pre-1990s geophysical reviews are just as relevant today as when they were written. The electromagnetic sounding data of the Apollo era is thoroughly reviewed by Sonnett (1982), much of the geophysics as reviewed by Hood (1986) is still highly relevant, and the paleomagnetism of the lunar samples has been comprehensively reviewed by Fuller and Cisowski (1987) and Collinson (1993). One suite of passive surface instruments deployed during the Apollo and Russian Luna missions that is still operational are the corner-cube retroreflectors. Laser ranging to these stations continues to the present day, and with the gradual accumulation of data, our knowledge of the deep lunar interior is slowly being revealed. The results of this experiment up to 1994 have been reviewed by Dickey et al. (1994). The remote sensing missions of the 1990s—Galileo, Clementine, and Lunar Prospector— for the first time obtained near-global compositional data sets that enable us to assess the nature of materials at the surface of the Moon. One of the major results of global remote sensing has been to locate regions of the lunar surface that differ from the Apollo landing sites and that are difficult to explain given the known compositions of the present lunar sample collection. In particular, the floor of the South Pole-Aitken basin, which is the largest recognized impact structure in the solar system, may contain rocks that formed deep in the crust and as yet, not sampled elsewhere. The mineralogy and petrology of known rock types, nevertheless, provide constraints on lunar petrogenetic processes, which then allow informed extrapolation to those areas that were not directly sampled. Near-global topography, magnetic data, and improved models of the lunar gravity field from these missions have further offered vastly improved datasets in comparison to the spatially limited Apollo observations. By combining general characteristics of the lunar samples and surface-derived geophysical constraints with these near-global orbital data, our understanding of the Moon’s interior structure has been much improved over the general sketch provided during the Apollo era. The Apollo and Luna samples were found to contain many different rock components brought together by impact processes. Any given sample of soil or breccia thus might contain a much wider diversity of rock types than whatever the local bedrock material might be. In fact, because of the potentially widespread effects of mixing together of widely separated materials by large impact events, a general (although incorrect) argument could be made that the Moon was adequately sampled by the Apollo and Luna missions. The discovery of lunar meteorites (Warren 1994), coupled with telescopic and global remote sensing, prove this argument wrong. Furthermore, recent studies of basin-ejecta deposits and deposition processes suggest that most of the Apollo sites were strongly influenced by material derived from one or a few late basin- forming events, especially Imbrium (e.g., Haskin 1998; Haskin et al. 1998). Thus, the feldspathic lunar meteorites provide our best samples of the lunar highland crust far removed from the effects of contamination by Imbrium ejecta. Several of the lunar meteorites also provide samples of basalt that differ from any of the basalt types known from the Apollo and Luna suites. 222 Wieczorek et al. Lunar Interior Constitution & Structure 223 The information presented and discussed in this chapter is set within the context of the new datasets. Key constraints based upon the samples, Apollo-era geophysical experiments, and pre-1990s remote sensing are reviewed and then coupled with recent results to reassess the makeup and structure of the lunar interior. One of the recurring themes of this chapter will be to demonstrate that the traditional “pie-wedge” cross-sectional view of the lunar interior can no longer be considered as being globally representative. Instead, the lunar crust and underlying mantle appear to be best characterized as being composed of discreet geologic terranes, with each possessing a unique composition, origin, and geologic evolution. This concept is best illustrated by the gamma-ray spectrometer data obtained from the Lunar Prospector mission, which shows that incompatible and heat-producing elements are highly concentrated in a single region of the Moon that was once volcanically very active. This recognition has led many workers to reassess the significance of the data derived from the Apollo era, particularly in light of the fact that all six Apollo landing sites straddle the border between two of the most distinctive terranes—the Procellarum KREEP Terrane and the Feldspathic Highlands Terrane (e.g., Jolliff et al. 2000a). The fact that a body as small as our Moon possesses vastly different geologic crustal provinces should be borne in mind when one attempts to assess the geologic history of a less understood and larger planetary body, such as Mercury and Mars. Because the concept of a magma ocean is so thoroughly ingrained in modern notions of how lunar rocks types are distributed as a function of depth in the crust and upper mantle (and for good reasons), we use it as a general framework for discussion. We do not maintain that we understand how it originated or solidified in detail, nor how deep it might have been and how it might have evolved. Indeed, fundamental questions remain that relate to these topics, and some of these are addressed in more detail later in this chapter and in Chapter 4 of this book. Nevertheless, we begin this chapter with a very brief summary of some of the key implications that a magma ocean has for the makeup and structure of the Moon, and what types of question we might address within the context of this model.