Understanding the Lunar Surface and Space-Moon Interactions Paul Lucey1, Randy L
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Reviews in Mineralogy & Geochemistry Vol. 60, pp. 83-219, 2006 2 Copyright © Mineralogical Society of America Understanding the Lunar Surface and Space-Moon Interactions Paul Lucey1, Randy L. Korotev2, Jeffrey J. Gillis1, Larry A. Taylor3, David Lawrence4, Bruce A. Campbell5, Rick Elphic4, Bill Feldman4, Lon L. Hood6, Donald Hunten7, Michael Mendillo8, Sarah Noble9, James J. Papike10, Robert C. Reedy10, Stefanie Lawson11, Tom Prettyman4, Olivier Gasnault12, Sylvestre Maurice12 1University of Hawaii at Manoa, Honolulu, Hawaii, U.S.A. 2Washington University, St. Louis, Missouri, U.S.A. 3University of Tennessee, Knoxville, Tennessee, U.S.A. 4Los Alamos National Laboratory, Los Alamos, New Mexico, U.S.A. 5Smithsonian Institution, Washington D.C., U.S.A. 6Lunar and Planetary Laboratory, Univ. of Arizona, Tucson, Arizona, U.S.A. 7University of Arizona, Tucson, Arizona, U.S.A. 8Boston University, Cambridge, Massachusetts, U.S.A. 9Brown University, Providence, Rhode Island, U.S.A. 10University of New Mexico, Albuquerque, New Mexico, U.S.A. 11 Northrop Grumman, Van Nuys, California, U.S.A. 12Centre d’Etude Spatiale des Rayonnements, Toulouse, France Corresponding authors e-mail: Paul Lucey <[email protected]> Randy Korotev <[email protected]> 1. INTRODUCTION The surface of the Moon is a critical boundary that shapes our understanding of the Moon as a whole. All geologic mapping and remote sensing techniques utilize only the outermost portion of the Moon. Before leaving the Moon for study in our laboratories, all lunar samples that have been studied existed at or very near the surface. With the exception of the deeply probing geophysical techniques, our understanding of the interior of the Moon is derived from surficial, but not superficial, information, coupled with geologic reasoning. While the surface is the upper boundary of the lunar crust, it is the lower boundary layer of the tenuous lunar atmosphere and constitutes both a source and a sink for atmospheric gases. The surface is also where the Moon interacts with the space environment, causing changes in the physical nature of lunar materials, and provides a laboratory for the study of processes that occur on all airless bodies. The data obtained remotely by the Galileo, Clementine, and Lunar Prospector missions, as well as data derived from lunar meteorites, have resulted in major changes to our understanding of global distributions of chemistry and rocks. This chapter summarizes the current understanding of this critical interface, the surface of the Moon, in its role as the lower boundary of the lunar atmosphere, the upper boundary of the crust, and the window through which we view, through remote sensing, the composition of the crust and the history of the Moon. In this post-Lunar Prospector time, the view of the Moon has changed, lending new perspectives to lunar samples and lunar processes. But the New View will likely remain in flux as we continue to digest the results from these recent space missions and move forward to a new era of lunar exploration. 1529-6466/06/0060-02$15.00 DOI: 10.2138/rmg.2006.60.2 84 Lucey et al. Lunar Surface and Space-Moon Interactions 85 Despite the freshness of our perspective, this is an important moment to capture, before the next generation of lunar scientists are forced to relearn old lessons. We can examine the new data through the lens of the Apollo era with its hard won lessons and years of anticipation of new data such as is now in our hands. We can attack these new data with tools forged for other missions that did not aim at the Moon. In addition to capturing a snapshot of rapidly evolving new scientific perspectives on the Moon, this Chapter will present an overview of the materials that constitute the surface of the Moon and its interior—its rocks and minerals and the regolith that constitutes the entire outer surface of the Moon. The new discoveries in the space weathering of the lunar soil aids the further understanding and quantification of the remotely sensed observations, and provide their ground-truth. Of necessity, this chapter is a snapshot of a field in flux, but not a new field, one lacking introspection. An integration of data obtained during the last 30+ years is revealed herein. It will become apparent to the reader that new views of the Moon have been accrued by the coalition of several planetary science disciplines focused on a common cause, the science of the Moon. Excellent and detailed reviews of lunar rocks, minerals, and soils have been presented in the Lunar Sourcebook (1991) and in Planetary Materials (1998). Readers seeking a more thorough treatment of these topics are referred to these monumental books. 2. THE LUNAR REGOLITH Regolith is the term for the layer or mantle of fragmental and unconsolidated rock material, whether residual or transported and of highly varied character, that nearly everywhere forms the surface. This definition applies to the surface of all heavenly bodies, including the Moon and Earth. The entire lunar surface consists of a layer of regolith that completely covers the underlying bedrock, except perhaps on some very steep-sided crater walls and lava channels, where bedrock may be exposed. All samples collected by the Apollo and Luna Missions, as well as the lunar meteorites, are from the regolith; no “bedrock” was sampled directly in place. The lunar regolith is the product of the more than 4 billion years of impacts of meteoroids, big and small, into the Moon. The lunar regolith is the boundary layer between the solid Moon and the matter and energy that fill the solar system. It contains information about both of these regions, and the complexities of studying the regolith are exceeded only by its importance to understanding the Moon and the space environment around it. The regolith is the source of virtually all our information about the Moon. All direct measurements of physical and chemical properties of lunar material have been made on samples collected from the regolith. Remotely sensed X-ray fluorescence, optical and infrared spectra, and gamma-ray signals come from the very top of the lunar regolith; in fact, from depths of no more than 20 µm, 1 mm, and 10–20 cm respectively (Adler et al. 1973; Metzger et al. 1973; Pieters 1983; Morris 1985). In addition, because of its surficial, unconsolidated, and fine-grained nature, it is likely that the regolith will be the raw material used for construction, mining, road building, and resource extraction when permanent lunar bases are established. As a resource, the lunar regolith is far more useful and accessible than the underlying “bedrock.” The lunar regolith also preserves information from beyond the Moon. Trapped in the solid fragments that make up the regolith are atoms from the Sun and cosmic-ray particles from be- yond the solar system. In the regolith, data about the nature and evolution of the Moon are mixed with other records. These records include the composition and early history of the sun, and the nature and history of cosmic rays. The regolith also contains information about the rate at which meteoroids and cosmic dust have bombarded the Moon and, by inference, the Earth. Unscram- bling these intertwined histories is a major challenge—and a major reward—of lunar research. 84 Lucey et al. Lunar Surface and Space-Moon Interactions 85 2.1. Some properties of lunar regolith and soil We review here a few first-order properties of lunar regolith that are important for subsequent discussion. Although soil and regolith are terms that are sometimes used interchangeably, lunar soil usually refers to the fine portion of the regolith, operationally, the <1-mm grain- size fraction. Lunar soil is somewhat cohesive and dark gray to light gray in color. The mean grain size of lunar soils ranges from 40 to 800 µm and averages between 60 and 80 µm. Using conventional terrestrial descriptions, most lunar regolith samples would correspond to pebble- or cobble-bearing silty sands. Sorting values (the standard deviation of Folk 1968) range from 1.99 to 3.73 φ. In other words, lunar soils are poorly to very poorly sorted because mechanisms such as wind and water that sort terrestrial sediments do not occur on the Moon. There is also an inverse correlation between mean grain size and sorting (standard deviation); the coarsest samples are the most poorly sorted. Soils from all the Apollo sites are nearly symmetrically to coarsely skewed (skewness = 0–0.3). Exceptions to these generalizations are soils dominated by pyroclastic (volcanic) ash, which are finer-grained (mean grain sizes of ~40 µm) and better sorted (σ = 1.6 to 1.7 φ) than impact-produced soils. The lunar regolith consists of fragments of igneous intrusive and extrusive rocks, crystalline impact-melt rocks that texturally resemble igneous rocks, various types of crystalline and glassy breccias produced by meteoroid impacts, mineral grains derived from rocks and breccias as a result of impact processes, glassy and crystallized spherules of volcanic origin, impact glass (spherules, ropy glass, glass coatings, glass fragments), meteorites, meteoritic metal, and agglutinates. Agglutinates are a special and common lithic component of the lunar regolith, one that was unanticipated and that has no terrestrial analog, a direct result of the lack of an atmosphere on the Moon, which does not slow impinging micrometeorites. (Fig. 2.1). Each agglutinate particle is produced during the collision between a micrometeoroid traveling at 15–30 km/s and lunar regolith. Agglutinates, sometimes called glass-welded aggregates in early literature, consist of lithic and mineral fragments welded together by the glass formed as the small volume of resulting impact melt quenches. Agglutinates are mostly glassy, with complex morphologies and high vesicularity.