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Space Studies Board Search: Jump to Top NewsJump to Science in the Subscribe to our FREE e- Headlines newsletter! NATIONAL ACADEMY OF SCIENCES NATIONAL ACADEMY OF ENGINEERING INSTITUTE OF MEDICINE NATIONAL RESEARCH COUNCIL June 18, 2004 Current Operating Status 5 The Moon PROGRESS The Moon is far more complicated than was generally appreciated when the 1978 report was written. This appreciation of the complexity of lunar history has come from continued study of lunar samples (including meteorites from the Moon), ground-based remote sensing data, sophisticated use of the Apollo orbital remote sensing data, and general advances in our understanding of geological and geophysical processes. Besides being intrinsically interesting in its own right, the Moon provides a unique window into solar system history. Its origin is intertwined with that of Earth, its craters preserve a record of meteoroid fluxes through time, and it preserves at least a fragmentary record of its early (pre-four-billion-year) evolution. It is one testing ground for our ideas about the origin and evolution of small planets. The Moon is an ideal body on which to study the processes, such as exogenic impacts, that have shaped the other solid bodies in the solar system and perhaps even caused extinctions of some forms of life on Earth. The Moon is also the only extraterrestrial body from which we have samples from a known geologic context, thereby providing a much more quantitative understanding of its history. The lunar soil preserves a four-billion-year-old record of the Sun's history. Finally, the Moon is a readily accessible body, making its scientific exploration easier to achieve. Lunar science has evolved in several stages. The late 1960s and early 1970s focused largely on surface exploration, centering on the Apollo program. The mid- 1970s through the 1980s have emphasized reflection on those results in the context of new concepts and continued analysis. The future will address unsolved problems in lunar science and prepare for advanced studies from a lunar base. During the mid-1970s, there was no consensus about how the Moon formed. Now the idea that it originated as a result of a giant impact on Earth has caught hold and has passed the tests given it so far. Nevertheless, this hypothesis is far from proven; assessing it requires a better understanding of the Moon's bulk chemical composition. http://www7.nationalacademies.org/ssb/innerch5.html (1 of 4) [6/18/2004 10:06:51 AM]
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Space Studies Board The idea that the primitive Moon was surrounded by an immense magma system known as the "magma ocean" continues to be a central tenet of lunar science, but vigorous debate is taking place about its nature and the processes that operated in it. Some investigators are even questioning whether there was a magma ocean. Proof of the magma ocean hypothesis hinges on the composition of the Moon's crust and the nature and ages of lunar anorthosites. Whether there was a magma ocean or not, it has become clear that there was a period prior to four billion years ago of intense igneous activity that modified the primordial lunar crust. In contrast to the narrow range of rock types defined during the mid-1970s, continued sample analysis has revealed a vast array of rock types in the lunar highlands, and remote sensing has shown that rock types rare in the Apollo collection are nevertheless abundant on the Moon. We need many more data from remote sensing and sample returns to determine the full range of rock types and how they relate to each other and to the products of the magma ocean. Our understanding of mare basalts has advanced tremendously. After the Apollo missions ended, it was generally believed that mare basalt volcanism took place 3.2 to 3.8 billion years ago. Subsequent photogeological and lunar sample studies have shown that this type of volcanism occurred over a much greater time period, from 4.3 to possibly 1.0 billion years ago. This discovery has great implications for the Moon's thermal history. Furthermore, we now know that we sampled only about half of the full range of mare basalts. Because basalts contain information about the interior, we have an incomplete knowledge of the nature of the lunar mantle. The time and rate of formation of large craters and the great lunar basins are still uncertain. During the mid-1970s, the consensus was that they formed during a narrow time interval 3.85 to 4.0 billion years ago—the so-called lunar cataclysm. The consensus now is that the bombardment rate declined gradually and that only a few basins, which we happened to sample during Apollo, formed during the period from 3.85 to 4.0 billion years ago. The question will remain open, however, until samples are obtained and dated from basins far removed from those on the near side. In addition, the compositions and sources of the projectiles that made large craters and basins are largely unknown. Understanding these factors will contribute to our understanding of the later stages of planetary accretion with implications for the early history of the Earth. Unanswered questions about the Moon abound. These problems can be addressed by global surveys from orbit, installation of a network of geophysical instruments, sample-return missions, and detailed field studies from a lunar base. SCIENTIFIC OBJECTIVES It is very important that the Moon's entire surface be adequately imaged and mapped geochemically, mineralogically, and geophysically. To meet this requirement, COMPLEX recommends that a spacecraft or series of them, be placed in a lunar polar orbit. The measurements would benefit greatly if two http://www7.nationalacademies.org/ssb/innerch5.html (2 of 4) [6/18/2004 10:06:51 AM]
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Space Studies Board spacecraft were in orbit simultaneously. The second spacecraft, which could be provided by another nation, would allow electromagnetic sounding of the interior and mapping of the far-side gravity field. Besides contributing to the solution of fundamental questions in lunar science, orbital measurements will provide critical information about where to locate a lunar base, regions containing potential resources, sites for sample-return missions and intensive field work, and emplacement of a network of geophysical stations. To contribute to significant advances in lunar research, orbital measurements ought to include the following: 1. Abundances of major rock-forming elements (O, SL Fe, Mg, Al, Ti, and Ca) and of selected minor and trace elements (K, U, and Th). This would yield the average composition of the surface and, if basins were used as natural drill holes, an estimate of the chemical composition of deeper crustal layers. 2. Spectroscopic .measurements to obtain mineralogical and chemical data at high spectral and spatial (<500-m) resolution. This would provide information on the distribution of lunar rock types. 3. Topographic data (combined with gravity data) to. address problems involving density distributions and lithospheric loading. 4. Measurement of the gravity field on both the near and far sides to allow calculation of crustal thicknesses and densities. 5. Measurements of the Moon's magnetic field to shed light on the characteristics and origins of magnetic anomalies and to place constraints on the size of the core. 6. Imaging data to obtain global, digital photographic coverage with a line-pair resolution of 25 m and high resolution (1 m) of selected areas. These data would provide information about impact cratering, volcanism, and tectonism and would provide the geologic context needed to interpret other types of data. 7. Measurement of the average global surface heat flow to constrain the Moon's thermal history and provide indirect measurement of the bulk content of heat- producing elements. A thorough understanding of the Moon will be impossible without knowledge of its interior. Better constraints on the size of the core will shed light on the origin of the remanent lunar magnetic field and, hence, on the origin of planetary dynamos in general. This will require the installation of a geophysical network of at least eight stations. Each station should include a seismometer, heat-flow probe, and atmospheric sensors. Such a seismic network would also be able to monitor the present meteoroid flux, using the entire Moon as a collecting surface. Deployment scenarios include automated landers or rovers, penetrators, or, eventually, astronauts. COMPLEX recommends that NASA develop the technology to deploy http://www7.nationalacademies.org/ssb/innerch5.html (3 of 4) [6/18/2004 10:06:51 AM]
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