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The Scientific Context for Exploration of the Moon Executive Summary We know more about many aspects of the Moon than about any world beyond our own, and yet we have barely begun to solve its countless mysteries. The Moon is, above all, a witness to 4.5 billion years (Ga) of solar system history, and it has recorded that history more completely and more clearly than has any other planetary body. Nowhere else can we see back with such clarity to the time when Earth and the other terrestrial planets were formed and life emerged on Earth. Planetary scientists have long understood the Moon’s unique place in the evolution of rocky worlds. Many of the processes that have modified the terrestrial planets have been absent on the Moon. The lunar interior retains a record of the initial stages of planetary evolution. Its crust has never been altered by plate tectonics, which continually recycle Earth’s crust; or by planetwide volcanism, which resurfaced Venus only half a billion years ago; or by the action of wind and water, which have transformed the surfaces of both Earth and Mars. The Moon today presents a record of geologic processes of early planetary evolution in the purest form. Lunar science provides a window into the early history of the Earth-Moon system, can shed light on the evolution of other terrestrial planets such as Mars and Venus, and can reveal the record of impacts within the inner solar system. By dint of its proximity to Earth, the Moon is accessible to a degree that other planetary bodies are not. For these reasons, the Moon is priceless to planetary scientists. It remains a cornerstone for deciphering the histories of those more complex worlds. But because of the limitations of current data, researchers cannot be sure that they have read these histories correctly. Now, thanks to the legacy of the Apollo program and subsequent missions, such as Clementine and Lunar Prospector, and looking forward to the newly established Vision for Space Exploration (VSE),1 scientists are able to pose sophisticated questions that are more relevant and focused than those that could be asked over three decades ago. Only by returning to the Moon to carry out new scientific explorations can we hope to narrow the gaps in understanding and learn the secrets that the Moon alone has kept for eons. The Moon is not only of intrinsic interest as a cornerstone of the Earth-Moon system science, but it also provides a unique location for research in several other fields of science. The Moon’s surface is in direct contact with the interplanetary medium, and the interaction of the Moon with the solar wind plasma flowing from the Sun forms a unique plasma physics laboratory. Astronomical and astrophysical observations as well as observations of Earth, its atmosphere, ionosphere, and magnetosphere may be made from the stable platform of the Moon. The absence of a significant ionosphere on the Moon should enable low-frequency radio astronomy to be carried out, particularly from the farside of the Moon where radio interference from terrestrial sources should be absent. 1 National Aeronautics and Space Administration (NASA), The Vision for Space Exploration, NP-2004-01-334-HQ, NASA, Washington, D.C., 2004.
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The Scientific Context for Exploration of the Moon NASA asked the National Research Council (NRC) to provide guidance on the scientific challenges and opportunities enabled by a sustained program of robotic and human exploration of the Moon during the period 2008-2023 and beyond as the VSE evolves. This report was prepared by the Committee on the Scientific Context for Exploration of the Moon (brief biographies of the committee are presented in Appendix F). The framework of the VSE was changing while this report was being prepared. However, the committee believes that its scientific rationale for lunar science and its goals and recommendations are independent of any particular programmatic implementation. It is the unanimous consensus of the committee that the Moon offers profound scientific value. The infrastructure provided by sustained human presence can enable remarkable science opportunities if those opportunities are evaluated and designed into the effort from the outset. While the expense of human exploration cannot likely be justified on the basis of science alone, the committee emphasizes that careful attention to the science opportunity is very much in the interest of a stable and sustainable lunar program. In the opinion of the committee, a vigorous near-term robotic exploration program providing global access is central to the next phase of scientific exploration of the Moon and is necessary both to prepare for the efficient utilization of human presence and to maintain scientific momentum as this major national program moves forward. PRIORITIES, FINDINGS, AND RECOMMENDATIONS According to the committee’s statement of task (see Appendix A): The current study is intended to meet the near-term needs for science guidance for the lunar component of the VSE…. [T]he primary goals of the study are to: Identify a common set of prioritized basic science goals that could be addressed in the near-term via the LPRP2 program of orbital and landed robotic lunar missions (2008-2018) and in the early phase of human lunar exploration (nominally beginning in 2018); and To the extent possible, suggest whether individual goals are most amenable to orbital measurements, in situ analysis or instrumentation, field observation or terrestrial analysis via documented sample return. Also outlined in the statement of task are the overall science scope for this study and several secondary tasks. Overarching Themes The committee identified four overarching themes of lunar science: early Earth-Moon system, terrestrial planet differentiation and evolution, solar system impact record, and lunar environment. The committee then constructed eight science concepts that address broad areas of scientific research. Each is multicomponent and is linked to different aspects of the overarching themes of lunar science. The committee approached the challenge of prioritization by developing a hierarchy of priority categories. It used the prioritization criteria adopted by the decadal survey New Frontiers in the Solar System: An Integrated Exploration Strategy3 as a guideline: the criteria are scientific merit, opportunity, and technological readiness. The committee thus structured the prioritization of goals called for in the statement of task along three lines: (1) prioritization of science concepts, (2) prioritization of science goals, and (3) specific integrated high-priority recommendations. Although the rationales for these three are linked throughout the discussion of this report, the implementation requirements are different. As requested in the statement of task, the priorities and recommendations presented in this report relate to the near-term implementation of the VSE, which includes the robotic precursors and initial human excursions on the Moon. Planning for and implementing longer-term scientific activities on the Moon are beyond the scope of this study. 2 The Lunar Precursor and Robotic Program (LPRP) was how robotic missions were identified in the NASA letter that requested this study. The LPRP terminology is no longer in use. 3 National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.
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The Scientific Context for Exploration of the Moon Prioritized Science Concepts The committee evaluated only the scientific merit of each science concept in order to rank the concepts. It should be noted that all concepts discussed are viewed to be scientifically important. The science concepts are prioritized below and discussed in more detail in Chapter 3. The bombardment history of the inner solar system is uniquely revealed on the Moon. The structure and composition of the lunar interior provide fundamental information on the evolution of a differentiated planetary body. Key planetary processes are manifested in the diversity of lunar crustal rocks. The lunar poles are special environments that may bear witness to the volatile flux over the latter part of solar system history. Lunar volcanism provides a window into the thermal and compositional evolution of the Moon. The Moon is an accessible laboratory for studying the impact process on planetary scales. The Moon is a natural laboratory for regolith processes and weathering on anhydrous airless bodies. Processes involved with the atmosphere and dust environment of the Moon are accessible for scientific study while the environment remains in a pristine state. Prioritization of Science Goals Within the 8 science concepts above, the committee identified 35 specific science goals that can be addressed, at least in part, during the early phases of the VSE. For these science goals, the committee evaluated science merit as well as the degree to which they can be achieved within current or near-term technical readiness and practical accessibility. Within their respective science concepts, the science goals are listed in the order of their overall priority ranking (a through e) in Table 3.1 in Chapter 3. All 35 specific science goals were also evaluated and ranked as a group, separately from the science concepts with which they are associated. The highest-ranking lunar science goals are listed in Table 5.1 in Chapter 5 in priority order. For this group of goals the committee identifies possible means of implementation to achieve each goal. FINDINGS AND RECOMMENDATIONS Principal Finding: Lunar activities apply to broad scientific and exploration concerns. Lunar science as described in this report has much broader implications than simply studying the Moon. For example, a better determination of the lunar impact flux during early solar system history would have profound implications for comprehending the evolution of the solar system, early Earth, and the origin and early evolution of life. A better understanding of the lunar interior would bear on models of planetary formation in general and on the origin of the Earth-Moon system in particular. And exploring the possibly ice-rich lunar poles could reveal important information about the history and distribution of solar system volatiles. Furthermore, although some of the committee’s objectives are focused on lunar-specific questions, one of the basic principles of comparative planetology is that each world studied enables researchers to better understand other worlds, including our own. Improving our understanding of such processes as cratering and volcanism on the Moon will provide valuable points of comparison for these processes on the other terrestrial planets. Finding 1: Enabling activities are critical in the near term. A deluge of spectacular new data about the Moon will come from four sophisticated orbital missions to be launched between 2007 and 2008: SELENE (Japan), Chang’e (China), Chandrayaan-1 (India), and the Lunar Reconnaissance Orbiter (United States). Scientific results from these missions, integrated with new analyses of existing data and samples, will provide the enabling framework for implementing the VSE’s lunar activities. However, NASA and the scientific community are currently underequipped to harvest these data and produce meaningful
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The Scientific Context for Exploration of the Moon information. For example, the lunar science community assembled at the height of the Apollo program of the late 1960s and early 1970s has since been depleted in terms of its numbers and expertise base. Recommendation 1a: NASA should make a strategic commitment to stimulate lunar research and engage the broad scientific community4 by establishing two enabling programs, one for fundamental lunar research and one for lunar data analysis. Information from these two recommended efforts—a Lunar Fundamental Research Program and a Lunar Data Analysis Program—would speed and revolutionize understanding of the Moon as the Vision for Space Exploration proceeds. Recommendation 1b: The suite of experiments being carried by orbital missions in development will provide essential data for science and for human exploration. NASA should be prepared to recover data lost due to failure of missions or instruments by reflying those missions or instruments where those data are deemed essential for scientific progress. Finding 2: Strong ties with international programs are essential. The current level of planned and proposed activity indicates that almost every space-faring nation is interested in establishing a foothold on the Moon. Although these international thrusts are tightly coupled to technology development and exploration interests, science will be a primary immediate beneficiary. NASA has the opportunity to provide leadership in this activity, an endeavor that will remain highly international in scope. Recommendation 2: NASA should explicitly plan and carry out activities with the international community for scientific exploration of the Moon in a coordinated and cooperative manner. The committee endorses the concept of international activities as exemplified by the recent “Lunar Beijing Declaration” of the 8th ILEWG (International Lunar Exploration Working Group) International Conference on Exploration and Utilization of the Moon (see Appendix D). Finding 3: Exploration of the South Pole-Aitken Basin remains a priority. The answer to several high-priority science questions identified can be found within the South Pole-Aitken Basin, the oldest and deepest observed impact structure on the Moon and the largest in the solar system. Within it lie samples of the lower crust and possibly the lunar mantle, along with answers to questions on crater and basin formation, lateral and vertical compositional diversity, lunar chronology, and the timing of major impacts in the early solar system. Missions to South Pole-Aitken Basin, beginning with robotic sample returns and continuing with robotic and human exploration, have the potential to be a cornerstone for lunar and solar system research. (A South Pole-Aitken Basin sample-return mission was listed as a high priority in the 2003 NRC decadal survey report New Frontiers in the Solar System: An Integrated Exploration Strategy.5) Recommendation 3: NASA should develop plans and options to accomplish the scientific goals set out in the high-priority recommendation in the National Research Council’s New Frontiers in the Solar System: An Integrated Exploration Strategy (2003) through single or multiple missions that increase understanding of the South Pole-Aitken Basin and by extension all of the terrestrial planets in our solar system (including the timing and character of the late heavy bombardment). Finding 4: Diversity of lunar samples is required for major advances. Laboratory analyses of returned samples provide a unique perspective based on scale, precision, and flexibility of analysis and have permanence and ready accessibility. The lunar samples returned during the Apollo and 4 See also National Research Council, Building a Better NASA Workforce: Meeting the Workforce Needs for the National Vision for Space Exploration, The National Academies Press, Washington, D.C., 2007. 5 National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.
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The Scientific Context for Exploration of the Moon Luna missions dramatically changed understanding of the character and evolution of the solar system. Scientists now understand, however, that these samples are not representative of the larger Moon and do not provide sufficient detail and breadth to address the fundamental science concepts outlined in Table 3.1 in this report. Recommendation 4: Landing sites should be selected that can fill in the gaps in diversity of lunar samples. Mission plans for each human landing should include the collection and return of at least 100 kg of rocks from diverse locations within the landing region. For all missions, robotic and human, to improve the probability of finding new, ejecta-derived diversity among smaller rock fragments, every landed mission that will return to Earth should retrieve at least 1 kg of rock fragments 2 to 6 mm in diameter separated from bulk soil. Each mission should also return 100 to 200 grams of unfractionated regolith. Finding 5: The Moon may provide a unique location for observation and study of Earth, near-Earth space, and the universe. The Moon is a platform that can potentially be used to make observations of Earth (Earth science) and to collect data for heliophysics, astrophysics, and astrobiology. Locations on the Moon provide both advantages and disadvantages. There are substantial uncertainties in the benefits and the costs of using the Moon as an observation platform as compared with alternate locations in space. The present committee did not have the required span and depth of expertise to perform a thorough evaluation of the many issues that need examination. A thorough study is required. Recommendation 5: The committee recommends that NASA consult scientific experts to evaluate the suitability of the Moon as an observational site for studies of Earth, heliophysics, astronomy, astrophysics, and astrobiology. Such a study should refer to prior NRC decadal surveys and their established priorities. RELATED ISSUES The committee identified several related issues pertaining to optimal implementation of science in the VSE. This effort was driven by the stark realization that more than 30 years have passed since Apollo and that the nature of the VSE itself warrants a major reconsideration of the basic approach to conducting lunar science. In those more than 30 years, robotic capability has increased dramatically, analytical instrumentation has advanced remarkably, and the very understanding of how to explore has evolved as scientists have learned about planetary formation and evolution. The VSE offers new opportunity: there is no longer the limitation of short-duration lunar stays of 2 or 3 days and “emplacement science”; scientists on the Moon can operate as scientists, doing analytical work and deciphering sample/source relationships; site revisit with follow-up science is possible (e.g., an outpost); robotic-capable equipment can be used between missions; geophysical equipment can be used in survey modes; time-consuming deep drilling is possible; high-grade lunar samples can be selected for return to Earth. Nurturing a new approach to lunar exploration must be fostered if the potential of the VSE is to be reaped. Finding 1R: The successful integration of science into programs of human exploration has historically been a challenge. It remains so for the VSE. Prior Space Studies Board reports by the Committee on Human Exploration (CHEX) examined how the different management approaches led to different degrees of success. CHEX developed principles for optimizing the integration of science into human exploration and recommended implementation of these principles in future programs.6 This committee adopts in Recommendation 1R the CHEX findings in a form appropriate for the early phase of VSE. Recommendation 1R: NASA should increase the potential to successfully accomplish science in the VSE by (1) developing an integrated human/robotic science strategy,7 (2) clearly stating where science fits in the 6 See p. 128 of the third report in a series by the Committee on Human Exploration: National Research Council, Science Management in the Human Exploration of Space, National Academy Press, Washington, D.C., 1997. 7 This CHEX Recommendation 1 refers to the development of science goals, strategy, priorities, and process methodology; CHEX Recommendation 3 (and this committee’s Recommendation 1R) refers strictly to the implementation of science in a program of human exploration.
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The Scientific Context for Exploration of the Moon Exploration Systems Mission Directorate’s (ESMD’s) goals and priorities, and (3) establishing a science office embedded in the ESMD to plan and implement science in the VSE. Following the Apollo model, such an office should report jointly to the Science Mission Directorate and the ESMD, with the science office controlling the proven end-to-end science process. Finding 2R: Great strides and major advances in robotics, space and information technology, and exploration techniques have been made since Apollo. These changes are accompanied by a greatly evolved understanding of and approach to planetary science and improvements in use of remote sensing and field and laboratory sample analyses. Critical to achieving high science return in Apollo was the selection of the lunar landing sites and the involvement of the science community in that process. Similarly, the scientific community’s involvement in detailed mission planning and implementation resulted in efficient and productive surface traverses and instrument deployments. Recommendation 2R: The development of a comprehensive process for lunar landing site selection that addresses the science goals of Table 5.1 in this report should be started by a science definition team. The choice of specific sites should be permitted to evolve as the understanding of lunar science progresses through the refinement of science goals and the analysis of existing and newly acquired data. Final selection should be done with the full input of the science community in order to optimize the science return while meeting engineering and safety constraints. Similarly, science mission planning should proceed with the broad involvement of the science and engineering communities. The science should be designed and implemented as an integrated human/robotic program employing the best each has to offer. Extensive crew training and mission simulation should be initiated early to help devise optimum exploration strategies. Finding 3R: The opportunity provided by the VSE to accomplish science, lunar and otherwise, is highly dependent for success on modernizing the technology and instrumentation available. The virtual lack of a lunar science program and no human exploration over the past 30 years have resulted in a severe lack of qualified instrumentation suitable for the lunar environment. Without such instrumentation, the full and promising potential of the VSE will not be realized. Recommendation 3R: NASA, with the intimate involvement of the science community, should immediately initiate a program to develop and upgrade technology and instrumentation that will enable the full potential of the VSE. Such a program must identify the full set of requirements as related to achieving priority science objectives and prioritize these requirements in the context of programmatic constraints. In addition, NASA should capitalize on its technology development investments by providing a clear path into flight development. Finding 4R: The NASA curatorial facilities and staff have provided an exemplary capability since the Apollo program to take advantage of the scientific information inherent in extraterrestrial samples. The VSE has the potential to add significant demands on the curatorial facilities. The existing facilities and techniques are not sufficient to accommodate that demand and the new requirements that will ensue. Similarly, there is a need for new approaches to the acquisition of samples on lunar missions. Recommendation 4R: NASA should conduct a thorough review of all aspects of sample curation, taking into account the differences between a lunar outpost-based program and the sortie approach taken by the Apollo missions. This review should start with a consideration of documentation, collection, and preservation procedures on the Moon and continue to a consideration of the facilities requirements for maintaining and analyzing the samples on Earth. NASA should enlist a broad group of scientists familiar with curatorial capabilities and the needs of lunar science, such as the Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM), to assist it with the review.
Representative terms from entire chapter: