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Introduction

WHY LUNAR SCIENCE?

We know more about many aspects of the Moon than we know about any world beyond our own, and yet we have barely begun to solve its countless mysteries. In the decades since the last Apollo landing on the Moon in 1972, there has been a widespread misperception that the Moon has already told us all the important things that it has to tell, that scientifically it is a “been there, done that” world. Nothing could be farther from the truth.

The Moon is, above all, a witness to 4.5 billion years 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—Mercury, Venus, an Mars—were formed and life emerged on Earth.

Planetary scientists have long understood the Moon’s unique significance as the starting point in the continuum of 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. Its airless surface also provides a continuous record of solar-terrestrial processes.

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 samples and data derived from them, researchers cannot be sure that they have read these histories correctly. Now, thanks to the legacy of the Apollo program, it is possible to pose sophisticated questions that are more relevant and focused than those that could be asked more than three decades ago. Only by returning to the Moon to carry out new scientific explorations can we hope to close the gaps in understanding and learn the secrets that the Moon alone has kept for eons.

OVERARCHING THEMES

Through NASA’s Vision for Space Exploration (VSE)1 the nation is embarking on a challenging and inspirational journey to the Moon, Mars, and beyond. This report focuses on the scientific context for exploration of

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 1 Introduction WHY LUNAR SCIENCE? We know more about many aspects of the Moon than we know about any world beyond our own, and yet we have barely begun to solve its countless mysteries. In the decades since the last Apollo landing on the Moon in 1972, there has been a widespread misperception that the Moon has already told us all the important things that it has to tell, that scientifically it is a “been there, done that” world. Nothing could be farther from the truth. The Moon is, above all, a witness to 4.5 billion years 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—Mercury, Venus, an Mars—were formed and life emerged on Earth. Planetary scientists have long understood the Moon’s unique significance as the starting point in the continuum of 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. Its airless surface also provides a continuous record of solar-terrestrial processes. 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 samples and data derived from them, researchers cannot be sure that they have read these histories correctly. Now, thanks to the legacy of the Apollo program, it is possible to pose sophisticated questions that are more relevant and focused than those that could be asked more than three decades ago. Only by returning to the Moon to carry out new scientific explorations can we hope to close the gaps in understanding and learn the secrets that the Moon alone has kept for eons. OVERARCHING THEMES Through NASA’s Vision for Space Exploration (VSE)1 the nation is embarking on a challenging and inspirational journey to the Moon, Mars, and beyond. This report focuses on the scientific context for exploration of 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 the Moon, especially in the early phases of implementation of the VSE. The exploration of the Moon is a rich and fruitful endeavor with many facets. The scientific context for the lunar science discussed throughout this report encompasses the four following overarching themes (see Figure 1.1), which are fundamentally important to solar system science, including the history of Earth: Early Earth-Moon System: The compositional and thermal histories of both the Moon and Earth were closely linked 4.5 billion years ago, after which each evolved separately. A prevailing scientific hypothesis is that the Moon was formed from debris of a collision of a Mars-sized body with the early Earth. How, when, and why did the two parts of the Earth-Moon system take different paths and how have they influenced one another? Terrestrial Planet Differentiation and Evolution: The Moon is a small planetary body that differentiated into crust, mantle, and core within a few hundred million years after formation. A magma ocean hypothesis describes this early process in terms of fractional differentiation of an initial globe-circling ocean of magma. What are the complexities of this fundamental process, and how can the lunar model be used to understand other rocky planets? Solar System Impact Record: Since its birth 4.5 billion years ago, the Moon has experienced the full force of early and late bombardment of solar system debris. Regarding early bombardment: A terminal cataclysm hypothesis holds that a burst of large impacts occurred on the Moon (and the inner solar system) about 4.0 billion years ago, which, if confirmed, provides important constraints on the evolution of terrestrial planets and the origin and evolution of life on Earth. Regarding late bombardment: After formation of the planets, the frequency of impacts gradually decreased, perhaps punctuated by occasional periods of increased impacts. The early impact record on Earth has been largely destroyed by erosion and plate tectonics, but it is well preserved on the Moon. What is the history of impact events throughout the past 4.0 billion years that is recorded on the Moon? Lunar Environment: The surface of the Moon is accessible and special. The lunar atmosphere, though tenuous, is the nearest example of a surface boundary exosphere, the most common type of satellite atmosphere FIGURE 1.1 Lunar science encompasses four overarching themes of solar system exploration.

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The Scientific Context for Exploration of the Moon in the solar system. In a near-vacuum environment with 1/6 the gravity of Earth, the Moon’s regolith accumulates products produced over billions of years by exposure to solar and galactic radiation and space plasmas. These products have scientific value and may well have practical value. The frigid lunar poles contain volatiles, perhaps abundant, that may provide information to characterize the sources of volatiles in the early solar system. All of the overachieving themes are involved in understanding the character and history of the environment at 1 astronomical unit shared by Earth and the Moon. Particularly relevant are implications for the evolution of Earth and the conditions that constrained the formation and evolution of life on Earth. STRUCTURE OF THIS REPORT The overarching themes presented above permeate the subjects of Chapters 2 through 5 of this report. An overview of the current understanding of the Earth-Moon system is provided in Chapter 2. The achievements of Apollo era exploration led to several hypotheses about the relation between Earth and the Moon. However, with small amounts of additional data and more sophisticated analytical and computational tools, which have become available in the past few decades, several paradigms have been (or are in the process of being) revised. The central science concepts addressed by lunar exploration are discussed in Chapter 3. Several specific science goals that can be addressed in the early phases of the VSE are identified for each concept. Implementation options and opportunities for addressing the science concepts are summarized in Chapter 4, as are plans of other nations for extensive lunar robotic exploration. Prioritization criteria for science that can be achieved in the early phases of the VSE are discussed in Chapter 5. While science concepts are prioritized on scientific merit, additional criteria—namely, the availability of opportunities for research and the technological readiness—are used to prioritize individual science goals. Findings and recommendations that envelop and support the prioritization of individual goals are also presented in that chapter. As the VSE proceeds, the Moon may also provide a unique location for research in several other fields of science, serving as a stable platform for astronomical and astrophysical observations as well as observations of Earth, its atmosphere, ionosphere, and magnetosphere. In addition, there will be opportunities for expanded activities from lunar orbit and at other locations as a result of new launch vehicles. Chapter 6 describes opportunities for research in astronomy and astrophysics and for observations of Earth and its magnetosphere that can take advantage of our return to the Moon. Since the VSE provides the focus for NASA’s activities over the next several decades, there are several additional but related concepts and goals that need attention so as to maximize the efficiency of human and robotic scientific interaction and to optimize the scientific returns from all aspects of lunar research. Issues such as program management and coordination, planning, operations, technology, and the development of facilities all affect the health of the overall VSE undertaking as well as science. Several findings and recommendations related to these issues are offered in Chapter 7. Concluding remarks and the principal finding of the report are presented in Chapter 8.