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Origin and Evolution of Earth: Research Questions for a Changing Planet Summary Modern science has its roots in fundamental questions about the origins of Earth and life. These grand questions are recorded in texts of the ancient Greeks, who laid the foundations of Earth science and whose language provides many of its terms. Analytical approaches to answering these questions date back to the 16th century for planetary science and the 18th century for geological science. Perhaps the first, and certainly one of the most controversial, of the more modern grand research questions in geology came from observations of sedimentary rocks. The thickness of sedimentary beds, their variable character and structures, and the presence of fossils within them led James Hutton to conclude that Earth must be very old (Hutton, 1788). The age of Earth became the ultimate grand question of the time. But not until almost 200 years later—after it was established that matter was made of atoms, that atoms had nuclei, and that some of those nuclei were unstable to radioactive decay—was it possible to establish the scale of geological time. The first accurate measurement of Earth’s age, 4.55 billion years, made in the mid-1950s (Patterson, 1956), was a major step in establishing a timescale for Earth, for life, and for the Universe. Until the 1960s, geological science was built almost entirely on the study of rocks and landforms on the continents; little was known about the seafloor. The grand research questions of the early 20th century were heavily influenced by this continent-centric view, as well as by a focus on mineral and water resources and discoveries in paleontology. There were grand questions about how volcanoes, mountain ranges, and sedimentary basins were created; why mineral deposits and petroleum deposits formed where and when they did; how fast mountains were built and eroded away; why fossils first became abundant only 500 million years ago; and what caused ice ages and earthquakes. An additional tantalizing question was why the Atlantic coastlines of South America and Africa looked like they were pieces of a puzzle that might once have been joined together. This seemingly unconnected set of grand questions of the mid-20th century were largely organized and linked by the advent of plate tectonics theory. In just half a decade, between 1963 and 1968, spurred largely by the first observations of the magnetism and depth of the seafloor, a grand picture of the dynamic behavior of the planet emerged. It was deduced that Earth’s surface consists of a dozen or so irregular, stiff plates that move a few centimeters per year and that the boundaries of these plates are the locations of earthquakes, volcanoes, and mountain ranges. The plate movements are connected to a planetwide system of solid-state convection deep within Earth, an idea that was inconceivable to most geologists a decade before. The plate tectonics model, including its corollaries of mantle convection, seafloor spreading, and continental drift, not only explained the pattern of earthquakes, volcanoes, and mountain ranges but also eventually provided possible mechanisms to create the continents and seafloor, to gradually shift Earth’s climate over geological time, and to influence the course of biological evolution. Toward the end of this watershed period of the 1960s, the United States landed the first astronauts
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Origin and Evolution of Earth: Research Questions for a Changing Planet on the Moon, who brought back rock samples that provided a glimpse of another planetary body much different from Earth. This new perspective ushered in the modern era where Earth is viewed as a planet and its constitution, history, and character are compared to those of other planets. In 1980 another breakthrough came from evidence that Earth was struck by a large meteoroid 65 million years ago and that the impact probably caused the extinction of dinosaurs and many of the other living things on the planet at the time (Alvarez et al., 1980). Within a few years it became evident that some meteorites found on Earth came from Mars (Bogard and Johnson, 1983). These two developments underscored the idea, which had begun with studies of impact craters on Earth and the Moon, that Earth must be viewed in its astronomical context; for example, life could be terminated by uninvited extraterrestrial objects or imported from other Solar System planets! Over the past 20 years the transformation of Earth science has continued. Major advances in technology that allow Earth to be observed much better at both large and small scales, continuing planetary exploration, and advanced computing have all contributed. We can now see into minerals and discern individual atoms, measure the properties of rocks at the immense pressures and temperatures inside Earth, watch continents drift and mountains grow in real time, and understand how organisms evolve and interact with Earth based on their DNA. We have also been able to extract new information from meteorites that tells us about how planets form and even about how the interiors of stars work. Armed with new tools, Earth science is turning to the deeper fundamental questions—the origin of Earth; the origin of life; the structure and dynamics of planets; the connections between life, climate, and Earth’s interior; and what the Earth may hold for humankind in the future. SCOPE AND PURPOSE OF THIS REPORT At the request of the U.S. Department of Energy, the National Science Foundation, the U.S. Geological Survey, and the National Aeronautics and Space Administration, the National Academies established a committee to propose and explore grand Earth science questions being pursued today. The charge to the committee, given below, provided unusual freedom in the selection of topics, without regard to agency-specific issues, such as mission relevance and implementation. The committee will formulate a short list of grand research questions driving progress in the solid-Earth sciences. The research questions will cover a variety of spatial scales and temporal scales, from subatomic to planetary and from the past (billions of years) to the present and beyond. The questions will be written in a clear, compelling way and will be supported by text and figures that summarize progress to date and outline future challenges. This report will not discuss implementation issues (e.g., facilities, recommendations aimed at specific agencies) or disciplinary interests. Our response to this charge has been to attempt to capture the scope and aspirations of what might best be referred to as geological and planetary science, which is another way of saying solid-Earth science. Research in this area draws on nearly every scientific discipline. However, research questions that are mainly the domain of other subdisciplines of Earth science—such as ocean, atmospheric, or space science—are discussed to the extent they are linked to solid-Earth science. The committee began by developing criteria for what constitutes a “grand” question. Our definition of grand questions was partly determined by the small number requested in the charge, which led us to aim for 7 to 10 questions, and partly by a desire for the questions to meet at least two of the following criteria: it transcends the boundaries of a narrow subfield of geological and planetary science; it deals with eternal issues, such as the origins of Earth and life; it is connected with phenomena that have significant impact on human well-being. Our ultimate objective was to capture in this series of questions the essential scientific issues that constitute the frontier of Earth science at the start of the 21st century. It is our hope that these questions and our descriptions of them are as compelling as we believe the science to be and that this short report is useful to those who would like to understand more about where Earth science stands, how it got there, and where it might be headed. We have attempted to make the text accessible to managers of scientific programs, graduate students, and colleagues in sister disciplines who have
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Origin and Evolution of Earth: Research Questions for a Changing Planet the technical or scientific background needed to comprehend what is discussed. Our most difficult problem in selecting the grand questions was to distill from a large number of topics and questions the “most worthy” candidates. To do so the committee canvassed the broad geological community and deliberated in meetings and telephone conferences. After arriving at 10 grand questions, the committee set about writing, as well as soliciting written contributions from other scientists. Some of our questions present truly awesome challenges and may not be fully understood for decades, if ever. Others seem more tractable, and significant progress may be made in a matter of years. Overall, we have included most of what the committee regards as the important issues and also most of what was suggested by the respondents to our canvassing effort. There was, in fact, a fair degree of consensus about what constitutes a grand question and which ones should be included here. GRAND RESEARCH QUESTIONS FOR THE 21ST CENTURY Although we started by simply identifying the overarching questions we believe to be driving modern Earth science, we found that these questions can be grouped into four broad themes. These themes constitute the four chapters of the report, and within each chapter are descriptions of the grand questions. Chapter 1 deals with origins—the origin of Earth and other Solar System planets, Earth’s earliest history, and the origin of life. Chapter 2 treats the workings of Earth’s interior and its surface manifestations and includes a question on material properties and their fundamental role in Earth processes. Chapter 3 addresses the habitability of the surface environment—climate and climate change and Earth–life interactions. Chapter 4 focuses on geologica10 hazards and Earth resources—earthquakes and volcanoes and modern environmental issues associated with water and other fluids in and on Earth. The following is a summary of the 10 grand research questions identified by the committee: How did Earth and other planets form? The Solar System, with its tantalizing geometric patterns and its wide variety of planets and moons, presents intriguing questions that become more nuanced as we make new observations from spacecraft and more exacting measurements on meteorites. While it is generally agreed that the Sun and planets all coalesced out of the same nebular cloud, it is still not known how Earth obtained its particular chemical composition, at least not in enough detail to understand its subsequent evolution or why the other planets ended up so different from ours and from each other. Earth, for example, has retained a life-giving inventory of volatile substances, including water, but Earth is far different from every other planet in this regard. Advanced computing capabilities are enabling development of more credible models of the early Solar System, but further measurements of other Solar System bodies and extrasolar planets and objects appear to be the primary pathway to furthering our understanding of the origin of Earth and the Solar System. What happened during Earth’s “dark age” (the first 500 million years)? It is now believed that in the later stages of Earth’s formation, a Mars-sized planet collided with it, displacing a huge cloud of debris that became our Moon. This collision added so much heat to Earth that the entire planet melted. Little is known about how this magma soup differentiated into the core, mantle, and lithosphere of today or how Earth developed its atmosphere and oceans. The so-called Hadean Eon is a critical link in our understanding of planetary evolution, but we have little information about it because there are almost no rocks of this age preserved on Earth. Clues about this time period are accumulating, however, as we learn more about meteorites and other planets and extract new information from ancient crystals of zircon on Earth. How did life begin? The origin of life is one of the most intriguing, difficult, and enduring questions in science. Because life in the Solar System arose billions of years ago, some of the most fundamental questions about its origin are geological. Our knowledge of the materials from which life originated, and where, when, and in what form it first appeared, stems from geological investigations of rocks and minerals that represent the only remaining evidence. When life first arose, the conditions at Earth’s surface may have been much different than today’s, and one critical challenge is to de-
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Origin and Evolution of Earth: Research Questions for a Changing Planet velop an accurate picture of the physical environments and the chemical building blocks available to early life. The quest to establish the origin of life is inherently multidisciplinary, spanning organic chemistry, molecular biology, astronomy, and planetary science, as well as geology and geochemistry. There is growing interest in studying Mars, where there is a sedimentary record of early planetary history that predates the oldest Earth rocks and other star systems where planets have been detected. How does Earth’s interior work, and how does it affect the surface? As planets age, they gradually cool, and this causes them to move through stages where their internal processes, their atmospheres, and their surface processes are gradually changing. The primary means by which heat is moved from the interior to the surface is planetwide solid-state and liquid convection. Although we know that the mantle and core are in constant convective motion, we can neither precisely describe these motions today nor calculate with confidence how they were different in the past. Core convection produces Earth’s magnetic field, which may have had an important influence on surface conditions. Mantle convection is the cause of volcanism, seafloor generation, and mountain building, and materials like water and carbon are constantly exchanged between Earth’s surface and its deep interior. Consequently, without detailed knowledge of Earth’s internal processes we cannot deduce what Earth’s surface environment was like in the past or predict what it will be in the future. Why does Earth have plate tectonics and continents? The questions regarding plate tectonics now have less to do with the soundness of the theory than with why Earth has plate tectonics in the first place and how closely it is related to other unique aspects of Earth—the abundant water, the existence of continents and oceans, and the existence of life. We do not know whether it is possible to have one aspect without the others or how they are interdependent. The existence and persistence of continental crust present problems as fundamental as those of plate tectonics. Continental crust makes the planet habitable by nonmarine life, and weathering of its surface plays a role in regulating Earth’s climate. But we still do not know when continents first formed, how they are preserved for billions of years, or exactly how they evolved to be what they are like today. New data and observations indicate that climate and erosion play a fundamental role in building and shaping mountain ranges and thus are fundamental to the formation as well as the destruction of continental crust. How are Earth processes controlled by material properties? Deciphering the secrets of the rock record on Earth and other planets begins with the understanding of large-scale geological processes. The keys to understanding these processes are the basic physics and chemistry of planetary materials. The high pressures and temperatures of Earth’s interior, the enormous size of Earth and its structures, the long expanse of geological time, and the vast diversity of materials and properties all present special challenges. These challenges are being met with new research tools based on synchrotron radiation, new measurements and simulation capabilities for large domains and heterogeneous materials, and quantum mechanics-based calculations of material properties under extreme conditions. New research areas are developing around the study of natural nanoparticles and the mediation of chemical processes by microorganisms. What causes climate to change—and how much can it change? Global climate conditions have been favorable and stable for the past 10,000 years, but we also know from geological evidence that momentous changes in climate can occur in periods as short as decades or centuries. Yet despite the numerous factors that can change climate, from the slowly changing luminosity of the Sun to the building of new mountain ranges and changes in atmospheric composition, Earth’s surface temperature seems to have remained within relatively narrow limits for most of the past 4 billion years. How does it remain well regulated in the long run, even though it can change so abruptly? Recent discoveries have highlighted periods of Earth history when the climate was extremely cold, was extremely hot, or changed especially quickly. Understanding these special conditions may lead to new insights about Earth’s climate, as will new geochemical observa-
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Origin and Evolution of Earth: Research Questions for a Changing Planet tions made on ancient sedimentary rocks and improved models for the climate system that will eventually enable us to predict the magnitude and consequences of climate changes. How has life shaped Earth—and how has Earth shaped life? Earth scientists have a tendency to view Earth’s geological evolution as a fundamentally inorganic process. Life scientists, in the same spirit, tend to regard the evolution of life as a fundamentally biological issue. Yet the development of life has clearly been influenced by the conditions of Earth’s surface, while Earth’s surface has been influenced by the activities of life forms. The atmosphere would not contain oxygen if it were not for life, and the presence of oxygen has enabled other types of life to evolve. We know that geological events and meteoroid impacts have caused massive extinctions in the past and influenced the course of evolution. But the exact ties between geology and evolution are still elusive. On the modern Earth we are interested in the role of life in geological processes like weathering and erosion. And we seek to understand how life may have manifested itself and left traces preserved in the geological records of other planets. Can earthquakes, volcanic eruptions, and their consequences be predicted? Thanks largely to sensitive new instrumentation and better understanding of causes, geologists are moving toward predictive capabilities for volcanic eruptions. For earthquakes, progress has been made in long-term forecasts, but we may never be able to predict the exact time and place an earthquake will strike. Continuing challenges are to deepen our understanding of how fault ruptures start and stop, to improve our simulations of how much shaking can be expected near large earthquakes, and to increase the warning time once a dangerous earthquake begins. Studies of volcanic activity have entered a new era as a result of real-time seismic, geodetic, and electromagnetic probes of active subsurface processes. But it remains a challenge to integrate such real-time data with field studies of volcanoes and laboratory studies of volcanic materials. The ultimate objective is to develop a clear picture of the movement of magma, from its sources in the upper mantle to Earth’s crust, where it is temporarily stored, and ultimately to the surface where it erupts. How do fluid flow and transport affect the human environment? Good management of natural resources and the environment requires knowledge of the behavior of fluids, both below ground and at the surface. The major scientific objectives are to understand how fluids flow, how they transport materials and heat, and how they interact with and modify their surroundings. New experimental tools and field measurement techniques, plus airborne and spaceborne measurements, are offering an unprecedented view of processes that affect both the surface and the subsurface. But we still have difficulty determining how subsurface fluids are distributed in heterogeneous rock and soil formations, how fast they flow, how effectively they transport dissolved and suspended materials, and how they are affected by chemical and thermal exchange with the host formations. Much better models of streamflow and associated erosion and transport are needed if we are to accurately assess how human impacts and climate change affect landscape evolution and how these effects can be managed to sustain ecosystems and important watershed characteristics. The ultimate objective—to produce mathematical models that can predict the performance of natural systems far into the future—is still out of reach but critical to making informed decisions about the future of the land and resources that support us.
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