Origin and Evolution of Earth: Research Questions for a Changing Planet (2008)

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Summary M odern science has its roots in fundamental sedimentary basins were created; why mineral deposits questions about the origins of Earth and and petroleum deposits formed where and when they life. These grand questions are recorded in did; how fast mountains were built and eroded away; texts of the ancient Greeks, who laid the foundations of why fossils first became abundant only 500 million Earth science and whose language provides many of its years ago; and what caused ice ages and earthquakes. terms. Analytical approaches to answering these ques- An additional tantalizing question was why the Atlan- tions date back to the 16th century for planetary science tic coastlines of South America and Africa looked like and the 18th century for geological science. Perhaps the they were pieces of a puzzle that might once have been first, and certainly one of the most controversial, of the joined together. more modern grand research questions in geology came This seemingly unconnected set of grand questions from observations of sedimentary rocks. The thick- of the mid-20th century were largely organized and ness of sedimentary beds, their variable character and linked by the advent of plate tectonics theory. In just structures, and the presence of fossils within them led half a decade, between 1963 and 1968, spurred largely James Hutton to conclude that Earth must be very old by the first observations of the magnetism and depth of (Hutton, 1788). The age of Earth became the ultimate the seafloor, a grand picture of the dynamic behavior of grand question of the time. But not until almost 200 the planet emerged. It was deduced that Earth’s surface years later—after it was established that matter was consists of a dozen or so irregular, stiff plates that move made of atoms, that atoms had nuclei, and that some a few centimeters per year and that the boundaries of of those nuclei were unstable to radioactive decay—was these plates are the locations of earthquakes, volcanoes, it possible to establish the scale of geological time. The and mountain ranges. The plate movements are con- first accurate measurement of Earth’s age, 4.55 billion nected to a planetwide system of solid-state convection years, made in the mid-1950s (Patterson, 1956), was a deep within Earth, an idea that was inconceivable to major step in establishing a timescale for Earth, for life, most geologists a decade before. and for the Universe. The plate tectonics model, including its corollaries Until the 1960s, geological science was built almost of mantle convection, seafloor spreading, and continen- entirely on the study of rocks and landforms on the tal drift, not only explained the pattern of earthquakes, continents; little was known about the seafloor. The volcanoes, and mountain ranges but also eventually grand research questions of the early 20th century provided possible mechanisms to create the continents were heavily influenced by this continent-centric view, and seafloor, to gradually shift Earth’s climate over geo- as well as by a focus on mineral and water resources logical time, and to influence the course of biological and discoveries in paleontology. There were grand evolution. Toward the end of this watershed period of questions about how volcanoes, mountain ranges, and the 1960s, the United States landed the first astronauts  

 ORIGIN AND EVOLUTION OF EARTH on the Moon, who brought back rock samples that mittee, given below, provided unusual freedom in the provided a glimpse of another planetary body much selection of topics, without regard to agency-specific different from Earth. This new perspective ushered in issues, such as mission relevance and implementation. the modern era where Earth is viewed as a planet and The committee will formulate a short list of grand its constitution, history, and character are compared to research questions driving progress in the solid-Earth those of other planets. sciences. The research questions will cover a variety of In 1980 another breakthrough came from evidence spatial scales and temporal scales, from subatomic to that Earth was struck by a large meteoroid 65 million planetary and from the past (billions of years) to the present and beyond. The questions will be written in a years ago and that the impact probably caused the clear, compelling way and will be supported by text and extinction of dinosaurs and many of the other living figures that summarize progress to date and outline fu- things on the planet at the time (Alvarez et al., 1980). ture challenges. This report will not discuss implemen- Within a few years it became evident that some me- tation issues (e.g., facilities, recommendations aimed at teorites found on Earth came from Mars (Bogard and specific agencies) or disciplinary interests. Johnson, 1983). These two developments underscored Our response to this charge has been to attempt to the idea, which had begun with studies of impact cra- capture the scope and aspirations of what might best be ters on Earth and the Moon, that Earth must be viewed referred to as geological and planetary science, which in its astronomical context; for example, life could be is another way of saying solid-Earth science. Research terminated by uninvited extraterrestrial objects or im- in this area draws on nearly every scientific discipline. ported from other Solar System planets! However, research questions that are mainly the do- Over the past 20 years the transformation of Earth main of other subdisciplines of Earth science—such as science has continued. Major advances in technology ocean, atmospheric, or space science—are discussed to that allow Earth to be observed much better at both the extent they are linked to solid-Earth science. large and small scales, continuing planetary exploration, The committee began by developing criteria for and advanced computing have all contributed. We can what constitutes a “grand” question. Our definition of now see into minerals and discern individual atoms, grand questions was partly determined by the small measure the properties of rocks at the immense pres- number requested in the charge, which led us to aim for sures and temperatures inside Earth, watch continents 7 to 10 questions, and partly by a desire for the ques- drift and mountains grow in real time, and understand tions to meet at least two of the following criteria: how organisms evolve and interact with Earth based on their DNA. We have also been able to extract new • it transcends the boundaries of a narrow subfield information from meteorites that tells us about how of geological and planetary science; planets form and even about how the interiors of stars • it deals with eternal issues, such as the origins of work. Armed with new tools, Earth science is turning Earth and life; to the deeper fundamental questions—the origin of • it is connected with phenomena that have sig- Earth; the origin of life; the structure and dynamics nificant impact on human well-being. of planets; the connections between life, climate, and Earth’s interior; and what the Earth may hold for Our ultimate objective was to capture in this series humankind in the future. of questions the essential scientific issues that constitute the frontier of Earth science at the start of the 21st SCOPE AND PURPOSE OF THIS REPORT century. It is our hope that these questions and our descriptions of them are as compelling as we believe At the request of the U.S. Department of Energy, the science to be and that this short report is useful to the National Science Foundation, the U.S. Geologi- those who would like to understand more about where cal Survey, and the National Aeronautics and Space Earth science stands, how it got there, and where it Administration, the National Academies established a might be headed. We have attempted to make the text committee to propose and explore grand Earth science accessible to managers of scientific programs, graduate questions being pursued today. The charge to the com- students, and colleagues in sister disciplines who have 

SUMMARY  the technical or scientific background needed to com- intriguing questions that become more nuanced as prehend what is discussed. we make new observations from spacecraft and more Our most difficult problem in selecting the grand exacting measurements on meteorites. While it is gen- questions was to distill from a large number of topics erally agreed that the Sun and planets all coalesced out and questions the “most worthy” candidates. To do so of the same nebular cloud, it is still not known how the committee canvassed the broad geological com- Earth obtained its particular chemical composition, at munity and deliberated in meetings and telephone least not in enough detail to understand its subsequent conferences. After arriving at 10 grand questions, the evolution or why the other planets ended up so different committee set about writing, as well as soliciting writ- from ours and from each other. Earth, for example, has ten contributions from other scientists. Some of our retained a life-giving inventory of volatile substances, questions present truly awesome challenges and may including water, but Earth is far different from every not be fully understood for decades, if ever. Others other planet in this regard. Advanced computing ca- seem more tractable, and significant progress may be pabilities are enabling development of more credible made in a matter of years. Overall, we have included models of the early Solar System, but further mea- most of what the committee regards as the important surements of other Solar System bodies and extrasolar issues and also most of what was suggested by the re- planets and objects appear to be the primary pathway spondents to our canvassing effort. There was, in fact, a to furthering our understanding of the origin of Earth fair degree of consensus about what constitutes a grand and the Solar System. question and which ones should be included here. 2. What happened during Earth’s “dark age” (the GRAND RESEARCH QUESTIONS FOR THE first 500 million years)? It is now believed that in the 21ST CENTURY later stages of Earth’s formation, a Mars-sized planet collided with it, displacing a huge cloud of debris that Although we started by simply identifying the over- became our Moon. This collision added so much heat arching questions we believe to be driving modern to Earth that the entire planet melted. Little is known Earth science, we found that these questions can be about how this magma soup differentiated into the grouped into four broad themes. These themes con- core, mantle, and lithosphere of today or how Earth stitute the four chapters of the report, and within each developed its atmosphere and oceans. The so-called chapter are descriptions of the grand questions. Chap- Hadean Eon is a critical link in our understanding ter 1 deals with origins—the origin of Earth and other of planetary evolution, but we have little information Solar System planets, Earth’s earliest history, and the about it because there are almost no rocks of this age origin of life. Chapter 2 treats the workings of Earth’s preserved on Earth. Clues about this time period are interior and its surface manifestations and includes a accumulating, however, as we learn more about mete- question on material properties and their fundamental orites and other planets and extract new information role in Earth processes. Chapter 3 addresses the habit- from ancient crystals of zircon on Earth. ability of the surface environment—climate and climate change and Earth–life interactions. Chapter 4 focuses 3. How did life begin? The origin of life is one of on geologica10 the most intriguing, difficult, and enduring questions in hazards and Earth resources—earthquakes and volca- science. Because life in the Solar System arose billions noes and modern environmental issues associated with of years ago, some of the most fundamental questions water and other fluids in and on Earth. about its origin are geological. Our knowledge of the The following is a summary of the 10 grand re- materials from which life originated, and where, when, search questions identified by the committee: and in what form it first appeared, stems from geologi- cal investigations of rocks and minerals that represent 1. How did Earth and other planets form? The the only remaining evidence. When life first arose, the Solar System, with its tantalizing geometric patterns conditions at Earth’s surface may have been much dif- and its wide variety of planets and moons, presents ferent than today’s, and one critical challenge is to de- 

 ORIGIN AND EVOLUTION OF EARTH velop an accurate picture of the physical environments Earth’s climate. But we still do not know when conti- and the chemical building blocks available to early life. nents first formed, how they are preserved for billions The quest to establish the origin of life is inherently of years, or exactly how they evolved to be what they multidisciplinary, spanning organic chemistry, molecu- are like today. New data and observations indicate that lar biology, astronomy, and planetary science, as well as climate and erosion play a fundamental role in build- geology and geochemistry. There is growing interest in ing and shaping mountain ranges and thus are funda- studying Mars, where there is a sedimentary record of mental to the formation as well as the destruction of early planetary history that predates the oldest Earth continental crust. rocks and other star systems where planets have been detected. 6. How are Earth processes controlled by material properties? Deciphering the secrets of the rock record 4. How does Earth’s interior work, and how does on Earth and other planets begins with the understand- it affect the surface? As planets age, they gradually ing of large-scale geological processes. The keys to cool, and this causes them to move through stages understanding these processes are the basic physics and where their internal processes, their atmospheres, chemistry of planetary materials. The high pressures and their surface processes are gradually changing. and temperatures of Earth’s interior, the enormous The primary means by which heat is moved from the size of Earth and its structures, the long expanse of interior to the surface is planetwide solid-state and geological time, and the vast diversity of materials and liquid convection. Although we know that the mantle properties all present special challenges. These chal- and core are in constant convective motion, we can lenges are being met with new research tools based on neither precisely describe these motions today nor synchrotron radiation, new measurements and simula- calculate with confidence how they were different in tion capabilities for large domains and heterogeneous the past. Core convection produces Earth’s magnetic materials, and quantum mechanics-based calculations field, which may have had an important influence on of material properties under extreme conditions. New surface conditions. Mantle convection is the cause of research areas are developing around the study of volcanism, seafloor generation, and mountain building, natural nanoparticles and the mediation of chemical and materials like water and carbon are constantly ex- processes by microorganisms. changed between Earth’s surface and its deep interior. Consequently, without detailed knowledge of Earth’s 7. What causes climate to change—and how internal processes we cannot deduce what Earth’s sur- much can it change? Global climate conditions have face environment was like in the past or predict what been favorable and stable for the past 10,000 years, but it will be in the future. we also know from geological evidence that momen- tous changes in climate can occur in periods as short 5. Why does Earth have plate tectonics and con- as decades or centuries. Yet despite the numerous fac- tinents? The questions regarding plate tectonics now tors that can change climate, from the slowly changing have less to do with the soundness of the theory than luminosity of the Sun to the building of new moun- with why Earth has plate tectonics in the first place tain ranges and changes in atmospheric composition, and how closely it is related to other unique aspects of Earth’s surface temperature seems to have remained Earth—the abundant water, the existence of continents within relatively narrow limits for most of the past and oceans, and the existence of life. We do not know 4 billion years. How does it remain well regulated in whether it is possible to have one aspect without the the long run, even though it can change so abruptly? others or how they are interdependent. The existence Recent discoveries have highlighted periods of Earth and persistence of continental crust present problems history when the climate was extremely cold, was ex- as fundamental as those of plate tectonics. Continental tremely hot, or changed especially quickly. Understand- crust makes the planet habitable by nonmarine life, ing these special conditions may lead to new insights and weathering of its surface plays a role in regulating about Earth’s climate, as will new geochemical observa- 

SUMMARY  tions made on ancient sedimentary rocks and improved begins. Studies of volcanic activity have entered a new models for the climate system that will eventually en- era as a result of real-time seismic, geodetic, and elec- able us to predict the magnitude and consequences of tromagnetic probes of active subsurface processes. But climate changes. it remains a challenge to integrate such real-time data with field studies of volcanoes and laboratory studies 8. How has life shaped Earth—and how has of volcanic materials. The ultimate objective is to de- Earth shaped life? Earth scientists have a tendency to velop a clear picture of the movement of magma, from view Earth’s geological evolution as a fundamentally its sources in the upper mantle to Earth’s crust, where inorganic process. Life scientists, in the same spirit, it is temporarily stored, and ultimately to the surface tend to regard the evolution of life as a fundamen- where it erupts. tally biological issue. Yet the development of life has clearly been influenced by the conditions of Earth’s 10. How do fluid flow and transport affect the surface, while Earth’s surface has been influenced by human environment? Good management of natural the activities of life forms. The atmosphere would not resources and the environment requires knowledge of contain oxygen if it were not for life, and the presence the behavior of fluids, both below ground and at the of oxygen has enabled other types of life to evolve. surface. The major scientific objectives are to under- We know that geological events and meteoroid im- stand how fluids flow, how they transport materials pacts have caused massive extinctions in the past and and heat, and how they interact with and modify their influenced the course of evolution. But the exact ties surroundings. New experimental tools and field mea- between geology and evolution are still elusive. On the surement techniques, plus airborne and spaceborne modern Earth we are interested in the role of life in measurements, are offering an unprecedented view geological processes like weathering and erosion. And of processes that affect both the surface and the sub- we seek to understand how life may have manifested surface. But we still have difficulty determining how itself and left traces preserved in the geological records subsurface fluids are distributed in heterogeneous rock of other planets. and soil formations, how fast they flow, how effectively they transport dissolved and suspended materials, and 9. Can earthquakes, volcanic eruptions, and how they are affected by chemical and thermal ex- their consequences be predicted? Thanks largely to change with the host formations. Much better models sensitive new instrumentation and better understand- of streamflow and associated erosion and transport ing of causes, geologists are moving toward predictive are needed if we are to accurately assess how human capabilities for volcanic eruptions. For earthquakes, impacts and climate change affect landscape evolution progress has been made in long-term forecasts, but we and how these effects can be managed to sustain eco- may never be able to predict the exact time and place systems and important watershed characteristics. The an earthquake will strike. Continuing challenges are to ultimate objective—to produce mathematical models deepen our understanding of how fault ruptures start that can predict the performance of natural systems and stop, to improve our simulations of how much far into the future—is still out of reach but critical to shaking can be expected near large earthquakes, and to making informed decisions about the future of the land increase the warning time once a dangerous earthquake and resources that support us.