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.

  1. 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.

  2. 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.

  3. 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.

  4. 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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