2
Science Opportunities

This chapter focuses on some specific topics in Earth science that appear to be ripe for major breakthroughs during the next decade. Six major areas are discussed, roughly organized according to proximity and scale: (1) the near-surface environment or “Critical Zone,” (2) geobiology, (3) Earth and planetary materials, (4) the continents, (5) the deep interior, and (6) the planets. The committee emphasizes that this is not intended to be a comprehensive list of exciting areas in Earth science, or to represent the full variety of research currently sponsored by the Earth Science Division (EAR) of the National Science Foundation (NSF), but rather to provide key examples of vigorous research areas from which some important programmatic directions can be discerned. The committee’s findings and recommendations regarding these directions can be found in Chapter 3 .

THE CRITICAL ZONE: EARTH’S NEAR-SURFACE ENVIRONMENT

The surface and near-surface environment sustains nearly all terrestrial life. The rapidly expanding needs of society give special urgency to understanding the processes that operate within this Critical Zone ( Box 2.1 , Figure 2.1 ). Population growth and industrialization are putting pressure on the development and sustainability of natural resources such as soil, water, and energy. Human activities are increasing the inventory of toxins in the air, water, and land, and are driving changes in climate and the associated water cycle. An increasing portion of the population is at risk from landslides, flooding, coastal erosion, and other natural hazards.



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Basic Research Opportunities in Earth Science 2 Science Opportunities This chapter focuses on some specific topics in Earth science that appear to be ripe for major breakthroughs during the next decade. Six major areas are discussed, roughly organized according to proximity and scale: (1) the near-surface environment or “Critical Zone,” (2) geobiology, (3) Earth and planetary materials, (4) the continents, (5) the deep interior, and (6) the planets. The committee emphasizes that this is not intended to be a comprehensive list of exciting areas in Earth science, or to represent the full variety of research currently sponsored by the Earth Science Division (EAR) of the National Science Foundation (NSF), but rather to provide key examples of vigorous research areas from which some important programmatic directions can be discerned. The committee’s findings and recommendations regarding these directions can be found in Chapter 3 . THE CRITICAL ZONE: EARTH’S NEAR-SURFACE ENVIRONMENT The surface and near-surface environment sustains nearly all terrestrial life. The rapidly expanding needs of society give special urgency to understanding the processes that operate within this Critical Zone ( Box 2.1 , Figure 2.1 ). Population growth and industrialization are putting pressure on the development and sustainability of natural resources such as soil, water, and energy. Human activities are increasing the inventory of toxins in the air, water, and land, and are driving changes in climate and the associated water cycle. An increasing portion of the population is at risk from landslides, flooding, coastal erosion, and other natural hazards.

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Basic Research Opportunities in Earth Science FIGURE 2.1 The Critical Zone includes the land surface and its canopy of vegetation, rivers, lakes, and shallow seas, and it extends through the pedosphere, unsaturated vadose zone, and saturated groundwater zone. Interactions at this interface between the solid Earth and its fluid envelopes determine the availability of nearly every life-sustaining resource.

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Basic Research Opportunities in Earth Science The Critical Zone is a dynamic interface between the solid Earth and its fluid envelopes, governed by complex linkages and feedbacks among a vast range of physical, chemical, and biological processes. These processes can be organized into four main categories: (1) tectonics driven by energy in the mantle, which modifies the surface by magmatism, faulting, uplift, and subsidence; (2) weathering driven by the dynamics of the atmosphere and hydrosphere, which controls soil development, erosion, and the chemical mobilization of near-surface rocks; (3) fluid transport driven by pressure gradients, which shapes landscapes and redistributes materials; and (4) biological activity driven by the need for nutrients, which controls many aspects of the chemical cycling among soil, rock, air, and water. Critical Zone processes are highly nonlinear and range across scales from atomic to global and from seconds to aeons ( Figure 2.2 ). The scientific challenges are illustrated by the problem of methane flux from wetlands and sediments, which reflects microbial processes and chemical gradients on small scales; the influence of vegetation and nutrient inputs on regional scales; and climatic factors such as precipitation and temperature on global scales. The scientific requirements thus include the development of process models that can capture the scale dependence (or invariance) and reconcile observations from one scale to another. Practical applications often rely on the predictive capability of such models. To engineer the safe disposal of radioactive wastes, geoscientists must be able to predict reliably the effects of hydrologic and geologic processes on underground disposal sites for thousands of years. The historical record of direct observations is far too short to capture the full range of possible behaviors in the Critical Zone, and extensive use of the geological record becomes necessary. For instance, biogeochemical cycles are studied over decades to centuries through high-precision geochemical analysis of ice and sediment cores and marine organisms, while the geologic forcing factors (e.g., volcanism, topography) are constrained through the analysis of sedimentary and volcanic rocks deposited over millions of years. Science Opportunities Processes in the Critical Zone control soil development, water quality and flow, and chemical cycling, and they regulate the occurrence of energy and mineral resources. A better understanding of Critical Zone is necessary to assess the impact of human activities on the Earth surface and to adapt to their consequences. It is not possible in this short report to do justice to all of the pressing scientific issues that bear on the near-surface environment, hence only some pertinent examples are given.

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Basic Research Opportunities in Earth Science Box 2.1. The Critical Zone The Critical Zone, depicted in Figure 2.1 , comprises the outermost layers of the continental crust that are strongly affected by processes in the atmosphere, hydrosphere, and biosphere. The upper boundary of the Critical Zone includes the land surface and its canopy of lakes, rivers, and vegetation, as well as its shorelines and shallow marine environments. On land, the shallower part is the vadose zone, in which unconsolidated Earth materials are intermixed with soil, air, and water. This porous medium is a host for many chemical transformations mediated by radiant energy, atmospheric deposition, and biological activity. Its storage capacity influences runoff and groundwater recharge, affecting both the flow and the quality of surface and subsurface waters. Within the vadose zone is the pedosphere, a collective term for soils at the land surface. The characteristic layering of the soil profile reflects the strong interaction of climate and biota in the upper portion and the accumulation of weathering, leaching, and decay products below. The water table marks the transition from the vadose zone to the deeper groundwater zone, where the pore space is filled by water. Like the vadose zone, the groundwater zone is a chemically reactive reservoir. The lower limit of the Critical Zone generally corresponds to the base of the groundwater zone, a diffuse boundary of variable depth extending a kilometer or more below the surface. The volume of water in the upper kilometer of the continental crust is an order of magnitude larger than the combined volume of water in all rivers and lakes. 1 The Critical Zone is perhaps the most heterogeneous and complex region of the Earth and the only region of the solid Earth readily accessible to direct observation. 1   NRC, Opportunities in the Hydrological Sciences. National Academy Press, Washington, D.C., 348 pp., 1991. Global Climate Change and the Terrestrial Carbon Cycle A significant amount of carbon is stored in soils and sedimentary rocks, thus the Critical Zone plays a key role in the global atmospheric CO2 balance. 1 Soil constitutes the third largest carbon reservoir, and work with carbon-14 tracers reveals that the distribution of soil organic matter strongly influences the means and rate of carbon uptake and release. The exchange of carbon among atmosphere, ocean, and terrestrial reservoirs is also affected by human land-use practices and land-use histories (e.g., agriculture, 1   A U.S. Carbon Cycle Science Plan. A Report of the Carbon and Climate Working Group, J.L. Sarmiento and S.C. Wofsy, co-chairs, U.S. Global Change Research Program, Washington, D.C., 78 pp., 1999 ( http://www.carboncyclescience.gov/planning.html#plan ).

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Basic Research Opportunities in Earth Science FIGURE 2.2 Spatial and temporal scales of Earth surficial processes, with those occurring in the Critical Zone enclosed in a solid frame. Modified from Opportunities in the Hydrological Sciences, National Academy Press, Washington, D.C., 348 pp., 1991. SOURCE: G. Sposito, University of California, Berkeley.

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Basic Research Opportunities in Earth Science reforestation), as well as by silicate weathering. However, the temporal and spatial variability of carbon sources and sinks is not well documented, especially on longer time scales. New evidence regarding the influence of microscopic and macroscopic organisms on the weathering of silicate minerals and the transfer of soluble carbonates to the substratum may provide constraints on weathering reactions and global climate change. The Interactions of Life, Water, and Minerals Organic molecules and microorganisms strongly affect the kinetics of important geological and pedological processes. However, these materials studies are only beginning to address the quantitative significance of microbial interactions in mineral weathering, soil formation, and the geochemical cycling of metals, nutrients, and other elements or isotopes ( Figure 2.3 ). For example, it is clear that organic compounds influence the burial of reduced carbon and help drive the decay or retention of toxins in natural soils and sediments, although mechanisms for the preservation of carbon are still being debated. The interactions of water with minerals and other materials in soils, sediments, and rocks are critical in the dispersal and concentration of chemical species, the migration of contaminants, and the accumulation of natural resources. The architecture of porous media influences transmission dynamics, sorption-desorption kinetics, and chemical fates. Models of physical transport in heterogeneous media are being developed to understand chemical and biological reactions, nutrient cycling, and the fates of contaminants. Such models are being constrained by imaging and analytical measurements developed over the past few years and will provide new insight into the physical, biological, and chemical influences on water quality and availability. The Land-Ocean Interface New technology is yielding vastly improved insights to the nature and dynamics of coastal sedimentary environments. 2 For example, scanning airborne lasers are measuring seafloor bathymetry and topography of coastal areas with unprecedented accuracy and spatial coverage allowing assessment and understanding of coastal change in ways that were not possible only a 2   Coastal Sedimentary Geology Research: A Critical National and Global Priority, results of a workshop held in Honolulu, Hawaii, November 9-12, 1999.

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Basic Research Opportunities in Earth Science FIGURE 2.3 Metabolic pathways affecting the degradation of organic carbon. In nature, pathways for organic matter degradation are interlinked, with the final step depending on available oxidants. Oxygen is the most efficient reagent for respiration, but it can quickly be depleted in wet environments. In the absence of oxygen, bacteria will employ nitrate, metal oxides, sulfate, or sulfur to convert organic compounds into CO2. Additional pathways include the conversion of complex compounds into simple substrates, fermentation, and the cycling of carbon dioxide, hydrogen, methane, and acetate by Archea. SOURCE: K. Pedersen, Göteborg University, http://pc61.gmm.gu.se/gmm/groups/pedersen/basic_research_at_the_dbl.htm .

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Basic Research Opportunities in Earth Science few years before. Using lasers and spectral imagers, it is now possible to map seafloor habitats and to determine whether the substrate is fixed or movable, living or fossil. These data also provide a better basis for integrated three-dimensional models of coastal ocean processes (e.g., tides, waves, currents). New models that integrate land-related processes (e.g., groundwater flow, sediment flux and storage, morphology of drainage systems and watersheds) are being used to understand nutrient loading in the coastal zone, as well as the ultimate fate of waste materials and pollutants in the ocean. Tectonics, Climate, and Weathering Topography, surface hydrology, sedimentation, and climate are intimately related. For example, erosion of the Tibetan-Himalayan plateau is related to precipitation and glacier development. Climate and hydrology interact to modify the surface expression of tectonic events, which in turn influence the rate and scale of fluvial responses. Topography exerts a major influence on erosion and the subsequent deposition of sedimentary rocks, as well as the formation and landscape diversity of soils. Similarly, removal of materials by surface processes influences tectonic uplift rates. Coupled geologic-hydrologic-climate studies promise a greater understanding of denudation rates, weathering processes, and the survivorship of mountain ranges. Earth History The history of the planet is recorded in soils, sediments, ice, water, and rocks. The geological record of environmental variations during the last several hundred thousand years provides the context for understanding the current climate system and its potential for future change. Important insights can also be gleaned from the behavior of the Critical Zone over even greater spans of geologic time: topographic relief, length of day, solar influx, and composition of the ocean and atmosphere have all varied significantly in the past. A good example, based on both careful geological field work and geochemical and isotopic observations, is the recent suggestion that the Earth went through a series of global glacial events (“snowball Earth”) about 750 million to 580 million years ago. The implications of such severe climatic conditions and other extreme events (e.g., extensive volcanism, meteorite impacts) for the evolution and maintenance of life on Earth are the subject of ongoing debate, stimulating new efforts to characterize extreme environmental conditions of the past.

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Basic Research Opportunities in Earth Science New Tools and Observations Many science disciplines—hydrology, geomorphology, biology, ecology, soil science, sedimentology, materials research, and geochemistry—are bringing new and powerful research tools to bear on the study of the Critical Zone as an integrated system of interacting components and processes. It is now possible to study the Critical Zone over a much greater range of length and time because of the wealth of data from satellites and aircraft that provide global information on scales from seconds to decades; advances in geochronology that extend the detailed record of near-surface environments to millions of years; imaging methods (e.g., electron and atomic force microscopes) and spectroscopic tools that probe soil materials to the atomic scale; and new information technologies, which permit the manipulation of large data sets and a variety of numerical simulations, from ab initio models of atomic and molecular interactions to global ocean and atmospheric circulation and mountain belt evolution. These technological advances have set up opportunities for novel cross-disciplinary activities. Synchrotron-based X-ray spectroscopy can be applied in tandem with computational geochemistry to elucidate the molecular-scale mechanisms of key aqueous geochemical reactions related to mineral weathering, contaminant sorption and desorption, and nutrient cycling. High-resolution electromagnetic and acoustic imaging of hydrological systems can be combined with molecular-scale mechanisms of aqueous geochemical reactions for accurate representations of transport in reactive systems. Such representations are essential for addressing silicate mineral weathering—a source of nutrients to the biosphere and a major control on the long-term CO2 budget—and contaminant transport. The techniques of isotope geochemistry and molecular biology reveal the pathways involved in biogeochemical cycles and the formation of secondary minerals in weathering environments. 3 New geochemical and stratigraphic tools and techniques for dating individual minerals are furnishing insights into the behavior of the Earth’s surface with increasing temporal resolution. Remote-sensing data, digital elevation models, and special dating techniques such as cosmogenic nuclide exposure ages can be used to validate a new generation of geomorphic transport models. 4 These models will allow a 3   Research Opportunities in Low-Temperature and Environmental Geochemistry, results of a workshop held in Boston, Massachusetts, June 5, 1999. 4   A Vision for Geomorphology and Quaternary Science Beyond 2000, Results of a workshop held in Minneapolis, Minnesota, February 6-7, 2000.

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Basic Research Opportunities in Earth Science more quantitative exploration of the dynamic interrelationships between tectonics, climate, soil diversity, and landscapes. Microprobe, X-ray diffraction, scanning and transmission electron microscopy, X-ray tomography, and microchemical probes can be used to map the architecture of soils and to investigate material properties at the atomic-scale resolution needed for understanding sorption-desorption kinetics and other equilibrium processes. 5 Ground-penetrating radar and three-dimensional seismic imaging of sedimentary deposits permit modeling and prediction of physical properties of heterogeneous sediments in three dimensions. 6 Chemostratigraphic techniques can be used to correlate sediments among sedimentary basins, particularly between onshore and offshore basins. In situ and aircraft sensors for measuring circulation patterns and mapping the bathymetry and substrate of the near-shore environment, combined with analysis of geochemical and sedimentological components and fluxes, can be used to quantify the variability of the geological, biological, and atmospheric components of coastal ecosystems. Need for Coordinated Field Work and Integrated Modeling The integration of disciplinary research is the key to future progress in the science of the Critical Zone. This theme permeates the discussion of many other aspects of Earth science in this report, but, in the case of the Critical Zone, it presents some special challenges, in part because of the sheer number of the disciplines and the diversity of their approaches, but more profoundly because of the spatial scales intrinsic to the scientific issues. Although Critical Zone processes often involve the global aspects of atmospheric and oceanic transport, many of the most intense interactions occur in relatively localized regions of the solid Earth—for example, over dimensions less than the characteristic horizontal variations in topography and near-surface geology (tens to hundreds of kilometers) or the thickness of the zone itself (about a kilometer). Indeed, much of the science to be done will require the in situ study of microscopic processes that are subject to numerous contingencies— physical, chemical, and biological—which vary from one surface environment to the next. Not surprisingly, disciplinary integration has proceeded more 5   Opportunities in Basic Soil Science Research, G. Sposito and R.J. Reginato, eds., Soil Science Society of America, Madison, Wisconsin, 129 pp., 1992. 6   Sedimentary Systems in Space and Time: High Priority NSF Research Initiatives in Sedimentary Geology, results of a workshop held in Boulder, Colorado, March 27-29, 1999.

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Basic Research Opportunities in Earth Science rapidly in the study of global-scale geosystems, which manifest a more obvious set of unifying concepts, diagnostic behaviors, and simple symmetries—for example, the dipolar magnetic field (core dynamo), the shifting mosaic of plate tectonics (mantle convection), or the largely zonal structure of global circulation (climate system). New mechanisms are thus needed to encourage multidisciplinary collaborations on Critical Zone problems, especially on local and regional scales. Modeling activities that employ conceptual and numerical tools to integrate different types of data are clearly important. However, the primary deficiency at this stage of the science is the difficulty in mounting field work to collect measurements that are sufficiently localized and simultaneous as well as dense and comprehensive enough to constrain process-based models of Critical Zone behaviors. Real progress will require some way to coordinate the field investigations of hydrologists, pedologists, geochemists, geobiologists, mineralogists, and other geoscientists in localized regions, often for extended periods of observation, and to encourage the integration of these data with controlled laboratory measurements and system-level models. One mechanism for encouraging this type of problem-focused, multidisciplinary field work is through the establishment of “natural laboratories” in which detailed, long-term observations can be made using a variety of disciplinary tools. As discussed in Chapter 3 , such a program would also provide new opportunities for scientific advancement in many other areas of Earth science. GEOBIOLOGY Life is inextricably linked to the Earth, so it is not surprising that some of the most challenging problems of geology and biology are intimately interwoven. The synthesis of these two sciences is geobiology, which addresses the interactions of biologic and geological processes, the evolution of life on Earth, and the factors that have shaped the current and past biospheres. Important issues include the following: origin of prebiotic molecules and life, and its early evolution; emergence and divergence of metabolisms and morphologies; effects of organisms on the physical and chemical characteristics of Earth and its fluid envelopes; nature of ecosystems and their response to environmental perturbations of many types; and the rules that govern biodiversity dynamics, including selectivity in

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Basic Research Opportunities in Earth Science FIGURE 2.15 USArray coverage of North America including the continental margins of the United States and potential cooperating stations in Canada and Mexico. SOURCE: P. Shearer, University of California, San Diego, http://mahi.ucsd.edu/shearer/USARRAY/usarray4.html . The current understanding of geomagnetism owes much to the geomagnetic observatories that have been maintained (in varying numbers) over the past century. Data from these observatories have been used to explore a wide range of physical phenomena, from the flow in the Earth’s liquid core, to the electrical conductivity of the solid mantle, to resonant hydromagnetic oscillations in the plasma environment of the ionosphere. From these studies much has been learned, not only about the geomagnetic field per se, but also about

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Basic Research Opportunities in Earth Science the physical state and dynamics of the solid Earth and the near-Earth space environment. Advances in theory and computational capability, together with modern satellite data, provide exciting new possibilities for extending the knowledge (and use) of geomagnetism on a number of fronts. However, achieving this promise will require a global array of geomagnetic observatories, both on the land surface and on the seafloor, in conjunction with ongoing satellite measurements. The need for better distribution of geomagnetic observatories with modern digital equipment is a well-known problem. 11 Geochemistry Studies Isotope and trace element geochemistry of mantle-derived rocks, including ultramafic xenoliths and lavas erupted at midocean ridges and ocean islands, documents the chemical composition of the deep Earth, as well as the time-integrated effects of four and a half billion years of planetary differentiation. The geochemistry of such samples provides unique insights to the formation of the Earth from the solar nebula, the processes and time scales of crust and atmosphere formation, the style of mantle convection and the origin of hotspots, and the fate of slabs subducted back into the mantle. New geochemical techniques have frequently arisen from the need to analyze mantle samples for previously inaccessible elements or with enhanced precision, speed, and spatial resolution. The results of these advances have shed new light on the deep interior, and the coming decade will continue this evolution. The new generation of thermal ionization mass spectrometers, offering enhanced precision and sensitivity for radiogenic isotope analyses, has been complemented with inductively coupled plasma mass spectrometry (ICPMS), which permits routine analyses of nearly every element in the periodic table on very small samples. In the past few years, results from the first multiple-collector ICPMS instruments have also begun to appear; this new technique permits rapid isotopic analyses with minimal chemical purification as well as analyses of elements not appropriate for thermal ionization. Advances have also occurred in the ability to map small-scale (tens of microns down to submicron) chemical and isotopic variability in crystals using both laser ablation ICPMS and secondary ion mass spectrometry. Of particular interest is the ability to analyze melt inclusions, which richly document previously unknown small-scale variability in the chemical and isotopic composition of the mantle. 11   For example, see The National Geomagnetic Initiative, National Academy Press, Washington, D.C., 261 pp., 1993.

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Basic Research Opportunities in Earth Science FIGURE 2.16 (A) Results of numerical simulations of the geomagnetic field of Glatzmaier and Roberts. Blue lines indicate flux into the core, and orange is outward-directed flux. External to the core, the field is approximately that of a dipole. Numerical simulations make predictions about field behavior (average intensity, secular variation, behavior during reversals) for different assumed boundary conditions. SOURCE: G.A. Glatzmaier and P. Roberts, A three-dimensional self-consistent computer simulation of a geomagnetic field reversal, Nature, v. 377, p. 203-209, 1995, Reprinted by permission from Nature. Copyright 1995 Macmillan Magazines Ltd. (B) Field model UFM1 of Bloxham and Jackson. Averages of geomagnetic field observations over the last 300 years are plotted as radial flux on the core mantle boundary. Color conventions are as in (A). Although the field is dominantly dipolar, there are significant departures (flux patches with the wrong color) from a dipole model that persist for hundreds of years. SOURCE: J. Bloxham and A. Jackson, Time-dependent mapping of the magnetic field at the core-mantle boundary, Journal of Geophysical Research B, v. 97, p. 19, 537-19,563, 1992. Copyright 1992 American Geophysical Union. (C) Averages of paleomagnetic data spanning the last 5 million years. There is loss of resolution relative to the geomagnetic field average shown in (B) due to the difficulty in getting global coverage with high-quality data. SOURCE: Reprinted by permission from C.L. Johnson and C. Constable, The time-averaged geomagnetic field; Global and regional biases for 0-5 Ma, Geophysical Journal International, v. 131, p. 643-666, 1997. Copyright 1997 Geophysical Journal International.

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Basic Research Opportunities in Earth Science High-Pressure Studies of Earth Materials With the advent of dedicated synchrotron beamlines and other modern technologies, it has become possible to document the properties of Earth materials in situ, at the high pressures and temperatures of the planet’s deep interior. Measuring the elastic properties that determine seismic wave velocities; the density, thermal conductivity, and rheological properties that control geodynamic motions; and the partitioning of minor and trace elements that produce the geochemical signatures observed in rock samples is now possible at deep-Earth conditions. First-principles quantum and statistical mechanical calculations are also offering significant theoretical insights into the chemical and physical properties of materials throughout the mantle and core. Thus, the seismic anisotropy of the inner core, the possible chemical and physical interactions at the core-mantle boundary, the distribution of radioactive heat-producing elements, and the initiation of melting (or crystallization) throughout the Earth are all subject to quantitative study over the coming decade. The abundance and distribution of water and other “volatile” molecules within the planet, as well as their cycling between the surface and interior over geological history, can be evaluated for the first time. These are specific examples of the broader question now being addressed through high-pressure studies; What is the current state of the Earth’s interior, and what are the processes by which it evolved to this state?

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Basic Research Opportunities in Earth Science THE PLANETS The Earth is only one member of a rapidly growing family of known planets, both within and beyond the solar system. The extraordinary pace of planetary exploration, driven by large national and foreign investments in spacecraft and telescopes, is expected to continue for at least the next decade. Current or planned space missions will provide unprecedented detail and coverage of the geology, topography, structure, and composition of many solar system bodies; in several cases this coverage will rival or exceed equivalent terrestrial data. Similarly, it is anticipated that by 2010 the first samples collected from Mars, a comet, an asteroid, and the Sun (via solar wind particles) will be returned to Earth for direct investigation of their composition and structure. Telescopic observations of primitive objects in the solar system (i.e., Kuiper belt objects, located beyond the orbit of Neptune) and of the planets orbiting distant stars promise to provide unique data regarding the origin and evolution of our solar system. Investigations of the solid Earth have usually been undertaken through studies of the Earth itself. However, the study of extraterrestrial materials has provided some of our most important insights regarding the Earth. For example, the age of the Earth, 4.56 billion years, was derived not from dated Earth material but by the study of meteorites (and subsequently confirmed by dating lunar samples). The baseline for discussions of the bulk composition of the Earth is likewise based on a meteorite (chondrite) reference. The richness of materials and information on extraterrestrial bodies ensuing from the next decade of exploration will provide important new opportunities for basic research into the origin, evolution, and structure of planets, including Earth. Effective use of these new data will require both a broad-based effort to promote interaction between the Earth science and planetary science communities, and a substantial enhancement of analytical capabilities. Promise of Planetary Exploration Robotic exploration of the solar system is increasing rapidly. The scientific opportunities provided by these missions are detailed in a number of National Research Council reports. 12 , 13 Here, the committee focuses on near- 12   An Integrated Strategy for the Planetary Sciences: 1995-2010. National Academy Press, Washington, D.C., 199 pp., 1994. 13   The Exploration of Near-Earth Objects. National Academy Press, Washington, D.C., 32pp., 1998.

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Basic Research Opportunities in Earth Science term missions of interest to the solid-Earth sciences. Within the decade, current or planned NASA or international missions will provide the following: Surface topography, gravity, and magnetic field data of unprecedented resolution for the terrestrial planets and for some satellites: For example, the Messenger Orbiter will define Mercury’s gravity field and Cassini flybys will refine the gravitational parameters of some Saturnian satellites. High-resolution geophysical observations, such as those collected by the Mars Global Surveyor (MGS) mission, are critical for identifying and understanding the geologic processes that shape solid planets. While much is known regarding these processes on Earth, existing data leave little doubt that conditions on other planets and satellites can yield fundamentally different results ( Figure 2.17 ). FIGURE 2.17 Active volcanism on Jupiter’s moon Io from the Galileo spacecraft. The true-color image on the left shows the ~1300-km-diameter red ring of sulfur surrounding the volcano Pele. The false-color infrared composite on the right reveals the glow of hot (up to 1027°C) magma in the volcano’s central crater. SOURCE: NASA Jet Propulsion Laboratory. In situ chemical and mineralogical analyses of the surfaces of Mars, satellites, asteroids, comets, and the solar wind (complemented by samples returned from these same sources): These data will open an exciting new

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Basic Research Opportunities in Earth Science research frontier documenting and understanding chemical processes that produced the solar system and that modify both the surfaces and the interiors of solar-system bodies. Such work was largely impossible from the earlier generation of orbiting spacecraft. The results will be important in a wide array of Earth and planetary science disciplines. Some key research areas benefiting from these observations include the composition and early evolution of the solar system, the climatic history of Mars, and the possible existence of life beyond the Earth. In addition, data collected from ongoing missions (e.g., the surprising diversity of Jovian satellite magnetic fields detected by the Galileo mission) will be analyzed over the next several years. A lengthy period of analysis of satellite data, combined with other geophysical observations, is important for understanding the chemical and physical structure and dynamics of planets. Science Opportunities Continuing exploration of the solar system, particularly Mars, will provide both new opportunities and challenges for the Earth science and planetary science communities in the coming decade ( Box 2.3 ). On the one hand, new high-resolution observations of key physical, geological, chemical, and mineralogical characteristics will permit insights to the processes occurring on and in other solar system bodies. As the quantity and quality of observations approach those for the Earth, the knowledge-base within the Earth science community will become increasingly important for interpretation and comparison. On the other hand, these observations will also contribute to an improvement in our understanding of Earth and the solar system as a whole: Geological and geophysical processes occurring on other planets are responses to the same basic forces as on Earth, but applied in different ways to materials with different properties and subject to different boundary conditions. As such, these systems will provide new environments for investigating basic planetary phenomena. Unlike the Earth, which is continually resurfaced and eroded, many bodies preserve a physical and chemical record from the earliest days of the solar system. Thus, they provide data regarding planetary evolution that no longer exists on Earth. For many fundamental processes that occur early in a planet’s history (e.g., formation of the first crust), the Earth’s record is extremely difficult to decipher. Distinctive chemical and isotopic composition is a first-order property of the solar system, and the new data likely to be obtained over the next

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Basic Research Opportunities in Earth Science decade are critical for furthering understanding of the processes of mixing, accretion, and differentiation of planets and meteorite parent bodies. Processing local resources to obtain various materials will be crucial to the establishment of self-sufficient planetary bases. 14 The study of soil formation and past aqueous weathering on planetary surfaces will be a prerequisite for developing extraterrestrial “soils” for growing plants. The factors that influence the activity, ecology, and population dynamics of microbes in soils are important for nutrient cycling, biodegradation, and regenerative life support on a planetary base. Box 2.3. Recent Studies of Mars NASA has identified exploration of Mars as a top research priority, and early results of this focused effort are now being obtained. These results illustrate some of the basic research opportunities and challenges posed by intensive solar system exploration: 1 , 2 Much of the interest in Mars has arisen from the study of SNC (Martian) meteorites, which permit detailed studies of rocks derived from a planetary body similar to Earth. In addition, one of these meteorites was initially believed to carry evidence of fossil life. Although the existence of fossils is not widely accepted, the exciting possibility of detecting and characterizing extraterrestrial life (or its fossil imprints) has invigorated study of the physical, chemical, and mineralogical composition of Martian rocks in a way analogous to what future returned samples will require. The lesson from these studies is that understanding detailed aspects of another planet (e.g., composition of the planet, petrogenesis and planetary evolution, climate history, possible existence of life) will require intensive and diverse studies with investigators derived from the planetary science and Earth science communities, and beyond. For studies at the micro- and nanoscale and with very limited amounts of material, new analytical techniques and instrumentation are critical. This broad effort to understand Mars has required specific initiatives from federal funding sources (i.e., NASA, NSF) because existing mechanisms within these organizations were not suited to such an effort. In 1998 the Mars Global Surveyor began orbiting Mars, and the first science returns were reported in early 1999. As with the SNC meteorites, the data already obtained from this mission illustrate important new linkages between the planetary and Earth sciences. For example, the first high-resolution magnetic survey of Mars has revealed regions of alternating magnetic polarity that provide exciting and unexpected insight to 14   Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies, National Academy Press, Washington, D.C., in press, 2000.

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Basic Research Opportunities in Earth Science the evolution of the Martian crust and interior ( Figure 2.18 ). That is, the strength and variability of magnetization demonstrate the unexpected existence of an Earth-like planetary dynamo within the core of Mars. Because Mars no longer has a strong magnetic field, its planetary dynamo must have ceased to operate sometime after the formation of the crust. These observations raise important questions about why the Martian dynamo behaved so differently from that of the Earth. In addition, the magnetizations have been interpreted as the equivalent of terrestrial “seafloor stripes” that are the hallmark of plate tectonics. Although this interpretation is controversial, the data are of first-order importance. Further effort in understanding how planetary dynamos and plate tectonics might work on Mars in comparison with the Earth is critical for accurately interpreting and understanding these data. Another important area being explored by MGS is the climate history of Mars. Photographic evidence has long suggested features derived from flowing water on Mars, but recent high-resolution images show unexpected complexity, raising questions regarding how the features were produced. Whereas some features look like flood deposits and channels, others look like subsurface flow. Further research using photogeologic techniques and other methods is necessary for understanding how flowing water under conditions very different from those on the Earth affects the surface morphology of planets. Similarly, recent data on surface mineralogy furnishes limited evidence for pervasive aqueous alteration of Martian soils. This clearly has bearing on the longevity of Martian surface waters, a key question for the history of climate and possible life on Mars. Additional work is necessary to understand basic soil formation and alteration processes. 1   An Integrated Strategy for the Planetary Sciences: 1995-2010. National Academy Press, Washington, D.C., 199 pp., 1994. 2   Review of NASA’s Planned Mars Program. National Academy Press, Washington, D.C., 29pp., 1996. Analytical Challenges In situ planetary studies will offer great benefits for Earth and planetary scientists, 15 but sample return offers the most promise for research funded by EAR. Samples brought back from various solar system bodies will share a common trait—the amount of material will be extremely small, ranging from perhaps a kilogram of Martian soil and rock to submilligram quantities of comet dust. In comparison, hundreds of kilograms of lunar materials were returned during the Apollo program. The small sample sizes, coupled with the 15   A Scientific Rationale for Mobility in Planetary Environments. National Academy Press, Washington, D.C., 56 pp., 1999.

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Basic Research Opportunities in Earth Science FIGURE 2.18 Clues to the ancient magnetic field of Mars from the Mars Global Surveyor spacecraft. This image documents the strength and variability of magnetization recorded by Martian crustal rocks. The pattern indicates the existence of an Earth-like planetary dynamo operating in the Martian core, but only very early in the history of the planet. The alternating polarity pattern records fundamental, yet previously unknown aspects of the formation of the early Martian crust, possibly by a process similar to seafloor spreading on Earth. SOURCE: Reprinted with permission from J.E.P. Connerney, M.H. Acuna, P.J. Wasilewski, N.F. Ness, H. Reme, C. Mazelle, D. Vignes, R.P. Lin, D.L. Mitchell, and P.A. Cloutier, Magnetic lineations in the ancient crust of Mars, Science, v. 284, p. 794-798, 1999. Copyright 1999 American Association for the Advancement of Science. need to investigate the samples for a tremendous diversity of purposes, place a new and extremely strict requirement for high-efficiency analyses. In addition, many types of analyses will require extremely high spatial resolution. For a

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Basic Research Opportunities in Earth Science number of critical measurements, present techniques and instruments are simply not adequate. An important challenge will be to construct appropriate instrumentation for these analyses. The necessary instrumentation is diverse, and includes advanced ion microprobes, mass spectrometers, and electron microscopes, as well as accelerator- and synchrotron-based analytical probes. This challenge, if met, is an important opportunity for the Earth science community. As evidenced by the blossoming of isotope geochemistry following the instrumentation campaign associated with the Apollo program, the creation of new techniques and capabilities can invigorate an entire field for many years.