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2 Earth Sciences- Status of Understanding INTRODUCTION The Earth is more fascinating and mysterious than ever, de- spite the great advances in knowledge achieved in the first three decades of the space age. The fascination is generated by the ex- traordinary complexity of the Earth and by the inherent inacces- sibility of many of its key processes. In the face of this complexity and inaccessibility, the notion of an orderly progression from re- connaissance to mapping becomes a myth. This is all the more reason to undertake a systematic and comprehensive program. By 1995 we will be ready to make an integrated study of the Earth as a planet, that is, to undertake a Mission to Planet Earth. Several developments, both recent and expected in the near fu- ture, make this timely. The Earth's complexity involves regimes of widely differing energy and time scales interacting in varied ways. For this reason problems of earth science cannot be reduced to fundamental elements analogous to the energetic particles of mod- ern physics or the DNA components of modern biology. Because the systems are "chaotic," it is usually impossible to predict the behavior of a regime within the Earth solely from first principles; attempts to do so are generally less successful than alternative ap- proaches, which may strike the basic scientist as crudely empirical. 16
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17 The geologic record of a long series of events that actually occurred can be used to forecast Earth's behavior. It is true that some of the interfaces between greatly differing regimes of the Earth are quite sharp, such as the ocean and atmosphere, the ocean and crust, or an organism and its environment. But one of the greatest weaknesses in our understanding concerns what happens at these interfaces. Across these interfaces chemical and energetic fluxes influence behavior on time scales ranging from seconds to millions of years. Earth is the only planet in our solar system on which life has come into existence and persisted. Why? Not only does Earth support life, it is influenced by life. Biological processes affect the Earth's atmosphere, oceans, and solid surfaces; systematic phenomena such as cInnate and the global cycling of chemicals respond to life. Conversely, living organisms are influenced directly by the climate system, by the distribution and flux of chern~cal compounds. As such, the earth system is strongly coupled with widely varying rate constants. The Earth is the only planet that supports plate tectonics. Why? The Earth is the only planet with a liquid ocean. Again, why? The complexity of the Earth has led to its examination being divided among several disciplines that speak imperfectly to each other. They range from geomagnetic theory to ecology, which is concerned with the interaction between organisms and their environments. One matter in common among these disciplines Is the problem of inference from incomplete data. In large part, this problem arises from the inaccessibility of key processes. Extreme examples are convective flows in the lower mantle generated by inhomogeneity of density on the one hand, and the secretion of calcium carbonate by foram~nifera in the ocean on the other. To help solve these problems of identifying and quantifying the forces behind sketchily sampled details, we need global, synoptic, and continuous data. The basic objectives of studies of Earth can be grouped as follows: ~ To understand the processes by which the Earth formed and evolved to its present state, and to determine the composition, structure, and dynamics of the solid planet. ~ To establish and understand the structure and dynamics of the oceans and atmosphere and their interactions with the
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18 solid Earth including the global hydrological cycle, weather, and cInnate. ~ To characterize the history and dynamics of living organ- isms, including mankind and their interactions with the environ- ment. To understand Earth in the context of the solar system, and the use of the Earth as a detector of cosmic events. There has to date been no systematic attack on these broad objectives. In this document the task group hopes to show how these objectives can be developed into "grand themes" to focus a systematic, global study a Mission to Planet Earth. To set the context, the following sections summarize the state of un- derstanding of the various subsystems of the Earth as the task group expects it to be in 1995. In many cases, the task group has recognized the existence of up-to-date reviews and recommenda- tions in previous reports, and has quoted summaries of these in appropriate contexts. THE EA1~H'S INTERIOR AND CRUST Our prunary objective is to understand the processes by which the Earth formed and evolved to its present state, and to determine the composition, structure, and dynamics of the solid planet. Since the synthesis of plate tectonics has given us a new understanding of Earth processes, the discussion will begin there. Plate Tectonics As has been pointed out in Part ~ of the CES strategy, geology has been revolutionized since the mid-196Os by the recognition of the plate structure of the lithosphere. According to the plate tectonic theory, the Earth's surface is divided into about 11 major and a large number of minor plates that behave as rigid units, are in continuous relative motion, and interact mainly at their edges. New plate material Is created at ocean ridges; old oceanic plate material is subducted or consumed at ocean trenches. Many active volcanoes are associated with plate boundaries. Earthquakes occur where plates are created or destroyed, and where plates move past one another. Earthquakes outline the worId's major plates and serve as energetic sources to probe the interior.
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19 The relative motion of the Earth's plates over approximately one hundred 100,000 year intervals is known for the last 200 mil- lion years from studies of magnetic lineations on the ocean floor. These motions give us some idea of the general rates of convective motions in the Earth's viscous mantle. There ts no ocean floor older than approximately 200 million years; most older oceanic crustal material has been subducted, some has been incorporated into continents. Inferences about plate interaction prior to this tune must be made from continental geology and especially pre- served pieces of ocean floor (ophiolite suites). Although we know the average relative velocities of the Earth's plates over a time scale of a million years, the Earth's magnetic field does not reverse polarity frequently enough to allow a finer resolution of the present rates of motion. We do not know what drives the plates. Earthquakes show that the motion of the plates at plate boundaries is episodic, but we do not yet know how strain accumulates at those boundaries. Nor do we know whether pre- cursory effects before major earthquakes are general phenomena, or diagnostic signals. The episodic motions at plate boundaries are thought to be damped out with distance from the boundary by stress relaxation in the viscous asthenosphere underlying the plates, so that the relative motions of plate interiors are steady; direct observations of plate motions over time scales of years are beginning to indicate that these rates are indeed steady. A major goal in plate dynamics is to understand the driving mechanism for the plate motions. This mechanism involves some form of thermal convection in the Earth's mantle, but the form of the motions is uncertain at levels deeper than the plates them- seives. We do not know whether the radial extent of the convection system involving the plates extends to the full depth of the mantle, or part way. Nor do we know the planform of the flow, that is, the pattern in plan view of upwelling and downwelling limbs of the convection system. further, the contribution to the driving energy for convection from secular cooling of the Earth's interior, including core-mantle differentiation, is uncertain. The detailed pattern of the convection flow is thought to be highly sensitive to the viscosity of the Earth's mantle and to its spatial variations. The history of mantle convection Is closely linked not only to the history of plate motions, but also to the removal of heat from the Earth's interior and to the chemical evolution of the crust and mantle.
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20 Gravitational and Magnetic Fields The longer wavelength variations of the geoid and gravity field provide information on the density Attribution in the man- tIe. Since these density inhomogeneities drive mantle convection, the measurements can be used to infer the structure of mantle convection. The interpretation of the long-wavelength features of the gravity field will be complemented by improvements in seismic resolution of density variations. Complete upper mantle coverage will be available from surface wave tomography. Lower mantle heterogeneity can be determined with more complete coverage and with an average resolution of about 200 km. To achieve this resolution worse requires a much denser global distribution of digital seismometers, including sea floor deployment. The long- wavelength part of the geoid shows a high degree of correlation with the lower mantle seismic heterogeneities. The inferred rela- tionship between density and velocity places constraints on the viscosity structure of the mantle and the resulting relief on the core-mantIe boundary. The intermediate wavelength part of the geoid correlates with the Attribution of slabs and upper mantle velocity variations, constraining the density variations in these regions. At shorter wavelengths, lithospheric contributions, to- gether with inherent limitations in seismological resolution, will make the interpretation much more patchy. There will still be uncertainty as to the relative contributions of convective or elastic support of geoid features. Understanding of the energetics of man- tie convection will probably continue to be limited by ambiguities in interpretation of heat flow data. High-resolution global maps of heat flow will never be available, but surface-wave tomography shows a high degree of correlation with heat flow. The direct inference of long-term (i.e., post-glacial) variation from gravity measurements began in 1983, with a determination of changes in long-wavelength harmonics. The geoid ~ not static! Estimates of changes in higher zonal harmonics can be expected by 1995, but determination of tesseral harmonic trends seems unfeasible. Determination of tidal effects on satellite orbits will be refined, and will help solve the problem of tidal dissipation. It also can be expected that the static gravity field will contribute to the understanding of post-glacial rebound. A notable recent achievement, the determination of the tune variation of J2 the oblateness—helps resolve the viscosity of the lower mantle.
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21 By 1995 the oceanic geoid should have an uncertainty of less than a meter and a spatial resolution of 10 to 20 km. The prin- cipal means to this resolution will be the DOD satellite Geosat; the task group assumes that its results will become available for scientific publication. Because of the variations introduced by ocean processes, improvements in knowledge of the ocean geoid will be slow, dependent on more and more complex analyses of growing data sets. Knowledge of the geoid over land areas, how- ever, is much more variable. In developed accessible areas, surface measurements provide low levels of uncertainty and good spatial resolution. Nevertheless, surface data are not now available in many areas because of either physical or political inaccessibility. By 1995 it is also hoped that the Geopotential Research Mis- sion (GRM) will provide gravity data over the continents with an accuracy of 2 mgal and a spatial resolution of 100 km. Be- cause of the lower limit on spacecraft altitudes it is not possible to significantly improve this resolution from satellites. The core interacts with the mantle in two unportant ways: it transfers heat into the base of the mantle, and it exerts torque on the mantle. The former contributes to and may even drive thermal convection in the deep mantle, and the latter causes changes in the length of day and in the orientation of the Earth's axis of rotation in space. Although it is widely accepted that the Earth's magnetic field is maintained against dissipative ohmic decay by self-excited dy- namo action in the liquid outer core, the details of the process remain obscure. It is uncertain whether there is (1) thermal con- vection driven by radioactive heating distributed throughout the outer core, or (2) slurry convection near the top of the core, or (3) chemical convection driven by compositional change and la- tent heat release at the boundary between the liquid outer core and the more solid inner core. We do not know (~) whether the core dynamo ~ lam~nar or turbulent, (2) whether there ~ a weak toroidal field, or (3) whether the toroidal field strongly dominates the poloidal field. Can the core magnetic field really change glow ally within an interval of less than 2 years as appears to have been the case during the Geomagnetic unpube of 1970?" Are such jerks rare or common, and how large can they be? Clearly, light would be shed on many of these questions if we could obtain data necessary to construct an acceptable mode] for the fluid motion beneath the core-mantle boundary (CMB).
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22 Probing magnetically more deeply into the fluid core does not yet appear to be feasible, but the construction of "synoptic weather maps" for the fluid near the CMB is technically realizable. The pattern of that motion could reveal the type of convection going on, and help determine the strength of the toroida] field, which is attenuated to unobservable levels at the Earth's surface. If the fluid motion is steady in time, it is uniquely determined by the tune-varying vertical component of the magnetic field at the top of the core. Currently, the major scientific challenge for the subject of core fluid dynamics is to develop sound methods for extracting horizontal fluid motions near the top of the core from magnetic measurements taken at and above the Earth's surface. Magsat resolved crustal magnetic anomalies and obtained an excellent snapshot of the main magnetic field, but gained hardly any useful instantaneous information on secular variation. Nev- ertheless, Magsat data were recently compared with observations at earlier epochs to determine, magnetically, the depth of the CMB supporting the frozen-flux mode} of the core and the nearly insulating mode} of the mantle. Geomagnetic secular variation, crucial for studies of core dy- nam~cs, is currently best measured by ground-based permanent magnetic observatories and repeat stations, which are sparsely and very unevenly distributed over the Earth's surface. They will continue to play an important supporting role, especially during the long intervals between magnetic main field missions, but they cannot provide an adequate data base for global studies on their own. The GRM will be of substantial importance for studies of core dynamics. Its low-altitude, carefully monitored, circular polar orbit will provide a significant improvement in our ability to resolve the crustal magnetic anomalies, which must then be removed from the data to expose the main field emanating from the core. The comparison between Magsat and GRM magnetic anomalies should go far toward establishing their repeatability, stability in time, and spatial scale of variation. However, snapshots of the main field at intervals of a decade and more cannot teach us about the continuous time evolution of the magnetic field, including the possible existence of short-term magnetic impulses. The Earth's outer core has long been thought to be a rela- tively homogeneous molten body. Seismic tomography studies of
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23 the lower mantle indicate the existence of mantle density varier tions, which are matched by long-wavelength features of the geoid and which require kilometer-scale relief on the core-mantIe bound- ary to support the mantle density variations. Relief at the top of the core may play a controlling role in core dynamo mechan- ics, and changes at the core-mantIe boundary may control such phenomena as westward drift of the geomagnetic field. Tomogra- phy can potentially be used to observe topographic variations in the boundary of the fluid outer core and variations in the core. Such observation will contribute to understanding the thermal be- havior of the core, chemical differentiation occurring there, and eventually the operation of the magnetic dynamo, one of the most significant and least understood processes of the Earth. The non-dipole terms of planetary magnetic fields extrapo- lated down to the generating regions are appreciable. These re- gional variations must be significant in the overall behavior, par- ticularly in reversals of dipole polarity evidenced by the Earth's remanent magnetism. Magnetic observations obtained over the last 150 years indicate a rate of change of the magnetic field such that the non-dipole terms would appear quite different in a few thousand years. This so-called secular variation can be inferred by magnetometers on appropriate satellites- ideally, small dedicated spacecraft orbiting for decades at altitudes of 1000 km or more. The Magsat satellite launched in 1978 established a baseline for measuring long-term changes. However, the orbit was far from op- t~mum for this purpose, and hence estimates of the field generated by the core are affected not only by solar-wind-induced variations, but also by the remanent magnetism of the crust. Furthermore, the duration of the mission was much too short to obtain any estimate of temporal variations in the field. The GRM will improve the resolution in determination of variations in remanent magnetism to about 100 km. An Explorer satellite with a magnetometer should greatly improve estimates of secular variations to about the tenth degree of harmonics. For an integrated approach to core fluid dynamics, we require long-term, nearly continuous vector magnetic data from nearly circular, polar orbits at sufficient height to minimize data contam- ination by crustal anomalies and ionospheric currents. It would probably suffice to turn on a satellite vector magnetometer for a week or two every 6 months, but long overall mission duration
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24 is vital. Magnetic signab diEuse downward through the conduct- ing mantle rather slowly, so long time spans of surface data are aLso required to probe deeply for the mantle conductivity profile. The Magnetic Field Explorer mission, currently under discussion, would be ideal for this purpose. It would be especially useful if it were in orbit during the period of the GRM, for it could then supply the excellent baseline main field mode! above which the GRM crustal anomalies stand out. Structure of the Earth's Interior It is a good first approximation to assume that the Earth's structure is radially symmetric. However, mantle convection in- evitably entails lateral heterogeneity. Recently, inversions of seis- mic travel times have been used to obtain mantle heterogeneities on a global scale. These "tomographic" studies can also be used to measure the topography of the core-mantIe boundary. First interpretations of the tomographic results indicate the importance of these studies toward understanding the dynarn~cs of the Earth. The results for the upper mantle show that the anomalies under the mid-ocean ridges vary significantly in their depth extent. Anomalies are associated with hotspots, shields, and back-arc basins and may provide important information on their origin. An illustration of the implications of this new geophysical ap- proach ~ the effort to interpret large-scale seismic velocity anoma- lies in terms of variations in density. The seismological results therefore can be integrated with interpretation of the gravity field and even the magnetic field (roughness of the core-mantle bound- ary is related to the westward drift of the non-dipole field), and can provide constraints on the rheology. Elements of the pattern of the flow in the mantle can be predicted and can be used to constrain the range of lateral variations in temperature and com- position. While some progress can be gained through refinements in analysis of the existing data base, it is clear that a major im- provement in the resolution can only be achieved by a significant increase in quality and quantity of seismic observations with a new global seismic network. Another unport ant application of such a global seismographic network is studying earthquakes. Quantitative determinations of the energetics and kinematics of earthquake sources are applied
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25 now systematically to several hundred events per year. Accumu- lation of such data for the past 7 or 8 years allows us for the first tone to monitor variations in the pattern of stress accumula- tion and release. In particular, indications of stress ~rugration and diffusion have been inferred for several subduction zones. Several interesting properties can be indirectly deduced from measurement of variations of Earth's orientation. Measurement of Earth's precession constant already provides the most accurate estimate of Earth's moment of inertia. Recent results from the Polaris very long baseline interferometry (V[Bl) network indi- cate that the amplitude of the annual notation differs from mode] predictions based on a hydrostatic Earth. The datum may indi- cate that the core-mantle boundary has a small non-hydrostatic ellipticity on the order of 300 m in amplitude that is somehow sup- ported by mantle convection. Detection of the free core rotation (namely, its frequency) or changes in other notation constants (semiannual and Midyear) could corroborate this interpretation. These measurements cad for a commitment to a long-term oh servation program utilizing Polaris, lunar laser ranging, and the Laser Geodynamic Satellites (LAGEOS). Another major structural parameter that may be inferred from these kinds of studies is inner core/fluid core density contrast. One method would be to attempt to observe the translational modes of the inner core by measuring the short-period variations in gravity at Antarctica. So far, this experiment has failed to observe any signal. An alternative would be to detect the inner core free wobble through its effect on the mantle wobble. The core wobble frequency is proportional to the core ellipticity and density contrast and requires core rigidity for time scales of at least a few years' duration. The existence of this mode requires an excitation mechanism (core dynamos. The detection of this mode may, therefore, provide new information related to dynamo processes at depth. Detection of the effect of inner core on mantle notation is another possibility, although this must be separated from the effect of core-mantle ellipticity. Data from laser ranging to reflectors placed on the Moon dur- ing the Apollo missions have been collected on a routine basis since 1969. Precise ranges have been obtained from Texas and Hawaii. This data set has proved extremely useful in determining variations in earth rotation and polar motion, tidal recession of the Moon, possible detection of a lunar core, and detection of lunar
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26 Chandler wobble. More operational ranging stations have recently been added in France and Hawaii. A refitted Australian system will soon be added to this list. The expectation is that these stations will range more frequently, with more accurate ranging systems, to obtain 5-cm or better normal point accuracy. The promise is that this system will complement the Polaris V[BI net- work in estimating earth orientation variations in earth rotation, polar motion, and notations. More precise measurements of lunar parameters are also expected from this growing network. Currently, V[BI and other techniques are greatly improving constraints on the rate of the Earth's rotation and the direction of the rotation axis (wobble). At the opposite end of the spectrum, we finally have two-cligit accuracy on the rate of tidal dissipation. It can be expected that techniques of instrumentation and analysis will continue to improve over the coming decade. The NAVSTAR satellites of the Global Positioning System (GPS), a carefully lo- cated set of strong sources, will make a significant contribution. By 1995 there should be an order-of-magnitude improvement in the sorting out of the contributions of the atmosphere, tides, and core-mantIe interaction to the spectra of rotation and polar wow ble, and we may have the first reliable determination of a change in the pole path due to an earthquake. History of l:arth's Crust The origin and early evolution of Earth might be considered so remote in time that there is no hope of gaining any mean- ingful information from measurements made today. However, the Earth has memories on various time scales and ancient rocks do exist. There are also objects in the solar system of various ages and at different stages of development that provide information complementary to earth-derived data. What constraints do we have? The most obvious constraint is the age and composition of the oldest rocks. These rocks show that water was present on the surface of the Earth and a magnetic field existed at about 3.8 eons BP. The presence of ultramafic komatiites in the Archaean suggests that the mantle was hotter at that time than it is now, or else that it was easier for high- density magmas to reach the surface. Isotopic data show that the Earth was separated into chemically distinct reservoirs in its early history. The decay of the radioactive isotopes of potassium,
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41 Relation of Physical and Biological Earth History From the perspective of the solar system, the Earth appears to be unique in its capacity to sustain life. Not only does the Earth sustain life today, it has done so for at least the last 3.5 bil- lion years. Organically preserved microfossiTs found in the oldest known unmetamorphosed sedimentary rocks document the early evolution of bacterial communities containing morphologically dii- verse organisms. Stromatolites (the sedimentary-free fossils of mi- crobial mat communities) further indicate that some early organ- isms were phototactic, and stable carbon isotope ratios in kerogen strongly suggest a carbon cycle driven by photosynthesis. Indeecl, the evidence available from paleontology, geochern~stry, and mi- crobiology suggests that anaerobic biogeochemical cycles were well established by the time the Earth's oldest surviving sedimentary rocks were deposited. Metabolism links biology closely to atmospheric science. Or- ganisms both produce and consume gases, thereby affecting the composition of the atmosphere. Not only is atmospheric oxygen re- lated to the evolution of cyanobacteria and (later) algae and green plants, but many other important gases such as nitrous oxide (a product of bacterial ammoniac oxidation and nitrate reduction) and methane (a product of bacterial methanogenesis) owe their presence to biogenic processes that track back to the Archean diversification of bacteria. Evolutionary innovations in biologi- cal structure and physiology may have had profound effects on the Earth's radiation balance, climate, rates of surface weathering and erosion, and the rates of deposition, diagnosis, and distribution of sediments. The extent to which our planet's surface, hydrosphere, and atmosphere have been altered by life throughout its history is a scientific problem of major theoretical and practical significance. Global Biota: Revelations Tom Space Nearly 4 billion years of evolution have produced a diverse biota estimated to include as many as 10 million distinct species. These species are not distributed randomly across the planetary surface; rather, they are organized into an ecological hierarchy based on biological interactions of species with similar physical
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42 tolerances. Local, recurrent associations of species form commu- nities that interact with the physical environment to form ecosys- tems. Congruent ecosystems can be grouped in bigger units called biomes, characterized by similarities in plant growth forms, com- munity structure, and productivity, among other things. Some biomes, such as the Great Plains grasslands, have a high capac- ity for primary production and so are crucial to the support of the human population. Others, such as the Arctic tundra biome, are less productive, but are believed to contain large quantities of organic carbon in their soils and may be sensitive indicators of global changes in temperature or pollution levels. The areal extent of biomes, gas fluxes into and out of them, and their mean primary productivity are very imprecisely known at present; however, be- cause blames have spectral properties that permit identification and analysis by remote sensing, the perspective from space makes possible the detailed global analysis of blame distribution and production. Global ecosystem analysis, especially of agriculturally impor- tant systems, will provide information prerequisite to the scientific estimation of the Earth's capacity to support the growing human population. Analysis of forested biomes, especially the tropical rain forests that are increasingly being destroyed, Is essential for efforts to understand the changing carbon dioxide content of the atmosphere. There can be no accurate quantitative models of bio- geochemical cycles and their interactions until such an ecosystem or blame inventory has been made. Once such data are produced, a host of hitherto insoluble geochemical and biological problems will be brought within our grasp. It is clear that we must continue to monitor the state of the Earth in this context for the indefinite future, to document changes in biotic regimes as they occur, and to aid in the development of quantitative models to assess the impact and origin of changes observed. Biogeochemical Cycles The state of knowledge and the crucial significance of water and the hydrological cycle have been discussed. But the movement of material through living organisms involves many more elements. The carbon, nitrogen, phosphorus, and sulfur cycles, to name just a few, are critical to the mechanisms and maintenance of life on Earth. The state of knowledge and major problem areas for each
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43 of these four major cycles are discussed below as a prelude to determining a strategy for studies in the 1995 to 2015 period. The task group emphasizes mainly the shorter time scales. However, the role of plate tectonics subduction and volcanism- in circulating and remobilizing these constituents may be crucial to the maintenance of life on Earth on the longer time scales. It is essential in this context to understand the nature of the forces responsible for volcanism for the recycling of critical elements such as carbon. To what extent are the changes in climate characteristic of past ages of our planet attributable to fluctuations in rates of cycling of carbon and associated variations in atmospheric carbon dioxide? The Mission to Planet Earth, with its interdisciplinary focus, must seek to provide answers to these questions. According to the NRC report Global Change in the Geosphere- Biosphere, There is abundant evidence for change at present. Most obvious perhaps are changes in the composition of the atmosphere—of CO2, CH4, CO, N2O, NO2, SOL and O3 and changes in the chemistry of precipitation. There are more subtle effects associated with altering practices of land and energy use, and of waste disposal. Anthropogenic changes are superimposer] on natural fluctuations, and it is difficult to separate the anthro- pogenic from the natural changes that are taking place today. There are clues, however, from the record of the past. "An impressive body of information has accumulated recently to suggest that fluctuations in CO2 may have played an important role in regulating at least some of the major changes in climate of the past. The level of CO2 was approximately 200 ppm during the last Ice Age. It rose by about 50 percent, to approximately its present value, in only a few thousand years, 10,000 to 12,000 years ago, ushering in the present interglacial period. "We can reconstruct the history of CO2 back to about 60,000 years before the present using air trapped in bubbles in ancient ice preserved in Greenland and in Antarctica. A more indirect technique, based on analysis of the isotopic composition of carbon in the carbonate skeletons of marine organisms In ocean sediments, has allowed us to extend the record even further, to about 400,000 years ago. The correlation with climate ~ striking. High CO invariably associated with warm conditions, low CO2 with cold; and indeed, changes in CO2 appear to precede changes in climate. Carbon dioxide is but one of several gases with the potential to rape the temperature of the Earth. Infrared radiation from
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44 the planetary surface is also absorbed and reradiated by methane (CH4), nitrous oxide (N2O), and O3 and by the industrial halocar- bons, CF2CI2 and CFCI3. On a molecule-per-molecuTe basis, these gases are much more efficient than CO2 in altering the radiative balance of the present Earth, and their concentrations are also changing. Their cumulative effect on climate over the past several decades may be comparable with that of CO2." The Carbon Cycle As was pointed out in Part IT of the CES strategy: "There are two central chemical processes in the carbon cycle: aerobic oxidation and anaerobic oxidation. Increases in the rate of aerobic oxidation are the probable cause of the observed increases in at- mospheric CO2; increases in the rate of anaerobic oxidation may be the cause of the observed buildup of CH4. The case of CO2 exemplifies many of the limitations in our current understanding of global cycles as well as important gaps in current data sets.... "The possible effects of human interference with the natural cycle of carbon by burning fossil fuels, harvesting forests, and converting land to agriculture are reflected most clearly by the phenomenon of increasing concentration of atmospheric CO2 [see Figure 2.44. If current trends continue, the atmospheric concen- tration will exceed 600 parts per million by volume by the year 204~more than 2 times the preindustrial level. The increase in CO2 is important because, in contrast to atmospheric O2 and N2, CO2 absorbs infrared radiation emitted by the Earth and prevents the escape of some of the normally outgoing radiation. This is known as the 'greenhouse' effect high efficiency on Venus. , a phenomenon operating with "At present, our ability to interpret the carbon cycle and thus predict future CO2 concentrations is confounded by unresolved im- balances in the carbon budget. Simply stated, the annual budget does not balance unless (1) fertilization effects, either terrestrial or aquatic, partly offset deforestation minus regrowth, (2) the im- balance diminishes from reductions in the estimate of the rate of deforestation or increases in the regrowth, (3) the oceanic uptake is underestimated, or (4) there are natural variations in the global rate of carbon uptake by the biota that are not yet recognized." Methane is the second most abundant form of carbon in the atmosphere. Its presence reflects the importance of localized media
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45 340 330 Q z IS z z o a 290 320 310 300 280 270 ~ 1 1 ~ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1700 1750 1800 1850 1900 1950 2000 MEAN GAS AGE (yr AD) FIGURE 2.4 Measured mean CO2 concentration plotted against the esti- mated mean gas age. The horizontal axis of the ellipses indicates the close-off time interval of 22 yr. The uncertainties of the concentration measurements are twice the standard deviation of the mean value, but not lower than the precision of 1 percent of the measurement. The dotted line represents the model-calculated back extrapolation of the atmospheric CO2 concentration, assuming only CO2 input from fossil fuel. Atmospheric CO2 concentrations measured in glacier ice formed during the last 200 years calibrated against the Mauna Loa record for the youngest gas sample. SOURCE: Neftel et al., Nature, volume 315, pages 45-57, 1985. where oxygen is deficient, as in swamps and the soil of rice paddies, for example, or in the digestive tracts of ruminants and a variety of other animals, including termites. Its abundance is now increasing at a rate of about 2 percent per year. The concentration in the atmosphere appears to have doubled since the sixteenth century. Why? How will it vary in the future? What was its level and scale of variation in the past? The Nitrogen Cycle The NRC report Global Change in the Geosphere-Biosphere noted that Nitrogen occurring in compounds as single atoms (fixed nitrogen) is chemically versatile and essential for life, with a range of oxidation states from -3 to +5. Processes that break the N-N bond (nitrogen fixation) are relatively slow, amounting
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46 to less than 0.2 x 10~5 g/yr of N. Recombination of fixed nitrogen to form N2 is also slow, owing largely to the kinetic stability of inorganic, fixed nitrogen (NH4+, NO2-, NO3-) in solution. The recombination reaction Is carried out biologically by bacteria using NO3- and NO2- as electron acceptors (denitrification). Denitrifi- cation takes place in anoxic, organic-rich locations such as flooded soils and estuarial sediments, bottom waters of some deep ocean basins and trenches, and in low-oxygen or anoxic waters at in- termediate depths in coastal upwelling regions. Denitrification is essential to the preservation of the present regret of atmospheric N2. In the absence of biological processes, the atmospheric nitrogen cycle would be open, leading to accumulation of NO2- and NO3- in the oceans. It is unclear how the global system acts to estate fish a balance between fixation and denitrification. Mechanisms directly coupling nitrogen fixation to denitrification have not been identified, and indirect connections are not obvious. Nitrogen is cycled through the biosphere at rates 10 to 100 times as large as the rate for fixation of N2. Inorganic fixed nitro- gen (NH4+, NO2-, NO3-) ~ assimilated into terrestrial biomass at a rate of about 3 x 10~5 g/yr of N. but this influx ~ balanced by decay of organic material. The rate at which inorganic fixed ni- trogen is consumed and recycled by biota in the oceans Is roughly 2 x 10~5 g/yr of N. with a large uncertainty. Internal cycles of mineral and organic nitrogen are essential links in the life-support system of the planet." The supply and distribution of fixed nitrogen thus affect not only the biosphere's productivity, but also the chemical and radi- ational environments for life. Changes in the abundances of atmo- spheric nitrous oxide and nitrogen oxides attest to the importance of contemporary changes in the biogeochemistry of nitrogen. How did levels and distributions of fixed nitrogen vary in the past and how did they relate or act to influence the climatic and biospheric condition of the planet? The Phosphorus Cycle Part II of the CES strategy pointed out that Phosphorus Is an essential element for life. It is relatively abundant in the crust of the Earth, but it exists principally as insoluble minerals (ap- atite, iron phosphates) or as absorbed phosphate. These forms are not available for biological uptake, and consequently, phosphorus
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47 is often a limiting nutrient in soils, lakes, and perhaps even ma- rine systems. Atmospheric transfer processes are unimportant for phosphorus, in contrast to carbon, nitrogen, and sulfur. Rather, the major phosphorus exchanges are associated with dissolved and particulate transport in rivers, and with weathering processes and diagensis in soils and sediments. There are thus important con- nections between the hydrologic cycle and the phosphorus cycle. "Most of the phosphorus in rivers is insoluble and biologi- cally unavailable, and there are major questions about the actual fraction of river-borne phosphorus that manages to participate in the biological cycle and the time scale for effective transfer from rivers to the oceans.... Additional uncertainty Is associated with storage of phosphorus in estuarine and coastal sediments. This latter issue is important since this phosphorus could be mobilized during epochs of low sea level (e.g., during glaciation) and delivered to the ocean where it could be responsible for an increase in biological productivity and a consequent drop in carbon dioxide. What determines the level of oceanic phosphorus? How has it varied in the past? What factors are responsible for change in oceanic phosphorus and what are their consequences? The Sulfur Cycle Sulfur is also an essential element for life, but unlike nitrogen and phosphorus it Is rarely limiting. It exists, like nitrogen, in a variety of oxidation states, from—2 in sulfides to +6 in sulfates, and is cycled among these states by the biota, by volcanoes, by combustion of fossil fuels, and by atmospheric reactions. As noted in Global Change in the Geosphere-Biosphere, "sulfur enters the atmosphere in two dominant ways. Combustion of fossil fuels adds sulfur in the form of SO2. Microorganisms in soils and in the surface waters of the ocean putatively contribute additional amounts in the form of (CHARS, H2S, and other reduced sulfur gases, but the precise amount is unknown and controversial. These reduced gases are oxidized to SO2 on time scales of hours to days. Anthropogenic and natural inputs of SO2 to the atmosphere are apparently comparable in amount. "The SO2 is oxidized to sulfate and in this way removed from the atmosphere on time scales of several days. The sulfur oxidization processes depend on atmospheric levels of the OH
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48 radical and thus on the abundances of atmospheric 03, H2O, nitrogen oxides, and hydrocarbons. din the soil and in the ocean photic zone, sulfate is taken up by plants and microorganisms. The sulfur is then recycled to the atmosphere through processes of decay; some accumulates in organic matter in ocean sediments. On geologic time scales, sedimentary sulfur is returned to the ocean-atmosphere system through volcanism." In addition, Part IT of the CES strategy states that "for the sulfur cycle, there is a need to identify and quantify the anthro- pogenic and biological fluxes of reduced sulfur gases and determine whether these fluxes are subject to change. A far better under- standing of the atmospheric chemistry of the reduced sulfur gases and the SO2 from combustion is also needed. Of particular con- cern in this chemistry are the roles of heterogeneous reactions, the coupling to atmospheric nitrogen and carbon chemnstry, and the mechanisms for dry and wet deposition. Finally, we require far more information on the manner in which sulfuric acid deposition affects the biology and geochemistry of terrestrial ecosystems." The external information needed to mode} these processes in- cludes the major biological sources and sinks of organic carbon and active nitrogen, and inputs of sulfur and other compounds from volcanic activity. Urban pollution is a topic all by itself, but is a major regional source of tropospheric ozone, oxides of nitro- gen, sulfur dioxide, and other ingredients of larger-scale problems like acid rain. Also required is information on the transport and mixing capacity of the atmosphere. Clouds play a key role in cat- alyzing certain reactions and in scavenging water-soluble products in precipitation. Although in principle the atmospheric transports and cloud fields are available as part of the modeling and data base of the physical climate system, in practice, considerable addi- tional effort is required to make them useful for chemical purposes. Just as unportant are the internal measurements, which give guid- ance as to which chemical processes are most significant, and give confidence that they are being modeled correctly. Human Activities There are concerns about both (1) human effects on the en- vironment, and (2) effects of natural phenomena on man. Both
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49 are exacerbated by growing population, and industrial and agri- cultural development. The basic scientific issues of biogeochemical interaction have been discussed above; the global issues and nec- essary measurements will be discussed in Chapter 3. PLANET EARTH IN THE SONAR SYSTEM A global understanding of the Earth entails explanation of why it is different from other planets. These differences arise from a relatively few fundamental properties mass, composition, distance from the Sun, rotation rate but often the secondary manifestations are greater than expected. Furthermore, the other planets must be considered in any meaningful consideration of the Earth's formation, which constitutes the starting conditions for the Earth's evolution. At least eight bodies in the solar system seven planets and one satellite have significant atmospheres whose dynamics and chemistry should be explained by any comprehensive theory of at- mospheres. Dynarn~cally, the atmosphere of Mars is most similar to the Earth's, in that it is a relatively thin fluid envelope around a rapidly rotating rocky planet, subject to marked seasonal varia- tions because of an appreciable tilt of the rotation axis to the orbit axis. Mars, like the Earth, has cyclonic systems of weather and, on a much longer time scale, appears to have undergone glacial waxing and waning. But beyond confirming a few fundamentals, Mars does not contribute significantly to the solution of prom lems regarding the Earth's atmosphere, most notably because it lacks an ocean. Compositionally, the atmospheres of both Venus and Mars could have been quite similar to the Earth's with one striking exception: the much greater complement of primordial inert gases in Venus, a difference that must be a consequence of circumstances of formation 4.5 billion years ago. Otherwise, the examples of Venus and Mars act as strong constraints on theories of atmospheric evolution: any worthwhile theory must account for the loss of water from the Venusian atmosphere (most likely by photodissociation, leading to loss of hydrogen and trapping of the free oxygen in surface rocks), resulting in the development of the greenhouse effect, so that the comparable portion of car- bon dioxide in Venus stays in the atmosphere rather than being incorporated in the ocean and thence in carbonate rocks. Pioneer Venus revealed that the high surface temperature of
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50 Venus arising from its massive carbon dioxide atmosphere has had important consequences for the solid planet. Venus undoubtedly has mantle convection, since it would have incorporated almost the same energy sources as the Earth. But its rocky surface is quite lacking in indicators of plate tectonics, such as an interconnected ridge system like the ocean rises on Earth. Hence the boundary layer of the mantle convection within Venus must be at depth, below a basaltic and sialic crust. The entire surface of Venus may be covered by a continent-like crust; the Pioneer altimetry indicates that there is only one predominant level of topography, rather than two, as on Earth. Venus ~ also significantly different in that it has no magnetic field. It certainly differentiated an iron core, but if the pressure is too low and the temperature too high no inner core will form, thus eliminating a possible energy source for a geodynamo. These marked differences of the planet most similar to the Earth in size and composition act as unportant constraints on models of the early evolution of the Earth including crustal formation, out- gassing, and other events of the early Archean, more than 2.5 billion years ago. Constraints of a somewhat different sort in understanding the Earth arise from consideration of its formation. Moon rocks and meteorites indicate, by their radiological ages, that formation of all the planets took place within a few 10 million years some 4.57 billion years ago. The retention of abundant hydrogen and helium by Jupiter indicates that it was quite massive before the forma- tion of the terrestrial planets was markedly advanced. Hence, the dynarn~cal circumstances of terrestrial planet formation were dominated by the gravitational influence of Jupiter, which prow ably was important in inducing growth to only four planets plus a satellite, rather than a larger number of smaller bodies. This growth pattern probably led to the terminal stages of formation being characterized by a few great impacts. Important evidence of that includes the differences in rotation rates and inert gas re- tention between Venus and the Earth, and the anomalously low iron content of the Moon. It is the current consensus that the Earth was probably hit by a very large body perhaps bigger than Mars which led to the lifting off of the material that made the Moon, and which removed virtually all of any primordial at- mosphere from the Earth. Consequently, the Earth formed very
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51 hot, leading to early core separation and outgassing of the atmo- sphere and ocean. A likely by-product of that was formation of a crust that was similar to the Moon's. Another consequence of the hot beginnings of the Earth was obliteration of any evidence of this crust. The lunar crust is 10 percent of the mass of the body. By contrast the terrestrial crust is less than 0.4 percent. Another application of comparative planetology to Earth his- tory is the record of cratering on the surfaces of the Moon, Mars, and Mercury. This record indicates that throughout this history there has been a sporadically declining infall of bodies, a few siz- able enough to have global effects catastrophic for major parts of the biosphere. Firm chemical evidence of such effects has only recently been deduced: most notably, the marked iridium spike at the horizon marking the end of the Cretaceous period 60 million years ago. The history of the Earth shares many common threads with the histories of one or more of the other inner planets, includ- ing early global differentiation of crust and core, outgassing and evolution of the atmosphere, early bombardment of the surface by a heavy flux of meteoroids, and development of a global mag- netic field and magnetosphere. The Earth has many attributes not shared, however, with any other known planet, including its oceans, the oxidized state of its atmosphere, its tectonic plate mo- tions and the consequent complex history of crustal deformation, and its life forms. A continuing challenge to the earth and plan- etary sciences is to account for the profoundly unique attributes of the Earth in the context of the common processes that have shaped the formation and evolution of the solar system. Various attempts have been made to use the Earth as a detec- tor of cosmic, stellar, and solar system events. In order to do this, however, the Earth itself must be better understood. Earth is a collector of extraterrestrial particles and thus can be used to estimate the current meteorite flux. Some have used extinctions throughout the geologic record to propose periodicity, or at least episodicity, in the influx rate of larger objects. Mete- orites falling to Earth are one guide to processes in the early solar system and processes in small disrupted objects.
Representative terms from entire chapter: