Mission to Planet Earth Space Science in the Twenty-First Century -- Imperatives for the Decades 1995 to 2015 (1988) / Chapter Skim
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2. Earth Sciences--Status of Understanding
2. Earth Sciences--Status of Understanding
Pages 16-51
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From page 16... ...
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 future, make this timely.
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From page 17... ...
As such, the earth system is strongly coupled with widely varying rate constants. The Earth is the only planet that supports plate tectonics.
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From page 18... ...
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.
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From page 19... ...
Nor do we know whether precursory 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.
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From page 20... ...
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.
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slurry convection near the top of the core, or (3) chemical convection driven by compositional change and latent heat release at the boundary between the liquid outer core and the more solid inner core.
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From page 22... ...
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.
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From page 23... ...
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 contamination by crustal anomalies and ionospheric currents.
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From page 24... ...
However, mantle convection inevitably entails lateral heterogeneity. Recently, inversions of seismic travel times have been used to obtain mantle heterogeneities on a global scale.
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From page 25... ...
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.
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From page 26... ...
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 meaningful information from measurements made today. However, the Earth has memories on various time scales and ancient rocks do exist.
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From page 27... ...
An understanding of the other terrestrial planets is of obvious importance, since they are all at different stages in their evolution because of differences in size and surface temperature. The Moon and Mars represent thermally old bodies because of their small size, but since they are relatively inactive they retain surface evidence of ancient processes.
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From page 28... ...
THE E:A1ITH'S AIR AND WATER The atmosphere, ocean, land, and biota form an interactive system that determines the current state and evolution of the portions of the Earth in which life evolved and on which we now live. A detailed understanding of this interactive system is necessary for prediction of climate on time scales ranging from months to centuries.
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From page 29... ...
It is possible that the dynamics of the atmosphere at the mesoscale level are basically unpredictable beyond 10 days, the small currently unmeasured m~croscale instabilities growing into macroscale perturbations in about 2 weeks. However, through recent experimentation on the global scale it appears that if accurate measurement of tropospheric winds, precipitation, and fluxes of water vapor from the surface into the atmosphere are made available, our ability to forecast weather for a period of a week to 10 days may improve substantially.
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From page 30... ...
Such a prediction therefore will have to await a more comprehensive understanding of the total interactive system. It now appears that the cycling of carbon, nitrogen, sulfur, and potassium through the atmosphere, biosphere, and oceans is interlinked, and that the tropospheric concentrations of molecules like methane, carbon monoxide, nitrogen oxides, ozone, and sulfur dioxide are interrelated by complex chemistry involving the OH radical, which has so far remained unmeasurable.
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From page 31... ...
Because of low temperature and presence of water vapor, once the gases arrive In the stratosphere they condense into small droplets of sulfuric acid and persist for up to several years. Through the solar energy absorbed by ozone and aerosob in the stratosphere and the role these constituents play in adding to the greenhouse effect in the troposphere, it is clear that the stratosphere plays a substantial role in modulating the energetice and dynamics of the lower atmosphere.
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From page 32... ...
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From page 33... ...
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From page 34... ...
Processes at work in the ocean are also unportant in the budgets of many chemically and radiatively important gases, such as carbon dioxide, nitrous oxide, methane, and many sulfur-containing gases. In the polar regions the ocean interacts with the atmosphere to produce sea ice, which is itself a sensitive indicator of global warming or cooling and which controls the rate at which heat can escape from the polar oceans into the overlying atmosphere.
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From page 35... ...
Existing weather satellites operated by NOAA, the Department of Defense, and non-U.S. space agencies provide some routine ocean surface observations, but we still lack crucial data on surface winds, ocean currents, biological productivity, and the gravity field of the Earth." What do we expect to have measured and learned about the ocean and its interaction with the atmosphere, solid Earth, and biosphere by the year 1995 with the planned satellite and in situ programs?
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From page 36... ...
of the CES strategy addressed the issue of fresh water and ice in some detail. The report observed that "water is the most abundant single substance contributing to the global biomass and cycling through the biosphere.
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From page 37... ...
37 ootC, _ ~ o_ In J Cal = O— 100,000 1,000 1 00 YEARS YRS.
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From page 38... ...
These changes all lag the climatic forcing factors by Mitering amounts, and they occur on top of natural variations that must be understood to adequately manage their immense consequence for food productivity and other human uses of the Earth. Likewise, the global hydrologic consequences of large-scale water diversion projects, urbanization, tropical deforestation, and regional irrigation projects need to be assessed.
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From page 39... ...
the problem of climate variability is linked to orbital changes of the Earth, to deep ocean circulation, and to continental drift arising from plate tectonics. The record of these climatic changes
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From page 40... ...
LIFE ON EARTH Our objective is to characterize the interactions between the biota and other components of the Earth most notably the atmosphere, oceans, and the solid Earth. These data will be used both to make predictions about the short-term future behavior of climate and selected ecosystems, and to better understand the past behavior of the biosphere and climate system, particularly as this relates to changes in the physical and chemical environment of planet Earth.
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From page 41... ...
further indicate that some early organisms 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 microbiology suggests that anaerobic biogeochemical cycles were well established by the time the Earth's oldest surviving sedimentary rocks were deposited.
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From page 42... ...
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, because 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.
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From page 43... ...
However, the role of plate tectonics subduction and volcanismin 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.
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From page 44... ...
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 atmospheric CO2; increases in the rate of anaerobic oxidation may be the cause of the observed buildup of CH4.
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From page 45... ...
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)
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From page 46... ...
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 radiational environments for life. Changes in the abundances of atmospheric nitrous oxide and nitrogen oxides attest to the importance of contemporary changes in the biogeochemistry of nitrogen.
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From page 47... ...
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.
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From page 48... ...
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 anthropogenic and biological fluxes of reduced sulfur gases and determine whether these fluxes are subject to change.
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From page 49... ...
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.
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From page 50... ...
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.
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From page 51... ...
The history of the Earth shares many common threads with the histories of one or more of the other inner planets, including 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 magnetic 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 motions and the consequent complex history of crustal deformation, and its life forms.
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