Space Science in the Twenty-First Century

Imperatives for the Decades 1995 to 2015

Overview

2

Earth Sciences: A Mission to Planet Earth

BACKGROUND

We now have the technology and the incentive to move boldly forward on a "Mission to Planet Earth." The steering group calls upon the nation to implement an integrated global program of fundamental research with space-borne and earth-based instrumentation. Such a program would probe the origin, evolution, and nature of our planet, its place' in our solar system, and its interaction with living things, including mankind.

For earth sciences it is particularly appropriate to focus on planning for the period from 1995 to 2015. This is because the science base of this discipline is well developed. Various observational systems have already been established, and programs extending into the last decade of this century have already been proposed. The long lead times associated with the development of spacecraft and sensors mean that recommendations adopted now will not affect current programs until at least the mid-1990s. Thus, a planning document at this time is particularly relevant.

During the past 2 or 3 years, there has been an enormous amount of planning for a study of Earth as a global system, and for an observing system to monitor global change. It is clear that such a system must be largely space-based, yet the earth-based part of the measurements is integral as well. Several recent reports have helped to set the scientific context for such global studies. These have come from the National Research Council (Committee on Earth Sciences, Committee for the International Geosphere-Biosphere Program, Space Applications Board), from NASA (Global Habitability, Earth Systems Sciences Committee), and from the International Council of Scientific Unions (Committee on Global Change). The technological context in which these studies will be carried out will depend largely on the pace of development of global observing systems. (For a general policy statement on cooperation, see Chapter 10.)

EARTH AS A GLOBAL SYSTEM

The records of the first human attempts to understand Earth are lost in antiquity, but we know that early man made exploration voyages in the Pacific, Atlantic, and Indian oceans. As early as the third century B.C. the Greeks knew that Earth was a finite globe and were able to estimate its circumference. Thus, from ancient times to the present, we have used exploration and physical reasoning to understand earth processes and to explore the Earth's place in the solar system.

Modern techniques and new integrated programs have yielded improved information about the state of the atmosphere, the ocean, and the land surface. We have been able to directly measure continental drift, and to probe Earth's crust by drilling; seismic and acoustic techniques have let us probe even deeper. In addition, we now possess improved weather forecasts and new information about agricultural conditions. Measurements of winds and waves on the ocean's surface, of ocean currents, of primary productivity in the ocean, and of the chemical constitution of the atmosphere have all added to our understanding of global systems.

Very recently, interest has focused on problems where advances could have important societal impacts. These problems include the prediction of earthquakes, volcanoes, and climatic anomalies such as El Niño, whose economic impact is measured in billions of dollars. The increase in the atmosphere of carbon dioxide and other gases that may contribute to a "greenhouse" effect has also focused attention among scientists. New tools and ideas will allow us to address such problems.

We also have much new information about the atmosphere and surface properties of the other planets that will help us in understanding our own. As we have learned about the other planets of the solar system, it has become evident that Earth is different in several remarkable ways. The blue and white of Earth contrast sharply with the red of dusty Mars, the dazzling whiteness of Venus, and the complex swirling colors of Jupiter. Continued exploration has shown other, fundamental differences between planet Earth and all other planets of the solar system. The most striking of these is that living creatures have existed on Earth for more than 3.5 billion years, evolving continuously from the simplest one-celled organism to the present diversity of life forms. In contrast, it is probable that biological activity is not—and perhaps never was—present on any of the other planets during the lifetime of the solar system.

Because liquid water is essential for life on Earth, the survival and evolution of biological organisms provide a convincing argument that Earth has always had water on its surface at a temperature to keep it liquid. Without the oceans, Earth's atmosphere would be profoundly different. For instance, we have only modest amounts of carbon dioxide in our atmosphere, thus avoiding the greenhouse effect experienced by Venus. It is believed that nearly all the carbon dioxide that has flowed from Earth's interior has been buried in ocean sediments as limestone or organic carbon. The presence of free oxygen would be impossible without the photodissociation of water and the consequent escape of hydrogen. Without the presence of oxygen, ozone would not exist in the stratosphere to shield surface life from destructive solar radiation. Most animals could not then exist, since they depend on oxygen-based metabolism.

In turn, other processes must limit these ocean effects to keep Earth habitable. Oxygen in moderate amounts is a necessity for animal life, but in higher concentrations it is toxic. If organic nutrients continued to accumulate in sediments, all nutrients would eventually return to insoluble forms. If limestone sediments continued to accumulate without a compensating inflow of carbon dioxide, photosynthesis would taper off as the carbon dioxide concentration fell.

Such a compensating inflow of carbon dioxide does, in fact, occur as part of the remarkable phenomenon of plate tectonics. This process of continual recycling of Earth's surface materials into the interior, and their reappearance in mid-ocean ridges and volcanoes, is probably essential to preserving Earth's benign environment. Moreover, motions deep in Earth's interior drive the plates and generate the magnetic field that partially shields it from the harsh environment of space.

Thus it is Earth's own inner life, together with the interactions of its unique surface phenomena, that has determined its history and our own. A convective process deep in Earth's core—fired by radioactive decay and the primordial heat of agglomeration—has joined the complex interplay of the atmosphere and oceans with the biosphere to forge the world we know. However, major questions remain unanswered. Why does the phenomenon of plate tectonics operate on Earth but not on Mars and not, perhaps, on Venus? What are the characteristics of Earth that make plate tectonic convection possible? What are the nature and rate of convection? What are the effects of changing rates of convection on atmospheric carbon dioxide concentration, and hence on climate and on the biosphere? What insights can we gain from studies of the variable magnetic field generated by Earth's interior dynamo? How do the ocean and the atmosphere interact to produce long-term climate change? What is the role of the biosphere in climate? And, finally, how does Earth work as a system?

Even the origins of life may be related to plate tectonics. We have discovered complex ecosystems around deep-sea vents in the mid-ocean ridges. In the vents' scalding water live anaerobic sulfide-oxidizing bacteria that provide the energy and organic compounds for the local animal inhabitants. This environment may have been the cradle of life on Earth, despite its inaccessibility to photosynthesis. High temperatures would have allowed rapid chemical reactions and reduced sulfur compounds for energy. The overlying water would have shielded organisms from destructive ultraviolet radiation.

Another unanswered question is the effect on Earth of asteroid and comet collisions. What has been their effect on the evolution of life? The "great dyings" in the biological record may be due to these collisions, stimulating, in turn, the rapid evolution of new life forms. A careful search for evidence of such collisions in the geologic record could throw a new light on evolutionary processes.

In more general terms, it is clear that a comprehensive study, from Earth's outer atmosphere to its inmost core, is essential to understanding the conditions for life. Advances in our ability to observe the planet both from space and from Earth itself now make such a global study possible. For example, we will soon possess computers that can model the turbulent flows typical of the oceans, the atmosphere, and molten materials. Between now and 1995 many of these earth-monitoring systems will be tested, and a number of research missions for remote sensing will be carried out. As the steering group looks to the period 1995 to 2015, it foresees the application of these results to the development of an ongoing observational system for the Earth. Understanding Earth as a complex whole will begin from such global studies.

SCIENTIFIC THEMES

Four overarching scientific themes (also called "grand themes" will guide the study of earth processes:

1. Determining the composition, structure, and dynamics of the Earth's interior and crust, and its evolution.

2. Establishing and understanding the structure, dynamics, and chemistry of the atmosphere, oceans, and cryosphere, and their interactions with the solid earth.

3. Characterizing the interactions of living organisms with the physical environment.

4. Understanding and monitoring the interaction of human activities with the natural environment.

The first of these themes is aimed at determining the composition, structure, and dynamics of Earth's interior and crust, and understanding the processes by which Earth evolved to its present state. Important properties of the mantle such as its composition, the spectrum of convective scales, and the relation between volcanism and tectonics are not understood. We will require measurements by seismic and other arrays of earth-based instruments, together with computer modeling and the monitoring of global gravity and magnetic fields, to fathom these processes.

The second theme is aimed at understanding the structure, dynamics, and chemistry of the oceans, the atmosphere, and the cryosphere. The interaction of these with the solid earth must then be detailed. Today we do not understand the factors that determine the global circulation of atmosphere and ocean, and the interaction of the atmosphere with surface geological and hydrological processes. The effects of biological processes on the hydrological cycle, climate dynamics, and geochemistry are major problems. We require satellite measurements, calibrated and validated from the ground, of these global-scale processes. For example, there is a pressing need for an instrument in orbit that can measure the rate of precipitation on the Earth-a major element in all models of the earth system.

The third theme deals with characterizing the interactions of living organisms among themselves and with the physical environment. This includes their effects on the composition, dynamics, and evolution of the ocean, atmosphere, and crust. The biosphere, for instance, controls the oxygen content and other aspects of the atmosphere, the oceans, and the solid earth. Yet land and ocean ecosystems are poorly understood or described today. Global measurements of biota from space, coupled closely with field experiments, are the key to better understanding in this realm. For example, ocean chlorophyll could be quantified by combining color measurements of the ocean with surface observations.

The fourth theme addresses human interaction with the natural environment. Human activity clearly affects the concentration of gases like carbon dioxide and methane in the atmosphere, as well as the amount of dust. Population increases and deforestation have uncertain implications for climate and genetic diversity. Conversely, many developments have made mankind more vulnerable to natural hazards. Some of these phenomena are best monitored from space, provided that proper calibration and validation are available.

RECOMMENDED PROGRAM: POST-1995

It is clear that to observe such an interactive and complex system as Earth we need both satellite and surface measurements. Satellites provide the global context for regional field studies, and most often are the only way to acquire global data. In particular, the steering group looks to a set of geostationary satellites to provide rapid synoptic images of the whole Earth. In addition, polar orbiters would provide high-resolution data and fill in the polar gaps. Special-purpose orbiters at various inclinations and altitudes would provide measurements as needed and communicate with instrumentation on the surface and in the atmosphere. A key requirement of these observations is their global completeness and simultaneity. Also, the observing system must be designed to assure continuous and consistent measurement over decades.

The volume of data collected by this many-faceted observing system will require faster, more automated, and more adaptable processing systems. Consistent formatting of different types of data from the atmosphere, oceans, and land will be essential. Better integration of modeling and observations will be another important aspect of future earth science systems. It is essential that data acquired over the globe be used both as inputs to these models and as tests for model predictions. This accomplished, scientists could use the entire Earth as a laboratory, following earth processes through their evolution. As always, advances in understanding require a mixture of empirical and fundamental approaches.

Specific recommendations given here, when implemented, will build on the results expected from the sensors and platforms of the NASA Earth Observing System (EOS), currently scheduled to fly as part of the Space Station complex in the mid-1990s. EOS, in turn, will build on its predecessor missions: the Upper Atmosphere Research Mission, the Navy's Remote Ocean Sensing System, the Ocean Topography Experiment, the Geopotential Research Mission, the Tropical Rainfall Mission, and the Magnetic Field Explorer. Other nations' missions, such as the European Space Agency's ERS-1 and Japan's Marine Observation Satellite-1, will also help define the specific parameters needed for adequate earth monitoring. EOS will be the next phase in the development of long-term measurement systems. But here the steering group looks beyond the initial deployment of EOS to lay out a series of specific recommendations for structure and programmatic content of a long-term mapping and monitoring system for Earth.

In this time period (1995 to 2015), the steering group suggests the following elements of an internationally sponsored program (U.S. responsibilities indicated):

1. A Satellite-Based Observing System

a. A set of five geostationary satellites (two provided by the United States) designed to carry a wide variety of instruments to cover the entire Earth for long-term measurements (replacement as required).

b. A set of two to six polar-orbiting platforms (two to three provided by the United States) to cover the polar areas above 60° and to provide platforms for instruments that must be closer to Earth.

c. A aeries of special missions that require other orbits.

Examples range from Shuttle-based instrument tests, to Explorer-type missions, to the Global Positioning System array of 18 satellites. With growing international interest in remote sensing of the Earth, the steering group expects an increasing proportion of joint or non-U.S. missions.

2. A Complementary Earth-Based Observing System

The steering group recommends the continuing development and deployment of a system of earth-based measuring devices to provide complementary data to the apace-based observing network. The data from the network should be transmitted in real time and integrated with observations from space. This earth-based system is an essential element of any observing system for Earth; it measures effects that cannot be detected through remote sensing from space, providing increased resolution in regional studies, as well as calibrating and validating space observations.

3. Theoretical Modeling

State-of-the-art computing technology must be utilized for data analysis and theoretical modeling of earth processes. Modeling earth systems will require the best data sets possible, the fastest computers, and imaginative ideas from research. In turn, modeling can set the context and give direction to future observations.

4. Data Systems

A coordinated system for both archiving and disseminating earth-related data must be established. This is a call not for a central archive, but for a central authority or data management unit. This authority would establish formats and other conventions, identify data location, and provide easy access to all data as required. The data rates from the earth-observing system will be high, on the order of 10¹⁴ to 10¹⁵ bits per day. This will require much selective averaging and heavy use of new data storage and retrieval technologies. Automation of some phases of the selection and averaging process will be required.

THE ROLE OF NASA IN EARTH SCIENCES

The National Aeronautics and Space Administration is to be commended for the strong role it has played to date in earth sciences. Its efforts have ranged from studies of atmospheric, oceanic, and land surface processes to studies in the field of the solid earth sciences. For a long time, satellites have been used not only to sense properties of the atmosphere, ocean, and land surface, but also to define more precisely the shape of the Earth and to investigate the distribution of mass in its interior. As the new Earth Observing System (EOS) is developed, NASA should continue to play this key role in the development not only of space-based technology, but of the necessary earth-based systems and data systems as well.

The steering group endorses the position of the Earth Observing System Science and Mission Working Group that in future NASA missions "satellite-obtained data must be used in concert with data from more conventional techniques." The steering group agrees that, in addressing multidisciplinary problems, "observational capabilities must be employed which range in scale from detailed earth-based and laboratory measurements to the global perspective offered by satellite remote sensing." Clearly, such studies must be carried out together with the other agencies that support basic research in earth sciences, notably NSF, USGS, and NOAA, as discussed below. But a strong program within NASA itself must be maintained.

In particular, the steering group notes the importance of a strong program in the solid earth sciences. NASA could play a major role in a comprehensive program that deals with all of the most exciting and important questions in that discipline today. These questions include the origin of magmas, the driving forces for plate tectonics, and the generation of Earth's atmosphere. Moreover, high-resolution mapping of Earth's gravity field is essential if ocean surface topography measurements are to reach their full potential for ocean circulation studies. NASA's engineering capability in state-of-the-art technology (e.g., advanced satellite systems and data base management) is essential to the accomplishment of these objectives.

NATIONAL COORDINATION

Communication among the heads of the Office of Science and Technology Policy, the Office of Management and Budget, and the federal agencies involved in the civilian earth science effort is needed to develop coordinated programs and budgets. This requires full cooperation among the agencies involved: NASA, NSF, NOAA, USGS, DOE, and others. The roles of these agencies relative to one another—NASA as a research and development agency, NSF as a supporter of basic research, and NOAA and USGS as operational, mission-oriented agencies in earth sciences—provide a test case for such cooperation. The steering group recognizes the importance of establishing clear roles, especially as researchers look to measurements on longer and longer time scales.

Coordination with the commercial sector is also essential. Plans are under way to operate the Landsat sensor package commercially, and the French are already flying a similar set of instrumentation on their Systeme Probatoire d'Observation de la Terre (SPOT satellite series. The data are available commercially. Opportunities to fly other sensor packages, such as meteorological sensors, on leased spacecraft may occur in the future. Thus, any comprehensive program must include the commercial sector as a major player.

CONCLUSIONS

We now have the technology and the incentive to mount a "Mission to Planet Earth." The United States should implement this integrated program of fundamental research on the origin, evolution, and nature of our planet, its place in our solar system, and its interaction with mankind. The mission's feasibility has been demonstrated. We now need to act.

In order to mount this mission we need to deploy a major observational system with arrays of satellites and earth-based instrumentation for long-term measurements. In addition, we must bring into play new supercomputers, establish comprehensive data systems, and fund scientists, engineers, and other participants who make the program possible. This broad program will require support from many federal agencies, private industry, and the international community. NASA will play a key role in the implementation of the program.