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Satellite Gravity and the Geosphere: Contributions to the Study of the Solid Earth and its Fluid Envelope Executive Summary The Earth is a dynamic system constantly undergoing change. As the processes of change affect the Earth's topography—the heights of land, ice, and ocean surfaces—they also modify the distribution of mass within the Earth and consequently alter its gravitational field. The unique contribution of gravity is to discriminate among causes of variation in topography—for example, between thermal expansion and mass inflow as sources of a rise in sea level. Studies of the Earth's static and temporally varying gravity field can yield improved understanding not only of the Earth's interior, but also of its external envelope—its ice, water, and air. However, time-varying effects are three to four orders-of-magnitude smaller than the static field variations, so dense temporal and spatial coverage and highly accurate measurements are necessary. These can be obtained only from space. The static (i.e., essentially constant in historic time) gravity field is affected by processes over a wide range of scales, from global (10,000 km) for mantle convection to regional (30 km) for tectonic, magmatic, and sedimentary processes. An improved knowledge of the static gravity field would help to resolve deep crustal and mantle structure and to understand plate tectonic processes, such as the structure and thermal state of ridges and trenches and the patterns of mantle convection. Additionally, a more accurately known geoid would lead to an improved mapping of the dynamic topography of the ocean, which would in turn allow an improved determination of ocean circulation and an enhanced understanding of ocean dynamics. An improved gravity field would also lead to better orbits and reference geoids for the lower-altitude-altimeters dating back to the 1985 Geosat mission. The much smaller time-dependent component of gravity is also affected by processes on a wide range of scales, most of them entailing lateral transfers of water. Determining and understanding these transfers is a high priority of geophysicists studying climate and related phenomena; indeed, second only to studies of the radiative balance of the atmosphere. Processes causing large enough transfers to be measurable include ocean circulation, annual cycles of snow pack and groundwater, post-glacial rebound, sea-level rise, and possibly melting of the ice sheets. In the past two decades, the earth-science community has called for improved measurements of the global gravity field (e.g., Nerem et al., 1995; National Aeronautics and Space Administration [NASA], 1987; National Research Council [NRC], 1979, 1982.) The Committee on Earth Gravity from Space concurs in that call and offers the following new findings, which address the committee's charge to examine new technological advances, the new scientific questions that could be addressed by a state-of-the-art gravity mission, and the benefits of complementary data (see Preface for details). Our findings were based on the consideration of five generic mission scenarios, new modeling results, and a literature review.
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Satellite Gravity and the Geosphere: Contributions to the Study of the Solid Earth and its Fluid Envelope BENEFITS OF A DEDICATED SATELLITE GRAVITY MISSION Fields of study that would be significantly advanced by a dedicated satellite gravity mission include the following: Ocean Dynamics. Ocean heights measured by current satellite altimetry are approaching ~10 mm accuracy, from which the horizontal pressure gradient uncertainty increases with decreasing length scale. Nevertheless, present geoid slope errors are much larger at resolutions shorter than about 3000 km. This prevents the accurate measurement of absolute surface pressure gradients from which ocean currents are computed A satellite gravity measurement can eliminate the geoid uncertainty in horizontal pressure gradients at much shorter scales (to about 300 km). It would also allow recomputation of accurate altimetric orbits for past satellites, back to 1985, permitting studies of long, global-sea-level time series. A time-varying gravity measurement would allow the determination of seafloor-pressure variations over the world's oceans at spatial scales of a few hundred kilometers or longer. This would be useful for inferring variable deep ocean currents and, when combined with altimetry measurements of sea-surface variations, would place a powerful constraint on models of ocean circulation. Continental Water Variation (including snow pack and groundwater). Seasonal and annual variations in groundwater and soil-moisture levels can potentially be measured with a high level of accuracy at subcontinental length scales. More accurate detection of these variations would be of great value in forecasting conditions for agriculture, as well as developing scientific insight into hydrologic cycles. Gravity data would also be valuable for monitoring secular water-level decline in aquifers. Sea-Level Rise. Measurement of the ocean geoid would enable determination of the nonsteric component (i.e., caused by the addition of water to the oceans) of sea-level rise. In addition, estimates from tide gauges would be much improved by more accurate knowledge of post-glacial rebound (see below). Changes in the masses of the Antarctic and Greenland ice sheets are the major unknown contributions to sea-level rise. Gravity measurements (particularly in combination with a laser-altimeter mission) would yield a much-improved determination of those contributions. Post-Glacial Rebound. Accurate gravity measurements, especially at long wavelengths, would lead to major improvement in the knowledge of mantle rheology and its lateral variations. Fields of study that would benefit from improved gravity data but that require ancillary data to increase the usefulness of the gravity data include the following: Structure and Evolution of the Crust and Lithosphere. Gravimetry is an important contributor, supplementing the findings of geology, seismology, and topography. Satellite gravity could help to validate and adjust conventional terrestrial gravity; however, GPS-located airborne gravimetry, which provides higher spatial resolution than satellite techniques, is more appropriate in areas lacking gravimetry. Mantle Dynamics and Plumes. The factors currently limiting progress in this field are our understanding of the relationship among gravity anomalies, seismic tomography, and mantle convection, and our ability to simulate realistic convection scenarios on a computer. However, refinement of the gravity field may eventually place tighter constraints on the problem. Atmospheric dynamics is an area of special consideration. Accurate measurement and modeling of surface atmospheric pressure, a direct measure of the atmospheric mass, is needed to unravel the atmospheric gravity signal from the hydrologic and glacial signals of interest. Although pressure measurements in most areas are sufficiently accurate to permit advances in hydrology and glaciology, some areas of interest (e.g., Antarctica) have few barometers. It is possible to use gravity measurements to learn more about the atmosphere, but extending barometric networks would provide the best return on available resources. The contributions that a gravity mission could make to the study of these processes are discussed in more detail below, following the discussion of mission designs considered in this report MISSION SCENARIOS AND MEASUREMENT TECHNIQUES All satellite gravity missions are constrained by fundamental trade-offs in temporal and spatial resolution that depend on orbital altitude, ground-track pattern, and mission lifetime. We considered three mission designs which could be built, launched, and operated at a reasonable cost (order $100M) today, and two mission designs which require further techno-
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Satellite Gravity and the Geosphere: Contributions to the Study of the Solid Earth and its Fluid Envelope logical development. These designs offer high resolution at lower cost than other systems described in the past, due to improved technologies and the fact that very low orbits and expensive "drag-free" designs are no longer called for. Two broad categories of mission designs, gravity gradiometry and satellite-to-satellite tracking, were considered. Both of the categories were subdivided into "generic" missions, based on the technology used and the mission duration. Gravity gradiometry, which measures the differences in acceleration of two masses within the same spacecraft, was divided into two missions: Spaceborne Gravity Gradiometry (SGG). An SGG mission would yield a significant improvement over current results to degree 155 (250 km wavelength) at an orbital height of 400 km and to degree 215 (180 km wavelength) at 300 km. However, currently estimated accuracies are poorer than for the satellite-to-satellite tracking missions for degrees less than 25 (1600 km wavelength). The launch vehicle allowable within a reasonable cost cap limits the size of the dewar for current gradiometer technology to one sufficient for only a year lifetime. Hence the SGG mission is of limited value for the study of temporal variability. Extended Spaceborne Gravity Gradiometry (SGGE). A mission using a larger launch vehicle and/or more miniaturized gradiometer could extend the lifetime to five years, and thus would permit the detection of temporal gravity variability on seasonal and interannual time scales. However, the accuracy would have to be improved to be competitive with the satellite-to-satellite tracking missions. Satellite-to-satellite tracking utilizes differential tracking of two satellites and thereby measures orbital perturbations; accelerometers are required to remove atmospheric drag effects. Three missions were considered: High-Low Microwave Tracking (GPS). In the immediate future, such a mission would depend on the Global Positioning System (GPS) for the high satellite. A mission flown at an altitude of 400-500 km would yield significant improvements over the best current Earth-gravity models at harmonic degrees less than 25 (1600 km wavelength), whereas a mission flown at 300 km would be useful for degrees up to 30 (1300 km wavelength). A system much more accurate than GPS would be of great value to geodesy and gravimetry and is technically feasible, but probably much more expensive than tolerable for these applications. Low-Low Satellite-to-Satellite Microwave Tracking (SST). The SST mission is highly accurate at long and moderate wavelengths (10,000 to 1600 km), at which it produces more accurate geoid heights and gravity anomalies than the SGG, SGGE, and GPS missions. Also, the mission lifetime (estimated to be five years) permits the effective determination of many important time-varying effects. Low-Low Satellite-to-Satellite Laser Interferometry (SSI). The anticipated results from the SSI mission are the best of the five scenarios studied. They are an order of magnitude better than the SST results at all wavelengths considered and two orders of magnitude better than SGG at long wavelengths. However, both SST and SGG involve mature technologies, whereas SSI requires additional development (e.g., order of magnitude improvements are needed in accelerometer accuracy and laser-cavity thermal noise at low frequencies) and proof-of-concept, which would delay a mission some years compared with the other techniques. OCEAN DYNAMICS AND HEAT FLUX The SGG, SST, and SSI missions at an orbital altitude of 400 km offer dramatic improvements in the knowledge of the absolute dynamic topography and surface circulation that can be obtained from satellite altimetry. The most significant improvement will come at basin scales (300-3000 km), at which the geoid determination will be virtually eliminated as an error source. The orbital height needed to resolve smaller-scale phenomena, such as western boundary currents, would need to be impractically low. In situ techniques, such as airborne or shipborne gravimetry, both with GPS positioning and controlled acceleration environment, appears more practical. Studies in ocean regions with a strong barotropic component will benefit from knowledge gained from the static geoid. These include the recirculation cells in the subtropical gyres of the western Atlantic, the Kuroshio Current, the Agulhas Current, and the Antarctic Circumpolar Current. Most of what is known about the ocean occurs in the upper 500 meters. Studies suggest that uncertainties in the deep circulation and heat and mass transport will be reduced by a factor of two or more in oceanographic regions that are data sparse. Part of this reduction comes from an improvement in estimates of
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Satellite Gravity and the Geosphere: Contributions to the Study of the Solid Earth and its Fluid Envelope surface currents. For example, the geostrophic advective terms in the mixed-layer heat budget would be resolvable with an uncertainty of less than 10 Wm–2. The combination of altimetry and gravity will allow the separation of the steric and mass components of sea-level rise. This separation will substantially increase the usefulness of sea-level measurements in testing ocean models and constraining ocean circulation. Interesting and detectable signals that indicate changes in sea-floor pressure averaged over spatial scales of a few hundred kilometers and larger are expected. These will allow the detection of abyssal ocean current variations with seasonal to interannual time scales. Detection of these phenomena requires a multi-year mission lifetime and high accuracies at long wavelengths. These requirements give priority to SST or SSI over an SGG-type mission. SOLID EARTH PROCESSES Satellite gravity measurements from four of the five generic missions (SGG, SGGE, SSI, and SST) would constrain properties of mantle convection on scales as small as 200 km (half wavelength). An accuracy of ~10 -2 mGal would be met for resolutions larger than 300-400 km, which would permit small, though important, variations in thermal structure to be characterized, thus helping to distinguish between various models of mantle structure. A one-mGal accuracy at length scales of 500-1000 km would resolve discrepant estimates of the depths of continental roots and would also help to distinguish between models of mantle flow. Gravity resolution of approximately 1 mGal over length scales of order ~120 km would help constrain the depths of origin of hotspot mantle plumes, which are a major source of intraplate volcanism and enhanced heat flow. The best satellite missions for regional tectonics are SGG, SSI, and SST. In a 300-km orbit, the SGG missions provide marginally better resolution than the SST mission for resolutions finer than 300 km. The recent availability of gravity data from former communist nations will help elucidate interesting geologic structures in remote regions such as the Himalayas and the Tibetan Plateau. Satellite gravity can help to put these data on a unified datum. Satellite gravity data could be used elsewhere also to calibrate existing terrestrial and marine gravity measurements, improving their continuity across political boundaries and shorelines by several milliGals, which would significantly improve the accuracy of the global terrestrial gravity database. The SST and SSI missions would provide the data needed to resolve differences between models of lower-mantle viscosity and to separate the effects of post-glacial rebound from the effects of other processes on sea-level rise, such as changes in ice sheets, groundwater, and surface water. These applications require the highest accuracy at the longest wavelengths. Only SST and SSI fulfill this need. A multi-year mission is essential. Improvements in the application of gravity data to studies of the crust and lithosphere require scales appreciably smaller than 200 km. Differential GPS measurements can contribute to these surveys by accurately controlling the altitudes of airborne gravimetry. WATER CYCLING Gravity missions can provide estimates of changes in water storage over spatial scales of several hundred kilometers and larger that would be accurate to 10 mm or better. These would benefit the Global Energy and Water Cycle Experiment (GEWEX) directly and would be useful to hydrologists for connecting hydrological processes at traditional length scales (tens of kilometers and less) to those at longer scales. The main measurables related to these processes are groundwater and snow pack. Improved knowledge thereof would enhance agricultural productivity by assessing water available for irrigation. Water storage is important also to meteorologists because of the effect of soil moisture on evapotranspiration. SST and SSI missions are more accurate than SGG missions at long wavelengths and thus are more useful than SGG for hydrology applications. SEA-LEVEL RISE AND GLACIOLOGY The sources of global sea-level rise (between 1.0 and 2.5 mm/yr over the last century) are uncertain; most, but not all, of the likely mechanisms involve the redistribution of mass from the continents to the ocean. Gravity measurements can help to discriminate between these sources through the continual monitoring of geoid changes, not only on global scales, but also on regional and basin scales. From an SST or SSI type mission (five-year mission assumed), an increasing mass of water in the ocean equivalent to 0.1 mm/yr of sea-level rise can be measured.
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Satellite Gravity and the Geosphere: Contributions to the Study of the Solid Earth and its Fluid Envelope Gravity changes in Greenland and Antarctica reflect changes in ice-sheet mass, but there are other phenomena that need to be evaluated to recover the mass loss to the oceans: secular post-glacial rebound, interannual variability in snowfall, and the effect of atmospheric pressure variations. (a) Post-glacial rebound could be partly separated from ice-mass changes with the aid of a network of GPS receivers on the land surface, models of rebound that use improved determinations of mantle viscosity provided by the gravity mission, and comparisons with satellite laser altimetry. (b) Separation of interannual mass changes from true secular changes will be aided greatly by the continually improving calculations of mass input to the ice-sheet surfaces from measurements of moisture-flux divergence around the perimeters of the ice sheets. (c) The removal of pressure effects over Antarctica and other remote areas will become more effective as the number of automatic weather stations in the interior of the continent increases. (d) Gravity measurements would be most effective in combination with the observations planned for NASA's Laser Altimeter Mission. Together, an accuracy of 0.1 mm/yr in the determination of the contribution of the ice sheets to sea-level change should be attainable. Satellite gravity measurements are capable of yielding valuable information about the mass balance of individual drainage systems within the Antarctic ice sheet, as well as of the ice sheet as a whole. Glaciologists could use such information to test models of ice dynamics, which are essential to the prediction of future sea-level change. Satellite gravity could be used to study secular, interannual, and seasonal changes in the mass of ice and snow in regions characterized by a large number of glaciers and ice caps. A prime example is the glacier system that runs from the Kenai Peninsula in southern Alaska down to the coastal ranges of the Yukon and British Columbia. Accurate evaluation of post-glacial rebound models, together with improved ocean circulation models, should remove significant errors from old tide-gauge records, thus gaining almost a century of sea-level data. THE DYNAMIC ATMOSPHERE The atmosphere is currently the best measured fluid of any the Earth's subsystems. This fact is key in unraveling the effects of the other subsystems (such as the hydrological cycle and the mass balance of the Antarctic ice sheet) involved in gravity variations. With increasing accuracy in gravity measurement, precise knowledge of the uncertainty in atmospheric surface pressure on seasonal, annual, and secular time scales becomes increasingly important. Reliable, extended-range forecasting, which would require interactive coupling between the atmosphere and the water in soils and the ocean, would benefit from hydrological constraints and improved understanding of ocean dynamics. Gravity measurements with high temporal and spatial resolutions may improve the atmospheric databases and aid in the verification of models in areas where atmospheric measurements are lacking. But it would be much more effective to have a global network of barometers, sufficient to remove the atmospheric signals from the gravity data. A TOOL FOR SCIENCE All five mission scenarios considered in this report offer significant improvements in the static gravity field that are necessary for several important applications. These include: An improved reference frame for defining position coordinates. Better calculation of orbits for other remote-sensing applications, such as altimetry and synthetic-aperture-radar interferometry. A more accurate geoid, the equipotential surface to which land elevations ideally refer and to which ocean circulation is referred. The adjustment and datum correction of regional terrestrial, marine, and airborne gravity-survey data. As shown in the examples above, satellite gravity measurements can provide unprecedented views of the Earth's gravity field and, given sufficient duration, its changes with time. Not only can they provide a truly global integrated view of the Earth, they have, at the same time, sufficient spatial resolution to aid in the study of individual regions of the Earth. Together with complementary geophysical data, satellite gravity data represent a "new frontier" in studies of the Earth and its fluid envelope.
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