Recent Developments in Gravity Surveys
The Earth’s gravity field is not constant with time, and the temporal changes can be diagnostic of important processes in its natural systems. Mass movements of air, water, ice and variations of temperature result in subtle variations of gravity in the same place over a period of time. Examples would include a decrease in gravity over regions going through drought, or where hot fluids are ascending towards the surface defining potential geothermal fields. A microgravity survey could aid the identification and delineation of such conditions/regions. These and similar applications have been shown to be practically feasible. The best way to monitor the temporal variation of gravity is to observe one satellite from another in a low-earth orbit. As the leading on comes up, for example, over a mountain, it would feel he pull of the mountain first and would drift away from the trailing one. As they leave the region of the mountain, they would come together again. This sort of ‘dance’ that the satellites do as they go around the earth will tell us what the gravity field underneath them is. The Earth’s gravity field is changing every day, every week, every month, because water is moving all around the earth (as liquid water, vapour and ice). It is raining in many places while many others are losing water on account of the drought conditions. Polar ice caps and mountain glaciers are melting. But what scientists really want to understand is not just what happens across the Earth in one year, they really want to monitor the changes occurring over many years or decades in order to understand what’s really happening in the climate system.
The GRACE Mission:
On March 17, 2002, the National Aeronautics and Space Administration (NASA) of the United States and the German Aerospace Center (DLR) launched the Gravity Recovery and Climate Experiment (GRACE) – a novel space mission that has given scientists a new understanding of variations of gravity over time and space, and over land and sea with unprecedented accuracy.
Launched from Northern Russia, on what was designed to be a three- to five-year mission, GRACE continues its task of making detailed measurements of Earth's gravity field. It does so by noting minute changes in gravitational pull caused by local changes in Earth's mass. Masses of ice, air, water and solid Earth can be moved by weather patterns, seasonal change, climate change and even tectonic events such as large earthquakes. To track these changes, the twin GRACE satellites, flying in formation 220 km apart in low Earth orbit (500 km), measure minute changes in Earth's gravity field by measuring micron-scale variations in the separation between the two spacecraft, designed and fabricated by NASA's Jet Propulsion Laboratory.
Measurements made by GRACE are used to produce monthly gravity maps that are more than 100 times more precise than previous models, providing the resolution necessary to characterize how Earth's gravity field varies spatially and temporally. Gravity data returned by GRACE in the 15 years since its launch has been processed to produce an amazing array of breakthrough science that is giving us a new understanding of changes in Earth's natural systems.
GRACE data have substantially improved the accuracy of techniques used by oceanographers, hydrologists, glaciologists, geologists and climate scientists. Through GRACE data we have been able to observe the critical indicators of climate change – sea level rise, ice loss from the polar ice sheets and mountain glaciers, changes in groundwater storage on land, and large-scale ocean circulation, to name but a few.
GRACE data reveal that, since 2011, the Sacramento and San Joaquin river basins in California decreased in volume by four trillion gallons of water each year (15 cubic kilometers). That's more water than California's 38 million residents use each year for domestic and municipal purposes. About two-thirds of the loss is due to depletion of groundwater beneath California's Central Valley. The study technique pioneered by GRACE is also being used by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission for mapping the gravitational field of the moon to learn about its internal structure and composition in unprecedented detail. GRAIL data will also provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved.
The great success of the GRACE Mission has inspired a follow on mission – GRACE-FO – a partnership between NASA and the German Research Centre for Geosciences (GFZ), of which construction of the first of the two satellites is already complete. The GRACE-FO Mission is scheduled for launch later this year. GRACE-FO will carry on the extremely successful work of its predecessor while testing a new technology designed to dramatically improve the already remarkable precision of its measurement system.
The GOCE Mission:
In the spring of 2009, the European Space Agency (ESA) launched a satellite mission – the Steady-state Ocean Circulation Explorer (GOCE) mission – to usher a whole new level of understanding of various aspects of oceanography, solid earth physics, geodesy, sea-level changes and climate change on the basis of variations of Earth’s gravity field. The sleek high-tech gravity satellite embodied many firsts in its design and used new technology in space to map Earth's gravity field in unprecedented detail. To gain the best possible gravity measurements, the sleek aerodynamic satellite was designed to fly in an extremely low orbit of 255 km above Earth – about 500 km lower than most Earth observation satellites – allowing improved sensitivity to earth’s gravity, thus returning data of unprecedented accuracy.
As the most advanced gravity space mission to date, GOCE data have realised a broad range of fascinating new possibilities for oceanography, solid Earth physics, geodesy and sea-level research, and significantly contributed to furthering our understanding of climate change.
GOCE mapped the variations in the gravity field with extreme detail and accuracy, resulting in a unique model of the 'geoid', which is the surface of equal gravitational potential defined by the gravity field. The geoid, in turn, is crucial for deriving accurate measurements of ocean circulation and sea-level change, both of which are affected by climate change. GOCE-derived data are also being used to understand more about processes occurring inside Earth and for use in practical applications such as surveying and levelling. The GOCE Mission ended in October 2013, when the satellite ran out of fuel and re-entered Earth’s atmosphere.
Applications of GOCE Data:
Using GOCE data, scientists have produced the most accurate model of ocean current speeds to date. The accurate estimate of ocean surface currents, as provided today by the combination of GOCE and altimetry data, is crucial for the better understanding of the ocean dynamics. In particular, the assimilation of this information into operational ocean monitoring and forecasting systems will provide highly valuable new insight into the present and future state of the ocean. The new ocean current speed map is of particular interest to UNESCO’s Intergovernmental Oceanographic Commission, which supports international cooperation and the understanding and management of oceans and coastal areas.
Gravity data from this super-low orbit satellite mission has improved our understanding of Earth’s interior, including identifying areas where oil and gas – the primary energy source for today’s civilisations – might be present. Models based on GOCE data show subsurface density and its vertical and lateral variability, which provides insight into varying geological compositions and temperatures. Estimating the structure and thermal state of Earth’s crust provides clues into the heat of shallower sedimentary rocks, and thus the potential location of oil and gas accumulations.
Data from the GOCE Mission are now being used to produce maps for geothermal energy development. Scientists from ESA and the International Renewable Energy Agency (IRENA) have used gravity measurements from the GOCE mission to produce an online tool that indicates areas likely to possess geothermal potential, narrowing the search for prospectors. This tool is available at: http://irena.masdar.ac.ae/?map=1046%20.
Although the Mission is over, the wealth of data from GOCE continues to be utilized to improve our understanding of ocean circulation, sea level, ice dynamics and Earth’s interior.
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