WE NOW have the most detailed map yet of the Earth鈥檚 gravitational field 鈥� and it shows subtle variations around the globe, including the added pull exerted by mountain ranges and oceanic ridges. Hailed as revolutionary by experts, the map should help reveal the movements of ocean currents and their effect on the climate, and could even predict drought or flood.
Twin satellites orbiting in formation collect the data used to produce the map. Dubbed GRACE (Gravity Recovery and Climate Experiment), the project is a joint effort by NASA and German space agency DLR.
The satellites, launched in March 2002, have been following an orbit that takes them over the poles, but in paths about 220 kilometres apart. As they circle the Earth, the precise speed of each and their distance apart is measured using GPS and a beam of microwaves bounced between them. This reveals their separation to a few micrometres 鈥� about a tenth the width of a human hair.
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Whenever the gravitational field changes, perhaps due to a mountain range, deep-sea trench or an accumulation of water created by strong winds or ocean currents, it slightly alters the orbital motions of each satellite. This changes the distance between them, from which the GRACE team can deduce the gravity fluctuation. The result, says project scientist Mike Watkins, is a map of our planet鈥檚 gravity that is hundreds of times more accurate than anything previously compiled.
The map should give important insights into how ocean currents swirl around the Earth. Until now, scientists have only had a few dozen sensors to measure deep-sea currents around the world. 鈥淔or the first time we鈥檒l have monthly maps of the Earth鈥檚 deep-sea movements,鈥� says Victor Zlotnicki, a NASA oceanographer.
Along with new data on glacier movement and the structure of the Earth鈥檚 mantle, GRACE will also provide far more precise maps of surface ocean currents, which affect short-range weather forecasts. NASA plans to release regular updates of the maps from GRACE, giving oceanographers and climatologists a whole new source of information.
Hydrologists are also lining up to use the new tool. To model how water moves in underground reservoirs, researchers usually rely on data from water wells and orbiting microwave sensors to estimate moisture levels in the ground. Unfortunately, wells are often scarce, and microwave sensors only work where there is no vegetation in the way.
According to NASA hydrologist Matt Rodell from the Goddard Space Flight Center in Greenbelt, Maryland, regular gravity measurements should make it far easier to monitor the amount of water held in aquifers in China, India and the Middle East. And by providing better estimates of evaporation from these stores, GRACE could help improve geological models that are crucial for predicting the risk of droughts and floods.