Formation flyers. The GRACE mission's twin satellites (inset) speed up over regions of high mass (red), leading to detailed gravity maps.

Hidden Water Seen From Space

SAN FRANCISCO--Twin satellites flying in tandem nearly 500 kilometers overhead have charted the most detailed map of the whole Earth's gravitational field yet seen. The image, displayed here on 9 December at a meeting of the American Geophysical Union, is the first step of a 5-year plan to track minuscule fluctuations in Earth's gravity--revealing movements in underground water, melting glaciers, and other processes vital to the planet's water cycle.

The distribution of Earth's mass shifts as water flows from place to place. Most of those changes are hidden, such as seasonal wetness in soils and currents that transport deep, cold seawater. But as water builds up it leaves a sign: The gravity over that region grows stronger. Ground-based surveys and previous satellites have exposed those subtle forces, but with fuzzy resolution.

To sharpen their gravitational vision, scientists at NASA and the German Aerospace Center in Bonn devised GRACE: the Gravity Recovery and Climate Experiment. Launched on 17 March, the mission consists of two identical satellites that fly about 220 kilometers apart. A microwave beam between the satellites gauges their distance with an accuracy of a few thousandths of a millimeter. Peaks and valleys in the planet's gravity push the satellites closer together or farther apart. Scientists can translate those oscillations into maps of Earth's gravity field.

The first such map, from just 89 days of in-flight testing, is 10 times better than 3 decades of gravity data from all other sources combined, says aerospace engineer Byron Tapley of the University of Texas, Austin, leader of the U.S. GRACE team. In early 2003, scientists will start to compare the mission's monthly maps. The most dramatic advances should come from monitoring the mass of Earth's polar regions, where existing gravity maps are poor. Small variations will expose the growth or wasting of ice caps, Tapley says, a more powerful method than measuring their heights with aerial surveys.

Hydrologists eagerly await GRACE's ability to track the contents of aquifers and river basins, especially in regions facing agricultural shortages or water-quality crises. The satellites will detect changes of less than a centimeter of water spread over areas a few hundred kilometers wide. That's a unique tool, says hydrologist Jay Famiglietti of the University of California, Irvine: "It's nearly impossible to accurately measure the changes in underground water storage with ground-based wells."

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