To lose weight fast, sail off the coast of Sri Lanka. That's one area of the earth where gravity is weakest, thanks to the deep Mid-Indian Basin. This geologic feature, among others--mountains, valleys, ridges, trenches and such--distributes mass unevenly about the planet's surface, thereby making the pull of gravity vary slightly. Recently a pair of satellites launched in March 2002 have yielded the most detailed map of the planet's gravitational field yet. These satellites make up the Gravity Recovery and Climate Experiment (GRACE), a joint effort by NASA and the German Aerospace Center.

The seas in particular draw interest because water sloshes around. "We can use these gravity measurements as a new way to track down water," says GRACE project scientist Michael Watkins. "It's exciting to have a new data type like that." At any given location, the gravitational field determines an ideal height where the water surface would rest if it weren't for winds, tides and other influences. By knowing this hypothetical rest position for the ocean, called the geoid, scientists can better understand how ocean currents behave, a key to predicting weather and climate. Gravity can also provide clues about underground water sources and help track the disappearance of the polar ice caps.

This article was originally published with the title Weight Watching.

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