A pair of satellites launched in 2002 has detected small, localized changes in Earth's gravitational field caused by seasonal variations in rainfall and soil moisture.
Earth's gravity can vary from place to place as large masses of air and water—such as weather systems and ocean tides—move across the planet's surface. Some of the largest gravitational variations result from water flow across land, says Byron D. Tapley of the University of Texas at Austin. The Gravity Recovery and Climate Experiment (GRACE) satellites, launched in March 2002, are designed to measure such gravity variations on a monthly basis (SN: 1/4/03, p. 6: Mapping with GRACE).
Analyses of GRACE data gathered between April 2002 and December 2003 suggest that scientists can track shifts in Earth's gravity caused by a change in mass equal to that of as little as 6 centimeters of water covering a 400-kilometer-by-400-km region.