Residents of sparsely populated northern Canada are experiencing a phenomenon that weight watchers the world over would love to deal with. That part of the country feels less gravitational pull than the rest of the planet, according to a new study, so residents weigh ever-so-slightly less. The difference isn't huge--only about four-thousandths of a percent--but it's enough to help scientists tease out some important clues about the last ice age and the dynamics of the mantle underlying Earth's crust.
About 20,000 years ago, much of North America lay under the Laurentide Ice Sheet, a massive glacier more than 3 kilometers thick in some locations. Its crushing weight caused some of the rock beneath to compress and sink, and in the process displace the underlying semifluid mantle. This effect was most severe in two regions of northern Canada flanking Hudson Bay, where the ice formed two huge domes. Theorists had predicted that Earth's gravity retained the echoes of the glacial domes as a pair of dimples where the force is reduced.
Now, researchers using NASA's twin Gravity Recovery and Climate Experiment (GRACE) satellites have created a map of the two gravity anomalies and how they are changing over time. The spacecraft use ultraprecise clocks to track irregularities in their orbits caused by anomalies in Earth's gravitational pull. In tomorrow's Science, a team led by geophysicist Mark Tamisiea of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, confirms that gravity is slightly weaker--about 1/25,000 less than normal--in the two regions. The tug of gravity is increasing slightly as the crust slowly rises and the mantle refills the resulting cavity underneath. Based on observations from the GRACE satellites over four years ending in 2006, Tamisiea says it will take up to 300,000 years for the regions' gravity to return to normal. The observations "tell us a great deal about the crust and mantle," he says, in terms of their rates of deformation and recovery.
These changes in gravity observed over time show that the planet's interior is "not too fluid and not too sluggish," says geophysicist John Wahr of the University of Colorado in Boulder. Based on these findings, Wahr adds, the GRACE satellites could help scientists tease out even more details about the behavior of massive ice age glaciers.