IF YOU think Canadians weigh less than Americans, you鈥檙e right. It鈥檚 because over a large swath of Canada gravity is weaker than elsewhere. And now we know why.
Geophysicist Mark Tamisiea of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and his team used the pair of satellites called Gravity Recovery and Climate Experiment (GRACE) to study the gravitational low over Canada鈥檚 Hudson Bay. GRACE can measure tiny changes in Earth鈥檚 gravity, allowing researchers to calculate the distribution of mass around the planet.
The Canadian low was noticed in the 1960s when Earth鈥檚 gravity fields were first mapped. Researchers have puzzled for years over whether it was due to the crust there rebounding slowly after the last ice age, a deeper issue involving convection in Earth鈥檚 mantle, or both.
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The distribution of mass around Hudson Bay, as revealed by GRACE, suggests that the region has not fully rebounded after the last ice age. This accounts for only about 50 per cent of the gravitational anomaly, though: the other half can be put down to the mantle dragging tectonic plates with it as it moves (Science, vol 316, p 881).
鈥淚t鈥檚 a very good piece of evidence that allows us to look beneath the surface of the Earth,鈥� says team member James Davis of the Harvard-Smithsonian Center.
