Canada's Low-Gravity Puzzle Solved, Scientists Say

May 11, 2007

A new satellite survey may have solved the mystery behind one of the world's strangest weight-loss methods: moving to a large area of northern Canada with unusually low gravity.

Though the weakness is slight—one part in 25,000, or a tenth of an ounce for a 150-pound (68-kilogram) person—scientists have argued for years about the cause.

One possibility is that underlying mantle rocks are slowly flowing downward.

It's like being aboard a raft in a rapids, said lead study author Mark Tamisiea, a geophysicist at the Proudman Oceanographic Laboratory in Liverpool, England.

"If the water was flowing downward in a sinkhole, [the raft] would be pulled down as well," he said.

But the new study supports an alternate theory: that 20,000 years ago, Ice Age glaciers pressed down on the area's crust like a person sitting on an extremely viscous waterbed.

The weight of all of that ice forced the mantle rocks to ooze slowly sideways. Then the ice melted—rapidly enough the crust hasn't yet fully bounced back. (Related: "Antarctic Ice Collapse Began End of Ice Age?" [March 17, 2003].)

Tamisiea and colleagues determined that the rebound indeed accounts for about half of the gravity loss. The research appears in today's issue of the journal Science.

Satellite Orbits

The study, conducted at the Harvard-Smithsonian Center for Astrophysics and the University of Toronto, relied on a sensitive pair of NASA satellites named GRACE.

These satellites follow the same orbit but remain about 130 miles (210 kilometers) apart. Microwaves are used to measure the distance between them with extreme precision.

As Earth's gravity varies, so does the distance between the satellites.

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