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Canada's Low-Gravity Puzzle Solved, Scientists Say

Rick Lovett
for National Geographic News
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.

"[It's as though] you have two automobile-size things, one in Los Angeles and one in San Diego, and you're measuring the distance between them to the size of a red blood cell," Michael Watkins of NASA's Jet Propulsion Laboratory in Pasadena said last fall at a scientific meeting. Watkins was not involved in the new study.

This allows scientists not only to map the Earth's gravity field from space, but also to look for small changes from one year to another.

That ability helped solve the Canadian gravity anomaly.

If a downwelling mantle plume was the culprit, then there wouldn't be any detectable changes, Tamisiea said, because mantle convection occurs on fairly long time scales.

There was a signal, however—the area's gravity is increasing at a rate comparable to that which would occur if you poured two inches (five centimeters) of water across the region each year.

"It's a fairly large signal," Tamisiea said.

It probably means that the region is rebounding around half an inch (about a centimeter) per year.

Global Warming Implications

The new GRACE study is also important for people studying global climate change, Watkins told National Geographic News.

GRACE was designed mostly to study changes in water, such as melting glaciers in Greenland and Antarctica. (Related: "Greenland Ice Sheet Is Melting Faster, Study Says" [August 10, 2006].)

"Water has weight and therefore it has gravitational attraction and GRACE can observe it," Watkins said.

The new measurements, however, helps show post-glacial rebound is also occurring in many places.

"In the case of Greenland it's not big," he said, "but in Antarctica, when GRACE sees a signal of so many centimeters of ice has changed, part of that is rebound."

Thus, Watkins said, "it also allows us to remove an error source for our climate measures."

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