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Massive Ice Sheet Partial Cause of Missing Canadian Gravity

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Hudson and James Bays, frozen over during the winter, as seen from space in a 2002 NASA image.NASA

A mysterious dip in gravity over northern Canada has long been a weighty topic for some scientists.

Satellite data indicates that that a massive ice sheet that once covered eastern North America is at least the partial cause of this gravitational anomaly.

Scientists have known that the area around Hudson Bay has lower gravity than the surrounding areas.

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Two theories have emerged to explain the strange phenomenon, but conclusive evidence has been elusive.

The first theory involves a change in the area's overlying glacial weight as the Laurentide Ice Sheet melted.

The new results, reported in the May 11 issue of the journal Science, provide a crude map of the ice sheet's structure as it was during the most recent ice age, which ended about 12,000 years ago.

The gigantic glacier left behind an imprint from which the Earth's surface is still rebounding, and that imprint contributes to the weird gravity.

"There are many uncertainties about the last ice age and its impact on the Earth," said Jerry Mitrovica, one of the study's researchers and a geophysicist at the University of Toronto. "We are able to show that the ghost of the ice age still hangs over North America."

Tag team

Mark Tamisiea of the Harvard-Smithsonian Center for Astrophysics in Massachusetts and his colleagues relied on gravity-hunting gear.

Between April 2002 and April 2006, they analyzed data collected by the twin Gravity Recovery and Climate Experiment (GRACE) satellites, which measure gravitational fluctuations caused by varying densities of terrestrial surface and subsurface matter.

GRACE's satellites fly in tandem in a polar orbit, constantly keeping track of the distance between them.

As the lead satellite passes over a gravitational anomaly, the tiny jolt changes the distance between it and its twin following about 135 miles behind.

Their instruments can measure the distance to within one micron — one millionth of a meter, slightly smaller than a red blood cell or 2,000 times tinier than a pinhead.

"If the Earth were a perfect sphere, gravity would be the same everywhere around the sphere," Tamisiea told LiveScience.

But the Earth is lumpy. Mass is distributed unevenly both inside the Earth and on its surface, resulting in varying gravity over different regions.

Missing gravity

Past studies and theoretical calculations have searched high and low for the missing gravity around Hudson Bay.

According to one explanation, convection within the Earth's mantle tugs the continental plate downward.

The main problem with this theory is that that process occurs on the order of a million years or so and wouldn't show up in the GRACE measurements, as they detect only geologically "quick" gravity changes on the order of years.

Fortunately, the glacial-rebounding theory occurs on a much shorter time scale.

During the last Ice Age, the two-mile-thick Laurentide Ice Sheet stretched from the down to New York and Chicago, and from the Atlantic to the Rockies, spanning 5 million square miles.

The massive sheet pressed down on the North American continental plate, deforming the crust like a sumo wrestler on a trampoline. Hudson Bay itself is a remnant of that imprint.

Even though the icy sheet has all but vanished, the Earth still feels the burden and, like a slowly rebounding memory-foam pillow, has yet to snap back to its ice-free shape.

Gravity measurements reveal that the slight deformation could explain about 25 to 45 percent of the unusually low gravity that has persisted over a large section of Canada.

The rest of the "missing gravity" can be explained by some sort of mantle tugging, the scientists say.

Two spots, Keewatin to the west and northern Quebec to the east, showed greater gravity changes compared with surrounding regions. That suggests at least two large ice domes jutted up from the Laurentide Ice Sheet during the Late Pleistocene Epoch, Tamisiea said.

The results will help scientists understand ice-sheet dynamics and how climate affects the mass and distribution of ice over the Earth.

"Knowing there are the two domes gives us a way to put tighter constraints on how the ice sheet develops and evolves with changing climate," Tamisiea said.

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