Canada's odd low gravity a relic of the ice age
DAWN WALTON
CALGARY -- For years, scientists have known that compared with the rest of the world, Canada is a low-gravity area - although nowhere close to the zero-G phenomenon of space travel - but now, researchers have discovered that this country's peculiar gravitational field is even more pronounced in two northerly regions.
While most of this country was blanketed by 3.2 kilometres of ice and snow 20,000 years ago (which is more than likely the culprit for Canada's overall low pull of gravity), two areas on either side of Hudson Bay, each covering land up to half the size of Quebec, were buried under an additional 500 metres of ancient ice.
Using satellites to measure minute variations in the Earth's gravity, researchers have mapped out what Canada's topography looked like underneath the Laurentide Ice Sheet, which vanished 10,000 years ago.
The two northern pieces of real estate have not rebounded from years of being weighed down by so much ice, according to a report in today's issue of the Journal Science. If the compressed ground were to return to its pre-ice-age levels, the gravity would return to the levels in the rest of Canada.
"I looked at the screen at this image and it gave me goose bumps," recalled study co-author Jerry Mitrovica, a geophysicist at the University of Toronto, "You're seeing the image of the remnants of the ice age."
The low-gravity environment on the moon made walking a challenge for astronauts and reduced them instantly to perhaps one-sixth their weight on Earth.
These homegrown spots of low gravity in Canada are not nearly that pronounced, but their detection does settle a long-running debate about what the ice that once covered Canada looked like. Some have argued that it was one massive dome like a tennis bubble covering the country. Others have suggested there was more than one bulge.
"What this picture suggests is that there was definitely at least these two domes that are continuing to cause this change in gravity that we see," said study co-author Mark Tamisiea, a research associate with the Proudman Oceanographic Laboratory in Liverpool, which is linked to the Harvard-Smithsonian Center for Astrophysics.
The findings are crucial for future climate change research, according to the authors. They also add to the modern-day understanding of global warming.
In 2002, NASA and the German Aerospace Center launched a pair of satellites to circle the planet from 500 kilometres above to map changes in mass and gravity. These so-called Gravity Recovery and Climate Experiment or Grace satellites collected data on ice sheets and glaciers over the next four years for a variety of researchers.
A pair of University of Colorado researchers discovered, for example, that Greenland, which is home to 10 per cent of all the ice on Earth, was losing between 150 and 250 cubic kilometres of ice a year, enough to push the world's oceans up by 0.5 millimetres annually.
If all of that ice disappears, experts say, sea levels would rise by six metres or more. But that's something that could take centuries.
The satellites not only can measure changes in the weight of ice, but detect regions with the greatest loss, according to Michael Watkins, a Grace project scientist with NASA's Jet Propulsion Laboratory in California.
The satellites found that the Earth's crust over Canada has not completed what scientists call the "post-glacial rebound." Now, it's clear that the ice age is still affecting the planet. |
