November 25th, 2007, 08:16 PM
Satellites solve mystery of low gravity over Canada
If it seems Canadians weigh less than their American neighbours, they do – but not for the reasons you might think. A large swath of Canada actually boasts lower gravity than its surroundings. Researchers have puzzled for years over whether this was due to the crust there rebounding slowly after the end of the last ice age or a deeper issue involving convection in the Earth's mantle – or some combination of the two.
Now, ultra-precise measurements taken over four years by a pair of satellites known as GRACE (Gravity Recovery and Climate Experiment) reveal that each effect is equally responsible for Canada's low gravity. The work could shed light on how continents form and evolve over time.
GRACE, a joint mission of NASA and the German Aerospace Center, was launched into space in 2002. The two spacecraft fly 500 kilometres above the Earth, 220 kilometres apart. Using a microwave ranging system, the two spacecraft can measure distance differences between them as tiny as a micron.
That allows them to measure tiny changes in the distribution of mass – and hence gravity – on the Earth. For example, if the leading spacecraft were to encounter an area with more gravity, it would be pulled ever-so-slightly closer to Earth than the trailing spacecraft, and that distance can be measured.
Crushing weight
A team led by geophysicist Mark Tamisiea, who performed the work while at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, has now used GRACE to study the gravitational low over Canada's Hudson Bay. Scientists first noticed this low in the 1960s, when the planet's first global gravity fields were mapped.
At first, researchers suspected it was due to an ice sheet called Laurentide that blanketed a sizeable chunk of North America during the last ice age. In places, the sheet was more than 3 kilometres thick, and it depressed the Earth's crust beneath it.
When the ice age ended about 20,000 years ago, the ice rapidly melted. But the crust has been springing back much more slowly, and it is rebounding today by about 12 millimetres per year.
But in the last decade or so, scientists have begun to suspect that convection in the Earth's mantle, a layer of hot, flowing rock beneath the crust, also plays a role.
The sludge-like mantle rises and falls in plumes as it is heated from below and cooled from above. The mantle can drag the overlying tectonic plates with it as it moves.
What lies beneath
GRACE cannot directly detect that movement since it is so slow. But scientists inferred the gravitational contribution of convection by subtracting the post-glacier effect from the region's overall gravity signal.
"It's a very good piece of evidence that allows us to look beneath the surface of the Earth," says team member James Davis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US. "It would be very hard for someone to say it's 100% post-glacial rebound."
Even after the Earth's crust rebounds completely from the glacier melting, there may still be a gravitational low over the area due to mantle convection. That would suggest that even parts of a continent away from the tectonic plate boundaries are affected by mantle convection.
Climate change
Ultimately, scientists hope to use such data to learn how continents form and evolve over time. "With this information, people could infer better whether the North America plate is actually predominantly stable," says C K Shum, an expert on the Earth's gravity at Ohio State University in Columbus, US, who was not a part of this study.
Using this same set of data, researchers have also found that there were two ice domes in the ice sheet on either side of Hudson Bay, given the features in the gravity field that have been left behind. The way the thickness of any ice cover changes at different periods could be used by climate modellers to understand past climate change.
"Where the ice formed, and how thick it was, is a clue to temperature, precipitation, and other weather/climate (for example, the jet stream) changes in the distant past," says team member Jerry Mitrovica of the University of Toronto in Canada.
Source
Satellites solve mystery of low gravity over Canada
- NewScientist.com news service
- Kelly Young
If it seems Canadians weigh less than their American neighbours, they do – but not for the reasons you might think. A large swath of Canada actually boasts lower gravity than its surroundings. Researchers have puzzled for years over whether this was due to the crust there rebounding slowly after the end of the last ice age or a deeper issue involving convection in the Earth's mantle – or some combination of the two.
Now, ultra-precise measurements taken over four years by a pair of satellites known as GRACE (Gravity Recovery and Climate Experiment) reveal that each effect is equally responsible for Canada's low gravity. The work could shed light on how continents form and evolve over time.
GRACE, a joint mission of NASA and the German Aerospace Center, was launched into space in 2002. The two spacecraft fly 500 kilometres above the Earth, 220 kilometres apart. Using a microwave ranging system, the two spacecraft can measure distance differences between them as tiny as a micron.
That allows them to measure tiny changes in the distribution of mass – and hence gravity – on the Earth. For example, if the leading spacecraft were to encounter an area with more gravity, it would be pulled ever-so-slightly closer to Earth than the trailing spacecraft, and that distance can be measured.
Crushing weight
A team led by geophysicist Mark Tamisiea, who performed the work while at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, has now used GRACE to study the gravitational low over Canada's Hudson Bay. Scientists first noticed this low in the 1960s, when the planet's first global gravity fields were mapped.
At first, researchers suspected it was due to an ice sheet called Laurentide that blanketed a sizeable chunk of North America during the last ice age. In places, the sheet was more than 3 kilometres thick, and it depressed the Earth's crust beneath it.
When the ice age ended about 20,000 years ago, the ice rapidly melted. But the crust has been springing back much more slowly, and it is rebounding today by about 12 millimetres per year.
But in the last decade or so, scientists have begun to suspect that convection in the Earth's mantle, a layer of hot, flowing rock beneath the crust, also plays a role.
The sludge-like mantle rises and falls in plumes as it is heated from below and cooled from above. The mantle can drag the overlying tectonic plates with it as it moves.
What lies beneath
GRACE cannot directly detect that movement since it is so slow. But scientists inferred the gravitational contribution of convection by subtracting the post-glacier effect from the region's overall gravity signal.
"It's a very good piece of evidence that allows us to look beneath the surface of the Earth," says team member James Davis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US. "It would be very hard for someone to say it's 100% post-glacial rebound."
Even after the Earth's crust rebounds completely from the glacier melting, there may still be a gravitational low over the area due to mantle convection. That would suggest that even parts of a continent away from the tectonic plate boundaries are affected by mantle convection.
Climate change
Ultimately, scientists hope to use such data to learn how continents form and evolve over time. "With this information, people could infer better whether the North America plate is actually predominantly stable," says C K Shum, an expert on the Earth's gravity at Ohio State University in Columbus, US, who was not a part of this study.
Using this same set of data, researchers have also found that there were two ice domes in the ice sheet on either side of Hudson Bay, given the features in the gravity field that have been left behind. The way the thickness of any ice cover changes at different periods could be used by climate modellers to understand past climate change.
"Where the ice formed, and how thick it was, is a clue to temperature, precipitation, and other weather/climate (for example, the jet stream) changes in the distant past," says team member Jerry Mitrovica of the University of Toronto in Canada.
Source
Executive Member
Posts: 9,534 
November 26th, 2007, 02:43 PM
Quoting missile
Yes, the Earth is slightly bulged at the equator and anyone on the equator would be slightly further from the center of mass and would weight a fraction less.
Is it true that we weigh less when at the [Not that I'm planning a move or anything]equator?
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November 26th, 2007, 05:11 PM
Quoting lone wolf
No, a little more than that. The difference is about 0.67%, according to my quick little back of the envelope calculation here. That'd be in the range of 10 to 20 ounces for most of us, or equivalent to about 300-600 grams.
I think you'll lose more weight by cutting an inch of one hair.
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Location: In the bush somewhere near Sudbury
Location: In the bush somewhere near Sudbury
November 26th, 2007, 05:21 PM
Woof!
Quoting Dexter Sinister
That's a pretty significant difference. I know the Lake Superior's north shore is rising somewhat faster than its south and the whole Hudson Bay basin will dry as rebound continues. Rockbursts are fairly common - even here on this four-billion-year-old rock. Makes sense there would be some magnetic disturbance too.
No, a little more than that. The difference is about 0.67%, according to my quick little back of the envelope calculation here. That'd be in the range of 10 to 20 ounces for most of us, or equivalent to about 300-600 grams.
Woof!
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Location: 50° 30' N, 104° 38' W
November 26th, 2007, 07:07 PM
Quoting triedit
I don't know anything specific about that one, but gravity anomalies like that generally indicate the presence of a larger, deeper valley in the bedrock that's filled with less dense unconsolidated sediment. In the Ohio Valley that's likely to be the outwash from the retreat of the most recent glaciation about 15,000 years ago.
Interesting. I wonder if they can explain an area of low gravity in the mid-Ohio valley.
