my recent articles |
| Sunken areas of Canada decrease gravity |
|
11 May 2007, for Nature Network Boston During the last ice age about 20,000 years ago, much of North America was buried under ice sheets as thick as three kilometers. Harvard researchers have found that parts of Canada haven’t fully rebounded from the effects of bearing all that weight. This could explain why the Hudson Bay area has unusually low gravity, according to the researchers’ paper in this week’s Science. Mark Tamisiea and James Davis of the Harvard-Smithsonian Center for Astrophysics, together with a colleague from the University of Toronto, describe how they used data from a pair of satellites called GRACE (Gravity Recovery and Climate Experiment).
The satellites communicate with each other by microwaves, allowing them to sense tiny changes in the distance between them due to changes in Earth’s gravity. Areas with higher gravity, like the Rocky Mountain range, tug the satellites a bit closer to Earth; over areas with less gravity, such as the depression in Canada, the satellites’ orbits are a little bit wider. Based on data collected around the globe from 2002 to 2006, the researchers saw a dip in gravity over Canada in two large spots to the west and east of Hudson Bay, a legacy of the ice sheet that weighed down the continent. But this ice-age depression causes only half of the decrease in gravity in the region, the researchers say. They modeled the flow of the mantle deep below the Earth’s crust. The mantle moves in convection cycles, rising toward the Earth’s surface, where it heats up, and then, as it cools, sinking toward Earth’s center. The researchers concluded that in the region around Hudson Bay, this convection process seems to be pulling down on the outer crust, accounting for half of the area’s decrease in gravity. |
