The two satellites Grace 1 and 2 can measure the annual increase in gravity over Canada very accurately. Illustration: NASA / Jet Propulsion Laboratory
Read more In the north of Canada, there are still after-effects of the ice age: as the kilometer-thick ice sheet pushed the earth's crust down at that time, gravity today is several thousandths of a percent weaker than the global average. Ultra-precise measurements of the Grace satellite pair (Gravity Recovery and Climate Experiment) show, however, that rock movements in the Earth's mantle are also responsible for the phenomenon. During the Ice Age, North America was covered by huge glaciers that were up to three kilometers thick. The weight of the ice shield weighed on the earth's crust, causing it to sink and displace the underlying rock. When the ice began to melt about 20, 000 years ago, the crust slowly rose again. The missing mass still manifests itself in the gravitational field of the earth as an extended minimum. However, the situation is slowly returning to normal.

With the help of the two Grace satellites, it was now possible for the first time to measure the slow increase in gravity in Canada. The two satellites, operated by NASA and the German Aerospace Center, are located 220 kilometers apart in a 500-kilometer-high orbit. Reflected microwaves allow us to measure changes in the distance between the two satellites on the order of a thousandth of a millimeter. This allows you to detect irregularly distributed masses in the earth. Grace's measurements are so sensitive that even a seasonally fluctuating groundwater level is noticed.

A research team led by Mark Tamisiea now analyzed data that Grace had registered between April 2002 and April 2006 to find out if the vanished ice was the sole cause of the gravity low over Canada. Some scientists have suspected for some time that sinking mantle rocks could also help. Since convective motions in the mantle are much slower than the rebound of the crust after the Ice Age, the researchers were able to distinguish the processes.

They found that post-glacial uplift contributes only between 25 and 45 percent to the gravity anomaly. In addition, the data show that the ice sheet did not have one but two peaks? one east and one west of Hudson Bay. According to the researchers, it will take another 300, 000 years before the ice age effect in the gravity data is no longer recognizable. Similar measurements, the researchers hope, can tell them much about the behavior of the glacial glaciers? but also about how yielding the hot, extremely viscous rock of the mantle is. display

Mark Tamisiea (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts) et al .: Science, Vol. 316, p. 881 Ute Kehse


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