The Earth is not a perfectly round sphere, nor is its internal composition exactly homogeneous. As a result, the Earth's gravitational field varies from place to place -- a ball dropped at different locations, for example, will fall at very slightly differing rates due to the different strengths of gravity at each place. The effects of these variations in the gravitational field are negligible in daily life, but they offer an innovative way for scientists to measure the shape and composition variations of the Earth.
SAO scientists Mark Tamisiea and Jim Davis, together with a colleague, used the twin GRACE satellites (Gravity Recovery And Climate Experiment) to measure the effects of the last Ice Age. At the peak of the last Ice Age (which ended about 20,000 years ago), the Laurentide ice sheet covered North America from the arctic through eastern Canada into the northeast US. It was so heavy that it compressed the surface of the Earth by hundreds of meters or more in places, and these distortions can be measured by changes in the local gravitational field. There has also been another geological effect at work to distort the shape of the Earth, the motion of the crust driven by the Earth's hot interior, and it is not easy to separate the effects of these two processes on the shape of the Earth.
The scientists realized that the GRACE satellite, with its high precision measurements of local gravity, could potentially distinguish between these two effects in eastern North America by seeing if the Earth's crust appeared to be rebounding upwards, as it would if the weight of the now-melted ice of the Lorentide sheet had been responsible. In last week's Science journal, the team reported the results of their analysis of satellite data from April 2002 through April 2006. Their conclusion: there were two large domes of ice on either side of the Hudson Bay region (one of two possible scenarios that had been suggested earlier), and the melting of that ice and the subsequent rebounding of the Earth contributes about 25 to 45 percent of
the changes in the Earth's local gravitational field. The results help
scientists understand the lingering effects of the Ice Age, and to refine their understanding of geological processes.