Weird gravity in Canada blamed on glaciers

A mysterious dip in gravity over Canada has been a weighty topic for some scientists. Now satellite data reveal a thick ice sheet that once cloaked the region partially resolves this so-called gravitational anomaly. Scientists have known that the Hudson Bay region features lower gravity than surrounding areas. While two theories have emerged to explain the strange phenomenon, conclusive evidence has been elusive. One theory involved a change in the area's overlying glacial weight as the Laurentide Ice Sheet melted.The new results, reported in the May 11 issue of the journal Science, provide a crude map of the ice sheet’s structure as it was during the most recent ice age. Turns out, the now-melted ice left behind an imprint from which the Earth is still rebounding, and that imprint contributes to the weird gravity. "There are many uncertainties about the last ice age and its impact on the Earth," said one of the study’s researchers Jerry Mitrovica, a physicist at the University of Toronto. “We are able to show that the ghost of the ice age still hangs over North America."

Mark Tamisiea of the Harvard-Smithsonian Center for Astrophysics in Massachusetts and his colleagues relied on gravity-hunting gear: Between April 2002 and April 2006, they collected data from the Gravity Recovery and Climate Experiment (GRACE). The twin GRACE satellites work by taking advantage of the fact that gravity’s pull on an area is proportional to the mass sitting atop that area.