Gravity Anomaly Solved

Gravity map.The University of Texas Center for Space Research, as part of a collaborative data analysis effort with the NASA Jet Propulsion Laboratory and the GeoForschungsZentrum Potsdam

About 1000 miles north of the Great Lakes, near Hudson Bay, Canada, the pull of gravity is a little weaker. A new study in the journal Science explains why.

Differences in gravity are not unusual on Earth: “If the Earth was a perfect sphere, the gravitational field would be the same everywhere,” says Mark Tamisiea, a geodetic geophysicist at the Proudman Oceanographic Laboratory in the United Kingdom and lead author on the study.

But our planet’s bumpy surface, the water flow on the planet, and the movement of plates below Earth's crust can all change the pull of gravity. “Gravity changes on many different time scales, from a matter of weeks to centuries,” says Tamisiea, adding that the variation in gravity can explain “quite a bit about what’s going on in the Earth.”

Differences in gravity can be mapped by tracking the movement of two orbiting satellites, Tamisiea says. As the Earth’s gravitational pull increases and decreases, the position of the satellites also changes.

For example, if two satellites were flying above a large mountain, Tamisiea explains, the first satellite would be pulled closer to the mountain. The mountain has more mass, so there is a stronger gravitational pull. As the first satellite is pulled toward the mountain, it moves farther away from the second satellite—the distance between the satellites increases. As the second satellite approaches the mountain it is pulled in and the distance between the satellites decreases again. The changes in distance between the satellites can be translated into a map of the Earth's gravitational field.

Using data like this, Tamisiea and his team mapped gravity, and how gravitational pull is changing over time, which allowed them to understand what is causing the Hudson Bay gravity anomaly.

The researchers created models that predicted 25 – 45 percent of the Hudson Bay gravitational signal was due to the last Ice Age—20,000 years ago, when a 2-mile-thick Laurentide ice sheet covered Canada, the land sank under the ice. “Although that ice sheet has long disappeared that ice sheet is still recovering,” Tamisiea says. The depression means less mass, which means less pull.

The scientists also discovered that the changes in gravity were larger in two spots, suggesting the Laurentide ice sheet was made up of two domes of ice — not one.

The primary driver of the lower gravity? The data suggests that plate tectonics and mantle convention is responsible. Mantle convection can cause plates inside the Earth to move vertically — in this case, Tamisiea says that convection probably dragged the crust down, which decreased the mass under Hudson Bay, decreasing the gravitational pull.

-Flora Lichtman (link)