Satellites map subtle variations in Earth’s gravitational field (3 of 3)

Grace in Space

A pair of satellites map subtle variations in Earth’s gravitational field, revealing secret craters, undersea mountains, and the impact of climate change.

by Sam Flamsteed

GRACE’s data are open to any scientist on the planet. “That,” says Byron Tapley, “led to a whole range of people outside the standard community who used GRACE results to do things that weren’t possible before.” In January 2005, for example, Ohio State University geophysicist Ralph von Frese and his colleagues noticed a concentration of higher-than-average-density material in the rock about a mile under the surface of the East Antarctic ice sheet. Mass concentrations like this often accumulate when giant impacts from space pound the crust. When the crust rebounds, it carries higher-density mantle materials up toward the surface and holds them there. Comparing the GRACE data with radar imagery of the icebound bedrock, von Frese found it was centered perfectly inside a ring some 300 miles wide—just what you’d expect from an impactor 30 or so miles across. “It just jumped out at us,” he says.

An asteroid that big would be about four to five times the diameter of the object that killed off the dinosaurs 65 million years ago. This crater is much older, arguably dating back to a time, some 250 million years ago, when something—perhaps a projectile from outer space—wiped out the majority of the species on Earth, including most reptiles, sponges, corals, starfish, clams, sea scorpions, and fish, thereby clearing the evolutionary decks for dinosaurs to become dominant. That was the greatest mass extinction in history, and thanks to GRACE, paleontologists and evolutionary biologists now have an idea of how it may have happened.

But GRACE’s greatest contribution comes from the fact that it remeasures the geoid every month or so. That enabled geologists to make before-and-after assessments of how the seafloor rearranged itself in the Sumatra-Andaman earthquake of December 26, 2004, which triggered the awful Indian Ocean tsunami. “When a major quake happens on land,” Watkins says, “you can go out and look at the changes. With GRACE, we can now look thousands of feet underwater as well.”

A gravity map of the world: Larger lumps and
red shading indicate regions of greatest
mass, and hence gravitational pull.

The satellites can also reveal movement of water itself, in ways never possible before. “It’s very cool, because water can go underground, it can move around the ocean, it can change from ice to liquid and runoff, but it can’t hide its mass from us,” says Watkins. Imagine, he says, a gigantic hockey puck made of water. “It could be in the form of an ice sheet, or an aquifer, or a piece of ocean. GRACE has the sensitivity to pick up a puck about a centimeter thick and 400 kilometers [half an inch and 250 miles] across.” All the water on Earth can be divided into hockey pucks, he says, and GRACE takes note of how they move around every 30 days.

Last March, geophysicists Isabella Velicogna and John Wahr at the University of Colorado at Boulder published a paper in Science Express that used GRACE data to show that the ice sheet covering Antarctica has shrunk by an average of 36 cubic miles of ice per year—surprising, given that many climate models predict a thickening of the ice as higher global temperatures lead to more evaporation and precipitation. “It’s very difficult for models to reproduce the physics of glaciers, and this shows that the models aren’t as good as we’d like them to be,” Velicogna says.

(Click here to enlarge.)
Grace captured images of Greenland each month in
2005 from January (top left) through December (bottom
right), revealing a giant loss of ice mass (dark blue and
purple).

Velicogna and her colleagues also measured a dramatic loss of Greenland ice, as much as 38 cubic miles per year between 2002 and 2005—even more troubling, given that an influx of fresh meltwater into the salty North Atlantic could in theory shut off the system of ocean currents that keep Europe relatively warm. (A separate group at the University of Texas published figures extrapolated from GRACE data showing that Greenland lost as much as 57 cubic miles of ice each year between 2002 and 2005; NASA shortly plans to publish data reconciling the two studies.) “It’s a wake-up call,” says Velicogna, “because there is a lot of water that can go from the ice sheets into the ocean. Both ice sheets are significantly losing mass, and that affects sea level. If sea level is going to rise, that will affect a lot of coastal areas.”

This past December an entire session of the American Geophysical Union’s fall meeting was devoted to movement of water in and out of giant watersheds all over the world. Speakers presented eight papers, on topics ranging from the hydrologic impact of the Three Gorges Dam in China to the impact of climate change on Siberian river systems. All the new findings were based entirely on data from GRACE. Notable results included a report from researchers at MIT that Alaska lost an average of 10 and a half cubic miles of ice each year from 2003 to 2005.

Oceanographers, geologists, and climatologists are scrambling to update their models of the planet based on the flood of GRACE data. But these will start to look positively primitive when a new, upgraded version of GRACE comes along in several years. Armed with laser interferometers more sensitive than the microwave type, GRACE scientists will be able to attain much better resolution, and thus to find even subtler gravity variations and more exquisite detail, or “smaller hockey pucks,” in Watkins’s words. Nevil Maskelyne never managed to make his own experiment work, but with GRACE his idea has been vindicated beyond even his wildest imaginings.