The Ross Ice Shelf, a thick, floating tongue of solid ice the size of Spain, is the biggest of the many such barriers that ring Antarctica and keep its ice sheets from sliding into the sea. Yet the shape of the sea floor beneath—a critical factor in how fast the shelf might melt—is virtually unknown. The ice keeps sonar-carrying ships out, and the water beneath it blocks radar. “It's the least known piece of ocean floor on our planet,” says Robin Bell, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York.
The Ross shelf, roughly 600 meters thick and sitting over about 200 meters of water, has been relatively stable in recent decades. However, ice shelves can be capricious: On the Antarctic Peninsula, an area of the Larsen B shelf the size of Rhode Island collapsed in a matter of months in 2002. Helen Fricker, a co-principal investigator for the survey and a glaciologist at the Scripps Institution of Oceanography in San Diego, California, wants to gather baseline parameters before Ross suddenly changes. “We don't know for sure that the Ross shelf won't change in the next 15 years,” she says.
Now, Bell and colleagues plan to fill in the giant blank spot. They have recently received a grant to survey the shelf with an ultrasensitive airborne gravity detector. The sensor detects tiny changes in gravity: the boosts caused by the extra mass of seafloor hills and the decreases from troughs. After a test flight over the mountains of Vermont next month, her team plans to crisscross the Ross shelf in 36 flights over two 3-week-long campaigns, one in November and a second in 2016. They hope to map features as small as 50 meters tall—dramatically better than the present map, which scientists pieced together in the 1970s by setting off small explosions on the ice every 50 kilometers and recording the echoes. Knowing the shape of the sea floor could give climate scientists important clues about how warm ocean water could melt the ice from below—a process with repercussions that could extend far beyond Antarctica. Floating ice does not affect global sea levels when it melts. But a thinned—or worse, collapsed—ice shelf could clear the way for more of Antarctica's continent-covering ice sheets to enter the ocean and push up sea levels. “Remove that plug, and the ice starts to flow faster,” says Helen Fricker.
Antarctica is ringed by the circum polar current, which carries a deep slug of warm water clockwise around the continent, generally at a safe distance from ice shelves. According to a study published in Science last December, the current has been warming since the 1970s and rising closer to the shelves, especially along the western peninsula of Antarctica. A new map could reveal whether that water has ready access to the underside of the Ross shelf. “Is there a deep pathway, a canyon, a valley that the water can run up like a road?” Bell asks. At a finer scale, knowing how rough or smooth the sea floor is will help ocean modelers gauge the threat from turbulent eddies that could bring that warm water up to the ice's underbelly. The rougher the bottom, the more vigorous the eddies. The 1970s-era map suggests that the sea floor is relatively smooth. But Bell says that a test flight with an older gravity sensor last November already revealed surprising roughness, suggesting that mixing is underestimated.
In May, Bell's team received a $2.2 million commitment from the Gordon and Betty Moore Foundation to fly the new gravity sensor aboard four-engine C-130 cargo planes, which fly longer distances in more types of weather than the Twin Otter, a workhorse of polar research. The gravity meter will be sensitive to changes as small as a milligal, which is the difference in gravity between the bottom and the top of a 3-meter stepladder. Bell is now waiting on an additional $3.4 million from the National Science Foundation that she needs to embark on the campaign.
Source: Science, www.sciencemag.org