Antarctica's Ross Sea is no place for swimming. Covered in ice in its southern half, the frozen bay's summer temperatures hover just enough above freezing to spark a yearly retreat of melting sea ice. So, what to do if you are a doughty team of oceanographic explorers trying to gather more information about conditions in its waters? Send in the robots.
"The biggest fear we had was what happens when (the robots) go under the ice," says oceanographer Vernon Asper of the University of Southern Mississippi in Hattiesburg. "But they were a lot cheaper than renting a ship." Leasing a scientific icebreaker ship runs at least $50,000 per day, with much of the time spent dangling strings of sensors into the water one location at a time. Asper and colleagues released two " Seaglider" robots in the Ross Sea in November and December, setting them off on days-long explorations of the icy bay, the ocean's southernmost reach. One of the most productive waters on Earth, the Ross Sea is filled with minke whales, Adelie penguins and phytoplankton, the tiny plants that form the base of the food chain.
Currents, temperatures and other basic oceanographic information about the sea are only spotty, however, due to its frigid state, requiring ice-breaker ships for normal exploration. "The beauty of the gliders is they can collect data for days without any human help," says fellow team oceanographer Walker Smith of the Virginia Institute of Marine Science in Glouster Point, Va. Made by the same iRobot company that makes the Roomba robot vacuum cleaner, the Seagliders last made science news last summer in the Gulf of Mexico, where they detected a subsea plume of oil particles streaming away from the Deepwater Horizon oil spill site.
The Ross Sea expedition was a proof-of-concept run for the underwater robots, funded by the National Science Foundation, to see if they could withstand the chill waters there, around 28 degrees Fahrenheit at depths reaching down to nearly 2,000 feet (the freezing temperature of seawater is lower because it has salt in it so it isn't all frozen at this depth), and evade ice patches when they surfaced. The goal was to map water conditions in the Ross Sea "polynya," a region of open water surrounded by sea ice, where the plankton blooms every Antarctic summer.
Things didn't start out looking promising for the first glider, dubbed GOE-2, Asper says. "We released it and it started heading the wrong way, right under the ice." The robots are true gliders — they move by pumping oil into a long beak on their front, which shifts their center of gravity forward and makes them dive underwater. They steer and adjust their altitude with their wings. When the robots need to surface, they pump the oil back into their interior to increase their buoyancy and steer upwards. But they still have to work with the current. The team retrieved the glider and decided they needed to launch them farther south.
So, they released the second one, GOE-3, from a stretch of sea ice closer to the open waters forming in the southern Ross Sea. "The only problem was the only hole we could find was a minke whale breathing hole," Asper says. "I thought we were out of luck when we dropped the glider in and 10 seconds later a minke surfaced to breathe right where it had just been. So much for that glider."
They didn't hear from the second, $150,000 glider for two-and-a-half days. The robot's computer programming included a navigation plan for avoiding ice above when it tried to surface. Essentially when the water temperature was freezing and it had stopped moving, the robot figured it was hitting ice and would have to dive again to find open water. Which is just what it did, and what had kept the glider from making contact. The unmapped currents of the sea had pushed the glider right under the glacial ice shelf, almost 850 feet thick in places, that extended for hundreds of miles across the southern Ross Sea.
Beating back against the ocean current by tacking like a sailboat, GOE-3 made its way into the polynya to perform its mapping mission. Reprogrammed to deal with the unexpectedly strong current, the first seaglider also made its way from the whale breathing hole to the polynya region. One glider intensively measured currents and temperature in a small part of the sea while the other followed a longer but less exhaustive path.
The data is still under analysis with colleagues at the University of Washington and United Kingdom's University of East Anglia, But Asper argues the robots have proven their worth. "The strength of the current there was a big surprise. This is supposed to be a placid, frozen sea. Just finding that out is interesting," he says.
The team would like to return with six gliders next year for a more thorough mapping of the currents of the Ross Sea, the place where only a century ago, the Norwegian explorer Roald Amundsen commenced his first successful expedition to the South Pole. "I think we are going to see oceanography transformed in the next three years or so by many more scientists using these sort of gliders," Smith says. "They just give us all sorts of new opportunities."