By sending sound waves into the warming seas around Antarctica, an underwater robot has given scientists a detailed new look at the melting undersides of the continent’s giant ice shelves, which matter hugely for global sea levels.
The images taken by the robot contain clues about how the shelves are thinning. They reveal several never-before-seen formations in the ice, including immense terraces with rounded, swirling edges, and teardrop-shaped divots, some that are hundreds of feet across, that appear to have been sculpted by turbulent currents.
The scientists behind the discovery described the structures as “enigmatic.”
“I couldn’t stop looking at it,” said Anna Wahlin, an oceanographer at the University of Gothenburg in Sweden, describing the first time she saw the data. “We had no idea it could look like this.”
It’s not just the transfixing beauty of these features that interests scientists. Ice shelves are the floating edges of glaciers, and they are critical to sea level rise. The shelves block Antarctica’s glaciers from shedding more of their ice into the ocean. As more warm water bathes their undersides, the shelves melt and weaken, causing glacial ice to flow more swiftly out to sea.
The new imagery comes from underneath the Dotson Ice Shelf, which sits on the Amundsen Sea off West Antarctica. Dotson is more stable than other shelves in the region. Dr. Wahlin and her colleagues were originally hoping to survey the bottom of the nearby Thwaites Glacier, which is losing mass quickly. Scientists have estimated that Thwaites’s total collapse would increase global sea levels by about two feet over several centuries.
When Dr. Wahlin and her team reached the frozen continent, however, Thwaites was inaccessible. Too much sea ice was in the way.
In the end, mapping the underbelly of Dotson gave the researchers a chance to test their equipment and methods, Dr. Wahlin said. Their findings were published Wednesday in the journal Science Advances. They are turning their attention to Thwaites next.
When Dr. Wahlin and her colleagues traveled to Dotson in early 2022, the key member of their team was more than 20 feet long and bright orange. Its name was Ran.
The scientists programmed Ran to travel back and forth beneath the shelf in neat rows, like a lawn mower, collecting data by pulsing the ice with sound waves. But the researchers couldn’t talk with Ran while it was under the ice. And Ran couldn’t use GPS to figure out where it was going. It had to navigate by dead reckoning, with the help of onboard motion sensors.
That made for plenty of heart-in-throat tension each time the scientists slid Ran into the water, Dr. Wahlin said. The shortest missions the robot completed beneath Dotson lasted four to five hours. The longest, 28 hours. “Just waiting, without any signs of life,” Dr. Wahlin said.
By the end of the scientists’ monthlong expedition, Ran had completed 14 undersea missions, but only about half of them were completely successful. The data it came back with covered 50 square miles of ice.
Another source of suspense: The scientists were far from sure what the robot would even find. The bottom of the ice shelf could have turned out to be flat and featureless, which would have been a letdown, Dr. Wahlin said. “So we were happy to see structures,” she said.
The structures were far more complex than anyone had expected, said Peter Davis, an oceanographer with the British Antarctic Survey who also contributed to the study. “It looks like a beach after the tide’s gone out,” he said.
In the past, scientists generally assumed that the melting process smoothed out the topography of underwater ice, Dr. Davis said. Now it’s becoming clear that these features and indentations can persist. Understanding how they affect overall melting in response to ocean warming and other changes will require further study, he said.
Earlier this year, Dr. Wahlin and her colleagues brought Ran back to Dotson. They wanted to repeat their surveys from 2022 and see how the ice had changed. But after one of its dives, Ran never showed up at its rendezvous point. The robot was lost.
Dr. Wahlin has two main theories about what happened. One is that Ran crashed in the grounding zone, the area hundreds of feet underwater where the ice shelf meets the bedrock. The other is that it had a fateful encounter with a curious seal.
The scientists spotted several seals living in a fracture in Dotson’s ice, Dr. Wahlin said. One of them could have started circling Ran underwater, causing it to become disoriented.
Scientists hope to have robots monitoring Antarctic ice more regularly, said David Holland, a professor of mathematics and ocean science at New York University who contributed to the research at Dotson. Only with more and better data could we forecast sea-level rise as precisely as we forecast the weather, Dr. Holland said.
“We’re not there yet,” he said. “It’s not going to happen unless we push it.”