reported on Thursday in the journal Science<\/a>.<\/p>\nThese quakes seemed to start near impurities in the ice that were deposited by volcanic eruptions, Dr. Fichtner said. Where these particles sit, the ice is weaker, more prone to cracking. Along these cracks, the ice sticks and slips and quivers as it moves, creating tiny seismic disturbances.<\/p>\n
This isn\u2019t what scientists usually imagine is going on inside the deep piles of ice that cover Earth\u2019s polar regions. Typically, they think of this ice as flowing like syrup: slowly, smoothly, fluidly.<\/p>\n
But if the ice were really moving like a uniform mass of honey, then Dr. Fichtner\u2019s cable would have picked up \u201ccomplete silence,\u201d he said. Instead, it recorded these \u201creally, really curious events,\u201d he said. \u201cThat was the surprise here.\u201d<\/p>\n<\/div>\n<\/div>\n
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By sending laser pulses through fiber-optic cable and measuring how they scatter, scientists can reconstruct fine movements along the cable\u2019s entire length. This has proved useful for monitoring seismic activity, deep-sea currents, glacial ice and more.<\/p>\n<\/div>\n<\/div>\n
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In Greenland, Dr. Fichtner and a colleague lowered a cable by hand nearly a mile down a borehole, one that other scientists had drilled to extract an ice core. There the cable lay for 14 hours, picking up vibrations.<\/p>\n
If winding and unwinding a cable doesn\u2019t sound especially challenging, let Dr. Fichtner be the first to inform you: It was \u201cserious physical work.\u201d The borehole was filled with a special kind of vegetable oil to keep it from closing up, so the cable was slow to sink and heavy to lift back out. Plus, the subzero cold made the cable brittle, meaning they had to handle it with utmost care.<\/p>\n
When Dr. Fichtner started looking at the readings that came back, he had to convince himself they weren\u2019t \u201crubbish.\u201d What if they showed vibrations coming from within the cable itself? Or from cracks forming in the borehole wall?<\/p>\n
In time, he and his team concluded that they\u2019d recorded something intrinsic to the ice. Still, Dr. Fichtner acknowledged that only by making more measurements in more places can scientists really say how commonly these quakes occur within ice sheets.<\/p>\n<\/div>\n<\/div>\n
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Getting enough measurements is a constant challenge for polar scientists, said H\u00e9l\u00e8ne Seroussi, an engineering professor at Dartmouth College in New Hampshire who wasn\u2019t involved in the new research. When oceanographers want to collect data, they can drop instruments into the deep sea in a matter of hours. Glacier researchers have to drill deep into the ice, which takes months, even years.<\/p>\n
\u201cThat\u2019s why we keep finding all these new principles and mechanisms that seem relatively fundamental,\u201d Dr. Seroussi said. \u201cEach time you have a new observation, a new ice core, a new way of measuring, you learn something new.\u201d<\/p>\n
Andy Aschwanden, a glaciologist at the University of Alaska Fairbanks, said Dr. Fichtner and his colleagues\u2019 discovery offered an interesting glimpse into the intricacies of ice physics. But he said it was too early to know whether it can help scientists better predict how quickly the melting ice sheets will lift global sea levels. The ice still holds other mysteries that, if unraveled, are likely to improve the modeling much more, Dr. Aschwanden said.<\/p>\n
The new findings could someday help scientists better understand the way ice sheets break apart at their edges, said Richard B. Alley, a professor of geosciences at the Pennsylvania State University.<\/p>\n<\/div>\n<\/div>\n