Crack Advances Across Antarctic Ice Shelf
In August 2016, the return of sunlight on the Antarctic Peninsula meant that the landscape became visible again in natural-color satellite imagery. That’s when scientists saw something interesting: a rift along Larsen C—the continent’s fourth-largest ice shelf—has grown considerably longer.
The scenario is similar to what occurred before a calving event and partial collapse of Larsen B in 2002. But exactly what’s in store for Larsen C remains to be seen. “We don’t know yet what will happen here,” said Ala Khazendar of NASA’s Jet Propulsion Laboratory.
The rift is visible in these images acquired on August 22, 2016, with the Multi-angle Imaging SpectroRadiometer (MISR) on NASA’s Terra satellite. The instrument has nine cameras that can be used in various combinations to get different perspectives of a landscape.
The top image was acquired with MISR’s downward-looking (nadir) camera. This natural-color image has a red tint due to the steep lighting angle, as the Sun does not reach far above the horizon in August. The ice shelf comprises the left half of the image, and thinner sea ice appears on the right.
The bottom image shows the same area in a composite image composed from MISR’s backward-, vertical-, and forward-pointing cameras. By combining these different angles in one image, you can discern surface roughness. Rougher surfaces appear pink and smoother areas appear purple. The ice shelf is generally smoother than the sea ice, with the exception of the crack—an indication that it is actively growing, according to the MISR team. Project MIDAS, a group in the United Kingdom that has been tracking the rift, reported that the crack grew 22 kilometers (13 miles) over the past six months. It now stretches 130 kilometers (80 miles).
Both images show other fissures as well, all of which terminate at about the same distance south of the lengthening crack. “People have been intrigued by this,” Khazendar said. “It’s quite a remarkable feature, how they open and then seem to stop opening.”
There are a few hypotheses as to why that happens. The cracks might come to a stop when they reach a suture zone—an area where sectors of ice feeding the shelf are advancing at different speeds, creating shear where they flow together. Ice in this zone is already so fractured that it halts further propagation of the big, crosswise cracks.
The cracks also could have reached an area where marine ice has formed on the bottom of the ice shelf. Marine ice is relatively warm and less stiff, so it can accommodate higher levels of strain without fracturing.
The crack that’s actively lengthening, however, has overcome those obstacles. “What’s happening now is different,” Khazendar said. “This crack goes farther and has started propagating northwards.”
Even before signs of the lengthening appeared at the surface, Khazendar and colleagues suspected something was going on. A study in 2011 that measured ice velocity showed a “line” across the shelf; everything between that line and front of ice shelf was flowing noticeably faster than everything upstream. They proposed that the line traced the location of a crevasse growing upward along the bottom of the ice sheet. Then in 2014, the MIDAS team first detected the rift growing at the surface.
“What might be happening is that there is enhanced melting at bottom of ice shelf, resulting in the removal of the softer marine ice, allowing fractures to be filled with ocean water,” Khazendar said. “When that happens, it could cause pre-existing bottom crevasses to propagate up through the ice shelf.”
Cracks and calving of ice from the front of an ice shelf is a normal process. Shelves are fed by ice coming from glaciers and ice streams from the interior of the continent. They advance into the ocean until a calving event takes place. The shelf front retreats and then advances again. The whole cycle can occur over the span of a few decades. “That’s just part of life for an ice shelf,” Khazendar said. “That’s how they behave.”
In the case of Larsen B, the big calving events took place with a frequency that did not allow enough time for the shelf to re-advance. As a result, the front of the shelf kept retreating in a run up to the big disintegration event that occurred in just six weeks in 2002.
“The growing crack on Larsen C could be the beginning of a process that will end up like Larsen B,” Khazendar said. “If a big calving event takes place, we will be interested to see how the shelf itself reacts. But all the indications so far are that it is relatively stable, albeit with intimations of change.”
NASA Earth Observatory images by Jesse Allen, using data provided by the NASA/GSFC/JPL, MISR Team. Caption by Kathryn Hansen.