NASA space lasers map meltwater lakes in Antarctica with striking precision

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STATUE: NASA researchers on the surface of the Antarctic Ice Sheet as part of the 88-South Traverse in 2019. The 470-mile expedition in one of the most barren landscapes on Earth… view more

Credit: Credit: NASA’s Goddard Space Flight Center/Dr. Kelly Brunt

From above, the Antarctic ice sheet could look like a calm, perpetual blanket of ice that has covered Antarctica for millions of years. But the ice sheet can be thousands of feet deep at its thickest, hiding hundreds of meltwater lakes where its base meets the continent’s bedrock. Deep below the surface, some of these lakes continuously fill and flow through a system of waterways that eventually empty into the ocean.

Now, with the most advanced laser Earth observation instrument NASA has ever flown into space, scientists have improved their maps of these hidden lake systems beneath the West Antarctic Ice Sheet — and have discovered two more of these active subglacial lakes.

The new study provides critical insight for spotting new subglacial lakes from space, as well as assessing how this hidden conduit system affects the rate at which ice slides into the Southern Ocean, adding freshwater that can alter circulation and ecosystems.

NASA’s Ice, Cloud and Land Elevation Satellite 2, or ICESat-2, allowed scientists to accurately map the subglacial lakes. The satellite measures the height of the ice surface, which despite its enormous thickness rises or falls as lakes fill or empty under the ice sheet.

The study, published July 7 in Geophysical Survey Letters, integrates elevation data from ICESat-2’s predecessor, the original ICESat mission, as well as the European Space Agency’s satellite dedicated to monitoring polar ice thickness, CryoSat-2.

Hydrological systems under the Antarctic ice sheet have been a mystery for decades. That began to change in 2007, when Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography at the University of California San Diego, made a breakthrough that helped update the classic understanding of subglacial lakes in Antarctica.

Using data from the original ICESat in 2007, Fricker first discovered that beneath Antarctica’s fast-flowing ice flows, an entire network of lakes is interconnected and actively fills and drains over time. It used to be thought that these lakes hold the melt water statically, without filling and draining.

“The discovery of these interconnected systems of lakes at the interface of the ice bed that displace water, with all these effects on glaciology, microbiology and oceanography – that was a big discovery of the ICESat mission,” said Matthew Siegfried, assistant professor of geophysics at the Colorado School of Mines, Golden, Colo. and principal investigator in the new study. “ICESat-2 is like putting on your glasses after using ICESat, the data is so accurate that we can really start mapping the lake’s boundaries on the surface.”

Scientists have hypothesized that the subglacial water exchange in Antarctica is the result of a combination of factors, including fluctuations in the pressure exerted by the enormous weight of the ice above, the friction between the ice sheet and the rocks below, and heat dissipated from the ice. The earth below is insulated by the thickness of the ice. That’s in stark contrast to the Greenland Ice Sheet, where lakes at the bottom of the ice fill with meltwater drained through cracks and holes on the surface.

To study the regions where subglacial lakes are more likely to fill and drain with satellite data, Siegfried collaborated with Fricker, who played a key role in designing how the ICESat-2 mission perceives polar ice from space.

The new study from Siegfried and Fricker shows that a group of lakes, including the Conway and Mercer Lakes beneath the Mercer and Whillans ice flows in West Antarctica, are going through a dry run for the third time since the original ICESat mission began measuring from elevation changes on the ice sheet’s surface in 2003. The two newly found lakes are also located in this region.

In addition to providing vital data, the study also revealed that the contours or boundaries of the lakes can gradually change as water enters and exits the reservoirs.

“We’re really mapping any elevation anomalies that exist right now,” Siegfried said. “If there are lakes filling and emptying, we will detect them with ICESat-2.”

‘Help us observe’ under the ice cap

Accurate measurements of basal meltwater are crucial if scientists are to gain a better understanding of Antarctica’s subglacial piping system, and how all that freshwater can alter the speed of the ice sheet above or the circulation of the ocean into which it eventually flows.

A huge dome-shaped sheet of ice that covers most of the continent, the Antarctic Ice Sheet slowly flows out from the central part of the continent like super-thick honey. But as the ice approaches the coast, the speed changes drastically, turning into river-like ice streams that quickly carry ice out to the ocean at speeds of up to several meters per day. How fast or slow the ice moves depends in part on how meltwater lubricates the ice sheet as it slides onto the bedrock below.

As the ice sheet moves, it encounters cracks, crevices, and other imperfections. When lakes under the ice gain or lose water, they also distort the frozen surface above. Big or small, ICESat-2 maps these elevation changes with a precision of just a few centimeters using a laser altimeter system that can measure the Earth’s surface with unprecedented detail.

Tracking those complex processes with lengthy satellite missions will yield crucial insights into the fate of the ice sheet. Much of what glaciologists have discovered about ice sheets over the past 20 years has come from observations of how polar ice changes in response to warming in the atmosphere and ocean, but hidden processes such as the way lake systems transport water beneath the ice will may also be key in future studies of the Antarctic ice sheet, Fricker said.

“These are processes going on under Antarctica that we would have no idea about if we didn’t have satellite data,” Fricker said, highlighting how her 2007 discovery allowed glaciologists to confirm that Antarctica’s hidden piping system transports water much faster than previously thought. “We are struggling to get good predictions about the future of Antarctica, and instruments like ICESat-2 help us observe at a process scale.”

‘A water system that is connected to the entire earth system’

How freshwater from the ice sheet can affect the circulation of the Southern Ocean and its marine ecosystems is one of Antarctica’s best-kept secrets. Because the continent’s subglacial hydrology plays a key role in moving that water, Siegfried also emphasized the ice sheet’s connection to the rest of the planet.

“It’s not just the ice sheet we’re talking about,” Siegfried said. “We’re really talking about a water system that’s connected to the entire system on Earth.”

Recently, Fricker and another team of scientists explored this connection between freshwater and the Southern Ocean — but this time by looking at lakes near the surface of an ice shelf, a large slab of ice that floats on the ocean like an extension of the ice. sheet. Their study reported that a large ice-covered lake collapsed abruptly in 2019 after a crack or rift formed from the bottom of the lake to the base of Amery Ice Shelf in East Antarctica.

Using data from ICESat-2, the team analyzed the rugged change in the ice shelf’s landscape. The event left a doline or sinkhole, a dramatic depression measuring about four square miles (about 10 square kilometers), or more than three times the size of New York City’s Central Park. The rift drained nearly 200 billion gallons of fresh water from the ice shelf’s surface to the ocean below in three days.

During the summer, thousands of turquoise meltwater lakes adorn the bright white surface of Antarctica’s ice shelves. But this abrupt event happened in the middle of winter, when scientists expect the water on the ice shelf’s surface to be completely frozen. Because ICESat-2 orbits Earth with precisely repeating ground trails, its laser beams can show the dramatic change in terrain before and after the lake drains, even during the darkness of the Arctic winter.

Roland Warner, a glaciologist with the Australian Antarctic Program Partnership at the University of Tasmania, and lead author of the study, first saw the scarred ice shelf in images from Landsat 8, a joint mission between NASA and the US Geological Survey. The drainage event was most likely caused by a hydraulic fracturing process in which the mass of the lake’s water led to a surface crack that was driven straight through the ice shelf to the ocean below, Warner said.

“Because of the loss of this weight of water on the surface of the floating ice shelf, the whole thing bends upward, centered on the lake,” Warner said. “That would have been hard to figure out just by staring at satellite images.”

Meltwater lakes and streams on Antarctica’s ice shelves are common during the warmer months. And because scientists expect these meltwater lakes to become more common as air temperatures warm, the risk of hydraulic fracturing could also increase in the coming decades. Still, the team concluded that it is too early to determine whether climate warming in Antarctica caused the demise of the observed lake on Amery Ice Shelf.

Witnessing the formation of a doline of altimetry data was a rare opportunity, but it’s also the kind of event glaciologists need to analyze to study all the ice dynamics relevant in models of Antarctica.

“We’ve learned so much about dynamic ice sheet processes from satellite altimeter measurement, it’s vital that we plan for the next generation of altimeter satellites to continue this record,” Fricker said.

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By Roberto Molar Candanosa NASA’s Earth Science News Team

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