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Polar firn aquifers: Why are we doing this?

November 16, 2018

Often when people envision Greenland and Antarctica, they see desolate, snow-filled lands; and in general, that is pretty accurate. Other than a few coastal towns and seasonal base camps, these lands are uninhabited, except for a few tough species of animals. And, of course, there is lots of snow. As snowfall settles, it compacts under the weight of new snow and the battering ram of the wind. This compacted older snow, at least one winter old, is called firn—still porous, but between fresh snow and glacial ice in density.

Where we are going to in Antarctica the firn has a special feature. These regions are known for very warm summer conditions, with lots of melting and lots of snowfall over the course of the year. The summer melt can percolate through the snow and firn grains to form a water-saturated layer that sits above the denser glacier ice. Like a well in sandstone, this is a kind of aquifer in the firn, almost like a natural snow cone. Without the syrup.

Firn aquifers have been observed on a few mountain glaciers, usually for just part of the year, but never on ice sheets until a discovery on the Greenland Ice Sheet in April of 2011. Since then, NASA Operation Ice Bridge data has mapped their extent over several regions of Greenland. In some parts of Greenland, they appear to persist for decades.

More recently, a mapping algorithm using satellite data correctly located the firn aquifers in Greenland. We applied the same method to Antarctica—and there they were, a signal in the data just like Greenland’s aquifer areas. Antarctica’s potential for firn aquifers is at present unconfirmed, yet application of a similar technique indicates that they likely exist in coastal and ice shelf regions that have climate conditions similar to firn aquifer areas in Greenland. That is what we are going to check out.

Firn aquifers are important because they could cause a kind of water-driven fracturing on ice sheets or ice shelves called hydrofracture, where water seeps into cracks in the ice and breaks them open, leading to a speed-up of a glacier or crumbling of an ice shelf. This kind of fracturing has led to some spectacular break-ups in Antarctica, or significant acceleration of glaciers in Greenland.

The objective of the Antarctic Firn Aquifer expedition is to verify the presence of firn aquifers on the Antarctic Ice Sheet by surveying two key sites on the Antarctic Peninsula: the Wilkins Ice Shelf and the southern George VI Ice Shelf. These field sites were identified using our mapping method and data from two satellite microwave instruments: a C-band radar scatterometer (EUMETSAT’s Advanced SCATterometer – ASCAT) and an L-band microwave radiometer (aboard NASA’s Soil Moisture Active Passive Satellite–SMAP). The longer wavelength of ASCAT and SMAP microwaves, and their sensitivity to the presence of liquid meltwater, allow them to see firn aquifers on ice sheets or ice shelves as deep as ~60 meters (200 feet). Over time, distinct patterns in the microwave signals can be used to distinguish firn aquifers from areas that do not store meltwater at depth.

 

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