Ghost Lake Found Beneath Antarctica's Ice Sheet
July 3, 2013

CryoSat Mission Uncovers Drained Crater Under Antarctica’s Icy Surface

April Flowers for - Your Universe Online

A vast crater has been found in Antarctica's icy surface by the European Space Agency's (ESA) CryoSat satellite. The crater was formed, scientists believe, when a lake lying under about 2 miles of ice drained suddenly.

There are lakes of fresh water trapped without a connection to the ocean, deep below the thick ice sheet that covers Antarctica. Scientists who are trying to understand water transport and ice dynamics beneath the frozen Antarctic surface are very interested in these lakes, but the information is not easy to obtain.

One method used so far is to drill holes through miles of ice to the water. In the harsh conditions of the polar regions, this is a very difficult and dangerous undertaking.

The European researchers involved in the new study, published in Geophysical Research Letters, looked up to the sky instead of down towards the ice, hoping to improve our understanding of subglacial water and its transport.

The team combined new measurements from CryoSat with data from NASA's ICESat to map the large crater left behind by a lake. They were even able to determine the scale of the flood that formed the lake.

"The crater's a big feature," said Dr Malcolm McMillan from the UK's University of Leeds.

"It covers an area of about 260 sq km, which is about the size of Edinburgh, and was as much as 70m deep," he told BBC News.

"We knew from the Icesat data there had been a big elevation change, but it's only now with Cryosat that we've been able to appreciate the true scale of what happened."

Approximately 1.5 cubic miles of water - about the same amount stored in Scotland's Loch Ness - drained from the lake beneath the ice between 2007 and 2008. This is the largest event of its kind ever recorded, as the 6 billion tons of water that drained equals a tenth of the annual melting that occurs beneath Antarctica.

The lake, located in the Cook Sub-Glacial Lake (SGL) region in the east of the continent, appears to be refilling since 2008, but approximately six times slower than it drained. It will take decades to refill at that pace.

CryoSat's unique capacity to map changes in the subglacial lakes of Antarctica in 3D allowed the researchers to shed new light on events at the base of the ice sheet. The radar altimeter onboard CryoSat can see through clouds and in the dark, which provides continuous measurements over areas like Antarctica that are prone to bad weather and long periods of darkness.

The radar can also measure the area and depth of ice craters in high resolution. This allows the researchers to calculate the volume of the crater accurately. This study is the first to use CryoSat's altimeter operating in Synthetic Aperture Radar interferometric (SARin) mode, which uses two antennas offset by about a meter to listen for the return echo of its radar pulse.

The two offset antennas allow the instrument to judge the angles of return in order to better sense the shape of the ice below, providing far more reliable information on slopes and ridges.

"Thanks to CryoSat, we can now see fine details that were not apparent in older satellite data records," said McMillan.

With the discovery of each subglacial lake, scientists hope to find prehistoric marine life. The rapid way this lake drained and refilled, however, suggests this was not the first time the lake had emptied.

"It seems likely that the flood water - and any microbes or sediments it contained - has been flushed into the Southern Ocean, making it difficult to imagine that life in this particular lake has evolved in isolation," said Prof. Andrew Shepherd.

So far, approximately 400 lakes have been discovered beneath the Antarctic ice sheet. As these lakes drain, they disrupt subglacial habitats and can cause the ice above to slide more quickly into the sea.

Scientists call them "ghost lakes." They are kept in a liquid state by heat rising from the rockbed below and the pressure of the enormous ice sheet above. They appear to be linked through a network of rivers. The lakes alternately fill from waters higher up in the hydrological system, then drain to lower elevations.

Glaciologists consider understanding this behavior key to their investigations. Water underneath the ice sheet will lubricate its movement and computer models that want to simulate how Antarctica will react to future changes in the climate will have to take this lubrication into account.

Scientists are currently unsure where Cook is being replenished from, but when it over-tops, the lake's waters may run all the way to the coast to enter the ocean.

"Further downstream, there was an inflation of the ice," explained team-member Hugh Corr from the British Antarctic Survey to the BBC. "But whether all that water reaches the ocean, or re-freezes onto the underside of the ice, or even melts more ice with its heat - we just don't know. It will, though, change the lubrication."

Helping to raise the global sea level, Antarctica is currently losing mass at the rate of 50-100 billion tons a year. The research team suggests a significant fraction of this mass loss could be the result of flood events like that seen at Cook SGL.

"This one lake on its own represents 5-10% of [Antarctica's] annual mass imbalance," Prof Shepherd told the news service.

"If there are nearly 400 of these sub-glacial lakes then there's a chance a handful of them are draining each year, and that needs to be considered," he concluded.