British
scientists have discovered rivers the size of the Thames in London
flowing hundreds of miles under the Antarctica ice shelf by examining
small changes in elevation, observed by ESA’s ERS-2 satellite, in the
surface of the oldest, thickest ice in the region, according to an
article published in Nature this week.



The
finding, which came as a great surprise to the scientists, challenges
the widely held assumption that subglacial lakes evolved in isolated
conditions for several millions of years and raises the possibility
that large floods of water from deep within the ice’s interior may have
generated huge floods that reached the ocean in the past and may do so
again.

Prof. Duncan Wingham, of the University College London, who led the
team said: "Previously, it was thought that water moves underneath the
ice by very slow seepage. But this new data shows that, every so often,
the lakes beneath the ice pop off like champagne corks, releasing
floods that travel very long distances."

The
team found anomalies in the ice-sheet surface elevation using
ultra-precise measurements from ERS-2’s radar altimetry and radar interferometry.
Close inspection of one anomaly revealed an abrupt fall in ice-surface
elevation with a corresponding abrupt rise some 290 kilometres away.
The scientists state the only possible explanation for these changes is
that a large flow of water was transferred beneath the ice from one
subglacial lake into several others.

Radar altimeters – originally designed to measure the sea surface
height by sending 1800 separate radar pulses down to Earth per second
then recording how long their echoes take to bounce back – have been
highly successful in measuring large-scale ice surface changes over
time. Radar interferometry, known as InSAR, is used to precisely
measure the spatial pattern of the changes. Together, they form a
powerful ‘instrument’ to investigate the physical causes of changes in
the ice surface.

According to Prof. Wingham, Director of
the Centre for Polar Observation and Modelling (CPOM), both sensors
were essential for the study. "With radar altimetry we can tell how
much the lake levels fell and rose, and that it occurred between late
1996 and early 1998, but we cannot determine the area, which is vital
for investigating the physics of the flood because we need to know the volume of water involved.

"Hence we turned to ERS-2 InSAR interferometry, which has excellent
spatial resolution. With it, we were able to image the area of at least
part of one lake, which was enough to give us an idea of the volume,
and with this we could get at the rate of the flow."

Subglacial lakes in Antarctica were first identified in the
1960s. Since then over 150 have been discovered but it is thought
thousands may exist, as much of the bed of Antarctica
remains unexamined. The team focused its study on the Dome Concordia
region in East Antarctica, where more than 40 lakes are known to be.

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