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A team of British and American scientists has successfully
deployed an autonomous robot submarine on six missions
beneath an Antarctic ice shelf using sonar scanners to
map the seabed and the underside of the ice as it juts
out over the sea.
The research is part of a larger, National Science Foundation-funded
project to study the dynamic Pine Island Glacier and to
understand how increasing ocean temperatures triggered
by a warming climate may affect the melting of the West
Antarctic Ice Sheet (WAIS) and global sea-level rise.
"There is still much work to be done on the processing
of the [submarine] data", said Adrian Jenkins of
British Antarctic Survey, "but the picture we should
get of the ocean beneath the glacier will be unprecedented
in its extent and detail. It should help us answer critical
questions about the role played by the ocean in driving
the ongoing thinning of the glacier."
Scientists hope to learn why the glacier has been thinning
and accelerating over recent decades. Pine Island Glacier
is in the Amundsen Sea, part of the Southern Ocean bordering
West Antarctica. Changes in its flow have been observed
since the early 1970's, and together with neighboring
glaciers it is currently contributing about 0.25 mm (less
than one-tenth of an inch) annually to global sea level
rise. But the ice shelf, like other ice shelves, serve
to hold back movement of the West Antarctic Ice Sheet.
The lead U.S. researcher on the project, Stan Jacobs,
of the Lamont-Doherty Earth Observatory at Columbia University,
is conducting long-term observations of the Pine Island
Glacier, most recently with International Polar Year (IPY)
funding from NSF. NSF is the lead U.S. agency for IPY
research. The World Meteorological Organization (WMO)
and the International Council for Science (ICSU), which
planned and sponsored IPY, has set aside the last two
weeks of March to recognize projects that focus on the
polar oceans.
One of NSF's IPY research goals is to better understand
the dynamics of the world's massive ice sheets, including
WAIS, which, if it were to melt all at once, would raise
global sea level by five meters (16 feet).
The most recent report of the Intergovernmental Panel
on Climate Change (IPCC) noted that because so little
is understood about ice-sheet behavior it is difficult
to predict how ice sheets will contribute to sea level
rise in a warming world. The behavior of ice sheets the
IPCC report said is one of the major uncertainties in
predicting exactly how the warming of the globe will affect
human populations.
Jacobs and Jenkins deployed the robot explorer, Autosub3,
from the NSF-chartered R/V Nathaniel B. Palmer, to investigate
the underside of the ice shelf and measure changes in
salinity and temperature of the underlying water.
Autosub3 was built and developed by the National Oceanography
Centre in the United Kingdom, a collaboration between
the University of Southampton and the Natural Environment
Research Council.
After a test mission in unusually ice-free seas in front
of the face of the glacier, the researchers started with
three 60-kilometer (37-mile) round-trip forays under the
floating ice shelf and extended the length of missions
to 110km (68-mile) round trip. In all, more than 500km
(310 miles) of the ocean beneath the ice was profiled.
Steve McPhail led the Autosub team during the ten-day
survey.
He said that "Autosub is a completely autonomous
robot: there are no connecting wires with the ship and
no pilot. Autosub has to avoid collisions with the jagged
ice overhead and the unknown seabed below, and return
to a pre-defined rendezvous point, where we crane it back
onboard the ship."
"Adding to the problems are the sub zero water temperatures
and the crushing pressures at 1000 meters depth. All systems
on the vehicle must work perfectly while under the ice
or it would be lost. There is no hope of rescue 60 km
in, with 500 meters of ice overhead."
Autosub3 has a maximum range of 400km (248 miles) and
is powered by 5,000 ordinary D-cell batteries. The batteries
are packed in bundles in pressure-tested housings. At
either end of the seven-meter (22-foot) sub there are
free-flooding areas where the payload of instruments are
installed.
Using its sonar, the Autosub3 picks its way through the
water, while creating a three-dimensional map that the
scientists will use to determine where and how the warmth
of the ocean waters drives melting of the glacier base.
Complementing the Autosub exploration, other work during
the 53-day cruise included setting out 15 moored instrument
arrays to record the variability in ocean properties and
circulation over the next two years, extensive profiling
of 'warm' and melt-laden seawater, sampling the perennial
sea ice and swath-mapping deep, glacially-scoured troughs
on the sea floor.
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NSF
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