FOR WHAT HAPPENED TO NOSE OF SUB CHECK LAST SECTION, BELOW "LUCK ESCAPE"
Robot sub explores deep beneath Antarctic ice shelf
18 March 2009
A UK-built robot submarine has successfully returned from a perilous 110-kilometre expedition under an Antarctic ice shelf - the Pine Island Glacier.
The ice shelf juts 60 kilometres into the ocean from the Antarctic mainland. The unmanned sub, known as Autosub, ventured as close to the grounding line - where the ice sheet hits the water - as the scientists who programmed it dared.
The high-risk expedition was necessary to find out why the glacier has been thinning and accelerating in recent decades.
The sub, designed by scientists and engineers at the National Oceanography Centre, Southampton, completed six missions under the ice over ten days.
Pine Island Glacier lies in the Amundsen Sea - part of the South Pacific bordering West Antarctica. The glacier is moving at a rate of four kilometres every year. Pine island and neighbouring glaciers are responsible for global sea level rises of about 0.25 mm a year.
Steve McPhail who led the Autosub team said, 'Autosub is a completely autonomous robot. There are no connecting wires with the ship and no pilot.'
The sub, which has a top speed of five kilometres an hour, uses sonar to pick its way under the ice. On the way it measures the salinity and temperature of the surrounding water and maps the jagged ice overhead and the unknown seafloor below. Scientists will use the sub's three-dimensional maps of the underside of the ice to determine where and how the warmth of the ocean waters melts the glacier base.
No hope of rescue
'All systems on the vehicle must work perfectly while under the ice. There is no hope of rescue 60 km in, with 500 metres of ice overhead,' added McPhail.
The international expedition was led by Dr Adrian Jenkins of British Antarctic Survey and Stan Jacobs of the Lamont-Doherty Earth Observatory, Columbia University, New York and used the American ice-strengthened research ship, the RVIB Nathaniel B Palmer.
After a test mission in unusually ice-free seas in front of the face of the glacier, they started with three 60 kilometre forays under the floating glacier before building up to 110-kilometre round-trips.
Jenkins said, 'The picture we should get of the ocean beneath the glacier will be unprecedented in its extent and detail.'
The ice shelf is 60 kilometres long and 35 kilometres wide. The shape of the seafloor and the ice shelf base are critical for working out how ocean circulations bring warm water under the ice causing it to melt.
'Autosub got as close to the grounding line as was possible,' said Jenkins. As the sub reached a bottle-neck, it was programmed to turn round once clearance above and below reduced to 100 metres.
'We cannot say precisely when the ice and seafloor would meet but we think we got within a few kilometres of the grounding line,' said Jenkins.
Autosub had a lucky escape returning from the grounding line on one of its 30-hour missions. It is programmed to follow the base of the ice shelf out to the ship. But towards the back of the ice shelf just a few kilometres from the grounding line it seems Autosub ascended suddenly, probably into a crevasse. Realising something was wrong, Autosub switched to abort mode and rapidly descended before following a programmed emergency ice exit strategy.
'We didn't know anything was wrong until it surfaced and we saw its damaged nose,' said Jenkins.
The lead US researcher on the project, Stan Jacobs, is researching the Pine Island Glacier with International Polar Year (IPY) funding from the National Science Foundation (NSF).
How the ice sheets respond as global temperatures rise is one of the remaining major uncertainties in climate research.