Robo drop: Autosub will be released next month to study the Arctic's ice from below. Credit: SOUTHAMPTON OCEANOGRAPHY CENTRE

Marine scientists are to take a second stab at a world first — the exploration of the dark waters below the permanent polar ice sheets.

Early next month, the robot submarine Autosub will slide beneath the fringes of the ice sheet that blankets Greenland. Researchers hope to use the vessel to study the underside of the permanent ice, to investigate rising sea levels.

The mission will be the vessel's second such attempt; Autosub was to be deployed under Antarctica's Pine Island Glacier in early 2003, but the ship on which it was being transported was blocked by rapidly drifting ice. Robot subs have previously journeyed under sea ice, which breaks up and reforms annually, but none has attempted to go under permanent ice.

Sea ice is often only a few metres thick, but the ice shelf that Autosub will explore is 200 metres deep and likely to contain narrow channels in which the sub could get irretrievably stuck or lost. In addition, unexpected changes in salinity could create buoyancy problems that might sink the vessel or force it up into the ice. Strong currents could also send the sub off course.

“This very much is undersea space exploration,” says Ken Collins of the Southampton Oceanography Centre and the expedition's scientific coordinator. “If anything goes wrong far out there, we won't see it again.” Collins and his colleagues on board the James Clark Ross have spent the past two weeks sending Autosub on only slightly less perilous missions under the transient sea ice, awaiting the return of the vessel — worth roughly £1.5 million (US$2.7 million) — like worried parents.

The 7-metre-long robot, powered by some 5,000 household batteries, is programmed to map the contours and biology of the sea bed, to measure the thickness of ice overhead and to collect sea water for analysis back on the ship. Together, the results should improve understanding of how fresh water from the ice is cycled into the ocean, contributing to rises in sea level.

“The effects of global warming are greatest and fastest in the Arctic,” says Peter Wadhams, a polar scientist from the University of Cambridge, UK, who is leading part of the expedition. “The Arctic ice has decreased in thickness by 40% over the past 20 years, so by 2080 the Arctic will be ice-free in summer.”

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