By Jonathan Amos
Science correspondent, BBC News, San Francisco
The summer's release was a proof of concept exercise
Tiny orange balls made from Christmas tree baubles could shed new light on how Greenland's Ice Sheet will respond to a warming world.
A Bristol University-led team has shown how the 4cm spheres can go through water channels that run under glaciers and be recovered afterwards.
If equipped with sensors, these balls could gather data on how this meltwater is lubricating the movement of the ice.
Currently, scientists are unsure about the significance of this effect.
And without better information, they will struggle to model the issue adequately.
In August this year, the team tested the baubles - sending them on their journey through a glacier and successfully retrieving many of their "E-tracers" when they emerged.
Next year, they plan to carry out experiments with more advanced data-gathering baubles.
Greenland holds the equivalent of 7m of global sea-level rise in its ice sheet, so its future evolution is a subject of considerable interest to researchers.
"We now have a technique to access the deeper parts of ice sheets and to recover data," said Dr Jemma Wadham from the Bristol Glaciology Centre.
"To predict an ice sheet's response to climate change, you need to know what's happening at the bed because this affects the speed of ice flow to the sea," she told BBC News.
Into the unknown
Dr Wadham has been describing her team's electronic tracers here at the American Geophysical Union's (AGU) Fall Meeting, the world's largest annual gathering of Earth scientists.
Several groups have tried to follow what happens to the meltwater that forms on the top of Greenland's glaciers in summer months.
The meltwater thunders along streams before draining down into the ice
Much of it is seen to drain down moulins, tubular cracks in the ice that go all the way to the rock bed.
And previous studies have shown how major drainage events will be accompanied by sudden accelerations in ice flow, as the descending water makes it easier for the ice to slip along its base.
Recent attempts to study this sub-glacial plumbing system have used coloured dyes, solid objects (rubber ducks) and sensored probes (tethered and autonomous), but the data return has been extremely limited.
The Bristol team, therefore, is thought to be the first group to send objects through the deep-ice channels and recover them.
Good return
The approach was very simple, but ingenious: the baubles contained a radio beacon that allowed them to be located after they had passed through the glacier, in a similar way to which animals can be tracked.
"The best shape to pass though a complex hydrology is a sphere, so we used the baubles as package for our radio beacons," explained Dr Steve Burrow from Bristol's department of Aerospace Engineering.
"It is a bit of a design challenge; it needs to be small and ideally have neutral buoyancy - you neither want it to sink to the bottom and get caught, nor float to the top and get trapped in an air pocket, but we also need to include the electronics and an efficient antenna."
The spheres were released into two moulins on Leverett Glacier near Kangerlussuaq, on the western edge of the Greenland Ice Sheet.
They took several hours to wend their way under the ice, before popping out into a stream at the side of the glacier. Some baubles travelled over 7km under the ice.
About 20% were recovered. Many more were located but could not be picked up because they were in places that were simply too awkward to reach.
For the proof of concept tests, the balls were equipped only with a tiny radio beacon so they could be found easily with a directional antenna once they had completed their subglacial adventure.
Listening for the "peep" noises coming from the baubles' onboard beacons
But the team now plans to incorporate a low-cost multi-sensor package inside the baubles, so they can log data as they travel through the drainage system.
"One of the key things that is difficult to model at the moment is the interaction between the speed of the ice flow and conditions at the ice sheet bed, e.g. how water flows," said Dr Wadham.
"The pressure at which this water flows will have a major influence on the speed of ice motion. For example, if water spreads out over the bed and flows at high pressure there would be a larger impact on ice flow than if the water is funnelled via low pressure channels.
"Even on Greenland glaciers that terminate in the ocean, where warming of ocean waters is thought to drive recent ice speed-up and thinning, an electronic tracking system introduced to the basal environment could provide further information on changes in the conditions at the ice sheet bed during this speed-up."
A proposal has gone into the UK's Natural Environment Research Council (Nerc) to fund a major deployment of more sophisticated spheres in 2010.
Also, a story can now be told about how August's expedition was nearly scuppered by Arctic wildlife.
Dr Burrow recalled: "One evening we'd stuck together our electronic tracers and gone to bed, and when we woke up they'd all gone. To cut a long story short, an Arctic fox had stolen them in the night from the hall of the tent.
"It must have thought they were a clutch of eggs and took them in its mouth and buried them."
