By Jonathan Amos
Science reporter, BBC News, San Francisco
The US space agency (Nasa) would like its rubber ducks back, please.
Ninety bathtub toys were hurled into a drainage hole on the Greenland ice in September - an experiment to see how melt waters find their way to the base of the ice sheet.
It was hoped the ducks would flow along subglacial channels and eventually pop out into the sea. They may still, but nothing has been seen of them so far.
"We haven't heard anything from them yet," said Nasa's Alberto Behar. "If somebody does find one, it will be a great breakthrough for us."
Dr Behar is a robotics expert with the agency at its Jet Propulsion Laboratory in Pasadena, California, US. He has been studying the tubular crevasses that appear on the surface of the Greenland ice known as moulins.
These "plug holes" can drain vast lakes of melt water that settle on the top of the ice during summer months. Scientists would like to know how and to what extent this water can help lubricate the base of the ice sheet, moving it faster towards the ocean.
If plastic fowl seem a very low-tech way for a man who has also worked on Mars rovers to investigate the problem, it should be stressed he also has a more sophisticated approach.
Huge quantities of melt water can go down the tubes
Dr Behar has been developing a Moulin Explorer, a probe that can travel through the chutes. He has been describing his work here at the American Geophysical Union's Fall Meeting 2008.
Dr Behar started out with a simple ice borehole camera on the end of a tether and then gradually built up the instrumentation and improved the packaging so that the probe moved more easily with the waters.
"We've found the moulins are complex; they're not just a shaft that goes all the way down to the bottom and creates a pipe out in a certain direction," he told BBC News.
"There are stair-steps and there are breaks in the ice where it seems the water goes in many directions and then comes back together."
This past summer season, he finally released an untethered Explorer into a moulin alongside the great Jakobshavn outlet glacier in West Greenland - together with those ducks.
The rubber ducks have Dr Behar's email address on them
The PVC cylinder - about the size of an American football - contained an accelerometer, a pressure transducer, GPS with satellite modem link, and an antenna; all powered by high-capacity lithium thionyl chloride batteries.
Like the ducks, the Explorer was supposed to pop out somewhere having gone through the subglacial plumbing. It was also supposed to call home.
"We did not hear a signal back so it probably got stuck under the ice somewhere," said Dr Behar. "It was a bit of a long shot but we thought it was worth a try. We've got to go back and scratch our heads and think about what we do next."
Even seeing a few ducks emerge from their subglacial adventure would give some clues as to what is happening down below.
These are early days. In the distant future, the descendents of the Moulin Explorer could be deployed by Nasa on icy moons in the outer Solar System that are thought to harbour subglacial oceans.
Jakobshavn and other outlet glaciers are a major focus of research
There seems little doubt that draining melt waters can lubricate the ice-rock interface at the bed of the Greenland sheet; surges in the ice have been recorded when large surface lakes have suddenly emptied through moulins.
But scientists are more circumspect about the waters' contribution to the acceleration noted in some of Greenland's outlet glaciers which - like Jakobshavn - have doubled their speed in recent years.
These outlet glaciers are the major route through which Greenland dumps its ice into the ocean. If there is to be concern about the Greenland ice sheet's contribution to future sea level rise in a warmer world, it is the behaviour of these glaciers which needs to be monitored most closely.
And recent research has given interesting perspectives on this issue.
Earlier this year, Ian Joughin from the University of Washington and colleagues analysed satellite observations and GPS data on ice motion from across a wide swathe of Greenland.
They found that summer surface melting was not producing large instabilities in the glaciers.
"In percentage terms, the speed-up of the ice sheet is somewhat large - 50 to 100% in summer months - but that part of the ice sheet isn't moving that much ice into the ocean. The area that's moving the ice into the ocean a lot is the big outlet glaciers and they're not being affected much by the melt lubrication."
In the case of Jakobshavn, Dr Joughin takes the view that the more significant influence is the presence of sea ice in front of the glacier. This has been largely absent in recent years, allowing the great mass behind to move much faster into the ocean, he says.
"The speed of the glacier is modulated by calving and sea ice seems to shut down calving," explained Dr Joughin.
The Moulin Explorer is unleashed
David Holland, from New York University, and colleagues are also presenting here in San Francisco.
They implicate the deep warm waters that now inhabit the fjords dominated by these big glaciers.
Dr Holland's team has been using a long data-set of thermometer readings gathered by fisheries researchers studying sea-bed conditions in the Jakobshavn region.
They have able to show how changes in atmospheric circulation in the North Atlantic could have driven deep warm waters around the southern coast of Greenland from as far away as Iceland.
"In the case of Greenland, the water around the coast at depth, from 200m to 600m - it was relatively cold until 1997 and then within six months the whole thing lit up on the west coast and became warm - almost dramatically so - and filled these deep fjords. It put things out of balance," Dr Holland told BBC News.
"I don't think we should be focusing on these moulins which have very little influence. The ocean is a massive heat source and these moulins are just not as important."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.