By Paul Rincon
BBC News Online science staff
Researchers are testing technology that could allow a lander to melt through the ice crust of Jupiter's moon Europa to reach the water ocean beneath.
Europa may hold a warm water ocean underneath its ice crust
Space scientists want to send a craft to the Jovian moon because its ocean might, in theory, harbour life.
Once through the 10-30km ice sheet, the probe could take a sample of water, to analyse it for microbial life.
But significant engineering challenges remain before the German Aerospace Centre lander could be sent to Europa.
"The idea is to land on Europa and somehow get through the ice," the centre's Dr Stephan Ulamec told BBC News Online.
"One needs some kind of melting probe which can melt through the ice and carry out investigation in the liquid ocean expected underneath."
The prototype being tested by Dr Ulamec is a 225cm-long hollow aluminium cylinder with a copper melting head powered by an electrical cable.
The probe's melting speed slows under low pressures
It is based on a design pioneered by researchers at the Alfred Wegener Institute for Polar Marine Research in Bremerhaven, Germany.
This melting probe, dubbed Susi, was originally envisaged as a tool to explore subglacial lakes on Earth, such as Lake Vostok in Antarctica. It could reach melting velocities of between one and two metres per hour and on one test melted to a depth of 220 metres in 75 hours.
Dr Ulamec believes that, with some improvements, the concept could provide the basis for a Europa lander.
Ultimately, he hopes that the melting head of the probe can be powered by a radioactive heating unit instead of an electrical cable linked to an external power source.
The prototype melter is currently being tested on ices with different impurities and in a vacuum chamber to simulate pressures it might encounter on Europa.
Low pressures on the Jovian moon mean that water would freeze behind the probe as it melted through the ice layer, drastically slowing its melting velocity.
Testing in this vacuum chamber will help simulate Europa conditions
One possible proposal to counter this problem is to distribute heating elements along the length of the probe. Another is to simply boost the power by 20% for a given melting velocity.
Once the probe is through the ice and into the ocean underneath, it could open a compartment to take a sample of the water and release some of its ballast to give it a positive buoyancy.
If the craft were based on a spherical shape it could turn round, float back up to the ice cover and begin melting back up to the surface.
But this could introduce a new set of complications for any mission to Europa, so Dr Ulamec hopes the craft could be furnished with an array of scientific instruments like those on Beagle 2 or the Mars rovers.
These could analyse the water while the craft floated in the ocean and the results could then be transmitted to Earth through the ice with a type of powerful transmitter used by submarines.
Dr Ulamec and his colleagues hope to soon test a version of the probe in Greenland or Antarctica.