Scientists are designing a nuclear-powered "tunnelbot" that can penetrate through the icy shell of Jupiter's moon Europa and search for signs of life in its sub-surface ocean.

Between 1995 and 2003, NASA's Galileo spacecraft made several flybys of Jupiter's moon, Europa. Several findings from observations of the moon pointed to evidence of a liquid ocean beneath Europa's icy surface.

Researchers believe that the ocean could harbour microbial life, or evidence of now-extinct microbial life.

While scientists generally agree on where to look -- underneath the thick, planet-wide ice shell where water is in contact with a rocky core and where biochemical ingredients for life may exist -- how to get there to collect samples remains a major tactical problem.

"Estimates of the thickness of the ice shell range between 2 and 30 kilometers, and is a major barrier any lander will have to overcome in order to access areas we think have a chance of holding biosignatures representative of life on Europa," said Andrew Dombard, associate professor at the University of Illinois at Chicago in the US.

Dombard and his team are tasked with designing technology and solutions for space exploration and science missions.

The group performed a concept study for a nuclear-powered "tunnelbot" that can penetrate the ice shell and reach the top of Europa's ocean while carrying devices and instruments that can be used to search for signs of life or extinct life.

The bot would also evaluate the habitability of the ice shelf itself.

"We didn't worry about how our tunnelbot would make it to Europa or get deployed into the ice. We just assumed it could get there and we focused on how it would work during descent to the ocean," said Dombard.

The bot would sample ice throughout the shell, as well as water at the ice-water interface, and would look at the underside of the ice to search for microbial biofilms.

The bot would also have the capability of searching liquid water "lakes" within the ice shell.

The researchers considered two designs for their bot: one powered by a small nuclear reactor, and the other powered by General Purpose Heat Source bricks -- radioactive heat source modules designed for space missions.

Heat from both these sources could be used to melt the ice shell. Communications would be provided by a string of "repeaters" connected to the bot by fibre optic cables, researchers said.