Some of the most interesting places in our solar system are also the most difficult to reach. Areas hidden under thick layers of ice such as the polar caps of Mars, Saturn's moon Enceladus and Jupiter's moon Europa are prime examples. Drilling through ice on Earth is complicated enough, but on another world the task becomes almost impossible.

Breaking the Ice
The notion of exploring thick sheets of ice isn't new. Probes built and used in the 1960s were strictly Earth-bound, tested in places like Greenland and Antarctica, and the theory behind them was fairly simple. A long and thin probe penetrates straight down into the ice.

A drill on the tip cuts through the ice, scientific equipment in the main body records data and a long cable trails out behind, all the way up to surface equipment. Heavy and complex equipment is needed on the surface to provide the vast amount of power needed by the large drill, which rules it out for all but the most ambitious missions to other worlds.

A different and more modern method of drilling uses a hot drill tip to melt ice, rather than cut through it. One such probe, called Cryobot, was recently tested in Antarctica. As the drill tip uses heat to melt ice, the probe sinks deeper and deeper.

Melting sounds good in theory, but if the probe hits something embedded deep in the ice, like a large rock, it will get stuck. Unable to melt through, the mission would come to an end.

The Best of Both Worlds
Peter Weiss is a scientist experienced in the field of sub-sea robotics. Together with his colleagues from the Hong Kong Polytechnic University and the Institut fuer Weltraumforschung in Graz, Austria, he has devised a novel way of combining drilling and melting methods.

The prototype "thermal drill" system they put together excelled in tests, as detailed in the July 2008 issue of the journal Planetary and Space Science. Armed with a series of blades and heaters in the tip, the thermal drill could be the answer to exploring below the ice on distant worlds.

But how does it work? Weiss explains, "Our thermal drill is like a 'classical' melting probe, equipped with two propellers that drill into the ice. We mechanically open up the hole in the ice, and by this move the ice particles backwards where they will be melted. The slurry of water and ice will be pushed backwards by the weight of the probe."

Weiss's thermal drill combines the best of drilling and melting techniques. "One advantage of melting is that you can produce heat directly to melt through the ice, so there are no losses due to the translation into mechanical power," he says.

As for the drill encountering layers of dust or other material that cannot be melted through, Weiss says that "Integrating a drilling mechanism will avoid your melting probe from simply getting stuck into a layer of sand while penetrating the ice - a scenario likely on the planet Mars, for example. A hybrid thermal drill will be able to penetrate even layers that cannot be melted."

The heat created also has the useful side-effect of sterilizing the probe, an essential consideration when exploring places where no one has been before. The environment should be kept pristine because contamination from the probe itself could ruin any experiments to such for certain chemistry or even signs of life. The constant production of heat also keeps the scientific equipment warm enough in very cold environments to work effectively.

Destined for the Solar System?
Many areas of our solar system are ideal candidates for thermal drill exploration. Says Weiss, "This study was done targeting the planet Mars and Jupiter's moon Europa. But since then spectacular new knowledge has been gained on worlds like Enceladus or even Titan where scientists speculate about sub-surface oceans."

Weiss and his colleagues have so far tested their prototype thermal drill using large blocks of ice in a lab. Testing the drill under vacuum conditions to simulate alien environments is a logical next step. They'll also want to test just how deep the probe could delve.

Despite high hopes for the thermal drill, Weiss isn't sure whether one will feature on any upcoming probes. He says, "ESA and NASA were discussing a future mission to icy Europa, but it is uncertain if there will be a landing or impacting probe onboard. But sending an orbiter without lander to Europa would be like going to a candy shop without bringing money to spend."