KentuckyFC writes: In the 1970s, NASA's Viking probes carried three experiments to Mars which were specifically designed to look for life. To everyone's amazement, these experiments sent back positive results. But the celebrations soon turned sour as scientists decided to discount the results on the grounds that they were caused by the severe oxidising environment on Mars. The result was a false positive. Now physicists say that a microbial fuel cell could do a better job because it can detect life in a way that is entirely independent of its chemical make up. The only assumption is that the life form in question must take chemical energy from the environment and use it to power the processes of life. In other words, it must metabolise. Their device consists of an anode and a cathode separated by a membrane through which protons (or cations) can pass. The anode is embedded in the medium under investigation, such as Martian soil. Metabolic processes, wherever they have evolved, must depend on redox reactions that generate electrons and protons (or cations). The anode in the fuel cell captures these electrons while the protons pass through the membrane, completing the circuit. So the amount of current that flows is a direct indicator of the amount of life present. Physicists have used the device to successfully detect creatures representing archaea, bacteria and eukarya, the three domains of life on Earth, including an extremophile living in conditions of extreme salinity, like those that may exist on Mars.