If life exists on other planets or moons in the Solar System, what kind of experiment should we send to detect it?
That turns out to be a tricky problem. 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 not by life but by the severe oxidising environment on Mars. The result was a false positive (although not everyone agrees on this).
Since then, no probe has carried an experiment to detect life. Instead, the emphasis has been on collecting evidence about the conditions in which microbial life might thrive.
Today, Ximena Abrevaya and buddies at the University of Buenos Aires in Argentina suggest a solution to this problem. They say that a microbial fuel cell 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, that it must metabolise.
Abrevaya and co have tested just such a fuel cell that they say can do the job. Their device consists of an anode and a cathode separated by a membrane through which protons can pass. the anode is embedded in the medium under investigation, such as Martian soil.
The idea is that the metabolic processes, wherever they have evolved, must depend on redox reactions that generate electrons and protons. The anode in the fuel cell captures the electrons generated in this process 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.
The Argentinian team tested the device by comparing the results it produces from life-containing soil with the same soil after it has been sterilised. And they’ve done it with creatures representing archaea, bacteria and eukarya, the three domains of life.
Of particular interest is the archea they tested–Natrialba magadii, a microorganism isolated from Magadii Lake in Kenya that survives in conditions of extreme salinity, like those that may exist on Mars and other places.
The team says the results were positive. The power and current densities were much higher when the anode was embedded in soil samples containing life compared to samples that had been sterilized, they say.
That makes microbial fuels cells an interesting candidate for exolife experiments. A Mars lander carrying a microbial fuel cell would simply take two samples of soil, sterilize one of them by heating it and then test both.
What’s cool about this approach is that the life needn’t even be carbon-based. “In contrast to the Viking experiments, our methodology does not require the existence of a carbon based life,” says the Argentinians
What the Argentinian team do not discuss in this paper is the conditions that could give false positives, however. It’s possible that heating an otherwise sterile sample of soil could change its chemistry in a way that reduces the power that flows through the fuel cell. Just how to eliminate this kind of false positive will need more work.
In the meantime, perhaps the best way to find evidence of microbial life on other planets is to study the composition of the atmosphere, as pointed out by James Lovelock in the 1960s. His idea is that over hundreds of millions of years any micro-organism will change the atmosphere of its planet.
This should produce an atmosphere with a distinctive chemical signature that is well out of ordinary thermodynamic expectations. This is certainly true on Earth: the oxygen and methane in our atmosphere is a sure sign of the life that produced it.
Lovelock is on record as saying that as soon as he saw the first chemical analyses of the Martian atmosphere, which showed that it is 95% carbon dioxide, he knew that it could not have been created by life.
Which means that the best place to test a microbial fuel cell exolife experiment is somewhere else entirely. Titan, anybody?
Ref: arxiv.org/abs/1006.1585: Microbial Fuel Cells Applied To The Metabolically-Based Detection of Extraterrestrial Life