How to detect life?

Last updated: 30/1/01 - entire text, new NASA policy

Assuming that you are physically looking for life on another planet (in other words, not using SETI) there are a number of well-established ways of detecting life.

Possibly the first serious attempt to detect life on another planet was conducted by the Viking probes sent to Mars in 1976, whose primary aim was in fact to find life. Scientists on the Viking mission were felt fairly certain that there were no large Martian life forms alive (large being anything bigger than the size of an ant) and on a planet with an average surface temperature of minus 26°C, you can understand why they came to this conclusion.

So they knew that if life existed on Mars, it would be in the form of microbes, or micro-organisms. The Viking landers (both identical) carried three different tests to look for life and a gas-chromatograph mass spectrometer (GCMS - to analyse the composition of chemicals found on Mars). The first was called the pyrolytic release experiment.

The pyrolytic release experiment involved taking a sample of Martian soil and also adding Martian atmosphere and radioactive carbon dioxide. The atmosphere of Mars is made up primarily of carbon dioxide, and the scientists believed that any organisms on Mars would have found a way to use it (just like the way that plants use the carbon dioxide in our atmosphere for photosynthesis). So once all of this was placed in a container in the probe, it was be left for 120 hours. After this time, the mixed atmosphere was flushed out and the soil heated to a high temperature. If the soil contained any radioactive carbon, that would mean that some process had pulled the radioactive carbon dioxide out of the atmosphere and combined it with the soil - and that process would probably be life (after all, that's what plants do on Earth).

The other two experiments depended on exposing Martian soil to nutrients in solutions of water, and monitoring them to see if there were any changes indicating life.

When the Viking landers finally touched down, the latter two experiments showed astonishing signs of life; so astonishing, in fact, that the scientists couldn't quite believe it. They had monitored huge changes in the soil/nutrient mixtures - too large for the small amounts of life they were expecting.

The first experiment, the pyrolytic release, took a little longer and returned evidence that something was going on, but the data was ambiguous. In other words, there was no strong evidence for the existence of life. Finally, the GCMS returned its results - there was no sign of any organic materials - no microbes, no waste products of microbes and no dead remains of microbes.

Scientists came to the conclusion that there was no life, and that the strange results had been caused by an oxidising chemical called hydrogen peroxide in the soil. Hydrogen peroxide (H₂O₂) does not exist on Earth in large quantities because it reacts with water violently, breaking down in. But since there's no liquid water on the surface of Mars, when the hydrogen peroxide in the soil had the water nutrients added to it, it reacted violently to produce oxygen - which people first mistook as being a sign of life.

So in the end, the Viking experiments didn't really prove anything, although they helped scientists understand more about Mars and how to design better experiments.

More information about the Viking experiments in the educational index of our sister site, Generation Mars

It is perhaps telling that NASA's next two Athena class rovers being sent to Mars will have no biological experiments whatsoever on them. In a recent interview with Matt Golombek, Mars Pathfinder Chief Scientist, the BBC science programme Horizon discovered that NASA's new policy was to discover more information about Mars itself - it's atmosphere, geology, composition and so on - before they start to consider constructing more refined and accurate experiments that will search for life.

THE MARTIAN WAY

		A photograph of the fully deployed Beagle 2. © Beagle 2.

The British Beagle 2 lander is set to touch down on Mars in 2003, and its primary aim, like the Viking landers, is to look for life. This time, the Beagle 2 has been armed with an entirely new set of tests for life.

One relatively easy way to tell whether life has existed on Mars in the past is looking for the remains of micro-organisms. These remains will be composed of organic material that is composed of carbon compounds. However, carbon compounds can be produced by other methods than biochemical reactions (which is one of the reasons why scientists dispute the fact that ALH 84001 showed there was life on Mars). Fortunately, by looking at the different isotopes of carbon that make up the carbon compounds, you can tell whether they were produced by life, since organisms use the ‘lighter’ carbon-12 preferentially over the ‘heavy’ carbon-13.

By burning carbon compounds that are taken from rocks on the surface and examining the carbon dioxide that is produced, scientists will be able to determine the isotopic composition of the carbon, and so discover whether life existed (this will be carried out by an incinerator and a mass spectrometer).

The one experiment that will attract the greatest attention from the public is the search for methane in the Martian atmosphere. Methane is solely produced by biological processes, and is quickly destroyed by light. Therefore, if any methane at all is found in the atmosphere, we can be almost certain that life exists on Mars right now. The equipment used to perform this experiment has already proven its worth in helping us understand the process of global warming, and perhaps it will make similarly ground-breaking discoveries on Mars.