What signs of life are scientists looking for on Mars?

Atmospheric gases could lead scientists to martian bacteria.© NASA

The smells of digestion. Life, as we know it, depends on chemicals built up from carbon and nitrogen, and whenever those chemicals break down they release gases like methane (CH₄) and ammonia (NH₃). Some bacteria on Earth get their energy by reacting carbon dioxide with hydrogen to make methane and water. Such 'methanogenic' bacteria are prime candidates for life on Mars, because they do not need sunlight or oxygen to survive.

How are these gases detected?

By looking for distinctive gaps in the heat spectrum coming from Mars's atmosphere. Different molecules absorb infrared radiation at characteristic wavelengths depending on their size, shape and composition. But the picture is complex, because many molecules absorb radiation at similar wavelengths. This overlapping can make it difficult to be absolutely sure that a particular chemical has been spotted.

Who is looking?

There are three main groups looking for these trace gases. Mars Express, the European Space Agency's probe, which is in orbit around the red planet, carries an instrument called the Planetary Fourier Spectrometer (PFS). Vittorio Formisano, from the Institute of Physics of Interplanetary Space in Italy, and his team are analysing the data sent back from the PFS.

Two independent US teams are attempting a similar feat from the ground. Although much further away, the telescopes they are using are of higher resolution than the instruments carried by Mars Express, enabling them to collect data of similar quality.

Michael Mumma, of NASA's Goddard Space Flight Centre near Washington DC, is leading a team that uses telescopes at Cerro Pachon in Chile and Mauna Kea in Hawaii. And Vladimir Krasnopolsky of the Catholic University of America in Washington DC, is using the Canada-France-Hawaii telescope, also on top of Mauna Kea.

What have they seen?

After preliminary reports in March this year that methane may have been detected in Mars's atmosphere, all three groups have confirmed that the gas is present. Methane can only survive for about 400 years in the martian atmosphere before completely breaking down, which means that the gas cannot be a remnant of ancient processes. It is almost certainly still generated on the planet today.

Mumma has confirmed that a major source of the gas is found just west of Meridiani Planum, the area that NASA's Mars rover Opportunity is currently exploring. Sadly, the robot is too far away to trundle over for a sniff, and it does not even have the right equipment on board to detect methane.

What does the presence of methane mean?

On Earth, most methane in the atmosphere comes from bacteria that live in the guts of animals. A little more comes from methanogenic bacteria that live independently. However, methane is also generated by geothermal activity. In the hot spots where huge plates of rock press against each other, minerals can break water molecules apart to make hydrogen gas. At the extreme temperatures and pressures present underground, this hydrogen gas combines with carbon dioxide to make methane.

Scientists simply do not know which is the most likely explanation for the methane on Mars. But either result is exciting, says Mumma. There is no other known process that could be generating methane on Mars, so even if the gas is not being generated by bacteria, it implies that Mars is a geologically active planet.

Have they found anything else?

Several groups have now reported seeing water vapour coming from certain parts of Mars, such as Arabia Terra and Elysium Planitia. Other researchers believe that water ice lies under the surface at these locations.

The presence of water vapour implies that ice under the martian soil in these locations is being melted by some heat source and evaporating into the air. The most likely heat source is volcanic, and, as we have learned from studies on Earth, where there's heat, there's life. Many extremophile bacteria live on the chemical energy found in minerals spewed out by volcanic vents under the sea.

Did they not also see ammonia?

Definitely not, despite recent stories to the contrary. Formisano has been hotly pursued by journalists since reports about Mars Express finding ammonia in the atmosphere began to circulate earlier this month. He now insists to news@nature.com that he has not detected ammonia, although in a recent conference abstract he had optimistically suggested that his team might have done so. This suggestion alone was enough to set tongues wagging, but some reporters were too eager to turn suggestion into certainty, Formisano says. "They want to shoot first, and they shoot the wrong statement."

Mike Mumma confirms that his own group has spent ten years looking for ammonia, "and we never saw it". He claims that Formisano could not have seen ammonia, because the Mars Express instrument does not have enough resolving power to distinguish ammonia from carbon dioxide.

If anyone did find ammonia, it would certainly be big news. In theory, ammonia molecules begin to fall apart in the martian atmosphere within an hour of forming. And the most likely source would be the breakdown of proteins, says Mumma. "I'm not aware of any geological processes that generate ammonia," he adds.

What would be convincing evidence of life?

Other chemicals such as methanol and formaldehyde could help to pinpoint where the methane comes from. They are potentially formed from the breakdown of methane in the atmosphere, and the relative amounts of these gases could reveal whether the source is volcanic or not.

But the best evidence would come from finding out what sort of carbon the methane is made from. Life on Earth tends to use more of a lighter form of carbon, called carbon-12, and less of the heavier carbon-13. Geological processes are not so discerning, so working out the ratio of these isotopes would pin down whether the methane was made by biology or not.

But there are still snags. There is no guarantee that any potential life on Mars uses carbon in the same way as Earthly inhabitants. And none of the instruments currently in use can pick out the tiny differences in these isotope ratios. The only answer is a mission to Mars with more advanced equipment, says Mumma.

But, he adds, at least when we do send a robot to Mars that can look for the chemistry of life, we will know where to start.