Published online 21 September 2004 | Nature | doi:10.1038/news040920-5

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Microbe population estimated, but space community is unconvinced.

Methane and water vapour seem to be seeping out from below the surface. © NASA

In the first published study to track methane on Mars, researchers have concluded that life is the only plausible source of the gas. The putative martians are hiding in a few isolated spots and the rest of the planet is totally sterile, they say.

Teams at conferences have already discussed finding martian methane. But Vladimir Krasnopolsky, an atmospheric scientist from the Catholic University of America in Washington DC, says that his study, to be published shortly in the peer-reviewed journal Icarus1, is the first hard evidence for methane on the planet.

He and his colleagues used the Canada-France-Hawaii Telescope on top of Mauna Kea in Hawaii to detect minute traces of methane in the red planet's atmosphere; they found levels of 10 parts per billion. This matches other researchers' estimates, and suggests that methane is being continually released from the surface.

Krasnopolsky has gone one step further, considering different possible explanations for the methane, and claims that no physical process can account for the amount of methane that he sees. He concludes that the gas must come from a biological source, and has worked out how many microbes it would take to produce it.

Certain bacteria on Earth generate methane, and scientists have long speculated that similar creatures live on Mars. Krasnopolsky thinks the presence of such bacteria is the best explanation of his results and, assuming they have similar biochemistry, he calculates that there could be 20 tonnes of methanogenic bacteria currently living on Mars.

If they were evenly distributed around the planet in a warm layer about 100 metres thick, each bug would have roughly 10 cubic centimetres of space to itself. But Krasnopolsky argues that the martians are probably concentrated in just a handful of oases, which would explain why NASA's Viking landers missed any signs of organic chemistry on Mars in 1975 and 1976.

Hunt the martian

An organized martian hunt would be difficult, Krasnopolsky says. Methane mixes into the atmosphere too quickly for us to follow it to a precise location. But tantalizing results just announced by another group of methane hunters could help narrow the search.

<mediar rid='m1'/>Vittorio Formisano of the Institute of Physics and Interplanetary Space in Italy uses data from the European Space Agency orbiter, Mars Express, to analyse gas emissions from Mars. He told scientists at the International Mars Conference in Ischia, Italy, on 20 September that his team had detected concentrated patches of both methane and water vapour coming from three distinct areas of Mars: Arabia Terra, Elysium Planum and Arcadia-Memnonia.

These locations all have stores of ice just below the surface, which might house a common source of the two gases. But scientists are still unsure whether that source is biological or geological.

Michael Mumma, of NASA's Goddard Space Flight Centre in Greenbelt, Maryland, leads a third team searching for methane, using telescopes at Cerro Pachon in Chile and Mauna Kea in Hawaii. He says he has seen similar overlaps between methane and water vapour in his own data.

"It certainly identifies key regions for further investigation on Mars," says Mumma.

He is sceptical, however, about Krasnopolsky's conclusion that life is the best explanation for the methane. "I don't think the community is that impressed," he says. He still believes that volcanic activity, somehow missed by NASA's orbiters, could be generating the methane. Another possibility is that the gas is being generated by rocks squeezing together deep inside the planet (see ""Earth's mantle can generate methane":http://www.nature.com/news/2004/040913/full/040913-5.html''), although Mumma adds: "If that were happening on Mars it would still be quite startling."

Infrared solution

Mumma does concede that it is perfectly possible that life exists on the planet today. "Below the permafrost, there may be active regions of life releasing methane right now," he says. Other researchers have found organisms living more than 500 metres below the Canadian permafrost, he adds.

He has just submitted a proposal to NASA for a space-based infrared telescope that should help resolve the matter. If geological processes are responsible for generating methane, it should also make other hydrocarbons such as ethane. Mumma believes that his telescope could easily find ethane on Mars. If none was found, the case for life would then be much stronger. The craft could launch in 2010, says Mumma, and would observe Mars from a point about 1.5 million kilometres from Earth.