Is the scent of life on Mars growing stronger?© NASA

Formaldehyde has been found in the martian atmosphere, according to a senior scientist working with the Mars Express orbiter. If correct, the discovery provides strong evidence that Mars is either extremely geologically active, or harbouring colonies of microbial life. But many experts are not yet convinced.

The claim comes from Vittorio Formisano, who is in charge of the Planetary Fourier Spectrometer on the European Space Agency's orbiter. The spectrometer analyses infrared light, whose frequencies carry the fingerprints of chemicals in the atmosphere.

The most likely source of formaldehyde (CH₂O) is the oxidation of methane (CH₄), which has already been identified in the martian skies (see 'Methane found on Mars'). So the presence of formaldehyde itself is not too surprising, says Michael Mumma, of NASA's Goddard Space Flight Centre near Washington DC, who studies the martian atmosphere. Any oxidizing atmosphere such as Mars's that contains methane should also have formaldehyde, he explains.

Big claim

The truly eye-opening part is the sheer quantity of formaldehyde that Formisano claims to have found: about 10 to 20 times more than there is of methane. This means that estimates of martian methane production must be revised upwards substantially, as most of the gas is oxidized as soon as it comes out of the ground, he says.

"If you consider formaldehyde as oxidized methane, then Mars is producing 2.5 million tonnes of methane a year," says Formisano.

This is simply too much to be accounted for by any known geological process, he says, so some other source (possibly life) must be involved. However, other planetary scientists say the planet alone could still be responsible.

"We don't know the intricacies of [martian] geochemistry," says Rocco Mancinelli, an astrobiologist from NASA's Ames Research Center in Moffett Field, California.

Formisano presented his results to a packed session of the Mars Express Science Conference at Noordwijk in the Netherlands on 24 February.

Unstable hope

The discovery of martian methane last year excited scientists, who said that there were two likely sources of the gas: active geological processes beneath the planet's surface or a population of methane-generating microbes. Because Mars was long thought to be a dead planet, devoid of both life and geothermal activity, either prospect came as a revelation.

However, a molecule of methane can typically survive for about 350 years in the atmosphere before being broken down by the Sun's ultraviolet radiation. So the possibility remained that the gas could have been delivered to the planet by a colliding comet, or by an occasional release from an underground reservoir.

Formaldehyde is far more unstable, surviving for just 7.5 hours or so before breaking apart. The majority of scientists agree that methane is the most likely precursor for formaldehyde on Mars, so this means that the planet's production of methane must be an ongoing, continuous process, says Formisano.

Going sceptic

Formisano is careful to point out that he has not proved there is life on Mars. "I do believe there is life inside the planet, maybe 50 to 100 metres below the surface, but there is a long way to go to demonstrate that."

"We all want to believe in something," says Yuk Yung, a planetary geologist from the California Institute of Technology, Pasadena. "Even as scientists we're not completely objective, especially about something we've worked on for ten years. There's enormous pressure to deliver, and under this pressure you can easily believe things that are unbelievable."

Many scientists are sceptical about the quality of Formisano's data. "The measurements are right on the borderline of the [instrument's ability]," says Mancinelli.

"I don't believe it," adds Yung. He explains that Formisano's infrared fingerprint of the formaldehyde in Mars's skies should match a laboratory sample of the gas, "and the match is just not convincing".

But Formisano argues that his martian spectrum tallies in 15 key places, which should be enough to convince anyone: "It's not a matter of opinion any more," he says. He adds that, since he presented his data at the conference, several sceptics have already changed their views. He points out that although rejected by Nature, the research will soon be published in the journal Planetary and Space Science.

Acid test

Formisano also announced at the conference that he has found traces of hydrogen fluoride (HF) and hydrogen bromide (HBr) in the atmosphere, which are probably produced when acids break down certain minerals in the soil.

Many scientists believe that Mars once had briny, acidic seas (see 'A picture of young Mars') that may have been conducive to life. "An acidic environment still exists," says Formisano. "On Earth, there are certain bacteria that prefer very acid conditions."

He also points out that he has found higher concentrations of methane directly above an area of Mars that seems to be covered in pack ice (see 'Mars may have frozen sea' ). This raises the tantalizing possibility of a microbial colony living beneath the surface.

Mumma says that more convincing evidence for life is needed. Continuous production of heavier hydrocarbons such as propane, which cannot come from geothermal processes, would be a key finding, he says. Better still would be a skew in the ratio of carbon isotopes in the air, as produced by organisms on Earth.

NASA and the European Space Agency are both planning Mars missions for the end of the decade that will look for precisely that. 

Principal investigator of the Planetary Fourier Spectrometer instrument on Mars Express

Planetary geologist from the California Institute of Technology, Pasadena