Maggie McKee , "Methane on Mars causes controversy" (2004)

Methane on Mars causes controversy

16:26 21 September 2004

NewScientist.com news service

Maggie McKee

Methane and water vapour are concentrated in the same regions of the Martian atmosphere, say scientists studying data from Europe's Mars Express orbiter. They say the link may point to a common source - possibly life - but others remain sceptical about the detection.

In March, scientists using the Planetary Fourier Spectrometer (PFS) on Mars Express announced they had found methane in the atmosphere at a level of just 10.5 parts per billion. Two other groups say they have also detected the gas with telescopes in Chile and Hawaii.

Ultraviolet sunlight takes about 300 years to destroy atmospheric methane. These detections suggest the gas is being replenished on Mars in the same way it is on Earth - by processes such as geothermal heating or by life forms, such as bacteria.

Now, the PFS has found that the methane overlaps with water vapour - long observed in the Martian atmosphere - over three broad equatorial regions, according to the European Space Agency (ESA).

Odyssey on ice

At low atmospheric altitudes, water vapour over Arabia Terra, Elysium Planum, and Arcadia-Memnonia appears two to three times’ more concentrated than at higher altitudes - 10 to 15 kilometres - where the molecule is evenly distributed.

The team says the methane-water vapour regions coincide with swathes of land that NASA's Mars Odyssey spacecraft, now orbiting the planet, suggests might contain ice.

The results were presented on Monday by PFS principal investigator Vittorio Formisano at a conference in Ischia, Italy.

ESA says further studies could find out if geothermal heat is acting on an underground "ice table" - if one exists - bringing methane and water vapour into the air. Or perhaps methane-producing bacteria may exist, living in water below the posited ice table.

But Bruce Jakosky, an expert on the Martian atmosphere at the University of Colorado in Boulder, US, says these scenarios rely on too many "ifs”.

"I think there are more questions than there are answers and it's premature to speculate about sources of methane and water or how they might be released," he told New Scientist.

He says only one of the three groups claiming to have detected methane has publicly written up its results - a paper co-authored by Vladimir Krasnopolsky of Catholic University of America in Washington, DC, will be published in a future issue of the journal Icarus. But even that research raises some questions about whether methane truly exists on Mars, says Jakosky.

Hydrogen atoms

And the evidence for a subsurface ice table around the Martian equator is "very controversial", he adds. It comes from Odyssey's detection of hydrogen atoms in the top metre of soil, which some scientists interpret as being locked in ice and others say could come from minerals affected by water in the past.

Furthermore, he says winds should spread water vapour through the atmosphere too quickly for it to be concentrated in certain spots. "It would take a tremendous source of water in the surface to pump water into the atmosphere faster than it would be redistributed," he says.

Krasnopolsky, standing by his methane detection, says winds should spread the trace amounts of methane around too. He believes the methane he detected is produced by bacteria that live in "oases" where liquid water can exist - however briefly - on the Martian surface, due to heating by sunlight or by a hydrothermal source.

He argues that a non-biological source of methane is unlikely because crater-counting methods suggest no surface lava on Mars is younger than 10 million years old.

But he will not rule out the possibility that underground bubbles of methane from ancient volcanism might somehow be brought to the surface to replenish the atmosphere.