Martian methane not meteoric

How’s that for an alliterative headline? A new career as a tabloid subeditor beckons.

Martian methane not meteoric

Scientists have ruled out the possibility that methane detected in the atmosphere of Mars is delivered through meteorite bombardment. To date there has been much debate between those who say that methane reveals the presence of microbial life on Mars, and others who point to an external origin for the gas, or argue that it is a by-product of non-biological chemical reactions catalysed by water.

Methane in the Martian atmosphere has a lifetime of only a few hundred years, owing to depletion by sunlight-accelerated chemistry. So how can the gas still be there? Martian methane must be constantly replenished, which means that it is either deposited via meteoric bombardment, or bubbles up from the planet’s surface. Previous studies have shown that methane cannot be delivered through volcanic activity.

Gases, water and even organic matter can in theory cross deep space locked up in meteors. But could methane survive the intense heat of entry into the Martian atmosphere? Not according to Imperial College geochemists Richard Court and Mark Sephton, who in a paper published today in the journal Earth and Planetary Science Letters show that the volumes of methane which could be released by meteorites are too low to maintain measured levels.

That leaves micro-biological processes, or chemical reactions between volcanic rock and water, as possible explanations for the continued presence of methane on Mars.

“Meteorites vaporising in the atmosphere are a proposed methane source,” says Court. “But when we recreate their fiery entry in the laboratory we get only small amounts of the gas. For Mars, meteorites fail the methane test.”

Court’s colleague Sephton adds…

“This work is a big step forward. As Sherlock Holmes said, eliminate all other factors and the one that remains must be the truth. The list of possible sources of methane gas is getting smaller and excitingly, extraterrestrial life still remains an option. Ultimately the final test may have to be on Mars.”

The researchers used a laboratory technique known as Quantitative Pyrolysis-Fourier Transform Infrared Spectroscopy to reproduce the conditions experienced by meteorites entering the Martian atmosphere. Fragments of methane-bearing rock were heated to 1,000 degrees, and the spectral signatures of gases released measured using infrared light. When combined with previously published figures of meteorite in-fall rates on Mars, the new experimental results show that only 10 kilogrammes of meteoric methane are produced each year. This is far below the 100 to 300 tonnes required to match observations from telescopes and Mars-orbiting spacecraft.

Further reading

Richard W Court & Mark A Sephton, “Investigating the contribution of methane produced by ablating micrometeorites to the atmosphere of Mars”, Earth and Planetary Science Letters (2009)