NASA scientists reported Thursday that the Mars rover Curiosity has not found any clear signs of methane, a gas that on our planet is a strong indicator of life.

Methane is the most abundant hydrocarbon in the Solar System. Previous reports of localized methane concentrations up to 45 parts per billion on Mars, which sparked interest in the possibility of a biological source on Mars, were based on observations from Earth and from orbit around Mars.

Using the Tunable Laser Spectrometer on Curiosity, the researchers performed six tests to search for traces of methane in Martian air from October 2012 through June 2013 and detected none.

Given the sensitivity of the instrument and not detecting the gas, they calculate the amount of methane in the Martian atmosphere today must be no more than 1.3 parts per billion. That is about one-sixth as much as some earlier estimates.

“It would have been exciting to find methane, but we have high confidence in our measurements, and the progress in expanding knowledge is what’s really important. We measured repeatedly from Martian spring to late summer, but with no detection of methane,” said Dr Chris Webster of NASA’s Jet Propulsion Laboratory in Pasadena, who is a lead author of the study published in the journal Science.

“There’s no known way for methane to disappear quickly from the atmosphere,” added study co-author Dr Sushil Atreya of the University of Michigan.

“Methane is persistent. It would last for hundreds of years in the Martian atmosphere. Without a way to take it out of the atmosphere quicker, our measurements indicate there cannot be much methane being put into the atmosphere by any mechanism, whether biology, geology, or by ultraviolet degradation of organics delivered by the fall of meteorites or interplanetary dust particles.”

“The highest concentration of methane that could be present without being detected by Curiosity’s measurements so far would amount to no more than 10 to 20 tons per year of methane entering the Martian atmosphere. That is about 50 million times less than the rate of methane entering Earth’s atmosphere,” Dr Atreya said.

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Bibliographic information: Christopher R. Webster et al. Low Upper Limit to Methane Abundance on Mars. Science, published online September 19, 2013; doi: 10.1126/science.1242902