22nd September 2013

No methane on Mars, according to NASA's Curiosity rover

NASA's Curiosity rover has revealed the Martian environment lacks methane. This is surprising to researchers, because previous data reported by U.S. and international scientists indicated positive detections.

The roving laboratory performed extensive tests to search for traces of Martian methane. Whether the Martian atmosphere contains traces of the gas has been a question of high interest for decades, because methane could be a potential sign of life, although it can also be produced without biology.

"This important result will help direct our efforts to examine the possibility of life on Mars," said Michael Meyer, NASA's lead scientist for Mars exploration. "It reduces the probability of current methane-producing Martian microbes, but this addresses only one type of microbial metabolism. As we know, there are many types of terrestrial microbes that don't generate methane."

Since October 2012, Curiosity has analysed samples of the Martian atmosphere for methane six times and detected none. Given the extreme sensitivity of the instrument used – the Tunable Laser Spectrometer – and not detecting the gas, scientists calculate the amount of methane in the Martian atmosphere today must be no more than 1.3 parts per billion, which 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," said Chris Webster of NASA's Jet Propulsion Laboratory. "We measured repeatedly from Martian spring to late summer, but with no detection of methane."

Webster is lead scientist for the Tunable Laser Spectrometer, which is part of Curiosity's Sample Analysis at Mars (SAM) laboratory. It can be tuned specifically for detection of trace methane. The laboratory can also concentrate any methane to increase the gas' ability to be detected. The rover team are using this method to check for methane at concentrations well below 1 part per billion.

Methane, the most abundant hydrocarbon in our Solar System, has one carbon atom bound to four hydrogen atoms in each molecule. Previous reports of localised methane concentrations up to 45 parts per billion, which sparked interest in the possibility of a biological source on Mars, were based on observations from Earth and from orbit around Mars. However, the measurements from Curiosity are not consistent with such concentrations, even if the methane had dispersed globally.

"There's no known way for methane to disappear quickly from the atmosphere," said co-author 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, Atreya estimated. That is about 50 million times less than the rate of methane entering Earth's atmosphere. Details of these findings were published in the Thursday edition of Science Express.

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