Sniffing methane on Mars

The quest to look for life on our nearest planetary neighbour, Mars, got a boost recently. NASA’s Curiosity rover, which has been trundling around on Mars since it landed 6 years ago to study the surface for the chemical fingerprints of life, suddenly detected a powerful whiff of the gas methane while it was sampling the Martian air. Methane matters because it might be a hallmark of microbial life, both past and present; but it can also be the product of other natural chemical processes. Katie Haylor spoke to David Rothery who’s a planetary geoscientist with the Open University where he’s been keenly following this story...

David - Curiosity inhales some of the Mars atmosphere into a chamber, and it bounces a laser beam to and fro across that chamber and looks at the specific absorptions of specific wavelengths of light and relate those to the concentration of gases. It's called a tunable laser spectrometer because it can change the wavelength of light they're using to tune it to exactly what the particular molecule they're looking for should be doing.

Katie - Okay. So this laser beam gives you an output related how much gas you’ve got?

David - Exactly. It's been well calibrated on the earth and they've got some on-board calibration as well, so the measurements it's making are pretty robust. I've spoken to team members, they're not doubting the numbers they're getting.

Katie - What are those numbers? How much methane was in this spike?

David - The recent spike got up to 21 parts per billion; that's not a lot of methane at all but it's a lot higher than the normal background. Normally we’ve been seeing methane, with the Curiosity rover at least, less than 1 part per billion.

Katie - When exactly was this seen and how long did it stick around for?

David - What excited people was when a plume was seen in the middle of June, but it disappeared very, very rapidly. It was gone by the 22nd/23rd of June and that's very crucial information, if it disappears so quickly it's telling you it wasn't a large volume of methane. You're not sniffing the edge of a really extensive plume, you're in the middle of a small plume which can very quickly be blown away or be diluted by the local atmosphere. This methane must have been released very very recently for it to be detectable. It could have been trapped underground for millions of years before escaping. Ultraviolet lights acting on carbon delivered by meteorites or micro meteorites can give you methane. Reactions between warm water and rock below ground can give you methane that can leak out. So methane can be produced by a variety of processes or it could be being generated today by methane-producing microbes, which we'd really like to be the case because we love to find some life on Mars.

Katie - What are scientists doing to better understand these outbursts? Are there any seasonal patterns emerging or is there anything that can be inferred?

David - There's a seasonal pattern to the methane at the Curiosity site. The Curiosity is moving around but not very far on the grand scale of things. Methane rises and falls at the 1 part per billion level with the seasons. Now Curiosity's seen, I think, three different spikes now with the latest one 21 parts per billion, and methane's also detected from spacecraft orbiting Mars. The most sophisticated spacecraft now is the trace gas orbiter with an instrument called Nomad on board and I understand that is looking very hard to see if it can find this plume that was seen on the surface. The trouble is from orbit you’re well equipped to see large plumes, a tiny plume covering a few square kilometres that was rapidly diluted by the wind, you won't see from orbit. But they’re looking, and that will help them pin down the nature of the plume.

Katie - As a planetary geosciences expert, what do you make of this? Should we be excited?

David - I'm interested because it's the biggest plume we've seen at the surface. So we need to build up a database of how quickly they appear, how quickly they can disappear and that will help us track down how locally the methane is sourced. I think it's going to turn out to be very locally to where the rover is. I think these are just small bursts of methane. We need something on the surface that can do an isotopic study of the methane because living processes will concentrate certain isotopes of carbon and hydrogen over other isotopes, and that won't be the case for a biotically produced methane. So we need to fingerprint the methane, which we’ve yet to do, with instruments we don't yet have Mars, and then we can really tell is it life doing this or is it something just chemical...