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Mars Express
The European Space Agency’s Mars Express orbiter, shown in this artist’s conception, has been circling Mars since 2003. (Spacecraft image credit: ESA / ATG Medialab; Mars: ESA / DLR / FU Berlin, CC BY-SA 3.0 IGO)

It may sound like an April Fool’s joke about flatulent aliens, but this is serious: The scientists behind the European Space Agency’s Mars Express orbiter think they know where a smelly outburst of Martian methane came from.

Any joke would fall flat, because the proposed explanation is purely geological rather than, um, biological.

The source of Martian methane has been a mystery for 15 years, thanks in part to earlier findings from Mars Express.

Back in 2004, readings from the orbiter’s instruments as well as ground-based observations picked up signs of the gas in Mars’ atmosphere. The findings raised eyebrows, because methane molecules wouldn’t persist for a long time in the air before breaking down.

That suggested there had to be some sort of source expelling the gas. And on Earth, some of the best-known sources are the digestive tracts of animals as well as methane-generating bacteria.

Identifying the source has been on the agenda for Mars missions launched since then, including ESA’s Trace Gas Orbiter and India’s Mars Orbiter Mission.

The mystery deepened in 2013 when NASA’s Curiosity rover picked up a stronger whiff of methane in the air, leading some to suspect that the concentrations changed on a seasonal cycle. Researchers were disappointed when they failed to detect the expected follow-up spike.

Today, in a study published by Nature Geoscience, the Mars Express team reports identifying what they believe is the same whiff that Curiosity picked up. The scientists based their findings on a review of data collected by the orbiter’s Planetary Fourier Spectrometer as it flew over the rover’s locale in Gale Crater.

“In general we did not detect any methane, aside from one definite detection of about 15 parts per billion by volume of methane in the atmosphere, which turned out to be a day after Curiosity reported a spike of about six parts per billion,” study lead author Marco Giuranna of the Institute for Space Astrophysics and Planetology in Rome said in a news release.

Ten other observations taken during the study period showed no methane concentrations high enough to register on the spectrometer.

Giuranna and his colleagues focused further in on Mars Express’ readings from Gale Crater to track the source of the methane: One group of researchers divided the area into a grid of 155-by-155-mile (250-by-250-kilometer) squares, and then ran a million computer simulations to take atmospheric circulation flows and likely gas seepage patterns into account.

Another group looked for geological features in the area that might be associated with the release of subsurface methane. The search took a decisive turn to an area east of Gale Crater in Mars’ Aeolis Mensae region.

Mars methane
This elevation map highlights Gale Crater, where NASA’s Curiosity rover measured a methane spike in 2013, as well as the area where scientists suspect the methane came from. The area marked with black dots is thought to contain shallow ice that could trap subsurface methane. Tectonic faults in the grid squares between that area and Gale Crater could have liberated the methane during episodes of seismic activity. Click on the graphic for a larger version. (ESA Graphic / Giuranna et al.)

“We identified tectonic faults that might extend below a region proposed to contain shallow ice. Since permafrost is an excellent seal for methane, it is possible that the ice here could trap subsurface methane and release it episodically along the faults that break through this ice,” said study co-author Giuseppe Etiope, a researcher at the National Institute of Geophysics and Volcanology in Rome.

“Remarkably, we saw that the atmospheric simulation and geological assessment, performed independently of each other, suggested the same region of provenance of the methane,” he said.

Co-author Frank Daerden of the Royal Belgian Institute for Space Aeronomy said the results “support the idea that methane release on Mars might be characterized by small, transient geological events rather than a constantly replenishing global presence.”

That’s the typical pattern for releases of methane from geological sources on Earth due to seismic events.

Looking ahead, the scientists on ESA’s team plan to reanalyze other readings recorded over past years, while continuing their monitoring efforts with Mars Express and the Trace Gas Orbiter. So if there are any any flatulent Martians trying to elude detection, they’re going to have to hold it in for quite a while longer.

In addition to Giuranna, Etiope and Daerden, the authors of the study in Nature Geoscience, “Independent Confirmation of a Methane Spike on Mars and a Source Region East of Gale Crater,” include Sébastien Viscardy, Lori Neary, Dorothy Oehler, Vittorio Formisano, Alessandro Aronica, Paulina Wolkenberg, Shohei Aoki, Alejandro Cardesín-Moinelo, Julia Marín-Yaseli de la Parra, Donald Merritt and Marilena Amoroso.

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