The mystery of methane on Mars has been wafting around for decades. Now, using data gathered by the ESA's Mars Express orbiter and NASA's Curiosity rover on the ground, a new study has not only confirmed that the gas is there, but identified its likely source.
Here on Earth, methane is released into the atmosphere in large quantities by all sorts of organisms (most notably livestock), but can also bubble up from geological sources. The problem is, it's an even more potent greenhouse gas than carbon dioxide. Luckily, it also tends to dissipate pretty quickly.
Those two factors make methane detection on Mars a pretty exciting prospect. There's a chance that it could have come from biological sources – and the fact that it doesn't linger long suggests it was released recently.
In the early days, Curiosity failed to sniff out much methane in the Martian air, to the disappointment of scientists. But by 2014 the rover picked up large spikes in the levels, indicating relatively local sources that were fluctuating over time.
One of these in particular took place on June 15, 2013, when Curiosity picked up a methane reading of six parts per billion. And now it turns out this was independently backed up by another spacecraft. When analyzing Mars Express data, the team on the new study realized the orbiter happened to detect a methane spike of 15 parts per billion when it flew over the area a day later.
"Although parts per billion in general means a relatively small amount, it is quite remarkable for Mars – our measurement corresponds to an average of about 46 tonnes of methane that was present in the area of 49,000 sq km (18,920 sq mi) observed from our orbit," says Marco Giuranna, lead author of the study.
The next step was to triangulate the source of the methane. At the time that Curiosity first caught a whiff of it, it was believed that the methane must have been coming down from the north, where the wind was blowing from that day.
To nail it down more precisely, the researchers ran millions of methane emission simulations across a wide region around Gale Crater, where Curiosity is exploring. These took into account the measured methane data and what kind of atmospheric circulation patterns would be expected, as well as the intensity and duration of geological methane release, based on what we know from Earth.
At the same time, the researchers examined the landmarks around Gale crater for the kinds of formations that would be the most likely sources. Interestingly, both of these techniques singled out the same source as the culprit – a large formation to the east known as Medusae Fossae.
"We identified tectonic faults that might extend below a region proposed to contain shallow ice," says Giuseppe Etiope, co-author of the study. "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."
The researchers plan to keep an eye out for more methane detections, from the various rovers and orbiters on and above Mars.
The research was published in the journal Nature Geoscience.
Source: ESA
