Last Wednesday, the Curiosity rover detected in Mars’ atmosphere a high concentration of methane, a gas which is usually associated with microbial life on Earth. While it’s too early to determine whether the gas was produced by Martian microbes or geological processes, the discovery prompted NASA scientists to reschedule Curiosity’s weekend to perform follow-up experiments. If the high concentration of methane is confirmed by Curiosity and orbiters around the Red Planet, the next step would be to locate the source of the gas and determine how it was produced.
This will require challenging data analyses, and possibly sending a new generation of instruments to Mars, so a definitive answer is unlikely to arrive anytime soon. But Curiosity’s observation has sparked a furor of speculation because it is perhaps the most promising avenue for determining if life once existed on our planetary neighbor.
It’s not the first time methane has been detected on Mars. This honor goes to the Mars Express, a European Space Agency orbiter that was sent to the planet in 2003. Shortly after arriving, an instrument called the Planetary Fourier Spectrometer detected the first evidence of methane in the atmosphere. Subsequent observations by Earth-based telescopes as well as spacecraft on or around Mars have confirmed the presence of small concentrations of methane in the Martian atmosphere.
In 2012, the Curiosity rover landed in Gale crater, a valley that is 96 miles in diameter with a large mountain rising near its center. The location promised to teach scientists a lot about the planet’s ancient past, including whether liquid water ever existed on its surface. NASA scientists got an unexpected bonus when, in 2013, Curiosity got a big whiff of methane in the Martian air. By terrestrial standards it wasn’t much—just seven parts per billion, whereas atmospheric methane concentrations on Earth are around 1,800 parts per billion—but it was 10 times more than the rover had detected when it first landed. Since then, planetary scientists have been trying to figure out where the spike in methane could have come from.
In April, a team of researchers led by Marco Giuranna, a planetary scientist at the Italian National Institute for Astrophysics, published a paper in Nature Geoscience that offered a tantalizing solution. By comparing models of atmospheric circulation on Mars with independent geological analyses, the researchers concluded with a high level of certainty that the methane originated in an area about 500 kilometers east of Gale crater. This region, called the Medusa Fossae Formation, is highly fractured and contains shallow ice. Since permafrost is an excellent seal for methane, it’s possible that the subsurface ice could trap the gas for millennia and release it when the faults break through the ice.
“The first step to understanding the ultimate question—what is the origin of the methane?—is to understand where it is released,” says Giuranna. “And now for the first time we have a likely source region of methane on Mars.”
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Determining whether the methane was produced by microbes or geological processes will be tricky, but that’s part of the reason why Giuranna and other scientists studying Mars are so excited about Curiosity’s new methane detection. On Sunday, NASA scientist Paul Mahaffy said that “with our current measurements, we have no way of telling if the methane source is biology or geology, or even ancient or modern.” But a suite of hyper-precise instruments orbiting Mars hundreds of miles above Curiosity might be able to help answer these questions.
In a case of cosmic good fortune, the Mars Express orbiter happened to be performing spot tracking observations of the Gale crater right around the time Curiosity detected the methane spike. Spot tracking focuses the satellite on a specific area as it passes overhead and allows its onboard instrumentation to perform hundreds of measurements during its brief flyby. This allows scientists to create data averages, rather than relying on single observations, to get a more accurate picture of what is happening in the Martian atmosphere. In this case, the Mars Express orbiter was doing spot tracking 20 hours before Curiosity’s methane detection, as well as 24 and 48 hours after the detection.
As an added bonus, the ExoMars Trace Gas Orbiter also was performing atmospheric observations at around the same time as Mars Express, but at a higher latitude. Together these measurements will be used to confirm Curiosity’s detection of a methane spike and add constraints that will help scientists determine things like when the methane release started and how long it lasted. But the real prize, says Giuranna, would be determining how the methane is produced.
This involves analyzing the ratio of methane and its isotopologues, which are methane molecules in which one of the constituent atoms—carbon or hydrogen—has a different number of neutrons from a ‘normal’ methane molecule. Based on the ratio of methane and its isotopologues, Giuranna says scientists can get an idea of the process that created it. Even though there is hardly any methane in the Martian atmosphere, he says the most recent spike observed by Curiosity, which registered 21 parts per billion, would be more than enough for the Trace Gas Orbiter to tease apart these methane ratios and point to their origin.
We’re probably going to have to wait a few weeks before scientists can complete their analysis of the data from the Mars Express and the Trace Gas Orbiter. Neither of the orbiters have relayed their data back to Earth yet and there’s a chance that the Trace Gas Orbiter didn’t register any methane. Indeed, the first major data dump from the Trace Gas Orbiter, published by Giuranna and his colleagues earlier this year, showed that the satellite hadn’t detected any methane in the Martian atmosphere. Giuranna says there are many possible explanations for such a discrepancy, such as a “destruction mechanism” capable of removing methane from the atmosphere.
If the Trace Gas Orbiter didn’t register a methane spike this time around, Giuranna says they’ll probably have to wait for the sensitive instruments aboard the ExoMars rover to hit the ground on the Red Planet in 2020 before he and his colleagues can dig deeper into the origin of Martian methane.