A small amount of seepage could explain conflicting measurements of methane in the Martian atmosphere.

For the last 20 years, scientists have been trying to determine if the Martian atmosphere contains methane. The presence of the molecule could reveal processes happening belowground, such as chemical reactions between liquid water and iron-rich minerals or even microbial activity. However, orbiters and even the Curiosity rover have provided conflicting measurements as to whether the gas is present at all.

Now, a study appearing in the August 20th Geophysical Research Letters proposes a solution that could resolve the dispute — though the debate isn’t over yet.

The Methane Debate

The quest for methane has become a source of heated debate among scientists as measurements obtained from different instruments have yielded conflicting results. On the one hand, NASA’s Curiosity rover has observed a seasonal pattern repeated over three Mars years, where methane abundance varies from 0.2 to 0.7 parts per billion by volume (ppbv). The rover has also detected here-and-gone peaks in methane concentration, known as plumes. In 2013 one of these plumes reached 5.78 ppbv. Moreover, the Planetary Fourier Spectrometer (PFS) onboard the European Space Agency (ESA) orbiter, Mars Express, confirmed the plume from orbit.

On the other hand, in April 2019, researchers operating the most sensitive spacecraft ever deployed to study the Martian atmosphere, the Exomars Trace Gas Orbiter, reported that they had failed to find signs of methane after several months of operations. A collaborative project between ESA and Roscosmos, the Russian space agency, Exomars TGO carries two independently operating spectrometers, the European NOMAD and the Russian ACS. They can detect vanishingly small concentrations of trace gases in the upper atmosphere (more than 5 kilometers off the ground), but neither one found any methane. Scientists on the Exomars team concluded that if any methane is present, it must be less than 0.05 ppbv.

What’s curious is that if methane were leaking into the Martian atmosphere at the rate found by Curiosity and Mars Express, it should build up over time. Current models predict that methane molecules can survive in the Martian atmosphere for about 300 years before being destroyed by sunlight. Therefore, even though instruments on the three craft explore different parts of the atmosphere, scientists can’t explain why TGO would see not even a hint of methane.

A Possible Resolution?

Scientists have now come up with an explanation that could reconcile the disparate measurements. Modeling the diffusion of gases in the Martian atmosphere over daily and seasonal cycles, they have determined that a tiny but constant methane release within the Gale Crater, where Curiosity roves, could explain its measurements. Curiosity’s instruments are designed to measure methane at night, when the atmosphere is relatively still. A more stable atmosphere could more easily hold onto any molecules that slip out from an underground source. This explains why the rover detected methane. TGO, on the other hand, measures methane abundance around sunset after a day of Sun-driven atmospheric mixing, when methane has already become too diluted to pick up.

John Moores (York University, Canada) and his colleagues have calculated that if local sources at Gale Crater were generating less than 2.8 kilograms of methane every day, that would explain Curiosity’s overnight measurements but wouldn’t increase global levels above TGO’s 0.05 ppbv detection limit. The researchers estimate that less than 27,000 square kilometers of Martian surface might be spewing methane at this low rate.

“This is indeed a very small amount of methane!” Moores says. “It’s about 20,000 times less than the smallest seeps found on Earth.”

It’s important to note that this tiny rate of methane production doesn’t rule out any possible explanations of its origin, Moores adds. “For solutions that involve degradation of organic carbon by whatever means, Mars has much less of this material than does the Earth, so you would anticipate that the amount of methane produced would be much smaller,” Moores says. “For water-rock reactions these too could proceed slowly, resulting in very little methane.”

Marco Giuranna (National Institute of Astrophysics, Italy), the principal investigator of the Mars Express PFS instrument, says these results are consistent with his own findings. He has suggested that the methane observed by Curiosity, and confirmed by Mars Express in 2019, was emitted from a region 500 km east of the crater. One possible mechanism he has proposed is that a layer of buried ice traps methane underground, then releases it in sudden bursts.

“From my perspective, the most befuddling aspect of the methane question is not, why is there so much, but why is there so little?” Moores says. Hundreds of tons of organic carbon fall on Mars every year in the form of interplanetary dust particles, he explains. “When we put this material under UV illumination in a lab here on Earth it generates tremendous amounts of methane, enough so that the atmosphere of Mars should contain 10 ppbv of methane all the time!” Considering that models show that any gas should be quickly distributed throughout the atmosphere, the small amount of methane is puzzling, though Giuranna argues that the daily changes in atmospheric mixing could help explain it.

The Debate Continues

The atmospheric mixing explanation only works for the long-term methane observations — the short-lived plumes are a different question. They could be unrelated to the constant seeping from underground. “I’m of the opinion that the plumes and the background are separate processes, so one does not preclude the other,” Moores says. “The background seepage is continuous and happens in the absence of other effects.”

But other scientists don’t think that the proposed solution to the methane question is feasible. “Curiosity is reporting levels between 0.4 and 0.5 ppbv, but that’s ten times more than we know there isn’t,” says Jose Juan Lopez-Moreno (Andalusian Institute of Astrophysics, Spain), co-principal investigator of the TGO’s NOMAD instrument. “Methane doesn’t hide.”

Instead, Lopez-Moreno proposes a different resolution to the debate: “There isn’t a Mars methane mystery because there isn’t any methane.”

Definitive answers will only come from more frequent measurements, notes Moores, preferably from landers or rovers. “My paper is based on 12 data points collected over 7 years on Mars,” he explains. “We once believed that methane concentrations changed on the order of centuries. Then we saw with TLS [instrument aboard Curiosity] that they change over the course of the seasons. Now it looks like methane concentrations can change over the course of the day.” Maybe the next generation of rovers, starting with Mars 2020, can settle the controversy.