If humans ever discover life on Mars, this is how it might start: with a breaking-news alert heralding a startling development well beyond Earth.
On a recent Saturday afternoon, The New York Times sent a bulletin: “Mars is belching a large amount of methane gas. It’s a sign of possible life on the red planet.”
NASA quickly published a press release acknowledging the detection, which, the Times had reported, marked the largest amount of methane ever registered by the Curiosity rover, a NASA mission that touched down on the red planet in 2012. But after that, the agency went quiet. The news had come from an email between scientists on the Curiosity team that had been leaked to the Times. It wasn’t supposed to be known, at least not yet. And there’s no room for nuance in a breaking-news alert.
Like the Times, NASA provided an important caveat: Many things can produce methane on Mars. Alien life is on that list, but other sources are far more likely.
After decades of exploration, spacecraft haven’t found any evidence of life on the surface of Mars. But some scientists say it may lurk beneath the surface, in the form of tiny organisms. And that’s why methane is so noteworthy. On Earth, microbes pump the natural gas into the planet’s atmosphere. Perhaps a similar arrangement exists on Mars.
Methane doesn’t last forever in the Martian atmosphere, however. Exposure to the sun’s radiation, combined with reactions with other gases, breaks down the gas molecules within a few centuries. This chemistry is what makes the spike that Curiosity found so intriguing. If methane is present in the Martian atmosphere right now, it must have been released fairly recently. Detectable quantities might be a sign that something is alive on Mars, capable of replenishing the supply.
Or not. Natural interactions between rock and water can also produce the gas. The methane might have been forged deep beneath the Martian surface—where reservoirs of ancient water chafe against sediment—escaping into the atmosphere through a narrow crack in the ground. The whiff Curiosity caught might have been billions of years old.
Scientists make sure to point out this dichotomy, as they did in the wake of the Times leak, whenever the question of life on Mars comes up. The story of methane on the red planet is complicated, and the search for the elusive gas was fraught well before Curiosity caught this spike. Ask one camp, and they’ll tell you that spacecraft have detected significant amounts of methane more than once. But ask another, and they’ll say NASA hasn’t found any at all.
The first evidence of methane in the Martian atmosphere came in 2004, from a spacecraft orbiting the planet and ground-based telescopes on Earth. There was debate right away. The scientists behind the discoveries said they were “99 percent confident” that the methane was there. Outside researchers said the signals weren’t strong enough. The ground observations, in particular, presented the uncomfortable possibility that molecules in Earth’s own atmosphere might have become scrambled in the measurements.
“The ground-based observations had been controversial, to say the least,” Paul Mahaffy, the NASA scientist who leads the instrument team on Curiosity that measured the recent uptick, said at a conference in June. “And so we were going to go to Mars and understand whether it was really there or not.”
To settle the debate, and even maybe solve the mystery, they needed to put instruments right inside the atmosphere. In its first year of operations, the Curiosity rover came up empty. But in 2013, it registered a spike. The puff seemed to remain for some weeks before vanishing. Subsequent detections followed the same pattern—short-lived signals that seemed to coincide with seasonal changes.
These were exciting findings, but some scientists weren’t convinced. “All the measurements that have been reported have been very tiny compared to the background signals that they have to sift through,” says Kevin Zahnle, a planetary scientist at NASA who studies Mars but isn’t part of the agency’s rover mission, and the field’s most vocal methane skeptic. “None of them really are convincing. But if you’re an investigator whose instrument it is, you’re much more likely to be convinced.”
Detecting methane on Mars is hard, even for a rover in the thick of it. Curiosity can’t just hold some litmus paper to the wind until it senses methane and changes color. In one method, the rover ingests some air into an analysis unit and scrubs out all the carbon dioxide, which makes up the majority of the Martian atmosphere. The unit concentrates the gases that remain, amplifying their signals, and teases out the methane.
The latest spike was found this way. It was three times the size of the 2013 detection, and Mahaffy said the team was “dumbfounded.” Scientists even scrapped the upcoming days’ rover experiments in favor of running the test again. When the data came back, the plume of methane had disappeared. The atmosphere around the rover appeared to have reverted to its usual state, with low background levels of methane. Curiosity scientists now believe that the spike is similar to other promising but short-lived detections.
The Curiosity rover is not equipped to further probe the mystery of methane on Mars, or the question of potential methane-producing beings. Although the rover’s instruments are designed to pick up methane, they can’t determine its source. NASA’s next rover, known for now as Mars 2020, won’t carry any methane-detecting instruments; when it arrives in early 2021, it will search for signs of ancient life, and any methane it produced would be long gone. Neither will Europe’s planned rover Rosalind Franklin, named for the chemist who helped unlock the secrets of DNA, when it touches down around the same time.
Methane can be detected from above, too, by orbiters circling Mars that study how the gas interacts with sunlight. Mars Express, a European Space Agency orbiter, picked up the same plume Curiosity registered in 2013. The Trace Gas Orbiter (TGO), another European spacecraft, is among the newest on and around Mars, and its instruments might be sensitive enough to provide some information about the source of a potential methane detection. “TGO may be able, if the gas comes in sufficient high concentration, but it is hard,” says Håkan Svedhem, the mission’s project scientist.
The TGO flew over the crater where Curiosity resides and made the latest detection, but Svedhem and European scientists haven’t responded to questions about whether the orbiter sensed it, too.
One of the best ways to answer the methane question is also one of the most ambitious: Drill deep into the Martian terrain, scoop up some material, and hurl it back toward home. Space agencies are still years away from this kind of sample-return mission, which would involve launching a rocket from the surface of Mars, something no one has ever tried before. And even if methane-releasing bacteria squirm beneath the surface, the most advanced technology might still miss the mark. “The problem with drilling a hole or two is, what if you miss?” says Paul Niles, a planetary geologist at NASA. “The bacteria might be concentrated in one area, and you might miss it if you drilled in the wrong place.”
The next development in the methane story will likely appear in the usual channels: a peer-reviewed paper in a scientific journal. For rover scientists, the results might represent another detection in a growing list. For the skeptics, they might not be worth all the fuss. But ask them all whether we could someday uncover life on Mars, whether methane leads us there or not, and they all invoke the Carl Sagan standard: “Extraordinary claims require extraordinary evidence.” And right now, neither side has it.