From the pasture to the swamp, methane emissions on Earth are the effluvia of life. So what are whiffs of the gas doing on barren Mars? Trace detections of the stuff, alongside glimpses of larger spikes, have fueled debates about biological and nonbiological sources of the gas. Last month, at a meeting of the American Geophysical Union (AGU) in New Orleans, Louisiana, NASA scientists announced a new twist in the tale: a seasonal cycle in the abundance of martian methane, which regularly rises to a peak in late northern summer.
"The thing that's so shocking here is this large variation," said Chris Webster, who leads the methane-sensing instrument on NASA's Curiosity rover. "We're left trying to imagine how we can create this seasonal variation," says Webster, who is at the Jet Propulsion Laboratory in Pasadena, California.
It is a variation on a very faint theme. Since landing in 2012, Curiosity has on 30 occasions opened a few valves to the martian night and taken a sniff of the thin, frigid air. In a small, mirrored chamber, it shines a laser through the air sample and measures the absorption at specific wavelengths that indicate methane. At the meeting, Webster reported vanishingly small background levels of the gas: 0.4 parts per billion (ppb), compared with Earth's 1800 ppb.
Where that whiff comes from is the heart of the mystery. Microbes (including those that live in the guts of cows and sheep) are responsible for most of Earth's methane, and Mars's could conceivably come from microbes as well—either contemporary microbes or ancient ones, if the methane they produced was trapped underground. But methane can also be made in ways that have nothing to do with biology. Hydrothermal reactions with olivine-rich rocks underground can generate it, as can reactions driven by ultraviolet (UV) light striking the carbon-containing meteoroids and dust that constantly rain down on the planet from space.
Now, add to the methane puzzle the seasonal variation Curiosity has detected, with levels cycling between about 0.3 ppb and 0.7 ppb over more than two martian years. Some seasonality is expected in an atmosphere that is mostly carbon dioxide (CO2), says François Forget, who models the climate of Mars at the Laboratory of Dynamical Meteorology in Paris. In the southern winter, some of that CO2 freezes out onto the large southern polar cap, making the overall atmosphere thinner. That boosts the concentration of any residual methane, which doesn't freeze, and by the end of northern summer this methane-enriched air makes its way north to Curiosity's location, Forget says. Seasonal variations in dust storms and levels of UV light could also affect the abundance of methane, if interplanetary dust is its primary source.
But, Webster said at the meeting, the seasonal signal is some three times larger than those mechanisms could explain. Maybe the methane—whatever its source—is absorbed and released from pores in surface rocks at rates that depend on temperature, he said. Another explanation, "one that no one talks about but is in the back of everyone's mind," is biological activity, says Mike Mumma, a planetary scientist at Goddard Space Flight Center in Greenbelt, Maryland. "You'd expect life to be seasonal."
The seasonal wiggles are a mystery within a larger mystery: claims of occasional methane spikes an order of magnitude or two higher than the background. Mumma and his colleagues reported one of the largest in 2009, when they detected spectral signs of a 45-ppb methane plume through a telescope in Hawaii. Curiosity, too, has detected a handful of spikes, to about 7 ppb. For these events, Webster favors the idea of a sudden release from a deep underground source.
Other scientists are looking skyward. Marc Fries, the cosmic dust curator at Johnson Space Center in Houston, Texas, says the source of methane spikes could be the hail of tiny meteors that falls when a planet crosses a comet's orbit and sweeps up carbon-rich dust and debris shed by the comet. Fries says that as the dust particles vaporize at altitudes of tens of kilometers, the same chemical reaction that produces methane from interplanetary dust at the surface would take place more quickly, driven by the stronger UV light at high altitudes. All the claimed methane spikes over the past 2 decades occurred within about 2 weeks of a known martian meteor shower, Fries and his colleagues found. "It could be a cause, and it could be a coincidence," he says.
Skeptics say the atmospheric reactions may not occur quickly enough and that meteor showers don't deposit much more material than the background flux of interplanetary dust. In 2014, when Mars nearly collided with comet Siding Spring, NASA's Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft was watching, monitoring magnesium ions as a proxy for dust dumped in the upper atmosphere. The MAVEN team reckons the encounter put 16 tons of material into the martian atmosphere—not much more than the 3 tons of interplanetary dust estimated to fall daily, and much less than the tens of thousands of tons that Fries says are needed to make a large methane plume. "I don't see how it's possible to produce the methane abundance he needs," says Matteo Crismani, a MAVEN science team member and postdoctoral researcher at the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder. But Fries contends that meteor showers are highly variable, and just because the Siding Spring encounter was close does not mean it was rich in dust and debris.
It happens that Fries will have a chance to test the hypothesis. On 24 January, Mars will have a close brush—less than a tenth of the Earth-moon distance—with the orbit of comet C/2007 H2 Skiff. Mumma is skeptical about Fries's idea, but he will nevertheless be watching for methane with his telescope in Hawaii in the days after the encounter. The MAVEN and Curiosity teams also plan to watch. "This is a great opportunity to test this hypothesis," Crismani says.
One spacecraft won't quite be ready to participate—even though it is best positioned overall to resolve the methane debate. In April, the European Space Agency's ExoMars Trace Gas Orbiter (TGO) will settle into its final orbit and begin science observations, mapping concentrations of methane across the planet. Atmospheric dust will probably prevent the orbiter from reaching its originally advertised sensitivity of several tens of parts per trillion, says Geronimo Villanueva, a science team member at Goddard. But he expects the TGO to approach Curiosity's sensitivity—and its ability to hunt for methane sources in space and time will be unrivaled. The "TGO will allow us to search for this molecule with new eyes," he says.