Methane on Mars could mean life. It could also be non-biological. The Hunt is on to figure out which.

On Mars, atmospheric methane-a sign of life on Earth-changes mysteriously with the seasons

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.

From article, (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.

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."

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.)