“The bottom line is that we’ve had the ability to look over the better part of a year,” said Mahaffy, a coauthor of the new report in Science. “We’re just not seeing methane.”
The search for methane on Mars, however, is not over, “and this does not rule out the possibility of life,” Atreya said.
There are, for instance, “many types of terrestrial microbes that don’t generate methane,” said Michael Meyer, NASA’s lead scientist for Mars exploration and an expert in the field known as astrobiology.
What’s more, scientists have theorized that methane vanishes more quickly on Mars than it does on Earth. Some researchers, for instance, have argued that methane decays quickly when it encounters Martian soil. Others believe that the dust devils that frequently scour the surface of Mars create strong electrical fields that can zap the gas away.
“That’s plausible,” Mahaffy said.
So the hunt continues. In 2014, Curiosity will likely receive new software that would allow it to search for methane in even higher fidelity than parts per billion.
Detecting trace amounts of methane would be one thing, but perhaps more importantly, it could allow Curiosity to show that those levels are changing. Proving that the levels vary could open the door, once again, to the possibility that there is a current source of methane on Mars.
Curiosity will get some help in 2016, when the ExoMars Trace Gas Orbiter is expected to reach the Red Planet. That craft, a joint project of the European and Russian space agencies, will scour the surface of Mars for methane — and if it finds it, will attempt to determine whether it is biological or geological in origin.
“The intriguing methane story continues,” Mahaffy said. “We’re still very motivated to keep looking. Methane seems to be a little elusive.”