The latest results from the Curiosity rover team are putting a new twist on the Martian methane mystery. Chris Webster of the Jet Propulsion Laboratory and colleagues published a paper in this week’s Science magazine in which they report that methane concentrations in the Martian atmosphere are generally quite a bit lower than reported from previous studies. But the rover also has observed sudden peak releases of methane at Gale Crater — from unidentified local sources.
On Earth methane is predominantly produced by biology, and given similar environmental conditions on Mars, either past or present biology would be the usual suspect to explain any large methane releases occurring beyond the natural (i.e., geological) background concentration.
But let’s first step back a moment. A few years ago methane was identified in the Martian atmosphere, at concentrations of up to a few tens of parts per billion, by several research groups, who used telescopes on Earth as well as spacecraft orbiting Mars. The mystery deepened when Curiosity initially did not detect any Martian methane, or at least not at the concentration levels previously measured. Various natural processes were proposed to account for the sudden disappearance of methane in the Martian atmosphere including the recent intriguing idea that quartz sand grains exposed to wind erosion could have gobbled up most of the methane on their reactive surfaces.
The new Curiosity results have added valuable insights into the methane puzzle. Background methane concentrations at Gale Crater are at a low concentration of 0.7 parts per billion. (Measuring such a low concentration of a gas on another planet is by itself an incredible feat.) The even more stunning result is that local sources of methane can raise these concentrations by at least tenfold.
That leads us to a much bigger question: What is causing the episodically occurring spikes of methane? Webster and colleagues don’t speculate much on this point, but they do say that an asteroid or comet impact in Gale Crater cannot be the source, because such an event would have produced a crater tens of meter in diameter. A fresh impact crater would have been observed by other Mars-orbiting spacecraft, and there has been no such sighting.
Other geological and atmospheric processes such as mineral weathering, degradation of organics, erosion of methane-containing rocks, or geothermal sources may be an explanation for the background concentration of 0.7 ppb, but these are unlikely to cause the peak releases seen by Curiosity. The source of the methane is difficult to pinpoint on the ground. It could either be nearby and weak, or more distant and quite strong. Either way, it pushes us closer and closer to the only remaining straightforward explanation for the source of the methane peaks: life on Mars.