In the search for life beyond Earth, NASA is following a new trail on Mars: methane gas.
For years, NASA has been diligently following the footsteps of water on Mars to try to figure out how the planet transformed from a warm and ocean-rich world to the cold and dry desert it is today. Now scientists have another trail to follow, one that could prove to be a more direct path to answering one of the most enduring and tantalizing questions of all time: Is Earth the only place with life?
The shift in strategy is being triggered by the realization that at least three distinct locations on Mars are venting methane on an ongoing basis.
The source could be an exhaled breath of microbial life, or a chemical reaction of water on rock. But even a geologic origin of the gas has implications for life; methane, on Earth anyway, serves as a food source for some colonies of microbes that live underground.
"Either way, I think we simply have to accept the fact that while we are currently developing a strategy to search primarily for ancient life during a wetter phase of Mars, we actually also need to think in terms of present-day Mars life still holding on somewhere in the subsurface," said Lisa Pratt, a geobiologist at Indiana University.
The first step of a revamped Mars exploration program would be a planet-wide map of the methane releases over time, said Michael Mumma, a NASA scientist at the Goddard Space Flight Center in Greenbelt, Md., and lead author of the paper in this week's Science magazine that details the methane discoveries.
"We need a way to identify where all the active vents are, to quantify their principal gases, to identify which might be dominated by biology and which might be dominated by geochemistry, and then ... establish which are repeated year after year," Mumma said.
"That permits you to decide that this particular site is where you really want to go," he added.
No existing spacecraft -- and none currently planned -- can produce detailed maps of Mars methane, but NASA's decision to postpone the launch of a sophisticated lander, a delay due to technical reasons, may turn out to be a fortuitous coincidence.
One of the potential landing sites for the Mars Science Laboratory is where Mumma's team found a methane plume. MSL, now slated for liftoff in 2011, has instruments that can detect and analyze extremely small concentrations of methane, improving the prospects that scientists will be able to tell if the gas came from living organisms.
The site, known as Nili Fossae, had been eliminated due to concerns the terrain was too risky for landing, but is now back under consideration in light of the methane discovery and the extra time for engineers to find ways to mitigate landing risks.
"We now have two more years to look at this. Adding more potential landing sites is certainly within the cards, and also we'll have a better idea of what really is the landing capability of MSL. So Nili Fossae is not ruled out," said Michael Meyer, the Mars program lead scientist at NASA headquarters in Washington, D.C.
Scientists will also return to their laboratories and reanalyze years of data collected about Mars in light of the new discovery.
"We may reconsider the suite of minerals that are likely to occur and be detected," Pratt said. "We have missed some candidates simply because we didn't think they were likely."