Scientists have long argued that a sample brought back from Mars will be the only way to confirm life. But that's not the only reason why returning rocks and soil from the Red Planet is the National Research Council's top priority for planetary science.
Until last year, NASA planned to follow up its Mars Science Laboratory Curiosity mission, which is designed to search for the ingredients for life, with a trio of missions in partnership with Europe that would culminate with a sample return in the 2020s.
But budget overruns in NASA science missions, including the $2.5 billion Curiosity program and the $8 billion James Webb Space Telescope, the successor to the Hubble observatory, prompted the United States to change course.
A new blueprint for Mars exploration is due this month, setting the stage for a potential mission or missions that would launch in 2016 and/or 2018 when Earth and Mars favorably align for flights.
"We have to address the overall goal that the (National Research Council) set for Mars exploration, which is sample return. I suspect there are other things that we may do on Mars, but if they don't help sample return they may be viewed as a non-starter," NASA's chief Mars scientist Michael Meyer told Discovery News.
Previous landers, rovers and orbiters sent to Mars have found strong evidence that the planet most like Earth in the solar system once shared Earth's propensity for water, believed to be a key ingredient for life.
Curiosity touched down Aug. 6 to take the next step in assessing Mars' suitability for life by looking for organic materials. The sophisticated roving chemistry laboratory is designed to spend two years studying a mound of what scientists believe is the layered remains of sediment that once completely filled a giant impact basin, one of the lowest points on Mars.
"Water flows downhill, so we chose to go to a low place," lead mission scientist John Grotzinger, with the California Institute of Technology, told reporters after Curiosity's landing inside Gale Crater.
Finding organics is much more challenging task that finding evidence for past water, mainly because the same processes that create rocks tend to destroy organic material. That is why a big part of the Curiosity's mission is to assess habitats that could have preserved organics, if any exist.
"The hope is that you had fine-grain sediments forming these layers, and as they settled out of the water, they trapped whatever the chemical information in that body of water was with them," Curiosity scientist Diana Blaney, with NASA's Jet Propulsion Laboratory in Pasadena, Calif., told Discovery News.
"The idea is that the rocks haven't been churned up and overheated, that we're actually looking at sediments that have trapped materials from these ancient bodies of water," she said.
Ultimately, scientists want to bring samples back to laboratories on Earth for a full analysis, but that is not the only motivation, nor even the primary one, behind a sample return mission. One of Mars' biggest mysteries is what happened to turn it from the warm, wet world that scientists believe existed early in its history into the cold, dry, acidic planet we find today.
"Finding life is not why most of the science community is interested in sample return. There's a reasonable proportion who figure 'Well, we don't see life on Mars now, so it's probably not there,' and that 'If the only payoff is finding life, you're wasting your time.' That's some of the science community," Meyer said.
"The fact that you could look at all the information that's in your samples, determine what environments it saw, is tremendous. There's a lot of things that would really boost our understanding that have nothing to do with life," he said.
Curiosity is just beginning its assessment of its landing site is expected to reach the three-mile high mound of sediment at the center of Gale Crater next year.
In the meantime, NASA hopes to get its Mars exploration plans back on track in time for the 2014 budget cycle.