NASA's Curiosity rover has discovered "tough" organic molecules in three-billion-year-old sedimentary rocks near the surface of Mars -- a finding that suggests the planet could have supported ancient life.
It has also found seasonal variations in the levels of methane in the atmosphere, a discovery that has relation to the search for current life on the Red Planet.
"With these new findings, Mars is telling us to stay the course and keep searching for evidence of life," said Thomas Zurbuchen, Associate Administrator for the Science Mission Directorate at NASA Headquarters, in Washington on Thursday.
"I'm confident that our ongoing and planned missions will unlock even more breathtaking discoveries on the Red Planet," Zurbuchen said.
While not necessarily evidence of life itself, these findings, detailed in two papers in the journal Science, are a good sign for future missions exploring the planet's surface and subsurface.
Organic molecules contain carbon and hydrogen, and also may include oxygen, nitrogen and other elements.
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While commonly associated with life, organic molecules also can be created by non-biological processes and are not necessarily indicators of life.
"Curiosity has not determined the source of the organic molecules," said Jen Eigenbrode of NASA's Goddard Space Flight Center in Greenbelt, Maryland, who is lead author of one of the two new Science papers.
"Whether it holds a record of ancient life, was food for life, or has existed in the absence of life, organic matter in Martian materials holds chemical clues to planetary conditions and processes," Eigenbrode said.
In the second paper, scientists described the discovery of seasonal variations in methane in the Martian atmosphere over the course of nearly three Mars years, which is almost six Earth years.
This variation was detected by Curiosity's Sample Analysis at Mars (SAM) instrument suite.
Water-rock chemistry might have generated the methane, but scientists cannot rule out the possibility of biological origins.
Methane previously had been detected in Mars' atmosphere in large, unpredictable plumes.
This new result shows that low levels of methane within Gale Crater repeatedly peak in warm, summer months and drop in the winter every year.
"This is the first time we have seen something repeatable in the methane story, so it offers us a handle in understanding it," said Chris Webster of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, lead author of the second paper.
"This is all possible because of Curiosity's longevity. The long duration has allowed us to see the patterns in this seasonal 'breathing," Webster added.
Launched in 2011, Curiosity was designed to assess whether Mars ever had an environment able to support small life forms called microbes.
Although the surface of Mars is inhospitable today, there is clear evidence that in the distant past, the Martian climate allowed liquid water -- an essential ingredient for life as we know it -- to pool at the surface.
Data from Curiosity reveal that billions of years ago, a water lake inside Gale Crater held all the ingredients necessary for life, including chemical building blocks and energy sources.
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