NASA's Curiosity rover has discovered potential building blocks of life in an ancient martian lakebed, advancing the case for possible life, past or even present, on the red planet, the US space agency said.
The new findings published in the journal Science describe "tough" organic molecules in three-billion-year-old sedimentary rocks near the surface, as well as seasonal variations in the levels of methane in the atmosphere.
Organic molecules contain carbon and hydrogen, and also may include oxygen, nitrogen and other elements. While commonly associated with life, organic molecules also can be created by non-biological processes and are not necessarily indicators of life.
These findings are a good sign for future missions exploring the planet's surface and subsurface, NASA said in a statement.
"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 here.
"I am confident that our ongoing and planned missions will unlock even more breathtaking discoveries on the red planet," Zurbuchen said.
"Curiosity has not determined the source of the organic molecules," said Jen Eigenbrode of NASA's Goddard Space Flight Center in the US.
"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.
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, NASA said.
Data from Curiosity reveals 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, it said.
"The martian surface is exposed to radiation from space. Both radiation and harsh chemicals break down organic matter," said Eigenbrode.
"Finding ancient organic molecules in the top five centimeters of rock that was deposited when Mars may have been habitable, bodes well for us to learn the story of organic molecules on Mars with future missions that will drill deeper," he said.
Scientists also discovered 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, NASA said.
"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 California.
To identify organic material in the martian soil, Curiosity drilled into sedimentary rocks known as mudstone from four areas in Gale Crater. This mudstone gradually formed billions of years ago from silt that accumulated at the bottom of the ancient lake.
The rock samples were analysed by SAM, which uses an oven to heat the samples ( 500 degrees Celsius) to release organic molecules from the powdered rock.
SAM measured small organic molecules that came off the mudstone sample - fragments of larger organic molecules that do not vapourise easily.
Some of these fragments contain sulphur, which could have helped preserve them in the same way sulphur is used to make car tires more durable, according to Eigenbrode.
The results also indicate organic carbon concentrations on the order of 10 parts per million or more. This is close to the amount observed in martian meteorites and about 100 times greater than prior detections of organic carbon on Mars' surface.
Some of the molecules identified include thiophenes, benzene, toluene, and small carbon chains, such as propane or butene.
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