NASA's Mars Curiosity rover, slowly making its way up the side of a towering mountain of sedimentary rock at the center of its Gale Crater landing site, has found fresh evidence for the red planet's past - and possibly present - habitability, scientists reported Thursday.
But it is "consistent with the past presence of biology", said Ken Williford, an astrobiologist at NASA's Jet Propulsion Laboratory.
Nasa announced that it had something exciting to share with us, adding that there would be a live discussion on Thursday 7 June to explain more about "new science results from Nasa's Mars Curiosity rover". Understandably, the authors of the two papers, published in the journal Science, are very careful not to make the claim that they have discovered life on Mars. Spikes of methane (CH4) were first noticed in the Red Planet's atmosphere several years ago, drawing intense debate over the hydrocarbon's possible source.
"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. But they're of interest to astrobiologists because they are the essential ingredients in all the chemistry that drives life on Earth.
NASA has good evidence that Gale Crater where Curiosity is rolling around used to be a lake.
While it could be produced by microorganisms under the surface of Mars, it could also be produced by non-biological processes such as chemical reactions in rocks, or the breakdown of organic matter in dust delivered by comets or meteors, by UV radiation. And, given the intensity of the radiation bombarding the planet's surface, it wasn't clear whether any relics from that warm, wet period could still be preserved in mudstones on the lake's dried-up floor.
Two rock samples taken by NASA's Curiosity rover were found to contain organic molecules.
The team estimates the ancient sediments, where the complex organic molecules were found, were actually the remains of a vast lakebed that existed more than 3bn years ago. But that explanation, too, suggests a provocative possibility; even if the organic molecules didn't come from life, they are exactly what life likes to eat. "It probably indicates more active water in the subsurface than we understood", scientist Kirsten Siebach, Martian geologist at Rice University not involved with the studies, told Gizmodo.
The methane study, spearheaded by JPL atmospheric scientist Chris Webster, is also intriguing for astrobiologists.
Water-rock chemistry might have generated the methane, but scientists can not rule out the possibility of biological origins. On Earth, living things (especially bacteria) produce lots of methane, though the gas also has plenty of non-living sources. "That's a big change", Dr Webster said.
"You'd expect life to be seasonal", Mumma noted. "Biological, geological and meteoritic sources are all possible", they wrote.
Curiosity's methane measurements occurred over four-and-a-half Earth years, covering parts of three Martian years.
A new analysis of data gathered by Curiosity has confirmed a long-term pattern of methane highs and lows, varying between 0.24 to 0.65 parts per billion.
In any case, Webster said the methane apparently works its way into the atmosphere from sub-surface reservoirs of some sort, places where non-biological geochemistry is going on or where microbial life might somehow flourish.