The Sample Analysis at Mars (SAM) arm of the rover found seasonal fluctuations of methane gas in the planet's atmosphere as well as organic material from shallow-soil sample analyses from Gale crater, which was a lake some 3 billion years ago.
But Curiosity's data are providing a clearer and more conclusive picture of the conditions and processes on Mars - and what it may have been like on the Red Planet billions of years ago, when conditions were more suitable for life. Today, scientists are announcing they've discovered conclusive evidence that several organic compounds are indeed found on the Red Planet.
Today, in a much-anticipated announcement live-streamed by NASA, it announced both an abundance of organic molecules and seasonal, recurrent releases of methane gas into the atmosphere of planet Mars. Researchers said they can't rule out a biological source.
The scientists hope to find better preserved organic compounds with Curiosity or other rovers that would allow them to check for chemical signatures of life.
The new results represent the longest systematic record of atmospheric methane, with measurements taken regularly over five years. To look for organics, Curiosity drills about two inches into a rock, collects the dust created, then lights it on fire to break the samples down to their chemical components (which is a similar process to Viking's).
That Mars possesses organic molecules is not surprising.
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For the previous mud stone samples that had produced the chlorinated molecules, scientists had heated the powdered rock to 200 degrees Celsius. And if life does exist elsewhere, it may be very different or even form differently from how we understand life on Earth. That doesn't necessarily mean that Curiosity discovered signs of past life.
The European Space Agency's ExoMars rover will also arrive in 2021 and begin measuring the atmosphere and drilling up to two metres below the surface. 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. "A rigorous approach based on available evidence starts with the scientifically responsible default explanation that Mars is and always has been lifeless", Fries says.
This is the first time we've seen something repeatable in the methane story, so it offers us a handle in understanding it.
"The question of whether life might have originated or existed on Mars is a lot more opportune now that we know that organic molecules were present on its surface at the time", wrote Utrecht University astrobiologist Inge Loes ten Kate of the Netherlands. But methane can also be produced by normal geologic processes.
The methane study, spearheaded by JPL atmospheric scientist Chris Webster, is also intriguing for astrobiologists. Curiosity can only drill a few centimeters into Martian rocks, and it lacks the advanced tools necessary to search for more complex markers of life.
In December 2012, the rover's two-year mission was extended indefinitely. "So way under the ground this methane is trapped".
MIT News checked in with SAM team member Roger Summons, the Schlumberger Professor of Geobiology at MIT, and a co-author on the Science paper, about what the team's findings might mean for the possibility of life on Mars.