The search for life on Mars over the past half century has been an exercise in lowering the bar combined with ever more sophisticated ways of seeking clues.
New Mars discoveries are advancing the case for possible life on the red planet, past or even present.
Nasa says it's found new evidence which suggests Mars could have supported ancient life.
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. That leaves open the possibility that microorganisms once populated our planetary neighbor and might still exist there.
This diagram shows how methane beneath the Martian ground might find its way to the surface, where its uptake and release could produce the large seasonal variation in the atmosphere that was observed by the Curiosity rover. Organic entails "containing carbon", as this NASA video says, but organic molecules can have biological or non-biological origins.
Arriving at Mars in 2012 with a drill and its own onboard labs, Curiosity confirmed the presence of organics in rocks in 2013, but the molecules weren't exactly what scientists expected.
To get firmer answers, researchers will need to get equipment to Mars that's sensitive enough to detect life's thumb on the chemical scales.
The compounds might have come from a meteorite, or from geological formations akin to coal and black shale on Earth, or some form of life, Eigenbrode said.
"This is the first time we've seen something repeatable in the methane story, so it offers us a handle in understanding it", Chris Webster, the lead author of the second paper, said in the NASA statement.
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Although they're often linked to life, they can also be created in other ways, so they're not evidence for life themselves.
The samples were drilled from the base of Mount Sharp, inside a basin called Gale Crater that is believed to have held an ancient Martian lake.
"It tells us that this ancient environment on Mars could have supported life", NASA biochemist Jennifer Eigenbrode said. Now with these new results, what does this all say about the possibility that there is, or was life on Mars? The rover's internal laboratory heated the powdered rock sample to 900 degrees Fahrenheit to release organic molecules.
The Curiosity rover, which has travelled 19.3 km since it landed in the Gale crater almost six years ago, detected a number of organic molecules in pieces of Martian mudstone it drilled from the lake bed and heated in its oven. She said the discoveries break down some of the strongest arguments put forward by life-on-Mars skeptics, herself included.
Curiosity's methane measurements occurred over 4 1/2 Earth years, covering parts of three Martian years.
As with methane, there could well be non-biological explanations for the presence of carbon-containing molecules on Mars, such as geologic processes or impacts by asteroids, comet, meteors and interplanetary dust. However, Webster says this is now ruled out by the cycle of methane variation.
There's also excitement because Curiosity found this stuff without having to look very hard: the organics turned up in some mud that looked likely.
"We don't know if that methane is ancient, we don't know if it's modern - it could be either", Webster said.