"I don't believe there's life on Mars at the present", Freeman says, because Mars is very dry, very cold and lacks much of an atmosphere.
Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters, in Washington, said: "With these new findings, Mars is telling us to stay the course and keep searching for evidence of life".
While the 1996 discovery has never been verified, it hasn't ever been conclusively disproved either.
No, NASA hasn't discovered life on Mars yet-but a new result makes it seem like maybe, at some point in the planet's history, the conditions were ripe for some extraterrestrial beings.
Mineral veins on Mars seen by Curiosity. Curiosity has had a somewhat rough go of things over the past few months, breaking its drilling mechanism and having to totally relearn how to use its tool in a new way so that it can continue to pierce Martian rocks.
Since it landed at the Gale Crater in 2012, the Curiosity Rover has been sniffing out methane in the area. Organic material can be produced without life.
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The mudstone rock was drilled from the top five centimeters (two inches) of the Martian surface and heated in a miniature analysis lab located on board the rover.
"The results convincingly show the long-awaited detection of organic compounds on Mars".
The complex organic matter was found buried - and preserved - in ancient, three-billion-year-old sediments, suggesting that the planet could have once been home to life. "Both radiation and harsh chemicals break down organic matter", Eigenbrode continued.
Excitingly, the material discovered on Mars is similar to terrestrial kerogen. "Organic matter" in this context doesn't mean anything we'd recognize from our lives on Earth. "We also don't know if that methane was created from rock chemistry or it was created by microbes".
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.
It "defines how questions will be asked and pursued in the next stage of Mars exploration", Anbar, who was not involved in the study, told AFP by email. The ESA's ExoMars rover, which is targeting the same time frame, will assist with the search for ancient life. Earth-based telescopes, spacecraft orbiting Mars and now Curiosity, have measured episodic sudden increases in the background methane content.
Gale crater on Mars. The planet's atmosphere-mostly carbon dioxide-is nearly nothing like our own. "We don't know, but these results tell us we are on the right track". He and his team also can't distinguish whether this methane is the product of geological or biological activity. The authors conclude that this must be related to surface temperature.
As winter falls, gases are once again trapped in ice cages, helping explain at least some of the vanishing methane.