That said, the agency hopes future missions to the red planet, NASA's Mars 2020 rover and ESA's ExoMars rover, could delve into these findings and reveal the complete history of our neighboring planet. The rovers performed experiments created to detect Martian life, but they largely found none.
The new evidence comes from a pair of rocks.
The authors of the two studies in Science agree with her, writing that there's no way to tell what produced the molecules And certain features of the molecules show they aren't the direct, unchanged remnants of anything living. The so-called "tough" molecules include carbon and hydrogen, and may also contain oxygen, nitrogen, and other building blocks of life.
"What we have detected is what we would expect from a sample from an ancient lake environment on Earth", said Eigenbrode, of NASA's Goddard Space Flight Center. Carbon-rich meteorites contain kerogen-like compounds, and constantly rain down on Mars.
And then there's the most intriguing possibility.
For his part Webster says he has no preference among the different explanations, and believes it will take a long time before any final conclusions can be drawn.
But the boffins are far from certain the the organic molecules or methane that Curiosity turned up have anything to do with life.
While we know that Mars was habitable in the past, the case demonstrates just how hard it will be to ever prove the existence of past life on its surface. "Whether there was in the past or not is certainly an open question". "While not necessarily evidence of life itself, these findings are a good sign for future missions exploring the planet's surface and subsurface", the space agency said. Sulfur may have helped protect the organics even when the rocks were exposed at the surface to radiation and bleach-like substances called perchlorates. There may be more material buried deeper. The drill that Curiosity is using is literally just scratching the surface; it goes 5cm deep. On Mars, where we only have a few molecules from a remote probe, this stuff is light years away from being conclusive. The mission's Trace Gas Orbiter arrived at Mars in late 2016, and it's now collecting data that will let scientists map Mars's methane-and maybe even pinpoint its sources. "There are locations, especially subsurface, where organic molecules are well-preserved".
In the first study, a team led by Christopher Webster, a chemist at NASA's Jet Propulsion Laboratory in California, describes how Mars' atmosphere appears to have low levels of fluctuating methane.
As winter falls, gases are once again trapped in ice cages, helping explain at least some of the vanishing methane.
"The idea that best fits our data is the idea of sub-surface storage", he said.
The methane is cool, whether it's linked to life or not.
Even though the TGO mission can't get as close to the source as the Curiosity Rover, Dr Webster said it could locate potential areas where methane is concentrated or coming from.