New Mars discoveries are advancing the case for possible life on the red planet, past or even present.
The rover, which has allowed scientists to explore whether Mars ever boasted conditions conducive to life, in 2014 made the first definitive detection of organic molecules, also in Gale crater rock formed from ancient lake sediment - but it was a much more limited set of compounds.
The organic molecules, discovered in three-billion-year-old sedimentary rocks near the surface, contain carbon and hydrogen, and might also include oxygen and nitrogen.
In a separate finding, Curiosity also detected that the small amount of methane present in the Martian atmosphere varies with the seasons. Again, without having a Mars rock in a laboratory on Earth for more detailed study, we can not say what processes formed these molecules and whether they formed on Mars or somewhere in the interstellar medium and were transported in the form of carbonaceous meteorites.
Freeman says finding organic molecules only a few centimeters below the surface of Mars is an encouraging sign for finding possible life.
Both discoveries emerged from Curiosity's Sample Analysis at Mars (SAM) instrument, a miniaturized chemistry lab and oven that roasts dollops of air, rock and soil to sniff out each sample's constituent molecules. Over time, a picture of the ebb and flow of methane on Mars has emerged.
"[Curiosity's] molecular observations do not clearly reveal the source of the organic matter in [Gale Crater]". The rock samples were analyzed by SAM, which uses an oven to heat the samples (in excess of 900 degrees Fahrenheit, or 500 degrees Celsius) to release organic molecules from the powdered rock. And if life does exist elsewhere, it may be very different or even form differently from how we understand life on Earth.
Ms Campbell said if life did produce the organic molecules it was on the "simpler end of the scale". She pointed out that the surface of Mars is regularly exposed to space radiation, and that radiation and chemicals typically break down organic matter.
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"Curiosity has not determined the source of the organic molecules", Jen Eigenbrode of NASA's Goddard Space Flight Center in Greenbelt, Maryland, said.
Methane is a strong greenhouse gas, and it could have supported a climate that sustained lakes on Mars.
The second paper, led by Christopher Webster of Jet Propulsion Laboratory, marks a major step toward solving the mystery of methane on Mars.
But it's getting easier to hypothesise that Mars once harboured life, because Curiosity's extended trundling on Mars has shown evidence of liquid water on the surface and found plenty of the chemicals that you'd expect as pre-cursors to, or by-products of, life. Seasonal peaks were detected in late summer in the northern hemisphere and late winter in the southern hemisphere.
By examining data spanning almost three Martian years (six Earth years), Webster and his colleagues discerned the first repeating pattern in Martian methane.
In December 2012, the rover's two-year mission was extended indefinitely. One of their most hard tasks is to prove that the carbon they find is biogenic, and not produced through non-living, geological processes. After drilling, Curiosity heats the rock samples, releasing the compounds.
Webster theorizes the methane created either now or long ago is seeping from deep underground reservoirs up through cracks and fissures in the crust.
"Are there signs of life on Mars?"