The discovery leaves open the possibility that microorganisms once populated the red planet - and still might..
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. According to the space agency, Curiosity rover discovered organic molecules after drilling mudstone, a type of sedimentary rock, in a handful of places in the Gale Crater.
What they can't say yet is whether there is, or ever was, life on the Red Planet. In two separate studies on data collected by the Mars rover over the last few years, scientists have identified an abundant source of organic matter in the ancient soil, and traced some of the planet's atmospheric methane to its roots.
It was to a great fanfare of publicity that researchers announced they had found evidence for past life on Mars in 1996.
Knowing that these molecules and compounds were present, then, gives new strength to the idea that life originated or existed on Mars and that more work by the Martian rovers can uncover the past. We might find even more evidence in future missions, too.
The amount of methane peaked at the end of summer in the northern hemisphere at about 2.7 times the level of the lowest seasonal amount. The magnitude of these seasonal peaks - by a factor of three - was far more than scientists expected.
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They may have also have been carried to Mars on an asteroid, for instance.
Still, "we're in a really good position to move forward looking for signs of life", NASA biogeochemist Jennifer Eigenbrode said in a study published Thursday in the peer-reviewed journal Science. "Both radiation and harsh chemicals break down organic matter", Eigenbrode continued.
Curiosity rover on Mars.
He and his colleagues think the methane is coming from underground. Here on Earth, we associate methane with life, but it's a mystery what could be causing it on Mars.
On Mars, organic molecules could have been produced by some form of either present or past lifeforms.
They hit pay dirt about 6.5 kilometres away, at two sites near Pahrump Hills at the base of Mt Sharp. "We also don't know if that methane was created from rock chemistry or it was created by microbes".
In four locations, including the spot nicknamed Mojave, which Curiosity explored in 2015, the rover discovered thiophenes (molecules that include a ring of carbon and sulphur atoms) and other substances that on Earth can be linked to biological activity. 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.