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In four locations, including the spot nicknamed Mojave (pictured here), the Curiosity 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.

NASA/JPL-Caltech/MSSS

Three-and-a-half billion years ago, the floor of Gale Crater on Mars was the muddy bottom of a large lake where sediment was deposited and, over time, turned into a fine-grained layered rock. Now, scientists working with NASA’s Curiosity rover are reporting that this rock still harbours organic molecules that must have been present in the water all those eons ago.

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. That doesn’t necessarily mean that Curiosity discovered signs of past life. The molecules could have been deposited on Mars by meteorites or formed in the lake through non-biological means.

However, if life was ever present on the red planet, the new results make the best case yet that evidence for it can persist and could one day be found by future probes that can penetrate deeper than the five-centimetre length of Curiosity’s drill.

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In a separate finding, Curiosity also detected that the small amount of methane present in the Martian atmosphere varies with the seasons. This could be a sign that there is a reservoir of methane somewhere under the surface that was generated by chemical reactions involving water or, possibly, by Martian microbes. The results were published Thursday in the journal Science.