After analyzing powdered rock samples collected from the surface of Mars by NASA’s Curiosity rover, scientists today announced that several of the samples are rich in a type of carbon that on Earth is associated with biological processes.

While the finding is intriguing, it doesn’t necessarily point to ancient life on Mars, as scientists have not yet found conclusive supporting evidence of ancient or current biology there, such as sedimentary rock formations produced by ancient bacteria, or a diversity of complex organic molecules formed by life.

“We’re finding things on Mars that are tantalizingly interesting, but we would really need more evidence to say we’ve identified life,” said Paul Mahaffy, who served as the principal investigator of the Sample Analysis at Mars (SAM) chemistry lab aboard Curiosity until retiring from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in December 2021. “So we’re looking at what else could have caused the carbon signature we’re seeing, if not life”.

In a report of their findings to be published in the Proceedings of the National Academy of Sciences journal on January 18, Curiosity scientists offer several explanations for the unusual carbon signals they detected. Their hypotheses are drawn partly from carbon signatures on Earth, but scientists warn the two planets are so different they can’t make definitive conclusions based on Earth examples.

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“The hardest thing is letting go of Earth and letting go of that bias that we have and really trying to get into the fundamentals of the chemistry, physics and environmental processes on Mars,” said Goddard astrobiologist Jennifer L. Eigenbrode, who participated in the carbon study. Previously, Eigenbrode led an international team of Curiosity scientists in the detection of myriad organic molecules — ones that contain carbon — on the Martian surface.

“We need to open our minds and think outside the box,” Eigenbrode said, “and that’s what this paper does.”

The biological explanation Curiosity scientists present in their paper is inspired by Earth life. It involves ancient bacteria in the surface that would have produced a unique carbon signature as they released methane into the atmosphere where ultraviolet light would have converted that gas into larger, more complex molecules. These new molecules would have rained down to the surface and now could be preserved with their distinct carbon signature in Martian rocks.

Read more: NASA