Researchers have discovered a type of chemical which can be produced by living organisms
Nasa's Perseverance rover has found possible signs of ancient life on the surface of Mars.
The vehicle has discovered complex carbon molecules on the Red Planet which were already under scrutiny for possible signs of ancient microbial life.
The rover's Sherloc instrument detected organic carbon in mudstones at the Bright Angel outcrop.
Bright Angel sits along Neretva Vallis, an ancient river channel that once flowed into the Jezero crater billions of years ago.
The type of carbon identified is called macromolecular carbon, or MMC - which can be produced by living organisms.
However, it can also form through purely geological processes, meaning it does not - yet - form definitive proof that life once existed on Mars.
Dr Ashley Murphy at the Planetary Science Institute in Arizona said MMC appears in various settings and rock types.
"It may originate from biological sources such as fossilised organic matter found in microbial mats and coal," she said.
The material could also come from chemical reactions between rocks and water.
Meteorites striking the planet's surface could also be a source of the complex chemical.
The presence of MMC alone cannot confirm whether it came from ancient Martian microbes - or whether more mundane geological activity was responsible.
Scientists must therefore consider multiple explanations for the carbon's origin.
The Bright Angel mudstones first attracted attention in 2024 when Perseverance spotted unusual surface spots and nodules.
These bore a striking resemblance to structures created by fossilised microbes on Earth.
When the scientific findings were published last year, Sean Duffy, the former acting head of Nasa, said: "This very well could be the clearest sign of life that we've ever found on Mars."
Murphy and her team used Perseverance's Sherloc ultraviolet laser to analyse the rocks, measuring light scattered back from the surface.
Nasa rovers have now discovered organic-bearing mudstones at locations more than 2,000 miles apart on Mars.
But Perseverance lacks the equipment needed to determine whether the carbon came from ancient life or non-biological processes.
Nasa had planned to bring Martian rock samples back to Earth for detailed testing, but the mission was effectively cancelled in January.
A revised mission is now being developed for the 2030s, while China aims to return its own samples by 2031.
"The science payload of the Perseverance rover was not designed to distinguish between organics formed via abiotic and biotic processes but was instead selected to identify compelling rocks to be collected for possible return to Earth for more rigorous testing," said Dr Kyle Uckert, a research scientist at Nasa's Jet Propulsion Lab.
Prof Mark Sephton, an organic geochemist at Imperial College London, said: "These treasure troves of information are puzzles that need to be solved. And that is best done back in Earth laboratories after sample return."