Picture a dried-up river on Mars, silent for billions of years, its bed now a ribbon of rust-coloured rock. It is here, in a place called Neretva Vallis, that NASA's Perseverance rover has been doing something extraordinary. As it rolled along the ancient channel leading into Jezero crater, its onboard Sherloc instrument fired an ultraviolet laser at mudstones from an outcrop called Bright Angel — and the light that bounced back carried a remarkable secret: complex carbon molecules, locked inside Martian rock. The carbon detected belongs to a family called macromolecular carbon, or MMC.

Here is where the story gets deliciously complicated. On Earth, MMC turns up in fossilised microbial mats, in coal, in the remnants of ancient life. But it can also be produced by purely geological reactions between rock and water, or delivered by meteorites crashing from space. "It may originate from biological sources such as fossilised organic matter found in microbial mats," said Dr Ashley Murphy of the Planetary Science Institute in Arizona, carefully leaving every door open.

The rocks are not confessing — not yet.