NASA’s Curiosity rover has found some of the best evidence yet that ancient life may have existed on Mars — and an answer for what could have wiped it out.

When drilling into rocks on Mount Sharp, the central peak of the Red Planet’s Gale Crater, the rover found evidence of siderite, an iron carbonate whose presence suggests Mars once had a carbon cycle. This hints that Mars once had potentially habitable conditions, and therefore possibly even life.

The finding, hidden from satellite scans, raises hopes that once samples collected by the Perseverance rover are brought to Earth, scientists may find evidence that ancient life once thrived on our now-desiccated neighbor. The researchers published their findings in the journal Science. “When it became apparent that these rocks contained siderite in such high quantities, I was unbelievably excited,” study lead-author Ben Tutolo, an associate professor with the department of earth, energy and environment at the University of Calgary, told Live Science. “One of the biggest questions in Mars science is: ‘Where are all the carbonates?’ So I knew right away how important this discovery was.”

For roughly the last 4 billion years, Earth’s carbon cycle has been key to its habitability — cycling carbon between the atmosphere, land and ocean, thus providing the key material for all living things and setting the atmospheric thermostat for them to thrive. The slow carbon cycle makes up half of this system. Spewed out from volcanoes, carbon dioxide is absorbed by calcium-rich oceans to form limestone rock that is subducted back into the mantle, heated and released once more.

Yet despite Mars showing plentiful signs that ancient rivers and lakes once crisscrossed the planet, neither rovers nor satellite scans had found any evidence of carbonate minerals that would imply a carbon cycle there. The Curiosity rover’s discovery changes all of that. Landing on Mars’ Gale Crater in 2012, the rover has traversed 21 miles of the 96-mile-wide meteor impact crater, dutifully investigating the geology within. In 2022 and 2023, Curiosity drilled four samples from rocks in the crater and analyzed the mineralogy using its onboard X-ray diffractometer before beaming the results back to Earth.

When Tutolo and his colleagues unpacked this analysis, they found that the rocks didn’t just contain traces of siderite, they were rich in it — making up between 5 % to 10% of the sample’s total weight. Mixed among the carbonate were other minerals, particularly highly water-soluble magnesium sulfate salts, which the researchers believe are acting to “mask” the siderite’s signal from satellite scans.