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 April 17 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.
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Yet despite Mars showing plentiful signs that ancient rivers and lakes once criss-crossed 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 (34 kilometers) of the 96-mile-wide (154 km) 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.
"Because similar rocks containing these salts have been identified globally, we infer that they, too, likely contain abundant carbonate minerals," Tutolo said. "Summing the carbonate that all of these deposits likely contain indicates that they may hold a substantial portion of the CO2 that was formerly implicated in warming Mars."
An 'imbalanced' cycle
The researchers believe that if their sample is representative of the whole planet, it likely points to Mars having an "imbalanced" carbon cycle. As Mars seemingly lacked Earth-like plate tectonics, toward the tail-end of its habitability Mars likely recycled its carbon into its atmosphere through chemical reactions with acidic water, a hypothesis supported by the presence of sulfate and iron-oxide minerals found within the sample.
But this process was top-heavy, pulling more carbon dioxide out of the atmosphere and into rocks than it released back. In the long run, this diminished the planet's ability to support an atmosphere, possibly snuffing out Mars' ancient life at the same time it began to flourish on Earth.
"Life may have been forming about that time on Earth. Our oldest fossils are about 3.5 billion years old and life must have formed before then," Janice Bishop, a senior research scientist at the Search For Extraterrestrial Intelligence (SETI) Institute and the writer of an accompanying perspective piece on the study, told Live Science. "As the atmospheric gasses [on Mars] were lost over time to space, the atmosphere became thinner and the planet became colder. Estimates of surface ages indicate that Mars has been cold and dry for at least 2 billion years."
Further revelations won't come from a direct retrieval of samples collected on Mars' surface any time soon — that mission is significantly over budget and delayed. But Curiosity is set to continue exploring the Martian surface to understand how the rocks there formed, and what it finds can be included in simulations of the planet's ancient climate.
