The carbon that once warmed Mars’ atmosphere has been locked in its rusty rocks for millennia.
That’s the story revealed by a hidden cache of carbon-bearing minerals unearthed by NASA’s Curiosity rover along its route up a Martian mountain. The finding is the first evidence of a carbon cycle on the Red Planet, but also suggests that Mars lost its life-friendly climate because that carbon cycle was slow, researchers report in the April 18 Science.
Many lines of evidence suggest that Mars once had abundant water and a warm, comfy climate, supported by a thick carbon dioxide atmosphere. But today, the planet next door is a cold, dry desert with barely any atmosphere at all.
All that carbon dioxide must have gone somewhere, says geochemist Benjamin Tutolo of the University of Calgary in Canada. The most likely place is locked up in carbonate minerals, which contain carbon and oxygen.
But despite decades of observations, planetary scientists hadn’t found enough carbonate to explain the planet’s dramatic drying.
“One of the biggest questions in the history of Mars is, where is all the carbonate?” Tutolo says.
Now, the Curiosity rover has found a carbonate mineral called siderite on a mountain in the ancient lakebed in the Gale crater. “We found it here, for the first time,” Tutolo says. “That really is the crux of what’s exciting here.”
Tutolo and colleagues studied data collected in 2022 and 2023, when Curiosity drove across a region where the rocks change from muddy clays to desiccated salty minerals called sulfates.
Reaching this spot has been one of the rover’s main goals ever since it landed in 2012.
“We believe this represents the great drying of Mars,” Tutolo says. “We knew it was going to be cool, but we didn’t know how cool until we got there.”
The rover drilled four samples from different rocks along an 89-meter stretch of terrain. Researchers analyzed the rocks’ contents with Curiosity’s onboard chemistry lab.
The research team identified siderite within the sulfate-bearing layers. This mineral probably formed as Mars dried out, through a combination of water-rock interactions and evaporation.
“This was a surprising discovery that nobody expected,” Tutolo says.
And there was a lot of it: The samples contained between 5 and 10 percent siderite by weight. If that much siderite is also hiding in other Martian sulfates, “we get a lot closer to figuring out where all the CO2 went that used to be in the atmosphere,” Tutolo says.
The rocks also contained different amounts of iron oxyhydroxides, which form when siderite dissolves in acidic water. That means some of the carbon would have returned to the atmosphere, creating a carbon cycle. But in contrast to Earth’s carbon cycle, which has been relatively stable for billions of years, Mars’ surface rocks absorbed far more carbon than they released.
“CO2 goes down, it doesn’t come back up,” says Tutolo, who got his Ph.D. studying carbon sequestration on Earth as a climate change solution. “This helps us understand why Mars was once habitable, and why it became inhabitable.”
This interpretation of the siderite “provides a great explanation for where the missing carbonate is and how the ancient Martian atmosphere could have been thick enough to support liquid water on the surface,” says planetary scientist Janice Bishop of the SETI Institute in Mountain View, Calif. She coauthored a companion piece in Science but was not involved in the new study.
Researchers should look carefully at orbital data to find more correlations between carbonates and other types of rock, she says. “Of course, though, the best way to characterize Martian samples in detail is by bringing the cached samples back to Earth.”
