A surprise discovery in Gale Crater is the component that was missing in the puzzle of Mars's climate history.

There, embedded in the bedrock, the Curiosity rover has identified a mineral called siderite that can only have formed from the precipitation of carbon from the Martian atmosphere. In other words, billions of years ago, Mars had an active carbon cycle.

It's the first in situ evidence of the carbon cycle on Mars, and it represents an important clue about whether or not the red planet could ever have supported life.

"It tells us that the planet was habitable and that the models for habitability are correct," says geochemist Benjamin Tutolo of the University of Calgary in Canada.

One of the biggest questions about ancient Mars involves its water. All evidence points to a planet that was rich in bodies of liquid water on its surface, with lakes and oceans that sloshed and lapped and crashed in waves upon shorelines.

In order to be warm and stable enough for this liquid water, the atmosphere of Mars would have needed a significant amount of carbon dioxide, belched into the sky by the active volcanoes that were once rampant on the surface.

Much of this carbon dioxide would have leaked out into space, but enough would have remained to warm Mars, and leave traces in the minerals on the surface.

There's just one itty bitty problem.

"Models predict that carbonate minerals should be widespread, but, to date, rover-based investigations and satellite-based orbital surveys of the Martian surface have found little evidence of their presence," Tutolo told ScienceAlert.

The Ubajara drillsite in Gale crater. (NASA/JPL-Caltech/MSSS)

The shock new discovery was found in data from 2022 and 2023, when the Curiosity rover, which has been beavering around Gale Crater for more than 10 years now, made X-ray diffraction analyses of minerals from different parts of the crater floor using its Chemistry and Mineralogy (CheMin) instrument.

Tutolo and his colleagues carefully analyzed the measurements made by Curiosity, and found remarkably pure crystalline siderite in three of the four drill holes bored by Curiosity. This siderite, mostly composed of iron and carbon trioxide, with trace amounts of magnesium, stunned the researchers.

"We were surprised to find carbonate minerals here because even the most detailed investigations of the orbital spectroscopy data acquired over these sedimentary rocks were unable to identify carbonate minerals," Tutolo said.

"It turns out that the presence of other minerals – particularly highly water-soluble magnesium sulfate salts – likely masks the signature of carbonate minerals in the orbital data. Because similar rocks containing these salts have been identified globally, we infer that they, too, likely contain abundant carbonate minerals."

The Tapo Caparo drillsite in Gale Crater. (NASA/JPL-Caltech/MSSS)

So, not only does the discovery finally pony up the carbonate minerals scientists expected to find, it reveals why scientists have been unable to find them previously, and how to look for more of them across the red planet.

The siderite identified in Curiosity data helps confirm and refine models of Mars's early warm period, more than 3.5 billion years ago. It confirms that carbon dioxide was abundant in the Martian atmosphere, and helped keep the planet warm enough for water; and that carbon was extracted from the atmosphere and trapped in minerals on the surface.

But the formation of siderite, while good news for scientists studying Mars today, was part of the end of an era for Mars itself.

"The important feature of the ancient Martian carbon cycle that we outline in this study is that it was imbalanced. In other words, substantially more CO2 seems to have been sequestered into the rocks than was subsequently released back into the atmosphere," Tutolo explained.

"Because Mars is further away from the Sun than Earth, it needs substantially more CO2 in its atmosphere to maintain habitable conditions. The observation that geochemical processes were capturing and sequestering that CO2 suggests that this imbalanced carbon cycle may have challenged Mars's ability to remain habitable."

A diagram of the team's proposed carbon cycle on ancient Mars. (Tutolo et al., Sci. Adv., 2025)

These results have several implications. Now that scientists know that siderite is effectively invisible to orbital instruments, they can go back over previous data and look for strange signs of its presence they may have overlooked. In addition, rover-collected data may have more evidence of carbonate minerals.

Now that researchers know mineral carbon sequestration took place on Mars, they can incorporate this information into models of the planet's climate history, and determine what role, if any, this capture played in the decline of Mars's habitability.

These minerals, so common and unremarkable on Earth, have opened up a whole new way of understanding Mars.

"I was trained as an aqueous geochemist and spent much of my career to date working on carbon sequestration as a solution for human-driven climate change. Working alongside the exceptionally talented and diverse expertise of the Mars Science Laboratory team, I was ultimately able to apply the knowledge I have gained from my climate change solutions work to interpret these mineralogical observations," Tutolo said.

"Frankly, if you told me about all of this when I was 15, I never would have believed it!"

The findings have been published in Science Advances.