Curiosity finally found Mars' missing carbon: "Surprising and important breakthrough"

Mars has long held our curiosity. Rust-colored, carbon-rich, and barren, it whispers tales of a very different past. Its dry riverbeds, crater lakes, and mineral-rich soil suggest that water once flowed across its surface. But something changed.

The thick atmosphere that could have supported that water is now gone. Where did it go? How did a once-wet Mars turn into the arid world we see today?

The answers may lie beneath the planet’s dusty surface. Thanks to NASA’s Curiosity rover, scientists now have evidence of a previously hidden carbon sink. It comes in the form of siderite – an iron carbonate mineral.

And its discovery could explain what happened to the carbon dioxide-rich atmosphere of ancient Mars.

Ancient Mars had carbon dioxide and water

Mars, in its ancient days, likely had a dense atmosphere filled with carbon dioxide.

This atmospheric pressure, combined with the greenhouse effect, would have made liquid water stable on the surface. That water interacted with volcanic rocks, setting off a chain of chemical reactions.

The expected result? The formation of carbonate minerals, which are stable compounds that trap carbon dioxide in stone.

Yet for decades, Mars missions failed to find enough carbonates to support this theory. This gap left a puzzle. If early Mars had so much carbon dioxide, why didn’t its rocks hold more evidence of this?

That’s what makes Curiosity’s new findings so important. It drilled deep into Mount Sharp, inside Gale Crater, and brought up powdered rock from ancient layers.

Among these layers, scientists found siderite – an iron carbonate that has never before been seen in such abundance on Mars.

“A surprising and important breakthrough”

“The discovery of abundant siderite in Gale Crater represents both a surprising and important breakthrough in our understanding of the geologic and atmospheric evolution of Mars,” said Benjamin Tutolo, assistant professor at the University of Calgary, and lead author of the Science paper.

Siderite is a particularly useful clue. It forms under water-limited conditions and suggests that ancient Mars experienced both water interaction and evaporation. Its presence points to a carbon cycle – carbon dioxide moved from the air, into the water, and then into rock.

Curiosity’s Chemistry and Mineralogy (CheMin) instrument helped confirm the identity of siderite. Meanwhile, the Sample Analysis at Mars (SAM) suite revealed its chemical signature.

The drill samples – taken from sites named Tapo Caparo, Ubajara, and Sequoia – showed siderite concentrations of between 4.8% and 10.5% by weight.

Mars’s carbon storage hidden in plain sight

Until now, orbital data failed to detect such minerals. Near-infrared satellite imagery can be obscured by surface dust or overwhelmed by brighter mineral signals.

Siderite’s location, which was buried beneath sulfate-rich deposits, may explain why previous missions missed it.

These sulfate-rich layers aren’t just located in Gale Crater. They exist across Mars. If they hide siderite in similar concentrations, then Mars may have stored a large portion of its atmospheric carbon dioxide underground.

Scientists estimate that these layers could hold between 2.6 and 36 millibars of CO₂. That’s as much, or more, than the planet’s current atmospheric CO₂ levels.

Such a reservoir could have once supported the warm, wet conditions needed for lakes and rivers to exist on the Red Planet. But what’s even more interesting is that not all of this stored carbon stayed locked in stone.

Some carbon escaped back into the Martian atmosphere

The drill samples don’t just contain siderite. They also hold various iron oxides – like hematite, goethite, and akageneite.

These oxides form when siderite breaks down under more oxidizing conditions. This process releases the previously trapped carbon dioxide back into the atmosphere.

“Drilling through the layered Martian surface is like going through a history book,” said Thomas Bristow, research scientist at NASA Ames. “Just a few centimeters down gives us a good idea of the minerals that formed at or close to the surface around 3.5 billion years ago.”

The story told by these layers is dynamic. Siderite formed during a time of evaporation and groundwater activity. Later, as the environment changed – possibly due to acidic fluids or shifts in pH – some of that siderite decomposed.

This released carbon dioxide and formed new minerals, completing a carbon loop.

Mars had an incomplete carbon cycle

Earth’s carbon cycle is balanced. Volcanic emissions, plant life, ocean chemistry, and weathering all work together to move carbon through the atmosphere, oceans, and rocks. Mars’s cycle, by contrast, appears broken and incomplete.

The presence of siderite shows that Mars once captured atmospheric CO₂ in mineral form. The iron oxides found in the same rocks reveal that some of that carbon later returned to the atmosphere. But not all of it. Some siderite remained preserved.

This incomplete cycle suggests that ancient Mars experienced a partial climate rebound. After trapping carbon dioxide, the planet may have warmed again temporarily. Then, over time, it cooled as more atmosphere escaped into space or got permanently buried.

Global clues and future searches

Sulfate-bearing layers like those in Gale Crater appear across the surface of Mars. If they also contain siderite, then this carbon sink might be global. But orbital instruments haven’t yet detected carbonate signatures in these areas.

The likely explanation is that these carbonates remain hidden under surface dust or mixed with other materials that mask their presence.

Future missions equipped with deeper drills or improved sensors could test this idea. If they confirm the widespread presence of siderite, we’ll have a clearer picture of Mars’s carbon history. We’ll also have a better understanding of how the planet lost its habitability.

Curiosity reveals a deeper Martian history

Curiosity’s discovery doesn’t just explain a mystery.

It paints a new picture of Mars – one where lakes once stood, minerals formed and reformed, and carbon danced between rock and sky. Each drill hole now tells a story that is millions of years old.

And with each story, we move closer to understanding whether Mars could once have supported life – and why it no longer does.

The study was published in the journal Science.

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