NASA’s Curiosity rover has discovered siderite, an iron carbonate mineral, hidden beneath Mars’ surface, offering the strongest clue yet in the long-standing puzzle of the planet’s missing carbonates.
NASA’s Curiosity Mars rover sees its tracks receding into the distance at a site nicknamed “Ubajara” on April 30, 2023. This site is where Curiosity made the discovery of siderite, a mineral that may help explain the fate of the planet’s thicker ancient atmosphere. Image Credit: NASA/JPL-Caltech/MSSS
In a recent NASA press release, the researchers detail this finding within the sulfate-rich layers of Gale Crater, where Curiosity’s instruments detected evidence that carbon dioxide from Mars’ ancient atmosphere may have been locked away underground. The study, based on data from Curiosity’s CheMin instrument, sheds new light on the planet’s geologic past and how it may have gradually lost its atmosphere, with major implications for understanding past habitability.
The Backstory: A Planet That Should Have Carbonates—but Doesn’t
For decades, scientists have been puzzled by the apparent absence of carbonate minerals on Mars. If the planet once had a thick carbon dioxide atmosphere, alongside liquid water, basic chemistry suggests carbonates should be abundant. But orbital scans and past rover missions kept coming up short.
Curiosity’s new find could explain why. Siderite, a type of iron carbonate, was discovered in the subsurface rock layers of Mount Sharp, suggesting that carbonates may be buried beneath sulfate deposits or exist in forms that are hard to detect with tools like near-infrared spectroscopy. This fits well with what we know about Gale Crater, a site that was once a lakebed. Here, water-rock-atmosphere interactions could have locked carbon dioxide into stable mineral forms like siderite.
These insights are already shifting how scientists model Mars’ climate history and reinforcing the value of digging just a little deeper.
How the Discovery Was Made
The CheMin instrument aboard Curiosity, designed to identify minerals by X-ray diffraction, picked up the signature of siderite in drill samples taken from three different spots in sulfate-rich strata. The iron carbonate showed up just three to four centimeters below the surface, which helps explain why it went undetected by orbiting instruments.
Geochemist Benjamin Tutolo, part of the research team, noted that siderite’s presence in such sulfate-heavy layers suggests carbonates might be more widespread on Mars, just hidden beneath the surface. The mineral likely formed around 3.5 billion years ago when iron-rich rocks interacted with slightly acidic water in a CO2-rich environment.
This formation process is notably different from Earth’s, where carbonates typically form in more neutral conditions. On Mars, the combination of iron, acidity, and carbon dioxide created a unique chemical pathway—one that implies the early atmosphere may have been thinner than expected, or that a significant amount of CO2 later escaped into space.
What This Means for Mars’ Climate Story
If Curiosity’s siderite discovery holds up across other regions, it could mean Mars didn’t need as much carbon dioxide to stay warm in its early days—or that the planet lost more of its atmosphere than models currently estimate. Either way, it challenges some existing ideas about how early Mars supported liquid water.
There’s also the question of whether other processes, like volcanic outgassing or asteroid impacts, might have temporarily warmed the planet in place of a dense atmosphere. The answers could come from upcoming missions like the European Space Agency’s Rosalind Franklin rover, which is set to explore other sulfate-rich regions.
Meanwhile, Curiosity continues to climb Mount Sharp, analyzing new layers and gathering more evidence to piece together how Mars transitioned from a wetter, potentially habitable world to the dry, dusty planet we see today.
Wrapping Up: Why This Matters
Curiosity’s detection of siderite beneath the surface offers a compelling explanation for where Mars’ missing carbonates went—and, by extension, what happened to its ancient atmosphere. By digging just a few centimeters into the rock, the rover revealed chemical clues that satellites can’t see, showing the power of direct, on-the-ground science.
Every new layer Curiosity studies adds to the evolving story of Mars’ climate and geology, and helps guide future missions in the search for life—or at least the chemical conditions that might have supported it.
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