NASA’s Curiosity rover finds elusive carbonate minerals on Mars

Minerals are like time capsules. Minerals are defined by chemical composition and crystal structure, characteristics that are achieved under specific environmental conditions. Now, researchers have identified minerals in Martian rock samples that provide clues about how the red planet’s climate—and habitability—changed billions of years ago (Science 2025, DOI: 10.1126/science.ado9966).

“We have abundant evidence for liquid water flowing on the Martian surface before 3.5 billion years ago or so,” says geochemist Ben Tutolo of the University of Calgary. For those wet conditions to exist, Mars must have been considerably warmer than it is today, blanketed back then by a CO₂-dense atmosphere. That ancient, long-gone atmosphere should be reflected by an abundance of carbonate minerals on Mars. “But until now, we hadn’t really seen them,” Tutolo says.

Tutolo joined the Mars Science Laboratory Curiosity rover mission as a participating scientist late in 2022, a decade after the NASA rover first touched down and began its trek across the Martian surface collecting rock samples. Within Tutolo’s first year on the mission, Curiosity collected three new samples—named Tapo Caparo, Ubajara, and Sequoia—in a section of Gale crater thought to contain elevated levels of sulfate minerals.

The rover’s onboard chemistry and mineralogy X-ray diffraction (CheMin) instrument allows the team to “do mineralogy on Mars in this breathtakingly elegant way,” Tutolo says. With CheMin, the team identified unexpectedly high concentrations of the iron carbonate mineral siderite in the three samples.

The discovery was a surprise: spectroscopy measurements from a satellite orbiting Mars didn’t find a trace of carbonate around the drill sites. The bountiful sulfate minerals effectively masked the carbonate signal when scanned from space, Tutolo says. But if the amount of siderite detected within these drilled samples is globally representative, similar geographic regions of Mars thought to be carbonate-free could actually contain an unidentified carbon reservoir.

The minerals in the samples also give researchers a clue about Mars’s climate a billion years ago. Siderite can form in multiple ways. But only evaporation would lead to highly pure siderite forming in the same geologic layer as the sulfate minerals the team identified, gypsum and starkeyite. Atmospheric CO₂ must have dissolved in Martian surface waters and become carbonate. As the waters dried up, carbonate and iron precipitated as siderite alongside precipitating sulfate minerals. “This is finally the evidence that justifies our necessary assumptions in the climate models that there had to be CO₂ in the atmosphere,” Tutolo says.

This discovery alone cannot account for the missing carbonate expected to form in a dense CO₂ atmosphere, says Tanja Bosak, a geobiologist at the Massachusetts Institute of Technology and scientist on the Mars Perseverance rover team. Around 1 bar of CO₂ must have been present to sufficiently warm the surface, and the Curiosity team can only extrapolate to tens of millibar by assuming that other geographic features contain the same amount of siderite. But Bosak says the new study provides evidence that ancient Martian waters could have been more habitable than some theories predict.

Carbonate can form only at a relatively hospitable pH level. The evidence that siderite formed in Gale crater suggests the presence of a long-dried-up ancient lake with clement conditions. “Finding any new minerals in these environments teaches us about the diversity of potentially habitable environments on Mars during the time when life may have been starting to spread,” Bosak adds.