A number of clues brought by Curiosity indicate that the concentration of iron carbonate deep in Gale Crater is astonishing, which means that ancient Mars once stored large amounts of carbon dioxide in its crust. By drilling into the sediments of a lost lake, scientists have discovered up to 10 percent siderite in sulfate-rich rocks—carbonates that orbital detection has eluded.

Images of Curiosity's components are combined into a self-portrait at the drilling target Unjana. Image source: NASA/JPL-Caltech/MSSS

This suggests that there once was an active carbon cycle on Mars, with carbon dioxide reacting with water and rocks to form minerals and then some being released back into the air, suggesting that Mars experienced dynamic climate changes and habitable areas in the past.

NASA's Curiosity Mars rover has uncovered a hidden chemical archive in the ancient Martian atmosphere, a new study says, suggesting large amounts of carbon dioxide were locked up in the planet's crust. The findings provide live evidence that carbon cycling occurred on ancient Mars and provide new insights into the planet's past climate.

The Martian landscape clearly shows that liquid water once flowed across its surface, which would have required a much warmer climate than today. Therefore, it is thought that Mars' past carbon dioxide atmosphere must have been thicker to maintain higher temperatures. A climate rich in liquid water and atmospheric carbon dioxide is expected to react with Martian rocks, triggering geochemical processes that produce carbonate minerals. However, while previous analyzes of Martian rocks have detected the presence of carbonates, the amounts found were lower than expected by geochemical models.

Using data from the Curiosity rover, Benjamin Tutuolo and colleagues studied carbonate minerals in a portion of Gale Crater that once contained an ancient lake. In 2022 and 2023, Curiosity drilled four rock samples from different stratigraphic units that represent the transition from lake bed to wind-blown environments and analyzed their mineralogy using the rover's onboard X-ray diffractometer.

Researchers found high concentrations of siderite (iron carbonate) in magnesium sulfate-rich formations, ranging from about 5% to more than 10% by weight. This was unexpected because orbital measurements did not detect carbonates in these formations. Given its origin and chemical properties, the authors concluded that siderite formed from water-rock reactions and evaporation, suggesting that carbon dioxide was chemically sequestered from the Martian atmosphere into sedimentary rocks. If the mineral composition of these sulfate layers is representative of sulfate-rich regions around the world, then these sediments contain a large, previously undiscovered reservoir of carbon.

Carbonate rocks have been partially destroyed in later processes, indicating that part of the carbon dioxide later returned to the atmosphere, forming the carbon cycle. "As orbiters and rovers explore Mars, our understanding of the details of Mars' geochemistry continues to deepen, and more clues about the diversity of potentially habitable environments are gradually revealed," Janice Bishop and Melissa Lane wrote in a related Perspectives article.

Compiled from /ScitechDaily