NASA’s Curiosity rover has uncovered a hidden chemical archive of ancient Mars’ atmosphere, which suggests that large amounts of carbon dioxide have been locked into the planet’s crust, according to a new study. The findings provide in situ evidence that a carbon cycle once operated on ancient Mars and offer new insights into the planet’s past climate. The Martian landscape shows clear signs that liquid water once flowed across its surface, which would have required a much warmer climate than the planet has today. It is therefore thought that Mars’ CO₂ atmosphere must have been thicker in the past, to maintain warmer conditions. A climate containing abundant liquid water and atmospheric CO₂ is expected to have reacted with Martian rocks, triggering geochemical processes that produce carbonate minerals. However, while previous analyses of Martian rock have detected the presence of carbonates, the quantities found were lower than expected from geochemical models.

Using data from the Curiosity rover, Benjamin Tutolo and colleagues investigated carbonate minerals in part of Gale crater – which once contained an ancient lake. In 2022 and 2023, Curiosity drilled four rock samples from different stratigraphic units representing transitions from lakebed to wind-blown environments and analyzed their mineralogy using the rover’s onboard X-ray diffractometer. Tutolo et al. identified siderite (iron carbonate) in high concentrations – ranging from approximately 5% to over 10% by weight – within magnesium sulfate-rich layers. This was unexpected, because orbital measurements had not detected carbonates in these strata. Given its provenance and chemistry, the authors infer that the siderite formed by water-rock reactions and evaporation, indicating that CO₂ was chemically sequestered from the Martian atmosphere into the sedimentary rocks. If the mineral composition of these sulfate layers is representative of sulfate-rich regions globally, those deposits contain a large, previously unrecognized carbon reservoir. The carbonates have been partially destroyed by later processes, indicating that some of the carbon dioxide was later returned to the atmosphere, forming a carbon cycle. “As details of Mars’ geochemistry are discovered through orbital and rover investigations around the planet, additional clues are revealed about the diversity of potentially habitable environments,” write Janice Bishop and Melissa Lane in a related Perspective.