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.

Based on data from over 1000 regional studies combined with machine learning, researchers estimate that as many as 1.4 billion people live in areas with soil dangerously polluted by heavy metals like arsenic, cadmium, cobalt, chromium, copper, nickel, and lead. The study reveals a global risk, but also a previously unrecognized high-risk, metal-enriched zone in low-latitude Eurasia, in particular. The growth in demand for critical metals means toxic heavy metal pollution in soils is only likely to worsen. “We hope that the global soil pollution data presented in this report will serve as a scientific alert for policymakers and farmers to take immediate and necessary measures to better protect the world’s precious soil resources,” say the authors. Toxic heavy metal pollution in soil, originating from both natural sources and human activities, poses significant risks to ecosystems and human health. Once introduced into soils, such metals can persist over decades. These pollutants reduce crop yields, affect biodiversity, and jeopardize water quality as well as food safety through bioaccumulation in farm animals. However, while previous studies have shown that toxic metal pollution is ubiquitous in soils, its worldwide distribution remains poorly understood. To address this knowledge gap, Deyi Hou compiled data from 1,493 regional studies encompassing 796,084 soil samples to assess the global distribution of toxic metals in agricultural soils and to identify where concentrations exceed safety thresholds. Using machine learning and modeling approaches, Hou et al. estimate that 14–17% of cropland globally – roughly 242 million hectares – is contaminated by at least one toxic metal, with cadmium being the most widespread, especially in South and East Asia, parts of the Middle East, and Africa. Nickel, chromium, arsenic, and cobalt also exceeded thresholds in various regions, largely due to a mix of natural geological sources and human activities such as mining and industrialization. Moreover, findings revealed a transcontinental “metal-enriched corridor” stretching across low-latitude Eurasia, which likely reflects the cumulative effects of ancient mining, weathering of metal-rich bedrock, and limited leaching over time. By superimposing these data with global population distribution, Hou et al. estimate that 0.9 to 1.4 billion people live in high-risk areas.