Carbonates identified by the Curiosity rover indicate a carbon cycle operated on ancient Mars.

Ancient Mars had surface liquid water and a dense carbon dioxide (CO2)–rich atmosphere. Such an atmosphere would interact with crustal rocks, potentially leaving a mineralogical record of its presence. We analyzed the composition of an 89-meter stratigraphic section of Gale crater, Mars, using data collected by the Curiosity rover. An iron carbonate mineral, siderite, occurs in abundances of 4.8 to 10.5 weight %, colocated with highly water-soluble salts. We infer that the siderite formed in water-limited conditions, driven by water-rock reactions and evaporation. Comparison with orbital data indicates that similar strata (deposited globally) sequestered the equivalent of 2.6 to 36 millibar of atmospheric CO2. The presence of iron oxyhydroxides in these deposits indicates that a partially closed carbon cycle on ancient Mars returned some previously sequestered CO2 to the atmosphere. Editor's summary: The Curiosity rover is gradually climbing a mountain located in Gale crater on Mars. Higher levels of the stratigraphy expose rocks that formed at later times. Tutolo et al. studied the composition of drill samples taken from a sulfate-rich layer (see the Perspective by Bishop and Lane). They found that the rocks contained abundant iron carbonate, which was invisible in previous orbital observations. If there are similar abundances of carbonate in other sulfate-rich layers across Mars, then those layers may contain a substantial reservoir of carbon dioxide extracted from the atmosphere. The carbonates found by the authors have partially decomposed, returning some carbon dioxide to the atmosphere: an ancient carbon cycle. —Keith T. Smith [ABSTRACT FROM AUTHOR]