Jezero rocks record at least three stages of water-driven alteration

Rocks bordering the ancient lake in Mars’ Jezero crater preserve a history more complex than the passage of a single body of water. A study published on September 21 in Communications Earth & Environment identifies at least three stages of fluid-driven alteration in the area called the Margin unit. Minerals and relationships between fractures allow the processes to be put in order, but not dated.

The team led by Candice Bedford of Purdue University combined images, chemical measurements, and mineralogical information from the Perseverance rover’s SuperCam instrument. One of its capabilities is firing a laser at rock: light emitted by the heated material reveals which elements are present. Other observations record reflected light and grain textures. More than 185 rock targets were analyzed across the unit; these are observation sites, not 185 samples brought back to Earth.

Comparisons across terrain at different elevations were crucial. In higher areas, crystalline grains of olivine, a mineral rich in magnesium and iron, preserve interlocking relationships and shapes consistent with slow cooling of molten material, with little subsequent alteration by water. Lower areas contain fractured grains, carbonates, and silica between grains. An igneous origin for the unit does not rule out sedimentary reworking in some areas: water and erosion may also have redistributed material that had already formed.

In the first reconstructed stage, carbon-dioxide-rich fluids circulated through the rock and promoted carbonate formation in fractures. This mineral filling resisted erosion better than the surrounding rock, leaving ridges visible today. Later, other fluids altered preexisting carbonates, opened spaces, and deposited silica. This second stage may be linked to the ancient lake or to chemical evolution of groundwater. The distribution of alteration helps assess these possibilities without by itself establishing which predominated.

The late stage left veins containing calcium sulfate and fluorite, a mineral containing calcium and fluorine. At the studied site, fluorite in a thick vein and the geological setting are consistent with heated water circulating underground. This is an interpretation of the combined evidence, not a temperature measured by the rover or a property that alone proves hot water was present. The paper itself notes that fluorite can also form under cold conditions.

The reconstruction changes the question about local habitability: rather than looking only for traces of a lakeshore, scientists will need to understand underground environments and successive water–rock reactions. Carbonates and silica can preserve signs of ancient processes, including possible biological activity, but the study does not identify life. It provides a geological sequence to guide interpretation of these rocks and the samples collected by Perseverance, still without absolute dates for the water episodes.