Imagine you buy an old house expecting drywall and find hand-laid stone instead — and then realize the stone has been repainted three times, each coat a different era of ownership. That's what Perseverance found at the Margin Unit. The team expected sedimentary lakebed rock and instead got igneous rock that had been chemically rewritten by water at least three separate times. The surprise isn't water on Mars — we knew that. The surprise is the complexity of the plumbing. The Margin Unit stretches along the inner rim of Jezero Crater, hugging what was once a lake shoreline. Orbital data had flagged strong carbonate signals, which on Earth typically mean shallow water environments capable of supporting life. The working hypothesis was straightforward: carbonates formed in the lake, sedimentary layers preserved the record. Perseverance's SuperCam — a mast-mounted laser spectrometer that can analyze targets up to 6.5 meters away — told a different story across 185+ bedrock targets spanning 265 meters of elevation. At higher elevations, the rover found coarse-grained olivine with virtually no water alteration. This rock formed deep underground in slowly cooling magma and only reached the surface after overlying material eroded away. Lower down, toward the old lakebed, the olivine was fractured and infiltrated with silica and carbonate — the chemical fingerprints of water-rock interaction. The olivine-to-carbonate reaction is significant because on Earth it releases hydrogen, a potential microbial food source, and deposits minerals (carbonate, silica) that are excellent at trapping biosignatures. The three water episodes read like chapters. First: CO₂-rich groundwater reacted with olivine, filling fractures with carbonate ridges that now stand proud as surrounding softer rock erodes. Second: a silica-enriching event likely tied to the crater lake itself, concentrated below the ancient waterline. Third: a late-stage hydrothermal pulse that deposited calcium sulfate and fluorite veins — roughly 25 cm thick — in the eastern Margin Unit. Fluorite is the smoking gun for hot water circulating through volcanic rock, pointing to subsurface heat long after the lake may have dried up. Lead author Candice Bedford (Purdue University) frames the result as rewriting the interpretation not just of Jezero but of one of the largest carbonate exposures on Mars. The carbonates weren't simply lake deposits — they record a crossroads of aqueous systems operating at different temperatures, chemistries, and times. This matters because carbonate-rich terrains are high-priority targets for biosignature searches, and the formation pathway determines what kind of life (if any) could have been preserved and how. The study, published in Communications Earth & Environment, draws on SuperCam spectroscopy co-led by Purdue, Los Alamos National Laboratory, and IRAP/CNES in Toulouse. The instrument's ability to profile rock chemistry at distance — laser-induced breakdown spectroscopy across hundreds of targets — is what made the spatial mapping of alteration gradients possible. Without that density of measurements, the three-episode story would likely have been invisible. What this doesn't do is date the episodes. Perseverance can sequence them (first, second, third) but not assign absolute ages. That constraint is load-bearing: the habitability window could span millions of years or billions, and the gap between water episodes matters enormously for any biological continuity argument. Resolving that requires returned samples — which ties this directly to the Mars Sample Return campaign's stakes.
