Nasa thought an ancient Martian lake shaped these rocks; Perseverance found evidence of three water events

When Nasa’s Perseverance rover reached the inner edge of Jezero Crater in September 2023, scientists expected to find sedimentary rocks shaped by an ancient Martian lake. Instead, the rover discovered igneous rocks whose minerals preserve a much more complicated history of water activity. A new study published in Communications Earth & Environment shows that the rocks in an area known as the Margin Unit interacted with water during at least three separate episodes. The evidence points to carbon dioxide-rich groundwater, later interaction with the ancient Jezero lake or changing groundwater conditions, and finally a hotter hydrothermal event that left mineral veins behind. The findings provide new clues about how water moved through early Mars and why the region remains important in the search for environments that may once have been habitable.Nasa Perseverance rover finds unexpected rocksThe Margin Unit runs along the inner edge of Jezero Crater and was previously interpreted from orbital observations as part of an ancient lake shoreline. Scientists had detected strong carbonate signatures from orbit, making sedimentary rocks a reasonable expectation because carbonates commonly form in watery environments.But when Perseverance reached the area, it found something different. Much of the rock was igneous and rich in olivine, indicating that it originally formed from magma deep underground before later being exposed at the surface by erosion.The discovery did not make the rocks less useful. Instead, their mineral crystals provided a record of what happened after the rocks formed. The rover's observations showed that water repeatedly altered the rock, leaving different chemical and mineral signatures from each episode.A rock formed deep undergroundPerseverance explored the Margin Unit across roughly 870 feet, or 265 metres, of elevation. At higher elevations, the rover encountered coarse-grained, crystalline rock dominated by olivine and showing little evidence of water exposure.The study indicates that this olivine-rich material cooled slowly inside a body of magma deep beneath the Martian surface. Slow cooling allowed relatively large mineral grains to form. Later erosion removed material above the rock and exposed it at the surface.Lower down, however, the rocks looked considerably different. Olivine grains had been fractured and silica was present between them, providing evidence that fluids had subsequently moved through the rock.The distinction between the relatively unaltered higher rocks and the heavily altered lower exposures helped researchers reconstruct the sequence of water-rock interactions.The first water event involved groundwaterThe first major alteration episode occurred when carbon dioxide-rich groundwater circulated through the olivine-rich bedrock.According to the study, the fluids were neutral to alkaline and reacted chemically with the rock. The process produced carbonate-rich material that accumulated within fractures. As softer surrounding material later eroded, some of these carbonate-filled fractures became visible as ridges.This is significant because carbonate minerals can preserve information about ancient environments. On Earth, interactions between water and olivine can also release hydrogen, which some microorganisms can use as an energy source.However, the Martian findings do not demonstrate that life existed at the site. They show that water-rock reactions created chemical conditions that are relevant to questions about ancient habitability.The ancient lake may have altered the rocks againThe second episode appears to have involved the ancient lake that once occupied Jezero Crater, although researchers say changing groundwater conditions may also have contributed.Some of the Margin Unit rocks contain silica, particularly in areas that were located below the former water line. The researchers propose that earlier reactions between olivine and water produced carbonate and left silica behind. Later exposure to lake water, or changes in groundwater chemistry, could have remobilised some of the carbonate and caused silica to precipitate in secondary spaces within the rock.This means the carbonate detected from orbit cannot simply be treated as evidence that the ancient lake directly created all of the deposits. Some of the carbonate had already formed through groundwater-driven alteration before later processes modified the rocks.A later event brought hot waterThe third episode was markedly different. Researchers found mineral veins in the eastern part of the Margin Unit containing calcium sulfate and fluorite. One of the veins was about 25 centimetres thick.Fluorite is particularly important because it can form when hot fluids circulate through volcanic rocks. Its presence therefore provides evidence that the Margin Unit experienced a later hydrothermal episode involving heated underground fluids.The researchers were able to establish the order of the three water-related events, but they cannot yet determine the precise ages of each episode. The sequence nevertheless shows that the Margin Unit was not simply a former lakeshore that was shaped by one period of surface water. It was affected by multiple fluid systems operating under different conditions.SuperCam reads the rocks from a distanceMuch of the evidence came from SuperCam, an instrument mounted on Perseverance's mast. It can study the chemistry and mineralogy of rocks by analysing the light produced when its laser strikes a target.The instrument can fire at rocks as far as 6.5 metres away. Perseverance has used SuperCam to analyse more than 185 bedrock targets across the Margin Unit, allowing researchers to compare the composition of rocks at different elevations and locations.The new study, led by Candice Bedford of Purdue University, uses those observations to reconstruct the alteration history of the area. The research was published in Communications Earth & Environment on September 21, 2026.What the discovery means for MarsThe findings could influence how scientists interpret carbonate-rich regions elsewhere on Mars. Jezero Crater lies within one of the planet's largest exposed carbonate deposits, so understanding how these minerals formed at the Margin Unit could help researchers reassess similar geological features.The discovery also adds another layer to the history of water at Jezero. Instead of a simple story involving an ancient lake, the rocks point to groundwater circulating through the crust, subsequent lake-related or groundwater alteration, and a later period of hydrothermal activity.For Perseverance, the broader goal remains astrobiology. The rover is studying Martian rocks and reconstructing the planet's ancient environments while collecting and caching samples for potential future return to Earth. The new results do not provide evidence that life existed on Mars, but they identify a geological setting where several different water-driven processes once operated, making the Margin Unit an important location for understanding the planet's past climate and potential habitability.