Summary
NASA’s Perseverance rover found that the Margin Unit in Jezero Crater records at least three separate episodes of water-rock interaction, including a later episode involving heated underground water. The findings revise the picture of how water moved through early Mars.
NASA’s Perseverance rover has found that a geologic area on the inner rim of Mars’ Jezero Crater was shaped by several different water systems rather than by a single ancient lake. The rocks in the area, known as the “Margin Unit,” record at least three episodes of water-rock interaction, including a later event in which hot underground water circulated through volcanic rock.
The findings, published in Communications Earth & Environment, come from more than 185 bedrock targets analysed by the rover’s SuperCam instrument. They provide a more complex account of how water moved through early Mars and may help scientists reconstruct the planet’s past climate and potential habitability.
An unexpected rock record
Perseverance reached the Margin Unit in September 2023. Because the area follows the shoreline of an ancient lake, scientists initially expected to find sedimentary rocks formed as layers of material accumulated on the lakebed. Orbiting spacecraft had also detected strong signals of carbonate minerals, which on Earth often form in shallow lakes and oceans.
Instead, the rover found igneous rock. These rocks formed from magma, either underground or through volcanic activity at the surface. Their mineral crystals can preserve details of when and how the rocks formed, as well as later chemical changes caused by water.
The Margin Unit extends across approximately 870 feet (265 metres) of elevation. At higher elevations, Perseverance found coarse-grained, crystalline rock dominated by olivine, a mineral containing magnesium and iron. The rock shows little evidence of contact with water. It likely formed slowly inside a deep underground body of magma and was exposed only after overlying material eroded away.
Lower down, near the ancient lakebed, the same broad rock unit appears heavily altered. Olivine grains have been fractured, and silica occurs between the fragments. This difference across the unit allowed researchers to reconstruct a sequence of water-related changes.
SuperCam helped make the analysis possible. Mounted on the rover’s mast, the instrument studies the light reflected by rocks to determine their mineralogy. It can also fire a laser at targets up to 21 feet (6.5 metres) away. The resulting plasma produces a spectrum that reveals the target’s chemical composition.
Three stages of water activity
The first identified water interaction involved carbon-dioxide-rich groundwater. It reacted with olivine and produced carbonate ridges inside fractures in the bedrock at lower elevations. As the softer surrounding rock eroded, the carbonate-filled fractures remained standing as ridges.
A second episode may have been connected to the lake that once occupied Jezero Crater. Some rocks contain silica, and the researchers report more silica in rocks that were below the ancient waterline. On Earth, reactions between water and olivine can release hydrogen, which some microbes use as an energy source, while also producing carbonate and silica that can preserve chemical traces of past environments.
The final identified event occurred in the eastern part of the Margin Unit. It created mineral veins about 10 inches (25 centimetres) thick, including minerals such as calcium sulfate and fluorite. Fluorite is an important clue because it typically forms when hot water circulates through volcanic rocks. Its presence indicates that the area later experienced a heated underground-water system, often described as hydrothermal activity.
The sequence matters because it shows that Jezero’s carbonate-bearing rocks were not shaped by one simple lake process. The site became a meeting point for groundwater, lake-related water and later heated fluids. Since Jezero lies within one of Mars’ largest exposed carbonate regions, the findings may also inform interpretations of carbonate deposits beyond this crater.
The observations concern mineral alteration and the history of water-rock interactions. Their significance for astrobiology is that they identify changing environments that could have affected habitability; the study does not report a detection of ancient microbial life. The team can determine the order of the water-related events, but the ages of those events cannot be established from the current observations.