Summary
NASA’s Perseverance rover found evidence that water altered rocks in Mars’ Jezero Crater on at least three separate occasions. The sequence includes carbon-dioxide-rich groundwater, a possible lake-related episode and later hot underground water.
NASA’s Perseverance rover has found evidence of at least three separate episodes of water activity in an area of Mars that scientists expected to contain lake sediments. Instead, rocks in the Margin Unit at the inner edge of Jezero Crater preserve a complicated history involving underground water, a possible lake-related episode and later hot water circulating through volcanic rock.
The findings were published in Communications Earth & Environment. They provide a more detailed record of how water moved through early Mars and altered its minerals over time.
Three episodes recorded in the Margin Unit
Perseverance reached the Margin Unit in September 2023. Because the region follows the shoreline of an ancient lake, scientists initially expected sedimentary rocks made from layers of sand and mud. Orbiting spacecraft had also detected strong signals of carbonate minerals, which on Earth often form in shallow lakes and oceans.
The rover instead found igneous rock. These rocks formed from magma, either underground or through volcanic activity, and their mineral crystals can preserve details about when and how they formed. Perseverance explored the unit across approximately 870 feet (265 metres) of elevation.
At higher elevations, it found coarse-grained, crystalline rock dominated by olivine, a mineral containing magnesium and iron. The rock showed little evidence of contact with water and appears to have formed from magma deep underground that cooled slowly before reaching the surface as overlying material eroded away.
Lower down, near the ancient lakebed, the olivine grains were fractured and surrounded by silica. The pattern records a sequence of chemical changes caused by water:
- Carbon-dioxide-rich groundwater first reacted with olivine, producing carbonate that filled fractures in the rock.
- A later episode may have been connected to the lake that once occupied Jezero Crater. Silica is more common in rocks that sat below the former waterline.
- A final event produced mineral veins in one eastern part of the unit. These veins are about 10 inches (25 centimetres) thick and include calcium sulfate and fluorite.
Fluorite is significant because it typically forms when hot water circulates through volcanic rocks. Its presence indicates that the area later experienced a heated underground-water system, distinct from the earlier groundwater and possible lake activity.
How Perseverance identified the minerals
The evidence came primarily from SuperCam, an instrument mounted on Perseverance’s mast. SuperCam analyses the light reflected from geological targets to help determine their mineralogy. When the rover identifies a target of interest, it can fire a laser at rocks up to 21 feet (6.5 metres) away. The resulting plasma emits a spectrum that reveals the target’s chemical composition.
Using this method, the rover team analysed more than 185 bedrock targets across the Margin Unit. The measurements allowed scientists to identify the mineral combinations and reconstruct the order in which water altered the rocks.
Carbonate and silica are important to astrobiology because water interacting with olivine on Earth can release hydrogen, which some microbes can use as an energy source. The same reactions also leave carbonate and silica that can preserve traces of past biological activity. In the Margin Unit, the minerals offer evidence about ancient water chemistry and environments that could have supported microbial life, while the rover’s reported discovery itself concerns the geological record of water.
The results also broaden the significance of the site beyond Jezero Crater. The Margin Unit is part of one of Mars’ largest exposures of carbonate-bearing rock, so its history may help scientists interpret other carbonate deposits across the planet.
The team can establish the sequence of the water-related changes, but it cannot yet determine the ages of the individual events. The connection between the second episode and Jezero’s former lake is also presented as a possibility rather than a settled conclusion.